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Learning Objectives
• Outline the movement of water and minerals in the xylem
Movement of Water and Minerals in the Xylem
Most plants obtain the water and minerals they need through their roots. The path taken is: soil -> roots -> stems -> leaves. The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells). Water and minerals enter the root by separate paths which eventually converge in the stele, or central vascular bundle in roots.
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf, or atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. However, this value varies greatly depending on the vapor pressure deficit, which can be insignificant at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata close and transpiration stops, the water is held in the stem and leaf by the cohesion of water molecules to each other as well as the adhesion of water to the cell walls of the xylem vessels and tracheids. This is called the cohesion–tension theory of sap ascent.
The cohesion-tension theory explains how water moves up through the xylem. Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to the internal air space and the water on the surface of the cells evaporates into the air spaces. This decreases the thin film on the surface of the mesophyll cells. The decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that form via a process called cavitation. The formation of gas bubbles in the xylem is detrimental since it interrupts the continuous stream of water from the base to the top of the plant, causing a break (embolism) in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water in a continuous column, increasing the number of cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
Control of Transpiration
Transpiration is a passive process: metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled. The atmosphere to which the leaf is exposed drives transpiration, but it also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.
Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.
Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant (xerophytes) and plants that grow on other plants ( epiphytes ) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.
Xerophytes and epiphytes often have a thick covering of trichomes or stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and other substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.
Key Points
• The cohesion – tension theory of sap ascent explains how how water is pulled up from the roots to the top of the plant.
• Evaporation from mesophyll cells in the leaves produces a negative water potential gradient that causes water and minerals to move upwards from the roots through the xylem.
• Gas bubbles in the xylem can interrupt the flow of water in the plant, so they must be reduced through small perforations between vessel elements.
• Transpiration is controlled by the opening and closing of stomata in response to environmental cues.
• Stomata must open for photosynthesis and respiration, but when stomata are open, water vapor is lost to the external environment, increasing the rate of transpiration.
• Desert plants and plants with limited water access prevent transpiration and excess water loss by utilizing a thicker cuticle, trichomes, or multiple epidermal layers.
Key Terms
• cohesion–tension theory of sap ascent: explains the process of water flow upwards (against the force of gravity) through the xylem of plants
• cavitation: the formation, in a fluid, of vapor bubbles that can interrupt water flow through the plant
• trichome: a hair- or scale-like extension of the epidermis of a plant | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.15%3A_Transport_of_Water_and_Solutes_in_Plants_-_Movement_of_Water_and_Minerals_in_the_Xylem.txt |
Learning Objectives
• Explain the transport of photosynthates in the phloem
Transportation of Photosynthates in the Phloem
Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly-developing plants. Once green shoots and leaves begin to grow, plants can produce their own food by photosynthesis. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.
Sources and Sinks
Sources are the structures that produce photosynthates for the growing plant. The sugars produced in the sources, such as leaves, must be delivered to growing parts of the plant. These sugars are transported through the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.
The products from the source are usually translocated to the nearest sink through the phloem. For example, photosynthates produced in the upper leaves will travel upward to the growing shoot tip, while photosynthates in the lower leaves will travel downward to the roots. Intermediate leaves will send products in both directions. The multidirectional flow of phloem contrasts the flow of xylem, which is always unidirectional (soil to leaf to atmosphere). However, the pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots during early development, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.
Translocation: Transport from Source to Sink
Photosynthates are produced in the mesophyll cells of photosynthesizing leaves. From there, they are translocated through the phloem where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through plasmodesmata to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H+ symporter.
Phloem STEs have reduced cytoplasmic contents and are connected by sieve plates with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs.
Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψs, which decreases the total water potential, causing water to move by osmosis from the adjacent xylem into the phloem tubes. This flow of water increases water pressure inside the phloem, causing the bulk flow of phloem sap from source to sink. Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth or converted to starch (for storage) or other polymers (for structural integrity). Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.
Key Points
• The products of photosynthesis are called photosynthates; they are usually in the form of simple sugars, such as sucrose.
• Photosynthates are produced by sources and are translocated to sinks.
• Photosynthates are directed primarily to the roots during early development, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development.
• Photosynthates are produced in the mesophyll cells of leaves and are translocated through the phloem; they are then transported to STEs and translocated to the nearest sink.
• The high percentage of sugar in phloem sap causes water to move from the xylem into the phloem, which increases water pressure inside the phloem, causing the sap to move from source to sink.
• Sucrose concentration in the sink cells is lower than in the phloem STEs, so unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration.
Key Terms
• source: structure that produces photosynthates
• photosynthate: any compound that is a product of photosynthesis
• sieve-tube element: a type of plant cell located in the phloem that is involved in the movement of carbohydrates
• sink: where sugars are delivered in a plant, such as the roots, young shoots, and developing seeds
30.17: Plant Sensory Systems and Responses - Plant Responses to Light
Learning Objectives
• Compare the ways plants respond to light
Plant Responses to Light
Plants have a number of sophisticated uses for light that go far beyond their ability to perform photosynthesis. Plants can differentiate and develop in response to light (known as photomorphogenesis), which allows plants to optimize their use of light and space. Plants use light to track time, which is known as photoperiodism. They can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Light can also elicit a directional response in plants that allows them to grow toward, or even away from, light; this is known as phototropism.
The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.
The red, far-red, and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.
Key Points
• Plants grow and differentiate to optimize their space, using light in a process known as photomorphogenesis.
• Plants grow and move toward or away from light depending on their needs; this process is known as phototropism.
• Photoperiodism is illustrated by how plants flower and grow at certain times of the day or year through the use of photoreceptors that sense the wavelengths of sunlight available during the day (versus night) and throughout the seasons.
• The various wavelengths of light, red/far-red or blue regions of the visible light spectrum, trigger structural responses in plants suited for responding to those wavelengths.
Key Terms
• photoreceptor: a specialized protein that is able to detect and react to light
• photoperiodism: the growth, development and other responses of plants and animals according to the length of day and/or night
• photomorphogenesis: the regulatory effect of light on the growth, development and differentiation of plant cells, tissues and organs
• phototropism: the movement of a plant toward or away from light | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.16%3A_Transport_of_Water_and_Solutes_in_Plants_-_Transportation_of_Photosynthates_in_the_Phloem.txt |
Learning Objectives
• Explain the response of the phytochrome system to red/far-red light
The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically-active form of the protein; exposure to red light yields physiological activity in the plant. Exposure to far-red light converts the Pfr to the inactive Pr form, inhibiting phytochrome activity. Together, the two forms represent the phytochrome system.
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.
The Phytochrome System and Growth
Plants use the phytochrome system to grow away from shade and toward light. Unfiltered, full sunlight contains much more red light than far-red light. Any plant in the shade of another plant will be exposed to red-depleted, far-red-enriched light because the other plant has absorbed most of the other red light. The exposure to red light converts phytochrome in the shaded leaves to the Pr (inactive) form, which slows growth. The leaves in full sunlight are exposed to red light and have activated Pfr, which induces growth toward sunlit areas. Because competition for light is so fierce in a dense plant community, those plants who could grow toward light the fastest and most efficiently became the most successful.
The Phytochrome System in Seeds
In seeds, the phytochrome system is used to determine the presence or absence of light, rather than the quality. This is especially important in species with very small seeds and, therefore, food reserves. For example, if lettuce seedlings germinated a centimeter under the soil surface, the seedling would exhaust its food resources and die before reaching the surface. A seed will only germinate if exposed to light at the surface of the soil, causing Pr to be converted to Pfr, signaling the start of germination. In the dark, phytochrome is in the inactive Pr form so the seed will not germinate.
Photoperiodism
Plants also use the phytochrome system to adjust growth according to the seasons. Photoperiodism is a biological response to the timing and duration of dark and light periods. Since unfiltered sunlight is rich in red light, but deficient in far-red light, at dawn, all the phytochrome molecules in a leaf convert to the active Pfr form and remain in that form until sunset. Since Pfr reverts to Pr during darkness, there will be no Pfr remaining at sunrise if the night is long (winter) and some Pfr remaining if the night is short (summer). The amount of Pfr present stimulates flowering, setting of winter buds, and vegetative growth according to the seasons.
In addition, the phytochrome system enables plants to compare the length of dark periods over several days. Shortening nights indicate springtime to the plant; lengthening nights indicate autumn. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this information to flower in the late summer and early fall when nights exceed a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring when darkness is less than a critical length (often 8 to 15 hours). However, day-neutral plants do not regulate flowering by day length. Not all plants use the phyotochrome system to adjust their physiological responses to the seasons.
Key Points
• Exposure to red light converts the chromoprotein to the functional, active form (Pfr), while darkness or exposure to far-red light converts the chromophore to the inactive form (Pr).
• Plants grow toward sunlight because the red light from the sun converts the chromoprotein into the active form (Pfr), which triggers plant growth; plants in shade slow growth because the inactive form (Pr) is produced.
• If seeds sense light using the phytochrome system, they will germinate.
• Plants regulate photoperiodism by measuring the Pfr/Pr ratio at dawn, which then stimulates physiological processes such as flowering, setting winter buds, and vegetative growth.
Key Terms
• phytochrome: any of a class of pigments that control most photomorphogenic responses in higher plants
• chromophore: the group of atoms in a molecule in which the electronic transition responsible for a given spectral band is located
• photoperiodism: the growth, development and other responses of plants and animals according to the length of day and/or night | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.18%3A_Plant_Sensory_Systems_and_Responses_-_The_Phytochrome_System_and_Red_Light_Response.txt |
Learning Objectives
• Differentiate among blue light responses of plants
Phototropism is the directional bending of a plant toward or away from a light source of blue wavelengths of light. Positive phototropism is growth toward a light source, while negative phototropism (also called skototropism) is growth away from light. Several proteins use blue light to control various physiological processes in the plant.
Phototropins and Physiological Responses
Phototropins are protein-based receptors responsible for mediating the phototropic response in plants. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore, which senses blue wavelengths of light. Phototropins belong to a class of proteins called flavoproteins because the chromophore is a covalently-bound molecule of flavin.
Phototropins control other physiological responses including leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.
Phototropism and Auxin
In 1880, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the the apical meristem (tip of the plant), but that the plant bent in response in a different part of the plant. The Darwins concluded that the signal had to travel from the apical meristem to the base of the plant, where it bent.
In 1913, Peter Boysen-Jensen conducted an experiment that demonstrated that a chemical signal produced in the plant tip was responsible for the plant’s bending response at the base. He cut off the tip of a seedling, covered the cut section with a permeable layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated even though the layer of gelatin was present. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend.
A refinement of Boysen-Jensen’s experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant still bent toward the light. Therefore, the chemical signal from the sunlight, which is blue wavelengths of light, was a growth stimulant; the phototropic response involved faster cell elongation on the shaded side than on the illuminated side, causing the plant to bend. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormones indole acetic acid (IAA) or auxin to accumulate on the shaded side. Stem cells elongate under the influence of IAA.
Cryptochromes
Cryptochromes are another class of blue-light absorbing photoreceptors. Their chromophores also contain a flavin-based chromophore. Cryptochromes set the plant’s circadian rhythm (the 24-hour activity cycle) using blue light receptors. There is some evidence that cryptochromes work by sensing light-dependent redox reactions and that, together with phototropins, they mediate the phototropic response.
Key Points
• In addition to phototropism, phototropins sense blue light to control leaf opening and closing, chloroplast movement, and the opening of stomata.
• When phototropins are activated by blue light, the hormone auxin accumulates on the shaded side of the plant, triggering elongation of stem cells and phototropism.
• Cryptochromes sense blue light-dependent redox reactions to control the circadian rhythm of plants.
Key Terms
• skototropism: growth or movement away from light
• phototropin: any of a class of photoreceptor flavoproteins that mediate phototropism in higher plants
• auxin: a class of plant growth hormones that is responsible for elongation in phototropism and gravitropism and for other growth processes in the plant life cycle
• cryptochrome: any of several light-sensitive flavoproteins, in the protoreceptors of plants, that regulate germination, elongation, and photoperiodism | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.19%3A_Plant_Sensory_Systems_and_Responses_-_Blue_Light_Response.txt |
Learning Objectives
• Describe the role of amyloplasts in gravitropism
Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, while roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.
Time-lapse of pea shoot and root growth: Time-lapse of a pea plant growing from seed, showing both the shoot and root system. The roots grown downward in the direction of gravity, which is positive gravitropism, and the shoot grows upward away from gravity, which is negative gravitropism.
The reason plants know which way to grow in response to gravity is due to amyloplasts in the plants. Amyloplasts (also known as statoliths ) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.
The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER). This causes the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone indole acetic acid (IAA) to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion and causes the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses, which involve the entire cell in the gravitropism effect, have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.
Key Points
• Positive gravitropism occurs when roots grow into soil because they grow in the direction of gravity while negative gravitropism occurs when shoots grow up toward sunlight in the opposite direction of gravity.
• Amyloplasts settle at the bottom of the cells of the shoots and roots in response to gravity, causing calcium signaling and the release of indole acetic acid.
• Indole acetic acid inhibits cell elongation in the lower side of roots, but stimulates cell expansion in shoots, which causes shoots to grow upward.
Key Terms
• amyloplast: a non-pigmented organelle found in some plant cells that is responsible for the synthesis and storage of starch granules through the polymerization of glucose
• statolith: a specialized form of amyloplast involved in graviperception by plant roots and most invertebrates
• gravitropism: a plant’s ability to change its growth in response to gravity | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.20%3A_Plant_Sensory_Systems_and_Responses_-_Plant_Responses_to_Gravity.txt |
Learning Objectives
• Differentiate among the types of plant hormones and their effects on plant growth
Growth Responses
A plant’s sensory response to external stimuli relies on hormones, which are simply chemical messengers. Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially, every cell in a plant can produce plant hormones. The hormones can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, acting alone.
Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins, cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors. The first three plant hormones largely affect plant growth, as described below.
Auxins
The term auxin is derived from the Greek word auxein, which means “to grow. ” Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, indole acetic acid (IAA) is the only naturally-occurring auxin that shows physiological activity. Apical dominance (the inhibition of lateral bud formation) is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.
Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping, which coordinates the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.
Cytokinins
The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally-occurring or synthetic cytokinins are known, to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds and cytokinins that promote bushier growth.
Gibberellins
Gibberellins (GAs) are a group of about 125 closely-related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.
GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods; they contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the incidence of mildew infection.
Key Points
• During phototropism and gravitropism, the plant hormone auxin controls cell elongation.
• The plant hormone cytokinin promotes cell division, controling many developmental processes in plants.
• Gibberellins control many aspects of plant physiology including shoot elongation, seed germination, fruit and flower maturation, seed dormancy, gender expression, seedless fruit development, and the delay of senescence in leaves and fruit.
Key Terms
• gibberellin: any of a class of diterpene plant growth hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation
• auxin: a class of plant growth hormones that is responsible for elongation in phototropism and gravitropism and for other growth processes in the plant life cycle
• cytokinin: any of a class of plant hormones involved in cell growth and division | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.21%3A_Plant_Sensory_Systems_and_Responses_-_Auxins_Cytokinins_and_Gibberellins.txt |
Learning Objectives
• Describe the roles played by ethylene and nontraditional hormones in plant development
Growth Responses
In addition to the growth hormones auxins, cytokinins, gibberellins, there are two more major types of plant hormones, abscisic acid and ethylene, as well as several other less-studied compounds that control plant physiology.
Abscisic Acid
The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more-recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds.
ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.
Ethylene
Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C2H4). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks, shedding their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps.
Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocados, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.
Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.
Nontraditional Hormones
Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.
Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.
Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury; they can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplify their physiological effect. Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids.
Key Points
• Under stress, abscisic acid accumulates in plants, inhibiting stem elongation and inducing bud dormancy.
• The plant hormone ethylene controls fruit ripening, flower wilting, and leaf fall by stimulating the conversion of starch and acids to sugars.
• Other nontraditional hormones such as jasmonates and oligosaccharins control defense responses from herbivores and bacterial/fungal infections, respectively.
Key Terms
• abscisic acid: a plant hormone that functions in many plant developmental processes, including bud dormancy, inhibition of seed germination, and plant stress tolerance.
• jasmonate: any of several esters of jasmonic acid that act as plant hormones
• ethylene: a plant hormone that is involved in fruit ripening, flower wilting, and leaf fall | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.22%3A_Plant_Sensory_Systems_and_Responses_-_Abscisic_Acid_Ethylene_and_Nontraditional_Hormones.txt |
Learning Objectives
• Compare the ways plants respond to directional and non-directional stimuli
The shoot of a pea plant wraps around a trellis while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.
The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism, implying “direction.” Tendrils are one example of this. A tendril is a specialized stem, leaf, or petiole with a threadlike shape that is used by climbing plants for support.The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.
A thigmonastic response is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin, fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.
Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain from wind, rain, or movement by other living things induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.
Key Points
• When subjected to constant directional pressure, such as a trellis, plants move to grow around the object providing the pressure; this process is known as thigmotropism.
• Thigmonastic responses include opening and closing leaves, petals, or other parts of the plant as a reaction to touch.
• Through thigmomorphogenesis, plants change their growth in response to repeated mechanical stress from wind, rain, or other living things.
Key Terms
• thigmotropism: plant growth or motion in response to touch
• thigmomorphogenesis: the response by plants to mechanical sensation (touch) by altering their growth patterns
• thigmonastic response: a touch response independent of the direction of stimulus
30.24: Plant Defense Mechanisms - Against Herbivores
Learning Objectives
• Identify plant defense responses to herbivores
Defense Responses Against Herbivores
Herbivores, both large and small, use plants as food and actively chew them. Plants have developed a variety of strategies to discourage or kill attackers.
Mechanical Defenses
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against herbivores. Other adaptations against herbivores include hard shells, thorns (modified branches), and spines (modified leaves). They discourage animals by causing physical damage or by inducing rashes and allergic reactions. Some Acacia tree species have developed mutualistic relationships with ant colonies: they offer the ants shelter in their hollow thorns in exchange for the ants’ defense of the tree’s leaves.
Chemical Defenses
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development.
Many metabolites are toxic and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides. Ingestion can cause nausea, vomiting, hallucinations, convulsions, or death.
Timing
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours. In addition, long-distance signaling elicits a systemic response aimed at deterring predators. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids: insects that spend their developing stages in or on another insect, eventually killing their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from herbivores.
• If herbivores breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the herbivore.
• When attacked by a predator, damaged plant tissue releases jasmonate hormones that promote the release of volatile compounds, attracting parasitoids, which use, and eventually kill, the predators as host insects.
30.25: Plant Defense Mechanisms - Against Pathogens
Plants defend against pathogens with barriers, secondary metabolites, and antimicrobial compounds.
Learning Objectives
• Identify plant defense responses to pathogens
Defense Responses Against Pathogens
Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against pathogens.
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic and can even be lethal to animals that ingest them.
Additionally, plants have a variety of inducible defenses in the presence of pathogens. In addition to secondary metabolites, plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens. Plants can close stomata to prevent the pathogen from entering the plant. A hypersensitive response, in which the plant experiences rapid cell death to fight off the infection, can be initiated by the plant; or it may use endophyte assistance: the roots release chemicals that attract other beneficial bacteria to fight the infection.
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from pathogens.
• If pathogens breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the pathogen.
• Plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens.
Contributions and Attributions
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_defense_against_herbivory. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/30%3A_Plant_Form_and_Physiology/30.23%3A_Plant_Sensory_Systems_and_Responses_-_Plant_Responses_to_Wind_and_Touch.txt |
Learning Objectives
• Describe how the nutritional requirements of plants are met
Introduction
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow. In order to develop into mature, fruit -bearing plants, many requirements must be met and events must be coordinated.
Take for example the Cucurbitaceae family of plants that were the first cultivated in Mesoamerica, although several species are native to North America. The family includes many edible species, such as squash and pumpkin, as well as inedible gourds. First, seeds must germinate under the right conditions in the soil; therefore, temperature, moisture, and soil quality are important factors that play a role in germination and seedling development. Soil quality and climate are significant to plant distribution and growth. Second, the young seedling will eventually grow into a mature plant with the roots absorbing nutrients and water from the soil. At the same time, the aboveground parts of the plant will absorb carbon dioxide from the atmosphere and use energy from sunlight to produce organic compounds through photosynthesis. Finally, the fruit are grown and matured and the cycle begins all over again with the new seeds.
There is a complex dynamic between plants and soils that ultimately determines the outcome and viability of plant life. The following sections of this chapter will discuss the many aspects of the nutritional requirements of plants in greater detail.
Key Points
• Nutrients and water are absorbed through the plants root system.
• Carbon dioxide is absorbed through the leaves.
• From seedling to mature plant, there is a complex dynamic between plants and their environment (soil and atmosphere).
Key Terms
• germinate: to sprout or produce buds
• photosynthesis: the process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts
• nutrient: a source of nourishment, such as food, that can be metabolized by an organism to give energy and build tissue
31.1B: The Chemical Composition of Plants
Plants are composed of water, carbon-containing organics, and non-carbon-containing inorganic substances such as potassium and nitrogen.
Learning Objectives
• Describe the chemical composition of plants
Key Points
• Water comprises a large percentage of a plant’s total weight and is used to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.
• Water is absorbed from the soil through root hairs and is carried to the rest of the plant through the xylem.
• Many essential organic and inorganic nutrients are required to sustain plant life.
Key Terms
• organic: relating to the compounds of carbon, relating to natural products
• inorganic: relating to a compound that does not contain carbon
• xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood
• transpiration: the loss of water by evaporation in terrestrial plants, especially through the stomata; accompanied by a corresponding uptake from the roots
Water
Since plants require nutrients in the form of elements such as carbon and potassium, it is important to understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight. Soil is the water source for land plants. It can be an abundant source of water even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves. Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.
Nutrients
Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An organic compound is a chemical compound that contains carbon, such as carbon dioxide obtained from the atmosphere. Carbon that was obtained from atmospheric CO2 composes the majority of the dry mass within most plants. An inorganic compound does not contain carbon and is not part of, or produced by, a living organism. Inorganic substances (which form the majority of the soil substance) are commonly called minerals: those required by plants include nitrogen (N) and potassium (K), for structure and regulation. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.01%3A_Nutritional_Requirements_of_Plants/31.1A%3A_Plant_Nutrition.txt |
Approximately 20 macronutrients and micronutrients are deemed essential nutrients to support all the biochemical needs of plants.
Learning Objectives
• Distinguish among the essential nutrients for plants
Key Points
• An element is essential if a plant cannot complete its life cycle without it, if no other element can perform the same function, and if it is directly involved in nutrition.
• An essential nutrient required by the plant in large amounts is called a macronutrient, while one required in very small amounts is termed a micronutrient.
• Missing or inadequate supplies of nutrients adversely affect plant growth, leading to stunted growth, slow growth, chlorosis, or cell death.
• About half the essential nutrients are micronutrients such as boron, chlorine, manganese, iron, zinc, copper, molybdenum, nickel, silicon, and sodium.
Key Terms
• micronutrient: a mineral, vitamin, or other substance that is essential, even in very small quantities, for growth or metabolism
• chlorosis: a yellowing of plant tissue due to loss or absence of chlorophyll
• macronutrient: any of the elements required in large amounts by all living things
Essential Nutrients
Plants require only light, water, and about 20 elements to support all their biochemical needs. These 20 elements are called essential nutrients. For an element to be regarded as essential, three criteria are required:
1. a plant cannot complete its life cycle without the element
2. no other element can perform the function of the element
3. the element is directly involved in plant nutrition
Macronutrients and Micronutrients
The essential elements can be divided into macronutrients and micronutrients. Nutrients that plants require in larger amounts are called macronutrients. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is, therefore, present in all macromolecules. On average, the dry weight (excluding water) of a cell is 50 percent carbon, making it a key part of plant biomolecules.
The next-most-abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. Hydrogen and oxygen are macronutrients that are part of many organic compounds and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Light energy is converted into chemical energy during photophosphorylation in photosynthesis; and into chemical energy to be extracted during respiration. Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain where hydrogen gradients are key in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.
Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, also contribute to the plant’s ionic balance.
In addition to macronutrients, organisms require various elements in small amounts. These micronutrients, or trace elements, are present in very small quantities. The seven main micronutrients include boron, chlorine, manganese, iron, zinc, copper, and molybdenum. Boron (B) is believed to be involved in carbohydrate transport in plants; it also assists in metabolic regulation. Boron deficiency will often result in bud dieback. Chlorine (Cl) is necessary for osmosis and ionic balance; it also plays a role in photosynthesis. Copper (Cu) is a component of some enzymes. Symptoms of copper deficiency include browning of leaf tips and chlorosis (yellowing of the leaves). Iron (Fe) is essential for chlorophyll synthesis, which is why an iron deficiency results in chlorosis. Manganese (Mn) activates some important enzymes involved in chlorophyll formation. Manganese-deficient plants will develop chlorosis between the veins of its leaves. The availability of manganese is partially dependent on soil pH. Molybdenum (Mo) is essential to plant health as it is used by plants to reduce nitrates into usable forms. Some plants use it for nitrogen fixation; thus, it may need to be added to some soils before seeding legumes. Zinc (Zn) participates in chlorophyll formation and also activates many enzymes. Symptoms of zinc deficiency include chlorosis and stunted growth.
Deficiencies in any of these nutrients, particularly the macronutrients, can adversely affect plant growth. Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis. Extreme deficiencies may result in leaves showing signs of cell death. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.01%3A_Nutritional_Requirements_of_Plants/31.1C%3A_Essential_Nutrients_for_Plants.txt |
Learning Objectives
• Explain soil composition
Soil Composition
Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. Soil is the outer, loose layer that covers the surface of Earth. Soil quality, a major determinant, along with climate, of plant distribution and growth, depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms.
Soil consists of these major components:
• inorganic mineral matter, about 40 to 45 percent of the soil volume
• organic matter, about 5 percent of the soil volume
• water, about 25 percent of the soil volume
• air, about 25 percent of the soil volume
The amount of each of the four major components of soil depends on the quantity of vegetation, soil compaction, and water present in the soil. A good, healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.
The organic material of soil, called humus, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure, providing plants with water and minerals. The inorganic material of soil is composed of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant particle size, containing a mixture of sand, silt, and humus; these soils are called loams.
Key Points
• The chemical composition of the soil, the topography, and the presence of living organisms determines the quality of soil.
• In general, soil contains 40-45% inorganic matter, 5% organic matter, 25% water, and 25% air.
• In order to sustain plant life, the proper mix of air, water, minerals, and organic material is required.
• Humus, the organic material in soil, is composed of microorganisms (dead and alive) and decaying plants.
• The inorganic material of soil is composed of rock, which is broken down into small particles of sand (0.1 to 2 mm), silt (0.002 to 0.1 mm), and clay (less than 0.002 mm).
• Loam is a soil that is a mix sand, silt, and humus.
Key Terms
• loam: soil with no dominant particle size that contains a mixture of sand, silt, and humus
• humus: a large group of natural organic compounds found in the soil composed of decaying plants and dead and living microorganisms
31.2B: Soil Formation
Learning Objectives
• Describe the five factors that account for soil formation
Soil Formation
Soil formation is the consequence of a combination of biological, physical, and chemical processes. Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, while the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms, bacteria, algae, or fungi, are dormant in dry soil, but become active once moisture is available.
Soil distribution is not homogenous because its formation results in the production of layers; the vertical section of the layers of soil is called the soil profile. Within the soil profile, soil scientists define zones called horizons: a soil layer with distinct physical and chemical properties that differ from those of other layers. Five factors account for soil formation: parent material, climate, topography, biological factors, and time.
Parent Material
The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil; therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay.
Climate
Temperature, moisture, and wind cause different patterns of weathering, which affect soil characteristics. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil.
Topography
Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steep soils are more prone to erosion and may be thinner than soils that are relatively flat or level.
Biological factors
The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices. Plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root in an area known as the rhizosphere. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.
Time
Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.
Key Points
• Parent material is the organic and inorganic material from which soil is formed.
• Climate factors, such as temperature and wind, affect soil formation and its characteristics; the presence of moisture and nutrients is also needed to form a quality soil.
• Topography, or regional surface features, affects water runoff, which strips away parent material and affects plant growth (the steeper the soil, the more erosion takes place).
• The presence of microorganisms in soil creates pores and crevices; plants promote the presence of microorganisms and contribute to soil formation.
• Soil formation takes place over long periods of time.
Key Terms
• rhizosphere: the soil region subject to the influence of plant roots and their associated microorganisms
• bedrock: the solid rock that exists at some depth below the ground surface
• horizon: a soil layer with distinct physical and chemical properties that differ from those of other layers
31.2C: Physical Properties of Soil
Learning Objectives
• Describe the physical properties or profile of soil
Physical Properties of the Soil
Soils are named and classified based on their horizons. The soil profile has four distinct layers:
1. The O horizon has freshly-decomposing organic matter, humus, at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients, enhancing soil moisture retention. Topsoil, the top layer of soil, is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas, such as the Mississippi River delta, have deep layers of topsoil. Topsoil is rich in organic material. Microbial processes occur there; it is responsible for plant production.
2. The A horizon consists of a mixture of organic material with inorganic products of weathering; it is the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface.
3. The B horizon, or subsoil, is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate.
4. The C horizon, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.
Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, depending on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers.
Key Points
• The O horizon, or topsoil, is made of decaying organisms and plant life; it is responsible for plant production.
• The A horizon is of a mixture of organic material and inorganic products of weathering; it is the beginning of true mineral soil.
• The B horizon, or subsoil, is a dense layer of mostly fine material that has been pushed down from the topsoil.
• The C horizon, or soil base, is located just above bedrock and is made of parent, organic, and inorganic material.
Key Terms
• topsoil: top layer of soil containing humus at its surface and decomposing vegetation at its base; the most fertile soil
• subsoil: dense layer of soil containing fine material that has moved downward; the layer of earth that is below the topsoil | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.02%3A_The_Soil/31.2A%3A_Soil_Composition.txt |
Learning Objectives
• Explain the process and importance of nitrogen fixation
Nitrogen is an important macronutrient because it is part of nucleic acids and proteins. Atmospheric nitrogen, which is the diatomic molecule N2, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to convert it into biologically useful forms. However, nitrogen can be “fixed.” It can be converted to ammonia (NH3) through biological, physical, or chemical processes. Biological nitrogen fixation (BNF), the conversion of atmospheric nitrogen (N2) into ammonia (NH3), is exclusively carried out by prokaryotes, such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture.
The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants, including many crops important to humans. The NH3 resulting from fixation can be transported into plant tissue and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, such as soybeans and peanuts, contain high levels of protein and are among the most important agricultural sources of protein in the world.
Soil bacteria, collectively called rhizobia, symbiotically interact with legume roots to form specialized structures called nodules in which nitrogen fixation takes place. This process entails the reduction of atmospheric nitrogen to ammonia by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and environmentally-friendly way to fertilize plants as opposed to chemical fertilization that uses a non-renewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically-available nitrogen. As in any symbiosis, both organisms benefit from the interaction: the plant obtains ammonia and bacteria obtain carbon compounds generated through photosynthesis, as well as a protected niche in which to grow.
Key Points
• Diatomic nitrogen is abundant in the atmosphere and soil, but plants are unable to use it because they do not have the necessary enzyme, nitrogenase, to convert it into a form that they can use to make proteins.
• Soil bacteria, or rhizobia, are able to perform biological nitrogen fixation in which atmospheric nitrogen gas (N2) is converted into the ammonia (NH3) that plants are able to use to synthesize proteins.
• Both the plants and the bacteria benefit from the process of nitrogen fixation; the plant obtains the nitrogen it needs to synthesize proteins, while the bacteria obtain carbon from the plant and a secure environment to inhabit within the plant roots.
Key Terms
• rhizobia: any of various bacteria, of the genus Rhizobium, that form nodules on the roots of legumes and fix nitrogen
• nitrogen fixation: the conversion of atmospheric nitrogen into ammonia and organic derivatives, by natural means, especially by microorganisms in the soil, into a form that can be assimilated by plants
• nodule: structures that occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria
31.3B: Mycorrhizae- The Symbiotic Relationship between Fungi and Roots
Learning Objectives
• Describe the symbiotic relationship of mycorrhizae and plant roots
Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
A nutrient depletion zone can develop when there is rapid soil solution uptake, low nutrient concentration, low diffusion rate, or low soil moisture. These conditions are very common; therefore, most plants rely on fungi to facilitate the uptake of minerals from the soil. Mycorrhizae, known as root fungi, form symbiotic associations with plant roots. In these associations, the fungi are actually integrated into the physical structure of the root. The fungi colonize the living root tissue during active plant growth.
Through mycorrhization, the plant obtains phosphate and other minerals, such as zinc and copper, from the soil. The fungus obtains nutrients, such as sugars, from the plant root. Mycorrhizae help increase the surface area of the plant root system because hyphae, which are narrow, can spread beyond the nutrient depletion zone. Hyphae are long extensions of the fungus, which can grow into small soil pores that allow access to phosphorus otherwise unavailable to the plant. The beneficial effect on the plant is best observed in poor soils. The benefit to fungi is that they can obtain up to 20 percent of the total carbon accessed by plants. Mycorrhizae function as a physical barrier to pathogens. They also provides an induction of generalized host defense mechanisms, which sometimes involves the production of antibiotic compounds by the fungi. Fungi have also been found to have a protective role for plants rooted in soils with high metal concentrations, such as acidic and contaminated soils.
There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae. Ectomycorrhizae form an extensive dense sheath around the roots, called a mantle. Hyphae from the fungi extend from the mantle into the soil, which increases the surface area for water and mineral absorption. This type of mycorrhizae is found in forest trees, especially conifers, birches, and oaks. Endomycorrhizae, also called arbuscular mycorrhizae, do not form a dense sheath over the root. Instead, the fungal mycelium is embedded within the root tissue. Endomycorrhizae are found in the roots of more than 80 percent of terrestrial plants.
Key Points
• Because nutrients are often depleted in the soil, most plants form symbiotic relationships called mycorrhizae with fungi that integrate into the plant’s root.
• The relationship between plants and fungi is symbiotic because the plant obtains phosphate and other minerals through the fungus, while the fungus obtains sugars from the plant root.
• The long extensions of the fungus, called hyphae, help increase the surface area of the plant root system so that it can extend beyond the area of nutrient depletion.
• Ectomycorrhizae are a type of mycorrhizae that form a dense sheath around the plant roots, called a mantle, from which the hyphae grow; in endomycorrhizae, mycelium is embedded within the root tissue, as opposed to forming a sheath around it.
• In endomycorrhizae, mycelium is embedded within the root tissue, as opposed to forming a sheath around it; these are found in the roots of most terrestrial plants.
Key Terms
• mycorrhiza: a symbiotic association between a fungus and the roots of a vascular plant
• hypha: a long, branching, filamentous structure of a fungus that is the main mode of vegetative growth
• mycelium: the vegetative part of any fungus, consisting of a mass of branching, threadlike hyphae, often underground | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.03%3A__Nutritional_Adaptations_of_Plants/31.3A%3A_Nitrogen_Fixation-_Root_and_Bacteria_Interactions.txt |
Learning Objectives
• Differentiate among the sources of plant nutrition
Nutrients from Other Sources
Some plants cannot produce their own food and must obtain their nutrition from outside sources. This may occur with plants that are parasitic or saprophytic: ingesting and utilizing dead matter as a food source. In other cases, plants may be mutualistic symbionts, epiphytes, or insectivorous.
Plant Parasites
A parasitic plant depends on its host for survival. Some parasitic plants have no leaves. An example of this is the dodder, which has a weak, cylindrical stem that coils around the host and forms suckers. From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host. The parasitic plant obtains water and nutrients through these connections. The plant is a total parasite (a holoparasite) because it is completely dependent on its host. Other parasitic plants, called hemiparasites, are fully photosynthetic and only use the host for water and minerals. There are about 4,100 species of parasitic plants.
Saprophytes
A saprophyte is a plant that does not have chlorophyll, obtaining its food from dead matter, similar to bacteria and fungi. (Note that fungi are often called saprophytes, which is incorrect, because fungi are not plants). Plants such as these use enzymes to convert organic food materials into simpler forms from which they can absorb nutrients. Most saprophytes do not directly digest dead matter. Instead, they parasitize mycorrhizae or other fungi that digest dead matter, ultimately obtaining photosynthate from a fungus that derived photosynthate from its host. Saprophytic plants are uncommon with only a few, described species.
Symbionts
A symbiont is a plant in a symbiotic relationship with other organisms, such as mycorrhizae (with fungi) or nodule formation. Root nodules occur on plant roots (primarily Fabaceae) that associate with symbiotic, nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA, as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones.
Fungi also form symbiotic associations with cyanobacteria and green algae; the resulting symbiotic organism is called a lichen. Lichens can sometimes be seen as colorful growths on the surface of rocks and trees. The algal partner (phyco- or photobiont) makes food autotrophically, some of which it shares with the fungus; the fungal partner (mycobiont) absorbs water and minerals from the environment, which are made available to the green alga. If one partner was separated from the other, they would both die.
Epiphytes
An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition; it is non-parasitic. The epiphyte derives its moisture and nutrients from the air, rain, and sometimes from debris accumulating around it instead of from the structure to which it is fastened. Epiphytes have two types of roots: clinging aerial roots (which absorb nutrients from humus that accumulates in the crevices of trees) and aerial roots (which absorb moisture from the atmosphere).
Insectivorous Plants
An insectivorous plant has specialized leaves to attract and digest insects. The Venus flytrap is popularly known for its insectivorous mode of nutrition and has leaves that work as traps. The minerals it obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the leaf immediately closes. Fluids and enzymes then break down the prey and minerals are absorbed by the leaf. Since this plant is popular in the horticultural trade, it is threatened in its original habitat.
Key Points
• Some plants are parasites, which acquire all of some of their nutrients from another host plant and are, therefore, entirely dependent upon it for their survival.
• Saprophytes acquire nutrients from dead matter, using enzymes to convert complex organic compounds into simpler forms from which the plant can absorb nutrients.
• A symbiont experiences a mutually-beneficial arrangement with a plant; both partners contribute necessary nutrients to the other.
• An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition; instead, it uses the other plant for physical support.
• Insectivorous plants have special adapatations for attracting and trapping insects, which they use to supplement their own nutrients, depleted in the surrounding soil.
Key Terms
• photosynthate: any compound that is a product of photosynthesis
• photobiont: a photosynthetic symbiont
• mycobiont: the fungus that is a component of a lichen
• saprophyte: any organism that lives on dead organic matter, as certain fungi and bacteria
• epiphyte: a plant that grows on another, using it as a physical support but neither obtaining nutrients from it nor causing it any damage if also offering no benefit
• insectivorous: capable of trapping and absorbing insects; such as the sundew, pitcher plant and Venus flytrap | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.03%3A__Nutritional_Adaptations_of_Plants/31.3C%3A_Nutrients_from_Other_Sources.txt |
Learning Objectives
• Differentiate among the ways in which plants reproduce
Introduction
Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce sexually while others reproduce asexually, in contrast to animal species, which rely almost exclusively on sexual reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is independent of these agents. Flowers are often the showiest or most strongly-scented part of plants. With their bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve their pollination needs. Other plants pollinate via wind or water; still others self-pollinate.
Asexual Reproduction
Vegetative reproduction is a type of asexual reproduction. Other terms that apply are vegetative propagation, clonal growth, or vegetative multiplication. Vegetative growth is enlargement of the individual plant, while vegetative reproduction is any process that results in new plant “individuals” without production of seeds or spores. It is both a natural process in many, many species as well as a process utilized or encouraged by horticulturists and farmers to obtain quantities of economically-valuable plants. In this respect, it is a form of cloning that has been carried out by humanity for thousands of years and by plants for hundreds of millions of years.
Sexual Reproduction and The Flower
The flower is the reproductive organ of plants classified as angiosperms. All plants have the means and corresponding structures for reproducing sexually. The basic function of a flower is to produce seeds through sexual reproduction. Seeds are the next generation, serving as the primary method in most plants by which individuals of the species are dispersed across the landscape. Actual dispersal is, in most species, a function of the fruit (a structural part that typically surrounds the seed).
Key Points
• Vegetative reproduction is a type of asexual reproduction that results in new plant individuals without seed or spore production.
• Vegetative reproduction is also utilized by horticulturists to ensure production of large quantities of valuable plants.
• Plants have flowers that produce seeds through sexual reproduction; seeds are dispersed to increase propagation of the next generation.
• Seeds are often dispersed by animals via ingestion of the fruits, which surround the seeds, promoting seed dispersal.
Key Terms
• vegetative reproduction: a form of asexual reproduction in plants
32.02: Plant Reproductive Development and Structure - Sexual Reproduction in Gymnosperms
Learning Objectives
• Describe the process of sexual reproduction in gymnosperms
Sexual Reproduction in Gymnosperms
As with angiosperms, the life cycle of gymnosperms is also characterized by alternation of generations. In conifers such as pines, the green leafy part of the plant is the sporophyte; the cones contain the male and female gametophytes. The female cones are larger than the male cones and are positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.
Male Gametophyte
A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts, known as microsporophylls, are the sites where microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on female cones.
Female Gametophyte
The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.
Reproductive Process
Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place because gymnosperm seeds have no covering.
Angiosperms Versus Gymnosperms
Gymnosperm reproduction differs from that of angiosperms in several ways. In angiosperms, the female gametophyte in the ovule exists in an enclosed structure, the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone and is not enclosed in an ovary. Double fertilization is a key event in the life cycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are a part of the flower. Finally, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.
Key Points
• In gymnosperms, a leafy green sporophyte generates cones containing male and female gametophytes; female cones are bigger than male cones and are located higher up in the tree.
• A male cone contains microsporophylls where male gametophytes ( pollen ) are produced and are later carried by wind to female gametophytes.
• The megaspore mother cell in the female cone divides by meiosis to produce four haploid megaspores; one of the megaspores divides to form the female gametophyte.
• The male gametophyte lands on the female cone, forming a pollen tube through which the generative cell travels to meet the female gametophyte.
• One of the two sperm cells released by the generative cell fuses with the egg, forming a diploid zygote that divides to form the embryo.
• Unlike angiosperms, ovaries are absent in gymnosperms, double fertilization does not take place, male and female gametophytes are present on cones rather than flowers, and wind (not animals) drives pollination.
Key Terms
• megasporophyll: bears megasporangium, which produces megaspores that divide into the female gametophyte
• microsporophyll: a leaflike organ that bears microsporangium, which produces microspores that divide into the male gametophyte (pollen) | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.01%3A_Plant_Reproductive_Development_and_Structure_-_Plant_Reproductive_Development_and_Structure.txt |
Learning Objectives
• Outline the components of a flower and their function
The lifecycle of angiosperms follows the alternation of generations. In the angiosperm, the haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms. Flowers contain the plant’s reproductive structures.
Flower Structure
A typical flower has four main parts, or whorls: the calyx, corolla, androecium, and gynoecium. The outermost whorl of the flower has green, leafy structures known as sepals, which are collectively called the calyx, and help to protect the unopened bud. The second whorl is comprised of petals, usually brightly colored, collectively called the corolla. The number of sepals and petals varies depending on whether the plant is a monocot or dicot. Together, the calyx and corolla are known as the perianth. The third whorl contains the male reproductive structures and is known as the androecium. The androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in the flower is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.
If all four whorls are present, the flower is described as complete. If any of the four parts is missing, the flower is known as incomplete. Flowers that contain both an androecium and a gynoecium are called perfect, androgynous, or hermaphrodites. There are two types of incomplete flowers: staminate flowers contain only an androecium; and carpellate flowers have only a gynoecium.
If both male and female flowers are borne on the same plant (e.g., corn or peas), the species is called monoecious (meaning “one home”). Species with male and female flowers borne on separate plants (e.g., C. papaya or Cannabis)are termed dioecious, or “two homes.” The ovary, which may contain one or multiple ovules, may be placed above other flower parts (referred to as superior); or it may be placed below the other flower parts (referred to as inferior).
Male Gametophyte
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, development of pollen takes place in a structure known as the microsporangium. The microsporangia, usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains.
Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain. An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores, contributing key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes. Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.
Female Gametophyte (Embryo Sac)
The overall development of the female gametophyte has two distinct phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte, or embryo sac. The polar nuclei move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo; the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization, protecting the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
Key Points
• A typical flower has four main parts, or whorls: the calyx ( sepals ), corolla (petals), androecium (male reproductive structure), and gynoecium (female reproductive structure).
• Angiosperms that contain both male and female gametophytes within the same flower are called complete and are considered to be androgynous or hermaphroditic.
• Angiosperms that contain only male or only female gametophytes are considered to be incomplete and are either staminate (contain only male structures) or carpellate (contain only female structures) flowers.
• Microspores develop in the microsporangium and form mature pollen grains (male gametophytes), which are then used to fertilize female gametophytes.
• During megasporogenesis, four megaspores are produced with one surviving; during megagametogenesism, the surviving megaspore undergoes mitosis to form an embryo sac (female gametophyte).
• The sperm, guided by the synergid cells, migrates to the ovary to complete fertilization; the diploid zygote develops into the embryo, while the fertilized ovule forms the other tissues of the seed.
Key Terms
• perianth: the calyx (sepals) and the corolla (petals)
• androecium: the set of a flower’s stamens (male reproductive organs)
• gynoecium: the set of a flower’s pistils (female reproductive organs) | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.03%3A_Plant_Reproductive_Development_and_Structure_-__Sexual_Reproduction_in_Angiosperms.txt |
Learning Objectives
• Determine the differences between self-pollination and cross-pollination, and describe how plants have developed ways to avoid self-pollination
Pollination: An Introduction
In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same or a different flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm that fertilize the egg.
Self-Pollination and Cross-Pollination
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators. These types of pollination have been studied since the time of Gregor Mendel. Mendel successfully carried out self-pollination and cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. An example is modern corn, which is a result of thousands of years of breeding that began with its ancestor, teosinte. The teosinte that the ancient Mesoamericans originally began cultivating had tiny seeds, vastly different from today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is minuscule.
Genetic Diversity
Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity is, therefore, required so that in changing environmental or stress conditions, some of the progeny can survive. Self-pollination leads to the production of plants with less genetic diversity since genetic material from the same plant is used to form gametes and, eventually, the zygote. In contrast, cross-pollination leads to greater genetic diversity because the male and female gametophytes are derived from different plants. Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose employs this technique. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower and the thrum-eyed flower. In the pin-eyed flower, anthers are positioned at the pollen tube’s halfway point, and in the thrum-eyed flower, the stigma is found at this same location. This allows insects to easily cross-pollinate while seeking nectar at the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumbers, have male and female flowers located on different parts of the plant, thus making self-pollination difficult. In other species, the male and female flowers are borne on different plants, making them dioecious. All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority of pollinators are biotic agents such as insects (bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.
Key Points
• Pollination, the transfer of pollen from flower-to-flower in angiosperms or cone -to-cone in gymnosperms, takes place through self-pollination or cross-pollination.
• Cross-pollination is the most advantageous of the two types of pollination since it provides species with greater genetic diversity.
• Maturation of pollen and ovaries at different times and heterostyly are methods plants have developed to avoid self-pollination.
• The placement of male and female flowers on separate plants or different parts of the plant are also barriers to self-pollination.
Key Terms
• pollination: the transfer of pollen from an anther to a stigma that is carried out by insects, birds, bats, and the wind
• heterostyly: the condition of having unequal male (anther) and female (stigma) reproductive organs
• cross-pollination: fertilization by the transfer of pollen from an anther of one plant to a stigma of another
• self-pollination: pollination of a flower by its own pollen in a flower that has both stamens and a pistil | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.04%3A_Pollination_and_Fertilization_-_Introduction.txt |
Learning Objectives
• Explain how pollination by insects aids plant reproduction
Bees
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees. The most common species of bees are bumblebees and honeybees. Since bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy -rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, which help guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair; when the bee visits another flower, some of the pollen is transferred to the second flower. Recently, there have been many reports about the declining population of honeybees. Many flowers will remain unpollinated, failing to bear seeds if honeybees disappear. The impact on commercial fruit growers could be devastating.
Flies
Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides energy while the pollen provides protein. Wasps are also important insect pollinators, pollinating many species of figs.
Butterflies and Moths
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which are usually found in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in a way to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship.
Key Points
• Adaptations such as bright colors, strong fragrances, special shapes, and nectar guides are used to attract suitable pollinators.
• Important insect pollinators include bees, flies, wasps, butterflies, and moths.
• Bees and butterflies are attracted to brightly-colored flowers that have a strong scent and are open during the day, whereas moths are attracted to white flowers that are open at night.
• Flies are attracted to dull brown and purple flowers that have an odor of decaying meat.
• Nectar guides, which are only visible to certain insects, facilitate pollination by guiding bees to the pollen at the center of flowers.
• Insects and flowers both benefit from their specialized symbiotic relationships; plants are pollinated while insects obtain valuable sources of food.
Key Terms
• nectar guide: markings or patterns seen in flowers of some angiosperm species that guide pollinators to nectar or pollen | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.05%3A_Pollination_and_Fertilization_-_Pollination_by_Insects.txt |
Learning Objectives
• Differentiate among the non-insect methods of pollination
Non-Insect Methods of Pollination
Plants have developed specialized adaptations to take advantage of non-insect forms of pollination. These methods include pollination by bats, birds, wind, and water.
Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored so that they can be distinguished from their dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally-large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Many species of small birds, such as hummingbirds and sun birds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in a way to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly-colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits. Botanists determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
Pollination by Wind
Most species of conifers and many angiosperms, such as grasses, maples, and oaks, are pollinated by wind. Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green, small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it. The flowers usually emerge early in the spring before the leaves so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower.
Pollination by Water
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water. When it comes into contact with the flower, it is deposited inside the flower.
Pollination by Deception
Orchids are highly-valued flowers, with many rare varieties. They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard; they have evolved different ways to attract the desired pollinators. They use a method known as food deception, in which bright colors and perfumes are offered, but no food. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent, which usually indicates food for a bee. In the process, the bee picks up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and, in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, but instead picks up pollen, which it then transfers to the next counterfeit mate.
Key Points
• Flowers that are pollinated by bats bloom at night, tending to be large, wide-mouthed, and pale-colored; they may also give off strong scents.
• Flowers that are pollinated by small birds usually have curved, tubular shapes; birds carry the pollen off on their heads and neck to the next flower they visit.
• Wind-pollinated flowers do not produce scents or nectar; instead, they tend to have small or no petals and to produce large amounts of lightweight pollen.
• Some species of flowers release pollen that can float on water; pollination occurs when the pollen reaches another plant of the same species.
• Some flowers deceive pollinators through food or sexual deception; the pollinators become attracted to the flowers with false promises of food and mating opportunities.
Key Terms
• food deception: a trickery method employed by some species of orchids in which only bright colors and perfume are offered to their pollinators with no food reward | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.06%3A_Pollination_and_Fertilization_-_Pollination_by_Bats_Birds_Wind_and_Water.txt |
Learning Objectives
• Describe the process of double fertilization in plants
Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. During this process, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac; it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm. Together, these two fertilization events in angiosperms are known as double fertilization. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
After fertilization, embryonic development begins. The zygote divides to form two cells: the upper cell (terminal cell) and the lower cell (basal cell). The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo. In dicots (eudicots), the developing embryo has a heart shape due to the presence of the two rudimentary cotyledons. In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially, but is then digested. In this case, the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they become crowded inside the developing seed and are forced to bend. Ultimately, the embryo and cotyledons fill the seed, at which point, the seed is ready for dispersal. Embryonic development is suspended after some time; growth resumes only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
Key Points
• Double fertilization involves two sperm cells; one fertilizes the egg cell to form the zygote, while the other fuses with the two polar nuclei that form the endosperm.
• After fertilization, the fertilized ovule forms the seed while the tissues of the ovary become the fruit.
• In the first stage of embryonic development, the zygote divides to form two cells; one will develop into a suspensor, while the other gives rise to a proembryo.
• In the second stage of embryonic development (in eudicots), the developing embryo has a heart shape due to the presence of cotyledons.
• As the embryo grows, it begins to bend as it fills the seed; at this point, the seed is ready for dispersal.
Key Terms
• double fertilization: a complex fertilization mechanism that has evolved in flowering plants; involves the joining of a female gametophyte with two male gametes (sperm)
• suspensor: found in plant zygotes in angiosperms; connects the endosperm to the embryo and provides a route for nutrition from the mother plant to the growing embryo
• proembryo: a cluster of cells in the ovule of a fertilized flowering plant that has not yet formed into an embryo | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.07%3A_Pollination_and_Fertilization_-_Double_Fertilization_in_Plants.txt |
Learning Objectives
• Name the three parts of a seed and describe their functions and development
Parts of a Seed
The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat, known as the testa, and inner coat, known as the tegmen. The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl. The embryonic axis terminates in a radicle, which is the region from which the root will develop.
Seed Growth
In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds.
Food Storage in the Seed
The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue. Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids. These products are absorbed by the scutellum and transported via a vasculature strand to the developing embryo.
In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons act as absorptive organs to take up the enzymatically-released food reserves, similar to the process in monocots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized, moving into the developing cotyledon for storage.
Seed Germination
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards; this plumule hook persists as long as germination proceeds in the dark. Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. During this time, the radicle is also growing and producing the primary root. As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system.
In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile: the covering of the shoot tip. Upon exposure to light, elongation of the coleoptile ceases and the leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This produces the fibrous root system of the monocot.
Depending on seed size, the time it takes a seedling to emerge may vary. However, many mature seeds enter a period of dormancy marked by inactivity or extremely-low metabolic activity. This period may last for months, years, or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to optimal conditions, seed germination takes place. These conditions may be as diverse as moisture, light, cold, fire, or chemical treatments. Scarification, the softening of the seed coat, presoaking in hot water, or passing through an acid environment, such as an animal’s digestive tract, may also be needed.
Key Points
• In angiosperms, the process of seed production begins with double fertilization while in gymnosperms it does not.
• In both monocots and dicots, food reserves are stored in the endosperm; however, in non-endospermic dicots, the cotyledons act as the storage.
• In a seed, the embryo consists of three main parts: the plumule, the radicle, and the hypocotyl.
• In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant, while in monocots, they remain below ground.
• In dicot seeds, the radicle grows downwards to form the tap root while lateral roots branch off to all sides, producing a dicot tap root system; in contrast, the end of germination in monocot seeds is marked by the production of a fibrous root system where adventitious roots emerge from the stem.
• Seed germination is dependent on seed size and whether or not favorable conditions are present.
Key Terms
• testa: the seed coat
• radicle: the rudimentary shoot of a plant that supports the cotyledons in the seed and from which the root is developed downward; the root of the embryo
• hypocotyl: in plants with seeds, the portion of the embryo or seedling between the root and cotyledons
• plumule: consisting of the apical meristem and the first true leaves of the young plant
• coleoptile: a pointed sheath that protects the emerging shoot in monocotyledons such as oats and grasses | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.08%3A_Pollination_and_Fertilization_-_Development_of_the_Seed.txt |
Learning Objectives
• Describe the development of a fruit in a flowering plant
After fertilization, the ovary of the flower usually develops into the fruit. Fruits are generally associated with having a sweet taste; however, not all fruits are sweet. The term “fruit” is used for a ripened ovary. In most cases, flowers in which fertilization has taken place will develop into fruits, while unfertilized flowers will not. The fruit encloses the seeds and the developing embryo, thereby providing it with protection. Fruits are diverse in their origin and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn) are all fruits. As the fruit matures, the seeds also mature.
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin. If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen in nuts and beans. An aggregate fruit is one that develops from numerous carpels that are all in the same flower; the mature carpels fuse together to form the entire fruit, as seen in the raspberry. A multiple fruit develops from an inflorescence or a cluster of flowers. An example is the pineapple where the flowers fuse together to form the fruit. Accessory fruits (sometimes called false fruits) are not derived from the ovary, but from another part of the flower, such as the receptacle (strawberry) or the hypanthium (apples and pears).
Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the seed is the edible part (the pit in this case is the endocarp). In many fruits, two, or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds.
Key Points
• Fruits can be classified as simple, aggregate, multiple, or accessory.
• Simple fruits develop from a single carpel or fused carpels of a single ovary, while aggregate fruits develop from more than one carpel found on the same flower.
• Multiple fruits develop from a cluster of flowers, while accessory fruits do not develop from an ovary, but from other parts of a plant.
• The main parts of a fruit include the exocarp (skin), the mesocarp (middle part), and the endocarp (inner part); these three parts make up the pericarp.
• Dehiscent fruits promptly release their seeds, while indehiscent fruits rely on decay to release their seeds.
Key Terms
• exocarp: the outermost covering of the pericarp of fruits; the skin
• simple fruit: fruit that develops from a single carpel or fused carpels of a single ovary
• endocarp: the inner part of the fruit
• mesocarp: middle part of the fruit
• accessory fruit: a fruit not derived from the ovary but from another part of the flower | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.09%3A_Pollination_and_Fertilization_-_Development_of_Fruit_and_Fruit_Types.txt |
Learning Objectives
• Summarize the ways in which fruits and seeds may be dispersed
Fruit and Seed Dispersal
In addition to protecting the embryo, the fruit plays an important role in seed dispersal. Seeds contained within fruits need to be dispersed far from the mother plant so that they may find favorable and less-competitive conditions in which to germinate and grow.
Some fruits have built-in mechanisms that allow them to disperse by themselves, whereas others require the help of agents such as wind, water, and animals. Modifications in seed structure, composition, and size aid in dispersal. Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits, such as the dandelion, have hairy, weightless structures that are suited to dispersal by wind.
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.
Animals and birds eat fruits; seeds that are not digested are excreted in their droppings some distance away. Some animals, such as squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Some fruits have hooks or sticky structures that stick to an animal’s coat and are then transported to another place. Humans also play a major role in dispersing seeds when they carry fruits to new places, throwing away the inedible part that contains the seeds.
All of the above mechanisms allow for seeds to be dispersed through space, much as an animal’s offspring can move to a new location. Seed dormancy allows plants to disperse their progeny through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species.
Key Points
• The means by which seeds are dispersed depend on a seed’s structure, composition, and size.
• Seeds dispersed by water are found in light and buoyant fruits, while those dispersed by wind may have specialized wing-like appendages.
• Animals can disperse seeds by excreting or burying them; other fruits have structures, such as hooks, that attach themselves to animals’ fur.
• Humans also play a role as dispersers by moving fruit to new places and discarding the inedible portions containing the seeds.
• Some seeds have the ability to remain dormant and germinate when favorable conditions arise.
Key Terms
• seed dormancy: a seed with the ability to delay germination and propagation of the species until suitable conditions are found
• dispersal: the movement of a few members of a species to a new geographical area, resulting in differentiation of the original group into new varieties or species
32.11: Asexual Reproduction - Asexual Reproduction in Plants
Learning Objectives
• Summarize methods of asexual reproduction in plants
Asexual Reproduction
Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. Asexual reproduction produces plants that are genetically identical to the parent plant because no mixing of male and female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when compared with plants produced from sexual reproduction because they carry genes identical to those of their parents.
Plants have two main types of asexual reproduction: vegetative reproduction and apomixis. Vegetative reproduction results in new plant individuals without the production of seeds or spores. Many different types of roots exhibit vegetative reproduction. The corm is used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples of this type of reproduction. A potato is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an adventitious root (a root arising from a plant part other than the main or primary root), and the strawberry plant has a stolon, which is also called a runner.
Some plants can produce seeds without fertilization. Either the ovule or part of the ovary, which is diploid in nature, gives rise to a new seed. This method of reproduction is known as apomixis.
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling. Asexual reproduction can take place by natural or artificial (assisted by humans) means.
Key Points
• Asexual reproduction produces individuals that are genetically identical to the parent plant.
• Roots such as corms, stem tubers, rhizomes, and stolon undergo vegetative reproduction.
• Some plants can produce seeds without fertilization via apomixis where the ovule or ovary gives rise to new seeds.
• Advantages of asexual reproduction include an increased rate of maturity and a sturdier adult plant.
• Asexual reproduction can take place by natural or artificial means.
Key Terms
• stolon: a shoot that grows along the ground and produces roots at its nodes; a runner
• apomixis: process of reproduction in which plants produce seeds without fertilization | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.10%3A_Pollination_and_Fertilization_-_Fruit_and_Seed_Dispersal.txt |
Learning Objectives
• Distinguish between natural and artificial methods of asexual reproduction in plants
Natural Methods of Asexual Reproduction
Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many plants, such as ginger, onion, gladioli, and dahlia, continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners (stolons) can give rise to new plants. In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; they may also start growing into independent plants if the leaf touches the soil. Some plants can be propagated through cuttings alone.
Artificial Methods of Asexual Reproduction
Artificial methods of asexual reproduction are frequently employed to give rise to new, and sometimes novel, plants. They include grafting, cutting, layering, and micropropagation.
Grafting
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. In grafting, two plant species are used: part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together. Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts producing shoots, eventually bearing flowers and fruits. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grafted onto root stock with specific resistance to disease.
Cutting
Plants such as coleus and money plant are propagated through stem cuttings where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept undisturbed in water for several weeks.
Layering
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are the preferred plant for this method. Jasmine and bougainvillea (paper flower) can be propagated this way. In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormone. After some time, roots will appear; this portion of the plant can be removed and transplanted into a separate pot.
Micropropagation
Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions. This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.
To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass, known as a callus, from which, after a period of time, individual plantlets begin to grow. These can be separated; they are first grown under greenhouse conditions before they are moved to field conditions.
Key Points
• In natural asexual reproduction, roots can give rise to new plants, or plants can propagate using budding or cutting.
• In grafting, part of a plant is attached to the root system of another plant; the two unite to form a new plant containing the roots of one and the stem and leaf structure of the other.
• Cutting is the process in which the stem of a plant is placed in moist soil or water to generate a new root system.
• In layering, part of the plant’s stem is bent down and covered with soil; this stem can generate a new root system and, therefore, an entirely new plant.
• Micropropagation is the process in which part of a plant is placed in plant culture medium and provided with all the hormones and nutrients it needs in order to generate new plants.
• When part of a plant is placed in plant culture medium and provided with all the hormones and nutrients it needs, it can generate new plants; this is known as micropropagation.
Key Terms
• layering: a method of plant propagation in which a bent stem is covered with soil in order to generate new roots
• grafting: process of attaching part of a stem from one plant onto the root of another plant
• micropropagation: practice of rapidly multiplying plant material to produce a large number of progeny plants using plant tissue culture methods
• cutting: placing part of a stem containing nodes or internodes in water or moist soil in order to produce new plants | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.12%3A_Asexual_Reproduction_-_Natural_and_Artificial_Methods_of_Asexual_Reproduction_in_Plants.txt |
Learning Objectives
• Explain the process of aging in plants
Plant Life Spans
The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds, and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4,500 years. Even as some parts of a plant, such as regions containing meristematic tissue (the area of active plant growth consisting of undifferentiated cells capable of cell division) continue to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems and the water-conducting tissue of the xylem, for example, are composed of dead cells.
Annuals, Biennial, and Perennials
Plant species that complete their life cycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials, such as carrots, complete their life cycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth and do not allow the plants to flower. Perennials, such as the magnolia, complete their life cycle in two years or more.
Monocarpic and Polycarpic Plants
In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime; examples of monocarpic plants include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually, accumulating a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this method, the plant does not require all its nutrients to be channeled towards flowering each year.
Genetics and Environmental Conditions
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue, such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency due to shading by upper leaves or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling. However, the complex pathways of nutrient recycling within a plant are not well understood
The aging of a plant and all the associated processes is known as senescence, which is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery, such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.
Key Points
• The life span of a plant is the length of time it takes from the beginning of development until death, while the life cycle is the series of stages between the germination of the seed until the plant produces its own seeds.
• Annuals complete their life cycle in one season; biennials complete their life cycle in two seasons; and perennials complete their life cycle in more than two seasons.
• Monocarpic plants flower only once in their lifetime, while polycarpic plants flower more than once.
• Plant survival depends on changing environmental conditions, drought, cold, and competition.
• Senescence refers to aging of the plant, during which components of the plant cells are broken down and used to support the growth of other plant tissues.
Key Terms
• annual: a plant which naturally germinates, flowers, and dies in one year
• biennial: a plant that requires two years to complete its life cycle
• perennial: a plant that is active throughout the year or survives for more than two growing seasons
• monocarpic: a plant that flowers and bears fruit only once before dying
• polycarpic: bearing fruit repeatedly, or year after year
• senescence: aging of a plant; accumulated damage to macromolecules, cells, tissues, and organs with the passage of time | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/32%3A_Plant_Reproductive_Development_and_Structure/32.13%3A_Asexual_Reproduction_-_Plant_Life_Spans.txt |
Learning Objectives
• Describe how form and function are related in an organism
Animal Form and Function
Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. The term body plan is the “blueprint” encompassing aspects such as symmetry, segmentation, and limb disposition. Body plans have been considered to have evolved in a geologically-sudden flash during the Cambrian Explosion (roughly 542 million years ago). However, there is also evidence of a more gradual development of body plans. With a few exceptions, most notably the sponges and Placozoa, animals have bodies differentiated into separate tissues, which in turn make up more complex organs and organ systems. These include tissues such as muscles, which are able to contract and control locomotion, and nerves, which send and process signals. Typically, there is also an internal digestive chamber with one or two openings. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. In addition, animal body plans have evolved in response to environmental pressures, as observed in fossil records, in order to enhance survival and reproductive success. Therefore, a large amount of information about the structure of an organism’s body (anatomy) and the function of its cells, tissues, and organs (physiology) can be learned by studying that organism’s environment. The arctic fox is an example of a complex animal that has adapted to its environment and illustrates the relationships between an animal’s form and function.
Key Points
• A body plan encompasses symmetry, segmentation, and limb disposition.
• Almost all animals have bodies made of differentiated tissues, which in turn form organs and organ systems.
• Animal bodies have evolved to interact with their environments in ways that enhance survival and reproduction.
Key Terms
• physiology: a branch of biology that deals with the functions and activities of life or of living matter (as organs, tissues, or cells) and of the physical and chemical phenomena involved
• body plan: an assemblage of morphological features shared among many members of a phylum-level group
• anatomy: the art of studying the different parts of any organized body, to discover their situation, structure, and economy; dissection
33.02: Animal Form and Function - Body Plans
Learning Objectives
• Describe the body plan of an animal
Body Plans
Animal body plans follow set patterns related to symmetry. They can be asymmetrical, radial, or bilateral in form. Asymmetrical animals are those with no pattern or symmetry, such as a sponge. Radial symmetry describes an animal with an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, such as a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility. Bilateral symmetry is illustrated in a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides.
In order to describe structures in the body of an animal it is necessary to have a system for describing the position of parts of the body in relation to other parts. For example, it may be necessary to describe the position of the liver in relation to the diaphragm or the heart in relation to the lungs. The most common terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Note that the terms superior and inferior are usually not used to describe animals. They are only used to describe the position of structures in the human body (and possibly apes) where the upright posture means some structures are above or superior to others.
Key Points
• Some animals have a body with no pattern or symmetry, making them asymmetrical.
• Animals (mostly aquatic) with an up-and-down orientation have a radial symmetry in which there is no definite right or left side, but any longitudinal plane cut produces equal halves.
• Animals, either aquatic or terrestrial, that have a high level of mobility usually have a body plan that is bilaterally symmetric.
• Terms such as anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach) are used to describe the position of parts of the body in relation to other parts.
Key Terms
• asymmetrical: having disproportionate arrangement of parts; exhibiting no pattern
• bilateral symmetry: having equal arrangement of parts (symmetry) about a vertical plane running from head to tail
• radial symmetry: a form of symmetry wherein identical parts are arranged in a circular fashion around a central axis | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.01%3A_Animal_Form_and_Function_-_Characteristics_of_the_Animal_Body.txt |
Learning Objectives
• Explain how the environment and skeletal structure can put limits on the size and shape of animals
Limits on Animal Size and Shape
Animals with bilateral symmetry that live in water tend to have a fusiform shape: a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. Certain types of sharks can swim at fifty kilometers an hour, while some dolphins can swim at 32-40 kilometers per hour. Land animals frequently travel faster (although the tortoise and snail are significantly slower than sharks or dolphins). Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. However, land-dwelling organisms are constrained mainly by gravity; drag is relatively unimportant. For example, most adaptations in birds are for gravity, not for drag.
Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles. As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer, such as chitin, and is often biomineralized with materials, such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton called apodemes function as attachment sites for muscles, similar to tendons in more advanced animals. In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually. It may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively-small size.
The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass. An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.
Key Points
• Aquatic animals tend to have tubular shaped bodies ( fusiform shape) that decrease drag, enabling them to swim at high speeds.
• Terrestrial animals tend to have body shapes that are adapted to deal with gravity.
• Exoskeletons are hard protective coverings or shells that also provide attachments for muscles.
• Before shedding or molting the existing exoskeleton, an animal must first produce a new one.
• The exoskeleton must increase thickness as the animal becomes larger, which limits body size.
• The size of an animal with an endoskeleton is determined by the amount of skeletal system required to support the body and the muscles it needs to move.
Key Terms
• fusiform: shaped like a spindle; tapering at each end
• exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda
• apodeme: an ingrowth of the arthropod exoskeleton, serving as an attachment site for muscles
• endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage
33.04: Animal Form and Function - Limiting Effects of Diffusion on Size and Development
Learning Objectives
• Describe how diffusion limits cell size and development
Limiting Effects of Diffusion on Size and Development
The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective at completing all of these tasks. The center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.
An important concept in understanding the efficiency of diffusion as a transportation mechanism is the surface-to-volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface-to-volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.
The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become less efficient at completing all tasks since they are now more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Surface-to-volume ratio also applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons or in the relationship between muscle mass and the generation of dissipation of heat.
Key Points
• Diffusion is effective over a specific distance, so it’s more efficient in small, single-celled microorganisms.
• Diffusion becomes less efficient as the surface-to-volume ratio decreases, so diffusion is less effective in larger animals.
• To overcome the limitations of diffusion, multicellular organisms have developed specialized tissues and systems that are responsible for completing a limited number of nutrient and waste tasks.
Key Terms
• surface-to-volume ratio: the amount of surface area per unit volume of an object or collection of objects; decreases as volume increases | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.03%3A_Animal_Form_and_Function_-__Limits_on_Animal_Size_and_Shape.txt |
Learning Objectives
• Differentiate among the ways in which an animal’s energy requirements are affected by their environment and level of activity
Animal Bioenergetics
All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, while others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient as an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively-constant body temperature, it is classified as a warm-blooded animal: an endotherm. The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat.
The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5-5 kcal/g, while fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600-1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day.
Energy Requirements Related to Body Size
Smaller endothermic animals have a greater surface area for their mass than larger ones. Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal.
Energy Requirements Related to Levels of Activity
The more active an animal is, the more energy is needed to maintain that activity and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR.
Energy Requirements Related to Environment
Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism, which allows animals to survive adverse conditions. Torpor can be used by animals for long periods. For example, animals can enter a state of hibernation during the winter months, which enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F).
If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals estivate to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface-to-volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day when they consume more energy to maintain their body temperature.
Key Points
• An animal is endothermic (warm-blooded) if it maintains a relatively-constant body temperature by conserving heat with the help of insulation.
• An animal is ectothermic if it does not have insulation to conserve heat and must rely on its environment for body heat.
• Metabolic rate is the amount of energy expended by an animal over a specific time; in endotherms, it is described as the basal metabolic rate (BMR), while in ectotherms, as the standard metabolic rate (SMR).
• Smaller endothermic animals have a higher BMR than larger endothermic animals because they lose heat at a faster rate and require more energy to maintain a constant internal temperature.
• More active animals have higher BMRs or SMRs and require more energy to maintain their activity.
• A long period of inactivity and decreased metabolism ( torpor ) that occurs in the winter months is hibernation; estivation is torpor that occurs in the summer months.
Key Terms
• endotherm: a warm-blooded animal that maintains a constant body temperature
• ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings
• hibernation: a state of inactivity and metabolic depression in animals during winter
• estivation: to go into a state of inactivity during the summer months | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.05%3A_Animal_Form_and_Function_-_Animal_Bioenergetics.txt |
Learning Objectives
• Describe the major body planes and cavities of animals
Animal Body Planes and Cavities
A standing vertebrate animal can be divided by several planes that can be used to as references to describe locations of body parts or organs. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the front (ventral) from the back (dorsal). A transverse plane (or, horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section; if the transverse cut is at an angle, it is called an oblique plane.
Vertebrate animals have a number of defined body cavities. The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior cavity, the cranial cavity houses the brain and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity.
The anterior cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is the more superior subdivision of the anterior cavity and is enclosed by the rib cage. The thoracic cavity contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The diaphragm forms the floor of the thoracic cavity, separating it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, the division that houses the digestive organs from the pelvic cavity, the division that houses the organs of reproduction.
Key Points
• A sagittal plane divides the body into right and left portions; a midsagittal plane divides the body exactly in the middle.
• A frontal or coronal plane separates the front from the back.
• A transverse or horizontal plane divides the animal into upper and lower portions; it is called an oblique plane if it is cut at an angle.
• The posterior (dorsal) cavity is a continuous cavity that includes the cranial cavity (brain) and the spinal cavity (spinal cord).
• The anterior (ventral) cavity includes the thoracic cavity and the abdominopelvic cavity.
• The thoracic cavity is divided into the pleural cavity (lungs) and pericardial cavity (heart); the abdominopelvic cavity includes the abdominal cavity (digestive organs) and the pelvic cavity (reproductive organs).
Key Terms
• transverse plane: divides a body into upper and lower portions
• frontal plane: divides a body into dorsal (back) and ventral (front) parts
• sagittal plane: divides the body into right and left halves
33.07: Animal Primary Tissues - Epithelial Tissues
Learning Objectives
• Differentiate among the types of epithelial tissues
Epithelial tissues cover the outside of organs and structures in the body. They also line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia.
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally-located nucleus. The cell outline is slightly irregular; cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple squamous epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs or the exchange of nutrients and waste at blood capillaries.
Cuboidal Epithelia
Cuboidal epithelial cells are cube-shaped with a single, central nucleus. They are most-commonly found in a single layer, such as a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer. They are most-commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells. These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer. This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which, in the case of the trachea, keep these irritants from getting into the lungs.
Transitional Epithelia
Transitional (or uroepithelial) cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it; the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Key Points
• Epithelium composed of only a single layer of cells is called simple epithelium, while epithelium composed of more than one layer of cells is called stratified.
• Squamous epithelial cells are round, flat, and have an irregular border; their function is usually to diffuse or filter substances across tissues.
• Cuboidal epithelial cells, as wide as they are tall, are cube shaped; they are usually found lining glands where they secrete substances.
• Columnar epithelial cells are taller than they are wide and function mostly in absorption, such as in the digestive tract.
• Pseudostratified columnar epithelia appear to be stratified because there seems to be more than one row of nuclei, but, in fact, it is a single layer of cells with the nuclei at different levels.
• Transitional epithelium has the ability to stretch; it usually lines the interior of organs such as the bladder.
Key Terms
• goblet cell: glandular simple columnar epithelial cells whose function is to secrete mucin, which dissolves in water to form mucus
• lumen: The cavity or channel within a tube or tubular organ. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.06%3A_Animal_Form_and_Function_-_Animal_Body_Planes_and_Cavities.txt |
Learning Objectives
• Distinguish between the different types of connective tissue
Connective Tissues
Connective tissues are composed of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is composed of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast, an immature connective tissue cell that has not yet differentiated. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues, while others may have specialized cells. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has a proportionally-less-dense matrix.
The organic portion, or protein fibers, found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length, returning to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers, the third type of protein fiber found in connective tissues, consist of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected.
Loose (Areolar) Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. Loose connective tissue has some fibroblasts, although macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel, helping to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly-arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue. The cells, called chondrocytes (mature cartilage cells), make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called “lacunae. ”
A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during some pre-birth developmental stages. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage, as do portions of the larynx, or voice box. In contrast, fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals, which must withstand a tremendous amount of stress. Cartilage can also transform from one type to another. For example, hyaline cartilage found in movable joints, such as the knee and shoulder, often becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage, resulting in “stiff” joints.
Key Points
• Fibroblasts are cells that generate any connective tissue that the body needs, as they can move throughout the body and can undergo mitosis to create new tissues.
• Protein fibers run throughout connective tissue, providing stability and support; they can be either collagen, elastic, or reticular fibers.
• Loose connective tissue is not particularly tough, but surrounds blood vessels and provides support to internal organs.
• Fibrous connective tissue, which is composed of parallel bundles of collagen fibers, is found in the dermis, tendons, and ligaments.
• Hyaline cartilage forms the skeleton of the embryo before it is transformed into bone; it is found in the adult body at the tip of the nose and around the ends of the long bones, where it prevents friction at the joints.
• Fibrocartilage is the strongest of the connective tissues; it is found in regions of the body that experience large amounts of stress and require a high degree of shock absorption, such as between the vertebrae.
Key Terms
• chondrocyte: a cell that makes up the tissue of cartilage
• motile: having the power to move spontaneously
• fibroblast: a cell found in connective tissue that produces fibers, such as collagen | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.08%3A_Animal_Primary_Tissues_-__Loose_Fibrous_and_Cartilage_Connective_Tissues.txt |
Learning Objectives
• Describe the structure and function of connective tissues made of bone, fat, and blood
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is materially similar to other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts, mostly calcium, that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. They deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone and assist in maintenance of the bone. Osteoclasts are active in breaking down bone for bone remodeling, providing access to calcium stored in tissues in order to release it into the blood. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons. A blood vessel and a nerve are found in the center of the osteon within a long opening called the Haversian canal, with radiating circles of compact bone around it known as lamellae. Small spaces between these circles are called lacunae. Between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae, which serve as struts, giving the spongy bone strength.
Adipose (Fat) Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix, and has only a few fibers. Adipose tissue is composed of cells called adipocytes that collect and store fat in the form of triglycerides for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic. They also function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing. The thin lines in the image are the cell membranes; the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix. The living cell types are red blood cells, also called erythrocytes, and white blood cells, also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte, responsible for transporting oxygen to body tissues. Erythrocytes are consistently the same size in a species, but vary in size between species. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are white blood cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.
Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells that participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte, which give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils, both of which help to facilitate the inflammatory response.
The slightly-granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.
Key Points
• Bone contains three types of cells: osteoblasts, which deposit bone; osteocytes, which maintain the bone; and osteoclasts, which resorb bone.
• The functional unit of compact bone is the osteon, which is made up of concentric rings of bone called lamellae surrounding a central opening called a Haversian canal, through which nerves and blood vessels travel.
• Compact bone, made of inorganic material that gives it strength and stability, is located on the shaft of long bones, while spongy bone, made of organic material, is found inside the ends of the long bones.
• Adipose (fat) tissue contains cells called adipocytes that store fat in the form of triglyerides; these can be broken down for energy by the organism.
• Blood is composed of erythrocytes (red blood cells), which distribute oxygen throughout the body; leukocytes (white blood cells), which mount immune responses; and platelets, which are involved in blood clotting.
Key Terms
• osteon: any of the central canals and surrounding bony layers found in compact bone
• canaliculi: plural form of canaliculus; any of many small canals or ducts in bone or in some plants
• trabecula: a small mineralized spicule that forms a network in spongy bone
• osteoblast: a mononucleate cell from which bone develops
• osteoclast: a large multinuclear cell associated with the resorption of bone | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.09%3A_Animal_Primary_Tissues_-__Bone_Adipose_and_Blood_Connective_Tissues.txt |
The function of muscle tissue (smooth, skeletal, and cardiac) is to contract, while nervous tissue is responsible for communication.
Learning Objectives
• Describe the structure and function of nervous tissue; differentiate among the types of muscle tissue
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body.
Smooth Muscle
Smooth muscle cells have a single, centrally-located nucleus and are spindle shaped. Constriction of smooth muscle occurs under involuntary, autonomic nervous control in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed primarily of smooth muscle. Contractions of smooth muscle move food through the digestive tracts and push blood through the blood vessels.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins, actin and myosin, that run throughout the muscle fiber. Skeletal muscle cells can contract by the attachment of myosin to actin filaments in the muscle, which then ratchets the actin filaments toward the center of the cells. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Stimulation of these cells by somatic motor neurons signals the cells to contract.
Cardiac Muscle
Cardiac muscle is found only in the heart. Similar to skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally-located nucleus; the muscle branches in many directions. Cardiac muscle is not under voluntary control, but is influenced by the autonomic nervous system to speed up or slow down the heart beat. An added feature to cardiac muscle cells is a line that extends along the end of the cell as it abuts the next cardiac cell in the row. This line, an intercalated disc, assists in passing electrical impulses efficiently from one cell to the next while maintaining the strong connection between neighboring cardiac cells, allowing the cardiac muscle cells to synchronize the beating of the heart.
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body organized into structures called nerves. A nerve consists of a neuron and glial cells. The main cell of the nervous system is the neuron. There is a large structure with a central nucleus: the cell body (or soma) of the neuron. Projections from the cell body are either dendrites, specialized in receiving input, or a single axon, specialized in transmitting impulses. Glial cells support the neurons. Astrocytes regulate the chemical environment of the nerve cell, while oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic, removing debris or damaged cells from the tissue.
Key Points
• Smooth muscle cells, spindle shaped with only one nucleus, contract involuntarily to push food through the digestive tract and blood through blood vessels.
• Skeletal muscle cells, long, striated, multinucleate cells under voluntary control, are responsible for the movement of skeletal muscles.
• Cardiac muscle cells, found only in the heart, are striated and branching (with one nucleus); they are joined by intercalacted discs which allow the cells to synchronize the beating of the heart.
• Nervous tissue is comprised of nerves, which are comprised of neurons, that send and receive signals, and glial cells, which support the neurons.
Key Terms
• intercalated disc: identifying features of cardiac muscle; these connect individual heart muscle cells to work as a single functional organ
• myosin: a large family of motor proteins found in eukaryotic tissues, allowing mobility in muscles
• oligodendrocyte: a cell that provides support and insulation to axons in the central nervous system of some vertebrates
• astrocyte: a neuroglial cell, in the shape of a star, in the brain
• actin: A globular structural protein that polymerizes in a helical fashion to form an actin filament (or microfilament).
33.11: Homeostasis - Homeostatic Process
Learning Objectives
• Give an example and describe a homeostatic process.
Homeostatic Process
The human organism consists of trillions of cells working together for the maintenance of the entire organism. While cells may perform very different functions, the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with everything that the cells need to survive (oxygen, glucose, mineral ions, waste removal, etc.) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.
Homeostasis
Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change. Adjustment of physiological systems within the body is called homeostatic regulation, which involves three parts or mechanisms: (1) the receptor, (2) the control center, and (3) the effector.
The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. The effector responds to the commands of the control center by either opposing or enhancing the stimulus. This ongoing process continually works to restore and maintain homeostasis. For example, during body temperature regulation, temperature receptors in the skin communicate information to the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin. As the internal and external environment of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the set point.
Purpose of Homeostasis
The ultimate goal of homeostasis is the maintenance of equilibrium around the set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to revert to it. A change in the internal or external environment (a stimulus) is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by moving the nutrient into tissues in the command center that require it, or to store it for later use.
Key Points
• Homeostasis is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.
• Homeostatic regulation is monitored and adjusted by the receptor, the command center, and the effector.
• The receptor receives information based on the internal environment; the command center, receives and processes the information; and the effector responds to the command center, opposing or enhancing the stimulus.
Key Terms
• homeostasis: the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium
• effector: any muscle, organ etc. that can respond to a stimulus from a nerve | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.10%3A_Animal_Primary_Tissues_-__Muscle_Tissues_and_Nervous_Tissues.txt |
Learning Objectives
• Discuss the ways in which the body maintains homeostasis and provide examples of each mechanism
Control of Homeostasis
When a change occurs in an animal’s environment, an adjustment must be made. The receptors sense changes in the environment, sending a signal to the control center (in most cases, the brain), which, in turn, generates a response that is signaled to an effector. The effector is a muscle or a gland that will carry out the required response. Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.
Negative Feedback Mechanisms
Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down; conversely, if a level is too low, the body does something to raise it; hence, the term: negative feedback. An example of negative feedback is the maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise, which is sensed by the nervous system. Specialized cells in the pancreas (part of the endocrine system) sense the increase, releasing the hormone insulin. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in a different group of cells in the pancreas: the hormone glucagon is released, causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of a feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys. The effects of PTH are to raise blood levels of calcium. Negative feedback loops are the predominant mechanism used in homeostasis.
Positive Feedback Loop
A positive feedback loop maintains the direction of the stimulus and possibly accelerates it. There are few examples of positive feedback loops that exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.
Set Point
Homeostasis is performed so the body can maintain its internal set point. However, there are times when the set point must be adjusted. When this happens, the feedback loop works to maintain the new setting. An example of changes in a set point can been seen in blood pressure. Over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal; there is no attempt made to return to the lower set point. The result is the maintenance of an elevated blood pressure which can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level through a process of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than one to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, while a light coat in summer assists in keeping body temperature from rising to harmful levels.
Key Points
• Negative feedback loops are used to maintain homeostasis and achieve the set point within a system.
• Negative feedback loops are characterized by their ability to either increase or decrease a stimulus, inhibiting the ability of the stimulus to continue as it did prior to sensing of the receptor.
• Positive feedback loops are characterized by their ability to maintain the direction of a stimulus and can even accelerate its effect.
• Acclimatization is characterized by the ability to change systems within an organism to maintain a set point in a different environment.
Key Terms
• acclimatization: the climatic adaptation of an organism that has been moved to a new environment
• endocrine: Producing internal secretions that are transported around the body by the bloodstream. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.12%3A_Homeostasis_-_Control_of_Homeostasis.txt |
Learning Objectives
• Outline the various types of processes utilized by animals to ensure thermoregulation.
Internal thermoregulation contributes to animal’s ability to maintain homeostasis within a certain range of temperatures. As internal body temperature rises, physiological processes are affected, such as enzyme activity. Although enzyme activity initially increases with temperature, enzymes begin to denature and lose their function at higher temperatures (around 40-50 C for mammals). As internal body temperature decreases below normal levels, hypothermia occurs and other physiological process are affected. There are various thermoregulation mechanisms that animals use to regulate their internal body temperature.
Types of Thermoregulation (Ectothermy vs. Endothermy)
Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, and are colloquially referred to as “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. Ectotherms are colloquially referred to as “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.
Ectotherm
An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments in which temperatures are constant, such as the tropics or ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.
Endotherms
In contrast to ectotherms, endotherms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is usually generated from the animal’s normal metabolism, but under conditions of excessive cold or low activity, an endotherm generate additional heat by shivering. Many endotherms have a larger number of mitochondria per cell than ectotherms. These mitochondria enables them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food more often. For example, a mouse (endotherm) must consume food every day to sustain high its metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.
Homeothermy vs. Poikilothermy
A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Poikilotherm’s internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs that have such stable ambient temperatures that their internal temperature remains constant.
Means of Heat Transfer
Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and from dry skin the same manner. When a mammal sweats, evaporation removes heat from a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.
Key Points
• In response to varying body temperatures, processes such as enzyme production can be modified to acclimate to changes in the temperature.
• Endotherms regulate their own internal body temperature, regardless of fluctuating external temperatures, while ectotherms rely on the external environment to regulate their internal body temperature.
• Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.
• Heat can be exchanged between environment and animals via radiation, evaporation, convection, or conduction processes.
Key Terms
• ectotherm: An animal that relies on external environment to regulate its internal body temperature.
• endotherm: An animal that regulates its own internal body temperature through metabolic processes.
• homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
• poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.13%3A_Homeostasis_-_Thermoregulation.txt |
Learning Objectives
• Describe some of the changes animals use in order to maintain body temperature
Heat Conservation and Dissipation
Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment, yet they maintain a constant, warm, body temperature. The arctic fox uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles create “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end; the loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.
Endotherms use their circulatory systems to help maintain body temperature. For example, vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. However, vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, thus, warming blood that returns to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption, which can be shut down in some animals to prevent overheating the internal organs, is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations (as in dolphin flukes and elephant ears) can help cool endotherms when needed.
Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. In cases of severe cold, a shivering reflex is activated that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.
Ecothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from becoming too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity, such as the activity of bees to warm a hive to survive winter.
Key Points
• Heat conservation is characterized by the ability to ensure blood remains in the core by undergoing vasoconstriction, reducing blood flow to the periphery (also known as peripheral vasoconstriction).
• Heat dissipation is characterized by the ability to undergo vasodilation which increases blood flow to the periphery, resulting in evaporative heat loss.
• Endothermic animals are defined by their ability to utilize both vasoconstriction and vasodilation to maintain internal body temperature.
• Ectothermic animals are defined by their change in behavior (lying in sunlight to warm up, hiding in shade to cool down) to regulate body temperature.
Key Terms
• endotherm: a warm-blooded animal that maintains a constant body temperature
• ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/33%3A_The_Animal_Body-_Basic_Form_and_Function/33.14%3A_Homeostasis_-_Heat_Conservation_and_Dissipation.txt |
Learning Objectives
• Summarize animal nutrition and the digestive system
Introduction to Animal Nutrition
All living organisms need nutrients to survive. While plants can obtain the molecules required for cellular function through the process of photosynthesis, most animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. The food consumed consists of protein, fat, and complex carbohydrates, but the requirements of each are different for each animal.
Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down to smaller components which will later be absorbed by the body.
Digestive System
The digestive system is one of the largest organ systems in the human body. It is responsible for processing ingested food and liquids. The cells of the human body all require a wide array of chemicals to support their metabolic activities, from organic nutrients used as fuel to the water that sustains life at the cellular level. The digestive system not only effectively chemically reduces the compounds in food into their fundamental building blocks, but also acts to retain water and excrete undigested materials. The functions of the digestive system can be summarized as follows: ingestion (eat food), digestion (breakdown of food), absorption (extraction of nutrients from the food), and defecation (removal of waste products).
The digestive system consists of a group of organs that form a closed tube-like structure called the gastrointestinal tract (GI tract) or the alimentary canal. For convenience, the GI tract is divided into upper GI tract and lower GI tract. The organs that make up the GI tract include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. There are also several accessory organs that secrete various enzymes into the GI tract. These include the salivary glands, the liver, and the pancreas.
Challenges to Human Nutrition
One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Imbalances can have serious health consequences. For example, eating too much food while not expending much energy leads to obesity, which in turn will increase the risk of developing illnesses such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases means that understanding the role of diet and nutrition in maintaining good health is more important than ever.
Key Points
• Animals obtain lipids, proteins, carbohydrates, essential vitamins, and minerals from the food they consume.
• The digestive system is composed of a series of organs, each with a specific, yet related function, that work to extract nutrients from food.
• Organs of the digestive system include the mouth, esophagus, stomach, small intestine, and the large intestine.
• Accessory organs, such as the liver and pancreas, secrete digestive juices into the gastrointestinal tract to assist with food breakdown.
Key Terms
• digestion: the process, in the gastrointestinal tract, by which food is converted into substances that can be utilized by the body
• macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins)
• alimentary canal: the organs of a human or an animal through which food passes; the digestive tract
34.02: Digestive Systems - Herbivores Omnivores and Carnivores
Learning Objectives
• Differentiate among herbivores, omnivores, and carnivores
Herbivores, Omnivores, and Carnivores
Herbivores are animals whose primary food source is plant-based. Examples of herbivores include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars. These animals have evolved digestive systems capable of digesting large amounts of plant material. The plants are high in fiber and starch, which provide the main energy source in their diet. Since some parts of plant materials, such as cellulose, are hard to digest, the digestive tract of herbivores is adapted so that food may be digested properly. Many large herbivores have symbiotic bacteria within their guts to assist with the breakdown of cellulose. They have long and complex digestive tracts to allow enough space and time for microbial fermentation to occur. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).
Omnivores are animals that eat both plant- and animal- derived food. Although the Latin term omnivore literally means “eater of everything”, omnivores cannot really eat everything that other animals eat. They can only eat things that are moderately easy to acquire while being moderately nutritious. For example, most omnivores cannot live by grazing, nor are they able to eat some hard-shelled animals or successfully hunt large or fast prey. Humans, bears, and chickens are examples of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish.
Carnivores are animals that eat other animals. The word carnivore is derived from Latin and means “meat eater.” Wild cats, such as lions and tigers, are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients; examples of obligate carnivores are members of the cat family. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores; dogs would be considered facultative carnivores.
Key Points
• Herbivores are those animals, such as deer and koalas, that only eat plant material.
• Omnivores are those animals, such as bears and humans, that can eat a variety of food sources, but tend to prefer one type to another.
• While most carnivores, such as cats, eat only meat, facultative carnivores, such as dogs, behave more like omnivores as they can eat plant matter along with meat.
• Facultative carnivores can eat meat as well as plant material while obligate carnivores eat meat all the time.
Key Terms
• omnivore: an animal which is able to consume both plants (like a herbivore) and meat (like a carnivore)
• obligate carnivore: an animal that necessarily subsists on a diet consisting mainly of meat because it does not possess the physiology to digest vegetable matter
• herbivore: any animal that eats only vegetation (i.e. that eats no meat)
• carnivore: any animal that eats meat as the main part of its diet | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.01%3A_Digestive_Systems_-_Introduction.txt |
Learning Objectives
• Explain the digestive process in invertebrates
Invertebrate Digestive Systems
Animals have evolved different types of digestive systems break down the different types of food they consume. Invertebrates can be classified as those that use intracellular digestion and those with extracellular digestion.
Intracellular Digestion
The simplest example of digestion intracellular digestion, which takes place in a gastrovascular cavity with only one opening. Most animals with soft bodies use this type of digestion, including Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones). The gastrovascular cavities of these organisms contain one open which serves as both a “mouth” and an “anus”.
Ingested material enters the mouth and passes through a hollow, tubular cavity. The food particles are engulfed by the cells lining the gastrovascular cavity and the molecular are broken down within the cytoplasm of the cells (intracellular).
Extracellular Digestion
The alimentary canal is a more advanced digestive system than a gastrovascular cavity and carries out extracellular digestion. Most other invertebrates like segmented worms (earthworms), arthropods (grasshoppers), and arachnids (spiders) have alimentary canals. The alimentary canal is compartmentalized for different digestive functions and consists of one tube with a mouth at one end and an anus at the other.
Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine and nutrients are absorbed. Because the food has been broken down exterior to the cells, this type of digestion is called extracellular digestion. The material that the organism cannot digest is eliminated as feces, called castings, through the anus.
Most invertebrates use some form of extracellular digestion to break down their food. Flatworms and cnidarians, however, can use both types of digestion to break down their food.
Key Points
• The simplest invertebrate digestive system in a gastrovascular cavity consists of only one opening that serves as both the mouth for taking in food and the anus for excretion.
• The gastrovascular cavity has cells lining it that secrete digestive enzymes to break down the food particles through a process called intracellular digestion.
• An alimentary canal is a long tube that begins with a mouth, then goes to the esophagus, then to the crop, gizzard, intestine, and finally, to an anus; this is used in the process of extracellular digestion.
• Most invertebrates use extracellular digestion; however, there are a few phyla that can use both intracellular and extracellular digestion.
Key Terms
• alimentary canal: the organs of a human or an animal through which food passes; the digestive tract
• intracellular digestion: Intracellular digestion is a form of digestion which takes place within the cytoplasm of the organism. Intracellular digestion takes place in animals without a digestive tract, in which food items are brought into the cell for digestion.
• extracellular digestion: Extracellular digestion is a process in which animals feed by secreting enzymes through the cell membrane onto the food. The enzymes break the food into molecules small enough to be taken pass through the cell membrane into the cell. These nutrients are transferred into the blood or other body fluids and distributed to the rest of the body.
• extracellular: occurring or found outside of a cell
• casting: the excreta of an earthworm or similar creature
• intracellular: Intracellular digestion is a form of digestion which takes place within the cytoplasm of the organism. Intracellular digestion takes place in animals without a digestive tract, in which food items are brought into the cell for digestion. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.03%3A_Digestive_Systems_-_Invertebrate_Digestive_Systems.txt |
Learning Objectives
• Differentiate among the types of vertebrate digestive systems
Vertebrate Digestive Systems
Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating un-masticated (un-chewed) food.
Monogastric: Single-chambered Stomach
As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach has an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. In the small intestine, enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels to the large intestine where water is absorbed and the drier waste material is compacted into feces that are stored until excreted through the rectum.
Avian
Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth, so their digestive system must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food while keeping their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine, while the waste is excreted through the cloaca.
Ruminants
Ruminants are mainly herbivores, such as cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them process vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue, and lips to tear and chew their food. From the mouth, the food travels through the esophagus and into the stomach.
To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum, the “true” stomach, is the equivalent of the monogastric stomach chamber. This is where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, while the large intestine aids in the elimination of waste.
Pseudo-ruminants
Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot break down cellulose, but microorganisms present in the digestive system can. Since the digestive system must be able to handle large amounts of roughage and break down the cellulose, pseudo-ruminants have a three-chamber stomach. In contrast to ruminants, their cecum (a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials) is large. This is the site where the roughage is fermented and digested. These animals do not have a rumen, but do have an omasum, abomasum, and reticulum.
Key Points
• Monogastric animals have a single stomach that secretes enzymes to break down food into smaller particles; additional gastric juices are produced by the liver, salivary glands, and pancreas to assist with the digestion of food.
• The avian digestive system has a mouth (beak), crop (for food storage), and gizzard (for breakdown), as well as a two-chambered stomach consisting of the proventriculus, which releases enzymes, and the true stomach, which finishes the breakdown.
• Ruminants, such as cows and sheep, are those animals that have four stomachs; they eat plant matter and have symbiotic bacteria living within their stomachs to help digest cellulose.
• Pseudo-ruminants (such as camels and alpacas) are similar to ruminants, but have a three-chambered stomach; the symbiotic bacteria that help them to break down cellulose is found in the cecum, a chamber close to the large intestine.
Key Terms
• peristalsis: the rhythmic, wave-like contraction and relaxation of muscles which propagates in a wave down a muscular tube
• proventriculus: the part of the avian stomach, between the crop and the gizzard, that secretes digestive enzymes
• cellulose: a complex carbohydrate that forms the main constituent of the cell wall in most plants | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.04%3A_Digestive_Systems_-_Vertebrate_Digestive_Systems.txt |
Learning Objectives
• Describe the parts of the digestive system from the oral cavity through the stomach
Parts of the Digestive System
The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms. The upper gastrointestinal tract includes the oral cavity, esophagus, and stomach.
Oral Cavity
The oral cavity, or mouth, is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food.
The extensive chemical process of digestion begins in the mouth. As food is chewed, saliva, produced by the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals. There are three major glands that secrete saliva: the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobulins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. In addition, saliva contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme, lipase, is produced by the cells in the tongue. It is a member of a class of enzymes that can break down triglycerides. Lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva shape the food into a mass called the bolus for swallowing. The tongue aids in swallowing by moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the stomach. The tracheal opening, the glottis, is covered by a cartilaginous flap, the epiglottis. When swallowing, the epiglottis closes the glottis, allowing food to pass into the esophagus, not into the trachea, preventing food from reaching the lungs.
Esophagus
The esophagus is a tubular organ connecting the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach. The peristalsis wave is unidirectional: it moves food from the mouth to the stomach; reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex, taking place in response to the act of swallowing.
Stomach
A large part of digestion occurs in the stomach. The stomach, a saclike organ, secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly- acidic environment is required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available.
The stomach is also the major site for protein digestion in animals other than ruminants. Protein digestion is mediated in the stomach chamber by an enzyme called pepsin, which is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Another cell type, parietal cells, secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly-acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially-digested food and gastric juice mixture is called chyme. Chyme passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter.
Key Points
• Mechanical and chemical digestion begin in the mouth with the chewing of food and the release of saliva, which starts carbohydrate digestion.
• The epiglottis covers the trachea so the bolus (ball of chewed food) does not go down into the trachea or lungs, but rather into the esophagus.
• The tongue positions the bolus for swallowing and then peristalsis pushes the bolus down the esophagus into the stomach.
• In the stomach, acids and enzymes are secreted to break down food into its nutrient components.
• The churning of the stomach helps to mix the digestive juices with the food, turning it into a substance called chyme.
Key Terms
• bolus: a round mass of something, especially of chewed food in the mouth or alimentary canal
• peristalsis: the rhythmic, wave-like contraction and relaxation of muscles which propagates in a wave down a muscular tube
• pepsin: a digestive enzyme that chemically digests, or breaks down, proteins into shorter chains of amino acids
• chyme: the thick semifluid mass of partly digested food that is passed from the stomach to the duodenum | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.05%3A_Digestive_Systems_-_Digestive_System-_Mouth_and_Stomach.txt |
Learning Objectives
• Describe the parts of the digestive system from the small intestine through the accessory organs
Parts of the Digestive System
The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms. The lower gastrointestinal tract includes the small and large intestines, rectum, anus, and accessory organs.
Small Intestine
Chyme moves from the stomach to the small intestine: the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly-folded surface containing finger-like projections: the villi. The apical surface of each villus has many microscopic projections: the microvilli. These structures are lined with epithelial cells on the luminal side to allow the nutrients from the digested food to be absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients.
The human small intestine, over 6 m long, is divided into three parts: the duodenum, the jejunum, and the ileum. The “C-shaped,” fixed part of the small intestine, the duodenum, is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum where it mixes with pancreatic juices. The alkaline solution is rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Absorption of fatty acids also takes place in there.
The second part of the small intestine is called the jejunum. Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.
The ileum is the last part of the small intestine. It is here that bile salts and vitamins are absorbed into blood stream. The undigested food is sent from the ileum to the colon through the ileocecal valve via peristaltic movements of the muscle. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.
Large Intestine
The large intestine reabsorbs water from undigested food material and processes waste material; although it is also capable of absorbing vitamins that are synthesized by the normal microflora housed herein. The human large intestine is much smaller in length than the small intestine, but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon. It is the receiving pouch for the waste matter. The colon, home to many bacteria or “intestinal flora” that aid in the digestive processes, can be divided into four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food and to store waste material. Due to their diet, carnivorous mammals have a shorter large intestine compared to herbivorous mammals.
Rectum and Anus
The rectum is the terminal end of the large intestine. Its primary role is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus, an opening at the far-end of the digestive tract, is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary, while the outer sphincter is voluntary.
Accessory Organs
The organs discussed above are those of the digestive tract through which food passes. Accessory organs are those that add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.
The liver, the largest internal organ in humans, plays a very important role in digestion of fats and detoxifying blood. It produces bile: a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats along with synthesizing many plasma proteins.
The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature; the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.
The gallbladder, a small organ, aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.
Key Points
• The small intestine is the primary site of enzyme activity and nutrient absorption during digestion.
• Enzymes from the liver and pancreas are added to the duodenum of the small intestine to aid with chemical breakdown; the remaining chyme is moved via peristalsis through the jejunum and the ileum into the large intestine.
• The large intestine reabsorbs water from the remaining food material and compacts the waste for elimination from the body by way of the rectum and the anus.
• The liver creates and secretes bile, which breaks down lipids; the pancreas secretes enzymes to assist with protein digestion.
Key Terms
• villus: a small projection from a mucous membrane, particularly those found in the intestines
• sphincter: a ringlike band of muscle that surrounds a bodily opening, constricting and relaxing as required for normal physiological functioning
• duodenum: the first part of the small intestine, starting at the lower end of the stomach and extending to the jejunum
• colon: part of the large intestine; the final segment of the digestive system, after (distal to) the ileum and before (proximal to) the anus | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.06%3A_Digestive_Systems_-_Digestive_System-_Small_and_Large_Intestines.txt |
Learning Objectives
• Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body
Food Requirements
What are the fundamental requirements of the animal diet? The animal diet should be well balanced and provide nutrients required for bodily function along with the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability.
Organic Precursors
The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.
Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted, as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.
Essential Nutrients
While the animal body can synthesize many of the molecules required for function from the organic precursors, there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients: they must be eaten as the body cannot produce them.
Vitamins and minerals are substances found in the food we eat. Your body needs them to be able to work properly and for growth and development. Each vitamin has its own special role to play. For example, vitamin D (added to whole milk or naturally-occurring in sardines), helps make bones strong, while vitamin A (found in carrots) helps with night vision. Vitamins fall into two categories: fat soluble and water soluble. The fat-soluble vitamins dissolve in fat and can be stored in your body, whereas the water-soluble vitamins need to dissolve in water before your body can absorb them; therefore, the body cannot store them.
Fat-soluble vitamins are found primarily in foods that contain fat and oil, such as animal fats, vegetable oils, dairy foods, liver, and fatty fish. Your body needs these vitamins every day to enable it to work properly. However, you do not need to eat foods containing these every day. If your body does not need these vitamins immediately, they will be stored in the liver and fat tissues for future use. This means that stores can build up; if you have more than you need, fat soluble vitamins can become harmful. Some fat-soluble vitamins include vitamin A, vitamin K, vitamin D, and vitamin E. Unlike the other fat-soluble vitamins, vitamin D is difficult to obtain in adequate quantities in a normal diet; therefore, supplementation may be necessary.
Water-soluble vitamins are not stored in the body; therefore, you need to have them more frequently. If you have more then you need, the body rids itself of the extra vitamins during urination. Because the body does not store these vitamins, they are generally not harmful. Water-soluble vitamins are found in foods that include fruits, vegetables, and grains. Unlike fat-soluble vitamins, they can be destroyed by heat. This means that sometimes these vitamins can often be lost during cooking. This is why it is better to steam or grill these foods rather then boil them. Some water-soluble vitamins include vitamin B6, vitamin B12, vitamin C, biotin, folic acid, niacin, and riboflavin.
The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to synthesize some membrane phospholipids. Many people take supplements to ensure they are obtaining all the essential fatty acids they need. Sea buckthorn contains many of these fatty acids and is also high in vitamins. Sea buckthorn can be used to treat acne and promote weight loss and wound healing.
Minerals are inorganic essential nutrients that must also be obtained from food. Among their many functions, minerals help in cell structure and regulation; they are also considered co-factors. In addition to vitamins and minerals, certain amino acids must also be procured from food and cannot be synthesized by the body. These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids. The rest must be obtained from food.
Key Points
• The animal diet needs to be well-balanced in order to ensure that all necessary vitamins and minerals are being obtained.
• Vitamins are important for maintaining bodily health, making bones strong, and seeing in the dark.
• Water-soluble vitamins are not stored by the body and need to be consumed more regularly than fat-soluble vitamins, which build up within body tissues.
• Essential fatty acids need to be consumed through the diet and are important building blocks of cell membranes.
• Nine of the 20 amino acids cannot be synthesized by the body and need to be obtained from the diet.
Key Terms
• nutrient: a source of nourishment, such as food, that can be metabolized by an organism to give energy and build tissue
• catabolism: destructive metabolism, usually including the release of energy and breakdown of materials
• vitamin: any of a specific group of organic compounds essential in small quantities for healthy human growth, metabolism, development, and body function | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.07%3A_Nutrition_and_Energy_Production_-_Food_Requirements_and_Essential_Nutrients.txt |
Learning Objectives
• Summarize the ways in which animals obtain, store, and use food energy
Food Energy and ATP
Animals need food to obtain energy and maintain homeostasis. Homeostasis is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment. For example, the normal body temperature of humans is 37°C (98.6°F). Humans maintain this temperature even when the external temperature is hot or cold. The energy it takes to maintain this body temperature is obtained from food.
The primary source of energy for animals is carbohydrates, primarily glucose: the body’s fuel. The digestible carbohydrates in an animal’s diet are converted to glucose molecules and into energy through a series of catabolic chemical reactions.
Adenosine triphosphate, or ATP, is the primary energy currency in cells. ATP stores energy in phosphate ester bonds, releasing energy when the phosphodiester bonds are broken: ATP is converted to ADP and a phosphate group. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration.
ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues. It also provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. When the amount of ATP available is in excess of the body’s requirements, the liver uses the excess ATP and excess glucose to produce molecules called glycogen (a polymeric form of glucose) that is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy is an evolutionarily-important step in helping animals deal with mobility, food shortages, and famine.
Key Points
• Animals obtain energy from the food they consume, using that energy to maintain body temperature and perform other metabolic functions.
• Glucose, found in the food animals eat, is broken down during the process of cellular respiration into an energy source called ATP.
• When excess ATP and glucose are present, the liver converts them into a molecule called glycogen, which is stored for later use.
Key Terms
• glucose: a simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism
• adenosine triphosphate: a multifunctional nucleoside triphosphate used in cells as a coenzyme, often called the “molecular unit of energy currency” in intracellular energy transfer
• phosphodiester: any of many biologically active compounds in which two alcohols form ester bonds with phosphate
34.09: Digestive System Processes - Ingestion
Learning Objectives
• Describe the process of ingestion and its role in the digestive system
Obtaining nutrition and energy from food is a multi-step process. For animals, the first step is ingestion, the act of taking in food. The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process is ingestion: taking in food through the mouth. Once in the mouth, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). Mastication, or chewing, is an extremely important part of the digestive process, especially for fruits and vegetables, as these have indigestible cellulose coats which must be physically broken down. Also, digestive enzymes only work on the surfaces of food particles, so the smaller the particle, the more efficient the digestive process. While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus.
Besides nutritional items, other substances may be ingested, including medications (where ingestion is termed oral administration) and substances considered inedible, such as insect shells. Ingestion is also a common route taken by pathogenic organisms and poisons entering the body.
Some pathogens transmitted via ingestion include viruses, bacteria, and parasites. Most commonly, this takes place via the fecal-oral route. An intermediate step is often involved, such as drinking water contaminated by feces or food prepared by workers who fail to practice adequate hand-washing. This is more common in regions where untreated sewage is prevalent. Diseases transmitted via the fecal-oral route include hepatitis A, polio, and cholera.
Key Points
• Food is ingested through the mouth and broken down through mastication (chewing).
• Food must be chewed in order to be swallowed and broken down by digestive enzymes.
• While food is being chewed, saliva chemically processes the food to aid in swallowing.
• Medications and harmful or inedible substances may be ingested as well.
• Pathogens, such as viruses, bacteria, and parasites, may be transmitted via ingestion, causing diseases like hepatitis A, polio, and cholera.
Key Terms
• ingestion: consuming something orally, whether it be food, drink, medicine, or other substance; the first step of digestion
• bolus: a round mass of something, especially of chewed food in the mouth or alimentary canal
• mastication: the process of chewing | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.08%3A_Nutrition_and_Energy_Production_-_Food_Energy_and_ATP.txt |
Learning Objectives
• Explain the processes of digestion and absorption
Digestion and Absorption
Digestion is the mechanical and chemical break down of food into small organic fragments. Mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the body can use.
It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across cell membranes. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance.
Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tracts of animals. Digestive enzymes are diverse and are found in the saliva secreted by the salivary glands, in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract.
Intestinal microflora benefit the host by gleaning the energy from the fermentation of undigested carbohydrates and the subsequent absorption of short-chain fatty acids. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols and xenobiotics.
Carbohydrates
The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The acidic environment in the stomach stops amylase from continuing to break down the molecules.
The next step of carbohydrate digestion takes place in the duodenum. The chyme from the stomach enters the duodenum and mixes with the digestive secretions from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose and other disaccharides. These disaccharides are then broken down into monosaccharides by enzymes called maltases, sucrases, and lactases. The monosaccharides produced are absorbed so that they can be used in metabolic pathways to harness energy. They are absorbed across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
Protein
A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking them down into peptides, short chains of four to nine amino acids. In the duodenum, other enzymes – trypsin, elastase, and chymotrypsin – act on the peptides, reducing them to smaller peptides. These enzymes are produced by the pancreas and released into the duodenum where they also act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). The amino acids are absorbed into the bloodstream through the small intestine.
Lipids
Lipid (fat) digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides, through emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances. Bile contains bile salts, which have hydrophobic and hydrophilic sides. The bile salts’ hydrophilic side can interface with water, while the hydrophobic side interfaces with lipids, thereby emulsifying large lipid globules into small lipid globules.
Emulsification is important for the digestion of lipids because lipases can only efficiently act on the lipids when they are broken into small aggregates. Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell, entering the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides, forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells, leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules, and are then coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids; they have proteins on their surface. The surface is also composed of the hydrophilic phosphate “heads” of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis, entering the lymphatic vessels. From there, they enter the blood in the subclavian vein.
Vitamins
Vitamins can be either water-soluble or lipid-soluble. Fat-soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.
Key Points
• In the mouth, carbohydrates are broken down by amylase into maltose (a disaccharide ) and then move down the esophagus, which produces mucus for lubrication, but no digestive enzymes.
• In the duodenum, disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases; the monosaccharides produced are then absorbed into the bloodstream and transported to cells to be used in metabolic pathways to harness energy.
• In the stomach, proteins are broken down into peptides, which are then broken down into single amino acids that are absorbed in the bloodstream though the small intestine.
• Lipids are digested mainly in the small intestine by bile salts through the process of emulsification, which allows lipases to divide lipids into fatty acids and monoglycerides.
• Monoglycerides and fatty acids enter absorptive cells in the small intestine through micelles; they leave micelles and recombine into chylomicrons, which then enter the bloodstream.
• Fat-soluble vitamins are absorbed in the same manner as lipids; water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.
Key Terms
• chemical digestion: The process of enzymes breaking down food into small molecules the body can use.
• lipase: Enzymes in the pancreatic juices that break down lipids.
• chylomicron: A microscopic globule of triglycerids and other lipids coated with proteins, found in blood and lymphatic vessels, that is associated with the digestion of fats.
• amylase: Any of a class of digestive enzymes present in saliva that break down complex carbohydrates, such as starch, into simpler sugars like glucose.
• mechanical digestion: The physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by enzymes. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.10%3A_Digestive_System_Processes_-_Digestion_and_Absorption.txt |
Learning Objectives
• Describe the process of elimination and problems that can occur
The final step in digestion is the elimination of undigested food content and waste products. After food passes through the small intestine, the undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.
Common Problems with Elimination
Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if not enough water is removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
Emesis
Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including, but not limited to, viruses, bacteria, emotions, trauma, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla.
Key Points
• Water is reabsorbed in the colon after undigested food enters it from the small intestine.
• Waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum.
• The rectum expands in response to the storage of fecal matter; neural signals are triggered, and the waste is eliminated from the anus by peristaltic movements of the rectum.
• Constipation is a condition where the feces are hardened because of excess water removal in the colon.
• Diarrhea results when large amounts of water are not removed from the feces.
• Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth caused by the strong contractions produced by the stomach muscles.
Key Terms
• emesis: the act or process of vomiting
• intestinal flora: the bacterial colonies that normally live in the digestive tract of animals
• constipation: condition where the feces are hardened because of excess water removal in the colon
34.12: Digestive System Regulation - Neural Responses to Food
Learning Objectives
• Summarize the neural responses to food
In reaction to the smell, sight, or thought of food, the first hormonal response is that of salivation. The salivary glands secrete more saliva in response to the stimulus presented by food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid to digest the food. Recall that the peristaltic movements of the esophagus and other organs of the digestive tract are under the control of the brain. The brain prepares these muscles for movement as well. When the stomach is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric control: the cephalic phase, the gastric phase, and the intestinal phase. Each requires many enzymes and is under neural control as well.
Digestive Phases
The response to food begins even before food enters the mouth. The first phase of ingestion, called the cephalic phase, is controlled by the neural response to the stimulus provided by food. All aspects, such as sight, sense, and smell, trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and salivary secretion in the cephalic phase can also take place at the thought of food. Right now, if you think about a piece of chocolate or a crispy potato chip, the increase in salivation is a cephalic phase response to the thought. The central nervous system prepares the stomach to receive food.
The gastric phase begins once the food arrives in the stomach. It builds on the stimulation provided during the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated by (1) distension of the stomach, (2) a decrease in the pH of the gastric contents, and (3) the presence of undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions.
The intestinal phase begins when chyme enters the small intestine, triggering digestive secretions. This phase controls the rate of gastric emptying. In addition to gastric emptying, when chyme enters the small intestine, it triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.
Key Points
• The cephalic phase is controlled by sight, sense, and smell, which trigger neural responses, including salivation and hydrochloric acid production, before food has even reached the mouth.
• Once food reaches the stomach, gastric acids and enzymes process the ingested materials in the gastric phase, which involves local, hormonal, and neural responses.
• The intestinal phase controls the rate of gastric emptying and the release of hormones needed to digest chyme in the small intestine.
Key Terms
• neural: of, or relating to the nerves, neurons or the nervous system
• salivary gland: any of several exocrine glands that produce saliva to break down carbohydrates in food enzymatically
• peristaltic: of, or pertaining to the rhythmic, wave-like contraction of the digestive tract that forces food through it | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.11%3A_Digestive_System_Processes_-_Elimination.txt |
Learning Objectives
• Describe hormonal responses to food
The endocrine system controls the response of the various glands in the body and the release of hormones at the appropriate times. The endocrine system’s effects are slow to initiate, but prolonged in their response, lasting from a few hours up to weeks. The system is made of a series of glands that produce chemicals called hormones. These hormones are chemical mediators released from endocrine tissue into the bloodstream where they travel to target tissue and generate a response.
One of the important factors under hormonal control is the stomach acid environment. During the gastric phase, the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin stimulates the release of stomach acid, or hydrochloric acid (HCl), which aids in the digestion of the majority of proteins. However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism.
In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.
Another level of hormonal control occurs in response to the composition of food. Foods high in lipids (fatty foods) take a long time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down the peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed.
Understanding the hormonal control of the digestive system is an important area of ongoing research. Scientists are exploring the role of each hormone in the digestive process and developing ways to target these hormones. Advances could lead to knowledge that may help to battle the obesity epidemic.
Key Points
• The presence and absence of hormones that are released into the bloodstream generate specific digestive responses; they either stimulate or discontinue digestive processes.
• In hormone control, a negative feedback mechanism takes place when the stomach is empty and its acidic environment does not need to be maintained; as a result, a hormone is released to stop the release of hydrochloric acid, which was previously activated to aid digestion.
• In some cases, hormones work in tandem and sequentially to achieve important digestive functions, such as in the breakdown of acidic chyme, where hormones act in releasing the appropriate secretions in the appropriate stages of digestion.
• When digesting certain types of foods, such as ones high in fat, hormones can control the speed of food digestion and, therefore, absorption.
Key Terms
• endocrine system: a control system of ductless glands that secrete hormones which circulate via the bloodstream to affect cells within specific organs
• chyme: the thick semifluid mass of partly digested food that is passed from the stomach to the duodenum
• secretin: a peptide hormone, secreted by the duodenum, that serves to regulate its acidity
• cholecystokinin: any of several peptide hormones that stimulate the digestion of fat and protein
• somatostatin: a polypeptide hormone, secreted by the pancreas, that inhibits the production of certain other hormones
• gastrin: a hormone that stimulates the production of gastric acid in the stomach
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44744/latest...ol11448/latest. License: CC BY: Attribution
• neural. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/neural. License: CC BY-SA: Attribution-ShareAlike
• peristaltic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/peristaltic. License: CC BY-SA: Attribution-ShareAlike
• salivary gland. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/salivary_gland. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Digestive System Regulation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44744/latest...e_34_04_01.jpg. License: CC BY: Attribution
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44744/latest...ol11448/latest. License: CC BY: Attribution
• Endocrine system. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Endocrine_system. License: CC BY-SA: Attribution-ShareAlike
• secretin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/secretin. License: CC BY-SA: Attribution-ShareAlike
• somatostatin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/somatostatin. License: CC BY-SA: Attribution-ShareAlike
• cholecystokinin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cholecystokinin. License: CC BY-SA: Attribution-ShareAlike
• gastrin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gastrin. License: CC BY-SA: Attribution-ShareAlike
• chyme. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/chyme. License: CC BY-SA: Attribution-ShareAlike
• endocrine system. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/endocrine_system. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Digestive System Regulation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44744/latest...e_34_04_01.jpg. License: CC BY: Attribution
• Endocrine Alimentary system en. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/File:Endocrine_Alimentary_system_en.svg. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/34%3A_Animal_Nutrition_and_the_Digestive_System/34.13%3A_Digestive_System_Regulation_-_Hormonal_Responses_to_Food.txt |
Learning Objectives
• Recall the differences in structure and function between the central and peripheral nervous systems
The Nervous System: Introduction
The nervous system coordinates the body’s voluntary and involuntary actions and transmits signals between different parts of the body. Nervous tissue first arose in wormlike organisms approximately 550 to 600 million years ago. In most types of vertebrate animals, it consists of two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. The PNS consists mainly of nerves, which are long fibers that connect the CNS to every other part of the body. The PNS includes motor neurons (mediating voluntary movement), the autonomic nervous system (comprising the sympathetic nervous system and the parasympathetic nervous system, which regulate involuntary functions), and the enteric nervous system (a semi-independent part of the nervous system whose function is to control the gastrointestinal system).
The nervous system performs several functions simultaneously. For example, as you are reading, the visual system is processing what is seen on the page; the motor system controls the turn of the pages (or click of the mouse); the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors, from eating to sleeping to finding a mate.
Nervous systems throughout the animal kingdom vary in structure and complexity. Some organisms, such as sea sponges, lack a true nervous system. Others, such as jellyfish, lack a true brain. Instead, they have a system of separate-but-connected nerve cells (neurons) called a “nerve net.” Echinoderms, such as sea stars, have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system, made up of a small “brain” and two nerve cords, and a peripheral nervous system containing a system of nerves that extend throughout the body. The insect nervous system is more complex, but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopuses may have the most complicated of invertebrate nervous systems. They have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.
Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS and a PNS. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally (near the abdomen), whereas the vertebrate spinal cords are located dorsally (near the back). There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.
The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire: it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glial cells support neurons, recent evidence suggests they also assume some of the signaling functions of neurons.
Key Points
• The central nervous system contains the brain and spinal cord; the peripheral nervous system consists of nerves, motor neurons, the autonomic nervous system, and the enteric nervous system.
• The nervous system coordinates the voluntary and involuntary actions of the body by transmitting signals from the brain to the other body parts and listening for feedback.
• Nervous systems vary across different animals; some invertebrates lack a true nervous system or true brain, while other invertebrates have a brain and a system of nerves.
• Unlike vertebrates, not all invertebrates have both a CNS and PNS; their nerve cords are located ventrally rather than dorsally.
• The functions of the nervous system are performed by two types of cells: neurons, which transmit signals between them and from one part of the body to another, and glia, which regulate homeostasis, providing support and protection to the function of neurons.
Key Terms
• neuron: cell of the nervous system that conducts nerve impulses; consisting of an axon and several dendrites
• nervous system: an organ system that coordinates the body’s voluntary and involuntary actions and transmits signals between different parts of the body
• glial cell: cell in the nervous system that supports and protects neurons | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.01%3A_Neurons_and_Glial_Cells_-_Introduction.txt |
Learning Objectives
• Describe the functions of the structural components of a neuron
The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors, from basic reflexes to more complicated behaviors such as finding food and courting mates. The ability of neurons to communicate with each other, as well as with other types of cells, underlies all of these behaviors.
Most neurons share the same cellular components. But neurons are also highly specialized: different types of neurons have different sizes and shapes that relate to their functional roles.
Parts of a Neuron
Each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, relative to most cells, which are required for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. While some neurons have no dendrites, other types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.
Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock, that integrates signals from multiple synapses and serves as a junction between the cell body and an axon: a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals, in turn, synapse on other neurons, muscles, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter: from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along these types of axons, there are periodic gaps in the myelin sheath. These gaps, called “nodes of Ranvier,” are sites where the signal is “recharged” as it travels along the axon.
It is important to note that a single neuron does not act alone. Neuronal communication depends on the connections that neurons make with one another (as well as with other cells, such as muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.
Types of Neurons
There are different types of neurons; the functional role of a given neuron is intimately dependent on its structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system (and across species).
While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates, but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain. Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal cord). The Purkinje cell, a multipolar neuron in the cerebellum, has many branching dendrites, but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single structure that extends from the soma (like a unipolar cell), which later branches into two distinct structures (like a bipolar cell). Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receives sensory information and another that transmits this information to the spinal cord.
Key Points
• Dendrites are the tree-like structures in neurons that extend away from the cell body to receive messages from other neurons at synapses; not all neurons have dendrites.
• Synapses enable the dendrites from a single neuron to interact and receive signals from many other neurons.
• Axons are tube-like structures that send signals to other neurons, muscles, or organs; not all neurons have axons.
• Neurons are divided into four major types: unipolar, bipolar, multipolar, and pseudounipolar.
• Unipolar neurons have only one structure extending from the soma; bipolar neurons have one axon and one dendrite extending from the soma.
• Multipolar neurons contain one axon and many dendrites; pseudounipolar neurons have a single structure that extends from the soma, which later branches into two distinct structures.
Key Terms
• dendrite: branched projections of a neuron that conduct the impulses received from other neural cells to the cell body
• axon: long slender projection of a nerve cell that conducts nerve impulses away from the cell body to other neurons, muscles, and organs
• synapse: the junction between the terminal of a neuron and either another neuron or a muscle or gland cell, over which nerve impulses pass | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.02%3A_Neurons_and_Glial_Cells_-_Neurons.txt |
Learning Objectives
• Describe the specific roles that the seven types of glia play in the nervous systems
While glia (or glial cells ) are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be disastrous; most brain tumors are caused by mutations in glia.
Types of Glia
There are several different types of glia with different functions. Astrocytes make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier: a structure that blocks entrance of toxic substances into the brain. They have been shown, through calcium-imaging experiments, to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells, protecting the brain from invading microorganisms. Oligodendrocytes form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes; one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as bridges for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal cord and the brain, and is a component for the choroid plexus.
Key Points
• Glia guide developing neurons to their destinations, buffer harmful ions and chemicals, and build the myelin sheaths around axons.
• In the CNS astrocytes provide nutrients to neurons, give synapses structural support, and block toxic substances from entering the brain; satellite glia provide nutrients and structural support for neurons in the PNS.
• Microglia scavenge and degrade dead cells, protecting the brain from invading microorganisms.
• Oligodendrocytes form myelin sheaths around axons in the CNS; Schwann cell forms myelin sheaths around axons in the PNS.
• Radial glia serve as bridges for developing neurons as they migrate to their end destinations.
• Ependymal cells line fluid-filled ventricles of the brain and central canal of the spinal cord which produce cerebrospinal fluid.
Key Terms
• satellite glia: glial cell that provides nutrients for neurons in the PNS
• radial glia: glial cell that serves as a bridge for developing neurons as they move to their end destinations
• astrocyte: a neuroglial cell, in the shape of a star, in the brain | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.03%3A_Neurons_and_Glial_Cells_-_Glia.txt |
Learning Objectives
• Explain the formation of the resting potential in neurons
Nerve Impulse Transmission within a Neuron
For the nervous system to function, neurons must be able to send and receive signals. These signals are possible because each neuron has a charged cellular membrane (a voltage difference between the inside and the outside). The charge of this membrane can change in response to neurotransmitter molecules released from other neurons and environmental stimuli. Any voltage is a difference in electric potential between two points; for example, the separation of positive and negative electric charges on opposite sides of a resistive barrier. To understand how neurons communicate, one must first understand the basis of charged membranes and the baseline or ‘resting’ membrane charge.
Neuronal Charged Membranes
The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations: open, closed, and inactive. Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential.
Resting Membrane Potential
For quiescent cells, the relatively-static membrane potential is known as the resting membrane potential. The resting membrane potential is at equilibrium since it relies on the constant expenditure of energy for its maintenance. It is dominated by the ionic species in the system that has the greatest conductance across the membrane. For most cells, this is potassium. As potassium is also the ion with the most-negative equilibrium potential, usually the resting potential can be no more negative than the potassium equilibrium potential.
A neuron at rest is negatively charged because the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV); this number varies by neuron type and by species. This voltage is called the resting membrane potential and is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell. The difference in the number of positively-charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential. When the membrane is at rest, K+ ions accumulate inside the cell due to a net movement with the concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). The negative charge within the cell is created by the cell membrane being more permeable to K+ movement than Na+movement.
In neurons, potassium ions (K+) are maintained at high concentrations within the cell, while sodium ions (Na+) are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. More cations leaving the cell than entering it causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium-potassium pump help to maintain the resting potential, once it is established. Recall that sodium-potassium pumps bring two K+ ions into the cell while removing three Na+ ions per ATP consumed. As more cations are expelled from the cell than are taken in, the inside of the cell remains negatively charged relative to the extracellular fluid.
Key Points
• When the neuronal membrane is at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.
• Potassium ions diffuse out of the cell at a much faster rate than sodium ions diffuse into the cell because neurons have many more potassium leakage channels than sodium leakage channels.
• Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential.
Key Terms
• ion channel: a protein complex or single protein that penetrates a cell membrane and catalyzes the passage of specific ions through that membrane
• membrane potential: the difference in electrical potential across the enclosing membrane of a cell
• resting potential: the nearly latent membrane potential of inactive cells | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.04%3A_How_Neurons_Communicate_-_Nerve_Impulse_Transmission_within_a_Neuron-_Resting_Potential.txt |
Learning Objectives
• Explain the formation of the action potential in neurons
Action Potential
A neuron can receive input from other neurons via a chemical called a neurotransmitter. If this input is strong enough, the neuron will pass the signal to downstream neurons. Transmission of a signal within a neuron (in one direction only, from dendrite to axon terminal) is carried out by the opening and closing of voltage-gated ion channels, which cause a brief reversal of the resting membrane potential to create an action potential. As an action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons.
Depolarization and the Action Potential
When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, voltage-gated ion channels open. At excitatory synapses, positive ions flood the interior of the neuron and depolarize the membrane, decreasing the difference in voltage between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV), and Na+ channels in the axon hillock open, starting an action potential. Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. The action potential travels down the neuron as Na+ channels open.
Hyperpolarization and Return to Resting Potential
Action potentials are considered an “all-or nothing” event. Once the threshold potential is reached, the neuron completely depolarizes. As soon as depolarization is complete, the cell “resets” its membrane voltage back to the resting potential. The Na+ channels close, beginning the neuron’s refractory period. At the same time, voltage-gated K+ channels open, allowing K+ to leave the cell. As K+ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K+ out of the cell hyperpolarizes the cell, making the membrane potential more negative than the cell’s normal resting potential. At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential. Eventually, the extra K+ ions diffuse out of the cell through the potassium leakage channels, bringing the cell from its hyperpolarized state back to its resting membrane potential.
Myelin and Propagation of the Action Potential
For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon, increasing the speed of action potential conduction. Diseases like multiple sclerosis cause degeneration of the myelin, which slows action potential conduction because axon areas are no longer insulated so the current leaks.
A node of Ranvier is a natural gap in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na+ and K+ channels. The flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon. Action potential “jumps” from one node to the next in saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly; Na+ and K+ channels would have to continuously regenerate action potentials at every point along the axon. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon.
Key Points
• Action potentials are formed when a stimulus causes the cell membrane to depolarize past the threshold of excitation, causing all sodium ion channels to open.
• When the potassium ion channels are opened and sodium ion channels are closed, the cell membrane becomes hyperpolarized as potassium ions leave the cell; the cell cannot fire during this refractory period.
• The action potential travels down the axon as the membrane of the axon depolarizes and repolarizes.
• Myelin insulates the axon to prevent leakage of the current as it travels down the axon.
• Nodes of Ranvier are gaps in the myelin along the axons; they contain sodium and potassium ion channels, allowing the action potential to travel quickly down the axon by jumping from one node to the next.
Key Terms
• action potential: a short term change in the electrical potential that travels along a cell
• depolarization: a decrease in the difference in voltage between the inside and outside of the neuron
• hyperpolarize: to increase the polarity of something, especially the polarity across a biological membrane
• node of Ranvier: a small constriction in the myelin sheath of axons
• saltatory conduction: the process of regenerating the action potential at each node of Ranvier | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.05%3A_How_Neurons_Communicate_-_Nerve_Impulse_Transmission_within_a_Neuron-_Action_Potential.txt |
Learning Objectives
• Describe the process of synaptic transmission
Synaptic Transmission
In a chemical synapse, the pre and post synaptic membranes are separated by a synaptic cleft, a fluid filled space. The chemical event is involved in the transmission of the impulse via release, diffusion, receptor binding of neurotransmitter molecules and unidirectional communication between neurons.
Chemical Synapse
Neurotransmission at a chemical synapse begins with the arrival of an action potential at the presynaptic axon terminal. When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated Na+ channels. Na+ ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca2+ channels to open. Calcium ions entering the cell initiate a signaling cascade. A calcium sensing protein binds calcium and interacts with SNARE proteins. These SNARE proteins are involved in the membrane fusion. The synaptic vesicles fuse with the presynaptic axon terminal membrane and empty their contents by exocytosis into the synaptic cleft. Calcium is quickly removed from the terminal.
Fusion of a vesicle with the presynaptic membrane causes neurotransmitters to be released into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft, binding to receptor proteins on the postsynaptic membrane.
The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open. The binding of a neurotransmitter to its receptor is reversible. As long as it is bound to a post synaptic receptor, a neurotransmitter continues to affect membrane potential. The effects of the neurotransmitter generally lasts few milliseconds before being terminated. The neurotransmitter termination can occur in three ways. First, reuptake by astrocytes or presynaptic terminal where the neurotransmitter is stored or destroyed by enzymes. Second, degradation by enzymes in the synaptic cleft such as acetylcholinesterase. Third, diffusion of the neurotransmitter as it moves away from the synapse.
Key Points
• In a chemical synapse, the pre and post synaptic membranes are separated by a synaptic cleft, a fluid filled space.
• The chemical event is involved in the transmission of the impulse via release, diffusion, receptor binding of neurotransmitter molecules and unidirectional communication between neurons.
• The neurotransmitter termination can occur in three ways – reuptake, enzymatic degradation in the cleft and diffusion.
35.07: How Neurons Communicate - Signal Summation
Learning Objectives
• Describe signal summation
Each neuron connects with numerous other neurons, often receiving multiple impulses from them. Sometimes, a single excitatory postsynaptic potential (EPSP) is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. Summation, either spatial or temporal, is the addition of these impulses at the axon hillock. Together, synaptic summation and the threshold for excitation act as a filter so that random “noise” in the system is not transmitted as important information.
One neuron often has input from many presynaptic neurons, whether excitatory or inhibitory; therefore, inhibitory postsynaptic potentials (IPSPs) can cancel out EPSPs and vice versa. The net change in postsynaptic membrane voltage determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential. If the neuron only receives excitatory impulses, it will also generate an action potential. However, if the neuron receives as many inhibitory as excitatory impulses, the inhibition cancels out the excitation and the nerve impulse will stop there. Spatial summation means that the effects of impulses received at different places on the neuron add up so that the neuron may fire when such impulses are received simultaneously, even if each impulse on its own would not be sufficient to cause firing. Temporal summation means that the effects of impulses received at the same place can add up if the impulses are received in close temporal succession. Thus, the neuron may fire when multiple impulses are received, even if each impulse on its own would not be sufficient to cause firing.
Key Points
• Simultaneous impulses may add together from different places on the neuron to reach the threshold of excitation during spatial summation.
• When individual impulses cannot reach the threshold of excitation on their own, they can can add up at the same location on the neuron over a short time; this is known as temporal summation.
• The action potential of a neuron is fired only when the net change of excitatory and inhibitory impulses is non-zero.
Key Terms
• temporal summation: the effect when impulses received at the same place on the neuron add up
• spatial summation: the effect when simultaneous impulses received at different places on the neuron add up to fire the neuron
• axon hillock: the specialized part of the soma of a neuron that is connected to the axon and where impulses are added together | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.06%3A__How_Neurons_Communicate_-_Synaptic_Transmission.txt |
Learning Objectives
• Distinguish between long-term potentiation and long-term depression
Synaptic plasticity is the strengthening or weakening of synapses over time in response to increases or decreases in their activity. Plastic change also results from the alteration of the number of receptors located on a synapse. Synaptic plasticity is the basis of learning and memory, enabling a flexible, functioning nervous system. Synaptic plasticity can be either short-term (synaptic enhancement or synaptic depression) or long-term. Two processes in particular, long-term potentiation (LTP) and long-term depression (LTD), are important forms of synaptic plasticity that occur in synapses in the hippocampus: a brain region involved in storing memories.
Short-term Synaptic Enhancement and Depression
Short-term synaptic plasticity acts on a timescale of tens of milliseconds to a few minutes. Short-term synaptic enhancement results from more synaptic terminals releasing transmitters in response to presynaptic action potentials. Synapses will strengthen for a short time because of either an increase in size of the readily- releasable pool of packaged transmitter or an increase in the amount of packaged transmitter released in response to each action potential. Depletion of these readily-releasable vesicles causes synaptic fatigue. Short-term synaptic depression can also arise from post-synaptic processes and from feedback activation of presynaptic receptors.
Long-term Potentiation (LTP)
Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection, which can last for minutes or hours. LTP is based on the Hebbian principle: “cells that fire together wire together. ” There are various mechanisms, none fully understood, behind the synaptic strengthening seen with LTP.
One known mechanism involves a type of postsynaptic glutamate receptor: NMDA (N-Methyl-D-aspartate) receptors. These receptors are normally blocked by magnesium ions. However, when the postsynaptic neuron is depolarized by multiple presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions are forced out and Ca2+ ions pass into the postsynaptic cell. Next, Ca2+ ions entering the cell initiate a signaling cascade that causes a different type of glutamate receptor, AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, to be inserted into the postsynaptic membrane. Activated AMPA receptors allow positive ions to enter the cell.
Therefore, the next time glutamate is released from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because the binding of glutamate to these AMPA receptors will allow more positive ions into the cell. The insertion of additional AMPA receptors strengthens the synapse so that the postsynaptic neuron is more likely to fire in response to presynaptic neurotransmitter release. Some drugs co-opt the LTP pathway; this synaptic strengthening can lead to addiction.
Long-term Depression (LTD)
Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic connection. One mechanism known to cause LTD also involves AMPA receptors. In this situation, calcium that enters through NMDA receptors initiates a different signaling cascade, which results in the removal of AMPA receptors from the postsynaptic membrane. With the decrease in AMPA receptors in the membrane, the postsynaptic neuron is less responsive to the glutamate released from the presynaptic neuron. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP. The weakening and pruning of unused synapses trims unimportant connections, leaving only the salient connections strengthened by long-term potentiation.
Key Points
• Short-term synaptic enhancement occurs when the amount of available neurotransmitter is increased, while short-term synaptic depression occurs when the amount of vesicles with neurotransmitters is decreased.
• Synapses are strengthened in long-term potentiation (LTP) when AMPA receptors (which bind to negatively-charged glutamate) are increased, allowing more calcium ions to enter the cell, causing a higher excitatory response.
• Long-term depression (LTD) occurs when the AMPA receptors are decreased, which decreases the amount of calcium ions entering the cell, weakening the synaptic response to the release of neurotransmitters.
• The strengthening and weakening of synapses over time controls learning and memory in the brain.
Key Terms
• long-term potentiation: a long-lasting (hours in vitro, weeks to months in vivo) increase, typically in amplitude, of the response of a postsynaptic neuron to a particular pattern of stimuli from a presynaptic neuron
• long-term depression: a long-term weakening of a synaptic connection
• plasticity: the property of neuron that allows it to be strengthened or weakened | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.08%3A__How_Neurons_Communicate_-_Synaptic_Plasticity.txt |
Learning Objectives
• Summarize the nervous system
Introduction to the Nervous System
The nervous system of higher vertebrates (the group that includes humans) is a widely-distributed communication and processing network that serves controlling functions over other organ systems. It possesses a key function in the orientation of the individual; controls its behavior, motor function, and sensory processing; and contains mechanisms to store information. A classification of the nervous system can be performed under different aspects. The anatomical compartmentalization of its components defines the classical approach. Two major divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS).
Central Nervous System
The vertebrate central nervous system (CNS) contains the brain and the spinal cord. The brain contains structurally- and functionally-defined regions. In mammals, these include the cortex (which can be broken down into four primary functional lobes: frontal, temporal, occipital, and parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem; although structures in some of these designations overlap. While functions may be primarily localized to one structure in the brain, most complex functions, such as language and sleep, involve neurons in multiple brain regions. The spinal cord is the information superhighway, connecting the brain with the rest of the body through the peripheral nerves. It transmits sensory and motor input and also controls motor reflexes.
The CNS is covered with three layers of protective coverings called meninges (from the Greek word for membrane). The outermost layer is the dura mater (Latin for “hard mother”). As the Latin name suggests, the primary function for this thick layer is to protect the brain and spinal cord. The dura mater also contains vein-like structures that carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater. The last layer is the pia mater (Latin for “soft mother”), which directly contacts and covers the brain and spinal cord like plastic wrap. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF). CSF is produced by a tissue called the choroid plexus in fluid-filled compartments in the CNS called ventricles. The brain floats in CSF, which acts as a cushion and shock absorber, making the brain neutrally buoyant. CSF also functions to circulate chemical substances throughout the brain and into the spinal cord.
Peripheral Nervous System
The peripheral nervous system consists of nerves that are connected to the brain (cranial nerves) and nerves that are connected to the spinal cord (spinal nerves). The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs, essentially serving as a communication relay between the brain and the extremities. Unlike the CNS, the PNS is not protected by the bone of spine and skull. Nor does it have a barrier between itself and the blood, leaving it exposed to toxins and mechanical injuries.
The autonomic nervous system, also part of the peripheral nervous system, controls internal body functions that are not under conscious control. For example, when a prey animal is chased by a predator, the autonomic nervous system automatically increases the rate of breathing and the heartbeat. It dilates the blood vessels that carry blood to the muscles, releases glucose from the liver, and makes other adjustments to provide for the sudden increase in activity. When the animal has escaped and is safe once again, the nervous system slows down all these processes and resumes all the normal body activities, such as the digestion of food.
Key Points
• The central nervous system consists of the brain, which controls complex body functions, and the spinal cord, which transmits signals from the brain to the rest of the body.
• The brain and spinal cord are covered by three layers of meninges, or protective coverings: the dura mater, the arachnoid mater, and the pia mater.
• Cerebrospinal fluid surrounds the brain, cushioning it and providing shock absorption to prevent damage.
• The peripheral nervous system is made up of nerves that originate within the brain and spinal cord; it serves to relay information from the central nervous system to all parts of the body.
• The autonomic system controls involuntary bodily functions, such as heart rate, breathing, digestion, and blood vessel dilation.
Key Terms
• central nervous system: in vertebrates, that part of the nervous system comprising the brain and spinal cord
• cerebrospinal fluid: a clear bodily fluid that occupies the subarachnoid space in the brain (between the skull and the cerebral cortex), and which acts as a cushion or buffer for the cortex
• meninges: the three membranes that envelop the brain and spinal cord
• autonomic nervous system: the part of the nervous system that regulates the involuntary activity of the heart, intestines and glands
• peripheral nervous system: the part of the nervous system which is not the central nervous system
Contributions and Attributions
• central nervous system. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/central_nervous_system. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44749/latest...ol11448/latest. License: CC BY: Attribution
• Anatomy and Physiology of Animals/Nervous System. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Anatomy...Nervous_System. License: CC BY-SA: Attribution-ShareAlike
• General Anatomy/Nervous System. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/General...Nervous_System. License: CC BY-SA: Attribution-ShareAlike
• Peripheral nervous system. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Periphe...nervous_system. License: CC BY-SA: Attribution-ShareAlike
• meninges. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/meninges. License: CC BY-SA: Attribution-ShareAlike
• peripheral nervous system. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/periphe...rvous%20system. License: CC BY-SA: Attribution-ShareAlike
• cerebrospinal fluid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cerebrospinal_fluid. License: CC BY-SA: Attribution-ShareAlike
• autonomic nervous system. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/autono...nervous_system. License: CC BY-SA: Attribution-ShareAlike
• Meninges-en. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Meninges-en.svg. License: Public Domain: No Known Copyright
• Central nervous system. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Ce...ous_system.svg. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.09%3A__The_Nervous_System.txt |
Learning Objectives
• Describe the structure and function of the cerebral cortex
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum.
Cerebral Cortex
The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex, which is folded into hills called gyri (singular: gyrus) and valleys called sulci (singular: sulcus ). The cortex is composed of two hemispheres, right and left, which are separated by a large sulcus. A thick fiber bundle, the corpus callosum, connects the two hemispheres, allowing information to be passed from one side to the other. Although there are some brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have relatively-undeveloped nervous systems.
In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain syndrome, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual fields, they may be unable to verbally name the object (and may claim not to have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and is unable to signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so, but will still be unable to vocally identify it.
The Four Brain Lobes
Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally- and spatially-defined lobes: frontal, parietal, temporal, and occipital. The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Areas within the motor cortex map to different muscle groups; there is some organization to this map. For example, the neurons that control movement of the fingers are next to the neurons that control movement of the hand. Neurons in the frontal lobe also control cognitive functions such as maintaining attention, speech, and decision-making. Studies of humans who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing risk.
The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and reading. Two of the parietal lobe’s main functions are processing somatosensation (touch sensations such as pressure, pain, heat, cold) and processing proprioception (the sense of how parts of the body are oriented in space). The parietal lobe contains a somatosensory map of the body similar to the motor cortex.
The occipital lobe is located at the back of the brain. It is primarily involved in vision: seeing, recognizing, and identifying the visual world.
The temporal lobe is located at the base of the brain by the ears. It is primarily involved in processing and interpreting sounds. It also contains the hippocampus (Greek for “seahorse”, which is what it resembles), a structure that processes memory formation. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his surgery and could learn new motor tasks).
Key Points
• The cerebral cortex is the outermost layer of the brain; it is easily recognizable by the grooves (sulci) and “hills” (gyri).
• The brain contains two hemispheres, the left and the right, which are connected by a bundle of nerve fibers called the corpus callosum that transmits information between them.
• The frontal lobe houses the olfactory bulb, which processes smells; the motor cortex, which controls movement; and it controls cognitive functions such as attention, speech, and decision-making.
• The parietal lobe is involved in speech and reading, as well as interpreting touch sensations such as pressure, pain, heat, cold, along with sensing where each part of the body is in relation to the others and its environment.
• The occipital lobe interprets visual cues, such as what we see and recognition of faces and objects.
• The temporal lobe processes and interprets sounds and is also involved in forming new memories, a task for which the hippocampus, a structure inside the temporal lobe, is responsible.
Key Terms
• corpus callosum: in mammals, a broad band of nerve fibers that connects the left and right hemispheres of the brain
• proprioception: the sense of the position of parts of the body, relative to other neighbouring parts of the body
• somatosensation: general senses which respond to stimuli like temperature, pain, pressure, and vibration
• gyrus: a ridge or fold on the cerebral cortex
• sulcus: any of the grooves that mark the convolutions of the surface of the brain | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.10%3A_The_Central_Nervous_System_-_Cerebral_Cortex_and_Brain_Lobes.txt |
Learning Objectives
• Explain the structure and function of the non-cerebral cortex portions of the brain
Basal Ganglia
Interconnected brain areas called the basal ganglia (or basal nuclei) play important roles in movement control and posture. Damage to the basal ganglia, which occurs in Parkinson’s disease, leads to motor impairments such as a shuffling gait when walking. The basal ganglia also regulate motivation. For example, when a wasp sting led to bilateral basal ganglia damage in a 25-year-old businessman, he began to spend all his days in bed and showed no interest in anything or anybody. But when he was externally stimulated, as when someone asked to play a card game with him, he was able to function normally. Interestingly, he and other similar patients do not report feeling bored or frustrated by their state.
Thalamus
The thalamus (Greek for “inner chamber”) acts as a gateway to and from the cortex. It receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states. A rare genetic disorder, fatal familial insomnia, causes the degeneration of thalamic neurons and glia. This disorder prevents affected patients from being able to sleep, among other symptoms, and is eventually fatal.
Hypothalamus
Below the thalamus is the hypothalamus. The hypothalamus controls the endocrine system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different hormones that affect other glands as well as other cells. This relationship means that the hypothalamus regulates important behaviors that are controlled by these hormones. The hypothalamus is the body’s thermostat: it makes sure key functions like food and water intake, energy expenditure, and body temperature are kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes called sleep cycles.
The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called the amygdala (Greek for “almond”). The two amygdale are important both for the sensation of fear and for recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.
Cerebellum
The cerebellum (Latin for “little brain”) sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks.
Brainstem
The brainstem connects the rest of the brain with the spinal cord. It consists of the midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem, allowing for the relay of signals between the brain and spinal cord. Ascending neural pathways cross in this section of the brain, allowing the left hemisphere of the cerebrum to control the right side of the body and vice versa. The brainstem coordinates motor control signals sent from the brain to the body. It also controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration.
Key Points
• The basal ganglia control movement and posture; they also appear to be involved in self-motivation.
• The thalamus communicates information from the cerebral cortex to the rest of the body and also helps regulate the states of consciousness versus sleep.
• The hypothalamus regulates the pituitary gland, which controls the release of hormones throughout the body; it indirectly regulates functions such as water intake, body temperature, and sleep cycles.
• The limbic system includes the amygdala, the hippocampus, and parts of the thalamus and hypothalamus; it regulates emotion, fear, and motivation.
• The cerebellum controls motor reflexes and is, therefore, involved in balance and muscle coordination.
• The brainstem connects and transmits signals from the brain to the spinal cord, controlling functions such as breathing, heart rate, and alertness.
Key Terms
• cingulate gyrus: a section of the cerebral cortex, belonging to the fornicate gyrus, which arches over the corpus callosum
• limbic system: part of the human brain involved in emotion, motivation, and emotional association with memory
• endocrine system: a control system of ductless glands that secrete hormones which circulate via the bloodstream to affect cells within specific organs | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.11%3A_The_Central_Nervous_System_-_Midbrain_and_Brain_Stem.txt |
Learning Objectives
• Describe the structure and function of the spinal cord
Connecting to the brainstem and extending down the body through the spinal column is the spinal cord: a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal cord is contained within the bones of the vertebral column, but is able to communicate signals to and from the body through its connections with spinal nerves (part of the peripheral nervous system). A cross-section of the spinal cord looks like a white oval containing a gray butterfly-shape. Myelinated axons (the part of neurons that send signals) compose the “white matter,” while neuron and glial cell bodies (neuronal “support” cells) compose the “grey matter.” Grey matter is also composed of interneurons, which connect two neurons, each located in different parts of the body. Axons and cell bodies in the dorsal (facing the back of the animal) spinal cord convey mostly sensory information from the body to the brain. Axons and cell bodies in the ventral (facing the front of the animal) spinal cord primarily transmit signals controlling movement from the brain to the body.
The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements, such as automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic connections. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one or two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey what happened (the knee jerked, or the hand was hot).
In the United States, there around 10,000 spinal cord injuries each year. Because the spinal cord is the information superhighway connecting the brain with the body, damage to the spinal cord can lead to paralysis. The extent of the paralysis depends on the location of the injury along the spinal cord and whether the spinal cord was completely severed. For example, if the spinal cord is damaged at the level of the neck, it can cause paralysis from the neck down, whereas damage to the spinal column further down may limit paralysis to the legs. Spinal cord injuries are notoriously difficult to treat because spinal nerves do not regenerate, although ongoing research suggests that stem cell transplants may be able to act as a bridge to reconnect severed nerves. Researchers are also looking at ways to prevent the inflammation that worsens nerve damage after injury. One such treatment is to pump the body with cold saline to induce hypothermia. This cooling can prevent swelling and other processes that are thought to worsen spinal cord injuries.
Key Points
• The spinal cord consists of a butterfly-shaped area of grey matter, containing neuronal and glial cell bodies, surrounded by white matter that contains the axons of the neurons.
• Neurons at the back of the spinal cord ( dorsal ) generally transmit information from the body to the brain, while neurons at the front of the spinal cord ( ventral ) primarily transmit information from the brain to the body.
• The spinal cord controls reflexes, which are incredibly fast reactions to stimuli; the speed at which they operate is due to the fact that they involve only a local connection between neurons and are not relayed through the brain.
• Spinal cord injuries often result in paralysis; they do not heal, as spinal nerves lack the ability to regenerate.
Key Terms
• grey matter: a collection of cell bodies and (usually) dendritic connections, in contrast to white matter
• synapse: the junction between the terminal of a neuron and either another neuron or a muscle or gland cell, over which nerve impulses pass
• axon: long slender projection of a nerve cell that conducts nerve impulses away from the cell body to other neurons, muscles, and organs
• white matter: a region of the central nervous system containing myelinated nerve fibers and no dendrites
• interneuron: a multipolar neuron that connects afferent and efferent neurons | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.12%3A_The_Central_Nervous_System_-_Spinal_Cord.txt |
Learning Objectives
• Explain the function of the autonomic nervous system
Autonomic Nervous System
The autonomic nervous system (ANS) serves as the relay between the central nervous system (CNS) and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands, largely without conscious control. It can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses. A preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
Sympathetic Nervous System
The sympathetic nervous system is responsible for the “fight or flight” response that occurs when an animal encounters a dangerous situation. One way to remember this is to think of the surprise a person feels when encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion, both of which help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator.
Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. This is both because one preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine (and the closely-related hormone epinephrine) into the blood stream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier for the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain (so the animal can think and run). The strength and speed of the sympathetic response helps an organism avoid danger. Scientists have found evidence that it may also increase long term potentiation in neurons, allowing the animal to remember the dangerous situation and avoid it in the future.
Parasympathetic Nervous System
While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs.
The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a ‘fight-or-flight’ event). Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
Key Points
• The autonomic nervous system controls the workings of internal organs such as the heart, lungs, digestive system, and endocrine systems; it does so without conscious effort.
• The sympathetic nervous system controls the body’s automatic response to danger, increasing the heart rate, dilating the blood vessels, slowing digestion, and moving blood flow to the heart, muscles, and brain.
• The parasympathetic nervous system works in opposition to the sympathetic; during periods of rest it slows the heart rate, lowers the blood pressure, stimulates digestion, and moves blood flow back to the skin.
Key Terms
• preganglionic: describing the nerve fibres that supply a ganglion
• sympathetic nervous system: the part of the autonomic nervous system that under stress raises blood pressure and heart rate, constricts blood vessels and dilates the pupils
• parasympathetic nervous system: one of the divisions of the autonomic nervous system, based between the brain and the spinal cord, that slows the heart and relaxes muscles
• acetylcholine: a neurotransmitter in humans and other animals, which is an ester of acetic acid and choline | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.13%3A_The_Peripheral_Nervous_System_-_Autonomic_Nervous_System.txt |
Learning Objectives
• Explain the role of the cranial and spinal nerves in the sensory-somatic nervous system
The sensory-somatic nervous system is composed of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the central nervous system (CNS). Motor neurons transmit messages about desired movement from the CNS to the muscles, causing them to contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, etc. ) and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse: one ending of the neuron is at the organ and the other directly contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses.
Cranial Nerves
Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve has a name. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information. The oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste (sensory) and swallowing (motor).
Spinal Nerves
Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each of the 31 spinal nerves (in humans) contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia. Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs, and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord, but are sometimes directly stimulated by sensory neurons.
Key Points
• The sensory and motor neurons of the sensory-somatic system have only one synapse between the organ and a neuron of the CNS; these synapses utilize acetylcholine to transmit signals across this synapse.
• The twelve cranial nerves either enter or exit from the skull; some transmit only sensory information, some transmit only motor information, and some transmit both.
• There are 31 spinal nerves that convey both sensory and motor signals between the spinal cord and the rest of the body.
Key Terms
• cranial nerve: any of the twelve paired nerves that originate from the brainstem instead of the spinal cord
• spinal nerve: one of 31 pairs of nerves that carry motor, sensory, and autonomic signals between the spinal cord and the body
• acetylcholine: a neurotransmitter in humans and other animals, which is an ester of acetic acid and choline | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.14%3A__The_Peripheral_Nervous_System_-_The_Sensory-Somatic_Nervous_System.txt |
Learning Objectives
• Distinguish between the neurodegenerative disorders of Alzheimer’s disease and Parkinson’s disease
Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are usually caused by neuronal death. These diseases generally worsen over time as more and more neurons die. The symptoms of a particular neurodegenerative disease are related to where in the nervous system the death of neurons occurs. Spinocerebellar ataxia, for example, leads to neuronal death in the cerebellum. The death of these neurons causes problems in balance and walking. Neurodegenerative disorders include Huntington’s disease, amyotrophic lateral sclerosis (ALS), Alzheimer’s disease, other dementia disorders, and Parkinson’s disease. In this section, Alzheimer’s and Parkinson’s disease will be discussed in more depth.
Alzheimer’s Disease
Alzheimer’s disease is the most common cause of dementia in the elderly. In 2012, an estimated 5.4 million Americans suffered from Alzheimer’s disease. Payments for their care are estimated at \$200 billion. Roughly one in every eight people age 65 or older has the disease. Due to the aging of the baby-boomer generation, there are projected to be as many as 13 million Alzheimer’s patients in the United States in the year 2050.
Symptoms of Alzheimer’s disease include disruptive memory loss, confusion about time or place, difficulty planning or executing tasks, poor judgement, and personality changes. Problems smelling certain scents can also be indicative of Alzheimer’s disease and may serve as an early warning sign. Many of these symptoms are also common in people who are aging normally, so it is the severity and longevity of the symptoms that determine whether a person is suffering from Alzheimer’s.
Alzheimer’s disease was named for Alois Alzheimer, a German psychiatrist who published a report in 1911 about a woman who showed severe dementia symptoms. Along with his colleagues, he examined the woman’s brain following her death and reported the presence of abnormal clumps, which are now called amyloid plaques, along with tangled brain fibers called neurofibrillary tangles. Amyloid plaques, neurofibrillary tangles, and an overall shrinking of brain volume are commonly seen in the brains of Alzheimer’s patients. Loss of neurons in the hippocampus is especially severe in advanced Alzheimer’s patients. Many research groups are examining the causes of these hallmarks of the disease.
One form of the disease is usually caused by mutations in one of three known genes. This rare form of early-onset Alzheimer’s disease affects fewer than five percent of patients with the disease and causes dementia beginning between the ages of 30 and 60. The more prevalent, late-onset form of the disease probably also has a genetic component. One particular gene, apolipoprotein E (APOE) has a variant (E4) that increases a carrier ‘s probability of developing the disease. Many other genes have been identified that may be involved in the pathology.
Unfortunately, there is no cure for Alzheimer’s disease. Current treatments focus on managing the symptoms of the disease. Because decrease in the activity of cholinergic neurons (neurons that use the neurotransmitter acetylcholine ) is common in Alzheimer’s disease, several drugs used to treat the disease work by increasing acetylcholine neurotransmission, often by inhibiting the enzyme that breaks down acetylcholine in the synaptic cleft. Other clinical interventions focus on behavioral therapies such as psychotherapy, sensory therapy, and cognitive exercises. Since Alzheimer’s disease appears to hijack the normal aging process, research into prevention is prevalent.
Parkinson’s Disease
Parkinson’s disease is also a neurodegenerative disease. It was first characterized by James Parkinson in 1817. Each year, 50,000-60,000 people in the United States are diagnosed with the disease. Parkinson’s disease causes the loss of dopamine neurons in the substantia nigra, a midbrain structure that regulates movement. Loss of these neurons causes many symptoms including tremor (shaking of fingers or a limb), slowed movement, speech changes, balance and posture problems, and rigid muscles. The combination of these symptoms often causes a characteristic slow, hunched, shuffling walk. Patients with Parkinson’s disease can also exhibit psychological symptoms, such as dementia or emotional problems.
Although some patients have a form of the disease known to be caused by a single mutation, for most patients, the exact causes of Parkinson’s disease remain unknown. The disease probably results from a combination of genetic and environmental factors, similar to Alzheimer’s disease. Post-mortem analysis of brains from Parkinson’s patients shows the presence of Lewy bodies, abnormal protein clumps, in dopaminergic neurons. The prevalence of these Lewy bodies often correlates with the severity of the disease.
There is no cure for Parkinson’s disease; treatment is focused on easing symptoms. One of the most-commonly prescribed drugs for Parkinson’s is L-DOPA, which is a chemical that is converted into dopamine by neurons in the brain. This conversion increases the overall level of dopamine neurotransmission and can help compensate for the loss of dopaminergic neurons in the substantia nigra. Other drugs work by inhibiting the enzyme that breaks down dopamine.
Key Points
• Neural death is the main cause behind neurodegenerative disorders.
• Symptoms of neurodegenerative disorders usually depend on the area within the nervous system where neuron deaths take place.
• Alzheimer’s disease, characterized by severe dementia, can appear in the form of disruptive memory loss, confusion, difficulty planning or executing tasks, poor judgement, and personality changes.
• A decrease in the activity of cholinergic neurons is commonly seen in patients with Alzheimer’s disease.
• In Parkinson’s disease, the loss of dopamine neurons results in symptoms that include tremors, slowed movement, speech changes, balance and posture problems, and rigid muscles.
• Neither Alzheimer’s nor Parkinson’s disease have cures, but there are drug treatments available to control symptoms.
Key Terms
• neurodegenerative: of, pertaining to, or resulting in the progressive loss of nerve cells and of neurologic function
• dementia: a progressive decline in cognitive function due to damage or disease in the brain beyond what might be expected from normal aging
• Parkinson’s disease: a degenerative disorder of the central nervous system
• Alzheimer’s disease: a disorder involving loss of mental functions resulting from brain tissue changes; senile dementia | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.15%3A_Neurodegenerative_Disorders_-_Introduction.txt |
Learning Objectives
• Distinguish between the neurodevelopmental disorders of autism and ADHD
Neurodevelopmental disorders occur when the development of the nervous system is disturbed. There are several different classes of neurodevelopmental disorders. Some, like Down Syndrome, cause intellectual deficits, while others specifically affect communication, learning, or the motor system. Some disorders, such as autism spectrum disorder and attention deficit/hyperactivity disorder, have complex symptoms.
Autism
Autism spectrum disorder (ASD, sometimes just “autism”) is a neurodevelopmental disorder in which severity differs from person to person. Estimates for the prevalence of the disorder have changed rapidly in the past few decades. Current estimates suggest that one in 88 children will develop the disorder. ASD is four times more prevalent in males than females.
A characteristic symptom of ASD is impaired social skills. Children with autism may have difficulty making and maintaining eye contact and reading social cues. They also may have problems feeling empathy for others. Other symptoms of ASD include repetitive motor behaviors (such as rocking back and forth), preoccupation with specific subjects, strict adherence to certain rituals, and unusual language use. Up to 30 percent of patients with ASD develop epilepsy. Patients with some forms of the disorder (e.g., Fragile X syndrome) also have intellectual disability. Because it is a spectrum disorder, other ASD patients are very functional and have good-to-excellent language skills. Many of these patients do not feel that they suffer from a disorder and instead just believe that they process information differently.
Except for some well-characterized, clearly-genetic forms of autism (e.g., Fragile X and Rett Syndrome), the causes of ASD are largely unknown. Variants of several genes correlate with the presence of ASD, but for any given patient, many different mutations in different genes may be required for the disease to develop. At a general level, ASD is thought to be a disease of “incorrect” wiring. Accordingly, brains of some ASD patients lack the same level of synaptic pruning that occurs in non-affected people. There has been some unsubstantiated controversy linking vaccinations and autism. In the 1990s, a research paper linked autism to a common vaccine given to children. This paper was retracted when it was discovered that the author falsified data; follow-up studies showed no connection between vaccines and autism.
Treatment for autism usually combines behavioral therapies and interventions, along with medications to treat other disorders common to people with autism ( depression, anxiety, obsessive compulsive disorder). Although early interventions can help mitigate the effects of the disease, there is currently no cure for ASD.
Attention Deficit Hyperactivity Disorder (ADHD)
Approximately three to five percent of children and adults are affected by attention deficit/hyperactivity disorder (ADHD). Like ASD, ADHD is more prevalent in males than females. Symptoms of the disorder include inattention (lack of focus), executive functioning difficulties, impulsivity, and hyperactivity beyond what is characteristic of the normal developmental stage. Some patients do not have the hyperactive component of symptoms and are diagnosed with a subtype of ADHD: attention deficit disorder (ADD). Many people with ADHD also show comorbidity: they develop secondary disorders in addition to ADHD. Examples include depression or obsessive compulsive disorder (OCD).
The cause of ADHD is unknown, although research points to a delay and dysfunction in the development of the prefrontal cortex and disturbances in neurotransmission. According to some twin studies, the disorder has a strong genetic component. There are several candidate genes that may contribute to the disorder, but no definitive links have been discovered. Environmental factors, including exposure to certain pesticides, may also contribute to the development of ADHD in some patients. Treatment for ADHD often involves behavioral therapies and the prescription of stimulant medications, which, paradoxically, cause a calming effect in these patients.
Key Points
• Disturbances in the development of the nervous system, genetic or environmental, may lead to neurodevelopmental diseases.
• Individuals affected by autism are believed to have one of many different mutations in genes required for the disease to cause disruptions in the nervous system that are generally observed; however, studies on specifics are still inconclusive.
• In ADHD, a strong genetic component may contribute to the disorder; however, no definitive links have been found.
• Individuals with ADHD may experience other psychological or neurological disorders in addition to their ADHD symptoms; this experience of having more than one disorder is termed comorbidity.
• The cause of both autism and ADHD are unknown and cures are unavailable; however, treatments to alleviate symptoms are accessible.
Key Terms
• autism: disorder observed in early childhood with symptoms of abnormal self-absorption, characterised by lack of response to other humans and a limited ability or disinclination to communicate and socialize
• attention deficit hyperactivity disorder: a developmental disorder in which a person has a persistent pattern of impulsiveness and inattention, with or without a component of hyperactivity
• fragile X syndrome: a particular, genetic syndrome, caused by the excessive repetition of a particular trinucleotide
• rett syndrome: a neurodevelopmental disorder of the grey matter of the brain that almost exclusively affects females, but has also been found in male patients
• comorbidity: the presence of one or more disorders (or diseases) in addition to a primary disease or disorder
• neurodevelopmental disorder: a disorder of brain function that affects emotion, learning ability and memoryand that unfolds as the individual grows | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.16%3A_Nervous_System_Disorders_-_Neurodevelopmental_Disorders_-_Autism_and_ADHD.txt |
Learning Objectives
• Distinguish between the disorders of schizophrenia and depression
Mental illnesses are nervous system disorders that result in problems with thinking, mood, or relating with other people. These disorders are severe enough to affect a person’s quality of life and often make it difficult for people to perform the routine tasks of daily living. Debilitating mental disorders plague approximately 12.5 million Americans (about 1 in 17 people) at an annual cost of more than \$300 billion. There are several types of mental disorders including schizophrenia, major depression, bipolar disorder, anxiety disorders, post-traumatic stress disorder, and many others. The American Psychiatric Association publishes the Diagnostic and Statistical Manual of Mental Disorders (or DSM), which describes the symptoms required for a patient to be diagnosed with a particular mental disorder. Each newly-released version of the DSM contains different symptoms and classifications as researchers learn more about these disorders, their causes, and how they relate to each other. A more detailed discussion of two mental illnesses, schizophrenia and major depression, is given below.
Schizophrenia
Schizophrenia is a serious and often-debilitating mental illness affecting one percent of the population in the United States. Symptoms of the disease include the inability to differentiate between reality and imagination, inappropriate and unregulated emotional responses, difficulty thinking, and problems with social situations. Symptoms of schizophrenia may be characterized as either “negative” (deficit symptoms) or “positive”. Positive symptoms are those that most individuals do not normally experience, but are present in people with schizophrenia. They can include delusions, disordered thoughts and speech, and tactile, auditory, visual, olfactory and gustatory hallucinations, typically regarded as manifestations of psychosis. Negative symptoms are deficits of normal emotional responses or of other thought processes, and commonly include flat or blunted affect and emotion, poverty of speech, inability to experience pleasure, lack of desire to form relationships, and lack of motivation.
Many schizophrenic patients are diagnosed in their late adolescence or early 20s. The development of schizophrenia is thought to involve malfunctioning dopaminergic neurons and may also involve problems with glutamate signaling. Treatment for the disease usually requires anti-psychotic medications that work by blocking dopamine receptors and decreasing dopamine neurotransmission in the brain. This decrease in dopamine can cause Parkinson’s disease-like symptoms in some patients. While some classes of anti-psychotics can be quite effective at treating the disease, they are not a cure; most patients must remain medicated for the rest of their lives.
Depression
Major depression (also referred to as just “depression” or “major depressive disorder”) affects approximately 6.7 percent of the adults in the United States each year and is one of the most common mental disorders. To be diagnosed with major depressive disorder, a person must have experienced a severely-depressed mood lasting longer than two weeks along with other symptoms that may include a loss of enjoyment in activities that were previously enjoyed, changes in appetite and sleep schedules, difficulty concentrating, feelings of worthlessness, and suicidal thoughts. The exact causes of major depression are unknown and probably include both genetic and environmental risk factors. Some research supports the “classic monoamine hypothesis,” which suggests that depression is caused by a decrease in norepinephrine and serotonin neurotransmission. One argument against this hypothesis is the fact that some antidepressant medications cause an increase in norepinephrine and serotonin release within a few hours of beginning treatment, but clinical results of these medications are not seen until weeks later. This has led to alternative hypotheses. For example, dopamine may also be decreased in depressed patients, or it may actually be an increase in norepinephrine and serotonin that causes the disease, and antidepressants force a feedback loop that decreases this release.
Treatments for depression include psychotherapy, electroconvulsive therapy, deep-brain stimulation, and prescription medications. Most commonly, individuals undergo some combination of psychotherapy and medication. There are several classes of antidepressant medications that work through different mechanisms. For example, monoamine oxidase inhibitors (MAO inhibitors) block the enzyme that degrades many neurotransmitters (including dopamine, serotonin, norepinephrine), resulting in increased neurotransmitter in the synaptic cleft. Selective serotonin reuptake inhibitors (SSRIs) block the reuptake of serotonin into the presynaptic neuron. This blockage results in an increase in serotonin in the synaptic cleft. Other types of drugs, such as norepinephrine-dopamine reuptake inhibitors and norepinephrine-serotonin reuptake inhibitors, are also used to treat depression.
Key Points
• Complications with thinking, mood, or problems relating to other people are issues that are commonly associated with those affected with neurodevelopmental disorders.
• Malfunctioning dopaminergic neurons and problems with glutamate signaling are thought to be potential causes of schizophrenia.
• Although no definitive answer yet exists, genetic and environmental risk factors are believed to be the main causes of depression.
• Exact cures do not exist for either schizophrenia or depression; however, schizophrenia may be treated with anti-psychotic medications while depression treatments include psychotherapy, electroconvulsive therapy, deep-brain stimulation, and prescription medications.
Key Terms
• norepinephrine: a neurotransmitter found in the locus coeruleus which is synthesized from dopamine
• serotonin: an indoleamine neurotransmitter that is involved in depression, appetite, etc., and is crucial in maintaining a sense of well-being, security, etc.
• mental disorder: any of the various diseases affecting the mind onset by brain damage or genetics
• schizophrenia: a psychiatric diagnosis denoting a persistent, often chronic, mental illness variously affecting behavior, thinking, and emotion
• depression: in psychotherapy and psychiatry, a period of unhappiness or low morale which lasts longer than several weeks and may include ideation of self-inflicted injury or suicide
• dopamine: a neurotransmitter associated with movement, attention, learning, and the brain’s pleasure and reward system
35.18: Nervous System Disorders - Other Neurological Disorders
Learning Objectives
• Distinguish between the neurological disorders of epilepsy and stroke
There are several other neurological disorders that cannot be easily placed into clean-cut categories. These include chronic pain conditions, cancers of the nervous system, epilepsy disorders, and stroke. Epilepsy and stroke are discussed below.
Epilepsy
Estimates suggest that up to three percent of people in the United States will be diagnosed with epilepsy in their lifetime. While there are several different types of epilepsy, all are characterized by recurrent seizures. Epilepsy itself can be a symptom of a brain injury, disease, or other illness. For example, people who have intellectual disability or autism spectrum disorder can experience seizures, presumably because the developmental wiring malfunctions that caused their disorders also put them at risk for epilepsy. For many patients, however, the cause of their epilepsy is never identified and is probably a combination of genetic and environmental factors. Often, seizures can be controlled with anti-convulsant medications. However, for very severe cases, patients may undergo brain surgery to remove the brain area where seizures originate.
Stroke
A stroke results when blood fails to reach a portion of the brain for a long enough time to cause damage. Without the oxygen supplied by blood flow, neurons in this brain region die. This neuronal death can cause many different symptoms, depending on the brain area affected, including headache, muscle weakness or paralysis, speech disturbances, sensory problems, memory loss, and confusion. Stroke is often caused by blood clots, but can also be caused by the bursting of a weak blood vessel. Strokes are extremely common; they are the third most-common cause of death in the United States. On average one person experiences a stroke every 40 seconds in the United States. Approximately 75 percent of strokes occur in people older than 65. Risk factors for stroke include high blood pressure, diabetes, high cholesterol, and a family history of stroke. Smoking doubles the risk of stroke. Treatment following a stroke can include blood pressure medication (to prevent future strokes) and (sometimes intense) physical therapy.
Key Points
• Although all types of epilepsy are characterized by recurrent seizures, the disorder itself can be a symptom of various factors, both genetic and environmental; the specific causes of epilepsy remain to be identified.
• Neural death, caused by a lack of oxygen for a prolonged period of time, is the main cause of stroke.
• Anti-convulsant medications and brain removal surgery are treatments for epilepsy, while anti-clotting medication and physical therapy are used in the treatment of stroke.
• Anti-convulsant medications and brain removal surgery are treatments for epilepsy while anti-clotting medication and physical therapy are used in the treatment of stroke.
Key Terms
• epilepsy: a medical condition in which the sufferer experiences seizures (or convulsions) and blackouts
• stroke: the loss of brain function arising when the blood supply to the brain is suddenly interrupted | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.17%3A_Nervous_System_Disorders_-_Neurodevelopmental_Disorders_-_Mental_Illnesses.txt |
Learning Objectives
• Explain the process of sensory reception
Introduction to Sensation
In more advanced animals, the senses are constantly at work, making the animal aware of stimuli, such as light or sound or the presence of a chemical substance in the external environment, while monitoring information about the organism’s internal environment. All bilaterally symmetric animals have a sensory system. The development of any species ‘ sensory system has been driven by natural selection; thus, sensory systems differ among species according to the demands of their environments. For example, the shark, unlike most fish predators, is electrosensitive (i.e., sensitive to electrical fields produced by other animals in its environment). While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.
Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (light, sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction.
There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment; this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure.
Reception
The first step in sensation is reception: the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away. For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances.
Key Points
• Reception is the process of activating a sensory receptor by a stimuli.
• Sensory transduction is the process of converting that sensory signal to an electrical signal in the sensory neuron.
• The process of reception is dependent on the stimuli itself, the type of receptor, receptor specificity, and the receptive field, which can vary depending on the receptor type.
Key Terms
• somatosensation: general senses which respond to stimuli like temperature, pain, pressure, and vibration
• reception: the act or ability to receive signals from stimuli | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.01%3A_Sensory_Processes_-_Reception.txt |
Learning Objectives
• Explain how stimuli are converted to signals that are carried to the central nervous system
Transduction
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor. The change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? As an example, a type of receptor called a mechanoreceptor possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. In the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.
Sensory receptors for the various senses work differently from each other. They are specialized according to the type of stimulus they sense; thus, they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.
Encoding and Transmission of Sensory Information
Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus. This segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system. The electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus: a sound.
The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials. Reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the central nervous system.
Perception
Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens, not at the level of the sensory receptor, but at the brain level. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus.
All sensory signals, except those from the olfactory system, are transmitted though the central nervous system: they are routed to the thalamus and to the appropriate region of the cortex. The thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex dedicated to processing that particular sense.
Key Points
• Sensory signals are converted to electrical signals via depolarization of sensory neuron membranes upon stimulus of the receptor, which causes opening of gated ion channels that cause the membrane potential to reach its threshold.
• The receptor potentials are classified as graded potentials; the magnitude of these potentials is dependent on the strength of the stimulus.
• The sensory system shows receptor specificity; although stimuli can be combined in processing regions of the brain, a specific receptor will only be activated by its specific stimulus.
• The brain contains specific processing regions (such as the somatosensory, visual, and auditory regions) that are dedicated to processing the information which has previously passed through the thalamus, the ‘clearinghouse and relay station’ for both sensory and motor signals.
• The four major components of encoding and transmitting sensory information include: the type of stimulus, the stimulus location within the receptive field, the duration, and the intensity of the stimulus.
Key Terms
• membrane potential: the difference in electrical potential across the enclosing membrane of a cell
• action potential: a short term change in the electrical potential that travels along a cell
• transduction: the translation of a sensory signal in the sensory system to an electrical signal in the nervous system | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.02%3A_Sensory_Processes_-_Transduction_and_Perception.txt |
Learning Objectives
• Describe the structure and function of mechanoreceptors
Somatosensory Receptors
Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. These categories are based on the nature of the stimuli that each receptor class transduces. Mechanoreceptors in the skin are described as encapsulated or unencapsulated. A free nerve ending is an unencapsulated dendrite of a sensory neuron; they are the most common nerve endings in skin. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.
Mechanoreceptors
There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically-gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound. There are four primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscle; two are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions.
Merkel’s disks are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin; that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, unencapsulated nerve endings, which respond to light touch. Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel’s disks are small, with well-defined borders. That makes them very sensitive to edges; they come into use in tasks such as typing on a keyboard.
Meissner’s corpuscles, also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. They are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly- adapting, fluid-filled, encapsulated neurons with small, well-defined borders which are responsive to fine details. Merkel’s disks and Meissner’s corpuscles are not as plentiful in the palms as they are in the fingertips.
Deeper in the dermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. They are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints; they provide valuable feedback for gripping objects and controlling finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also detect warmth. Note that these warmth detectors are situated deeper in the skin than are the cold detectors. It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli.
Pacinian corpuscles, located deep in the dermis of both glabrous and hairy skin, are structurally similar to Meissner’s corpuscles. They are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. They are rapidly-adapting mechanoreceptors that sense deep, transient (not prolonged) pressure, and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed which stimulates their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel’s disks and Meissner’s corpuscles.
Key Points
• The four major types of tactile mechanoreceptors include: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles.
• Merkel’s disk are slow-adapting, unencapsulated nerve endings that respond to light touch; they are present in the upper layers of skin that has hair or is glabrous.
• Meissner’s corpuscles are rapidly-adapting, encapsulated neurons that responds to low-frequency vibrations and fine touch; they are located in the glabrous skin on fingertips and eyelids.
• Ruffini endings are slow adapting, encapsulated receptors that respond to skin stretch and are present in both the glabrous and hairy skin.
• -Pacinian corpuscles are rapidly-adapting, deep receptors that respond to deep pressure and high-frequency vibration.
Key Terms
• dendrite: branched projections of a neuron that conduct the impulses received from other neural cells to the cell body
• glabrous: smooth, hairless, bald | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.03%3A_Somatosensation_-_Somatosensory_Receptors.txt |
Learning Objectives
• Describe how the density of mechanoreceptors affects the receptive field
Integration of Signals from Mechanoreceptors
The configuration of the different types of receptors working in concert in the human skin results in a very refined sense of touch. The nociceptive receptors (those that detect pain) are located near the surface. Small, finely-calibrated mechanoreceptors (Merkel’s disks and Meissner’s corpuscles) are located in the upper layers and can precisely localize even gentle touch. The large mechanoreceptors (Pacinian corpuscles and Ruffini endings) are located in the lower layers and respond to deeper touch. Consider that the deep pressure that reaches those deeper receptors would not need to be finely localized. Both the upper and lower layers of the skin hold rapidly- and slowly-adapting receptors. Both primary somatosensory cortex and secondary cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of mechanoreceptors.
Density of Mechanoreceptors
In the somatosensory system, receptive fields are regions of the skin or of internal organs. During the transmission of sensory information from these fields, the signals must be conveyed to the nervous system. The mechanoreceptors are activated, the signal is conveyed, and then processed. Some types of mechanoreceptors have large receptive fields, while others have smaller ones. Large receptive fields allow the cell to detect changes over a wider area, but lead to a less-precise perception. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm). Thus, the fingers, which require the ability to detect fine detail, have many, densely-packed (up to 500 per cubic cm) mechanoreceptors with small receptive fields (around 10 square mm), while the back and legs, for example, have fewer receptors with large receptive fields. Receptors with large receptive fields usually have a “hot spot”: an area within the receptive field (usually in the center, directly over the receptor) where stimulation produces the most intense response. Tactile-sense-related cortical neurons have receptive fields on the skin that can be modified by experience or by injury to sensory nerves, resulting in changes in the field’s size and position. In general, these neurons have relatively large receptive fields (much larger than those of dorsal root ganglion cells). However, the neurons are able to discriminate fine detail due to patterns of excitation and inhibition relative to the field, which leads to spatial resolution.
The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if they feel one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point. The size of the receptive field of a single receptor could be estimated from that distance.
Key Points
• The various types of receptors, nociceptors, mechanoreceptors (both small and large), thermoreceptors, chemoreceptors, and proprioreceptors, work together to ensure that complex stimuli are transmitted properly to the brain for processing.
• The distribution of mechanoreceptors within the body can affect how stimuli are perceived; this is dependent on the size of the receptive field and whether single or multiple sensory receptors are activated.
• A large receptive field allows for detection of stimuli over a wide area, but can result in less precise detection; a small receptive field allows for detection of stimuli over a small area, which results in more precise detection.
• The two-point discrimination test can be used to determine the density of receptors within various locations by measuring whether a two-point stimulus (such as thumb tacks) is detected as one or two points.
Key Terms
• mechanoreceptor: any receptor that provides an organism with information about mechanical changes in its environment, such as movement, tension and pressure | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.04%3A_Somatosensation_-_Integration_of_Signals_from_Mechanoreceptors.txt |
Learning Objectives
• Describe the various types of receptors used for thermoreception: Krause end bulbs, Ruffini endings, free nerve endings
Thermoreception
Thermoception or thermoreception is the sense by which an organism perceives temperatures. The details of how temperature receptors work are still being investigated. Mammals have at least two types of sensors: those that detect heat (i.e., temperatures above body temperature) and those that detect cold (i.e., temperatures below body temperature). A thermoreceptor is a sensory receptor or, more accurately, the receptive portion of a sensory neuron that codes absolute and relative changes in temperature, primarily within the innocuous range. The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate; cooling results in a decrease in warm receptor discharge rate. For cold receptors, their firing rate increases during cooling and decreases during warming. The types of receptors capable of detecting changes in temperature can vary.
Types of Thermoreceptors: Capsule Receptors
Some of the receptors that exhibit the ability to detect changes in temperature include Krause end bulbs and Ruffini endings. Krause end bulbs are defined by cylindrical or oval bodies consisting of a capsule that is formed by the expansion of the connective-tissue sheath, containing an axis-cylinder core. End-bulbs are found in the conjunctiva of the eye, in the mucous membrane of the lips and tongue, and in the epineurium of nerve trunks. They are also found in the penis and the clitoris; hence, the name of genital corpuscles. In these locations, they have a mulberry-like appearance, being constricted by connective-tissue septa into two to six knob-like masses.
The Ruffini endings, enlarged dendritic endings with elongated capsules, can act as thermoreceptors. This spindle-shaped receptor is sensitive to skin stretch, contributing to the kinesthetic sense of and control of finger position and movement. Ruffini corpuscles respond to sustained pressure and show very little adaptation. Ruffinian endings are located in the deep layers of the skin where they register mechanical deformation within joints as well as continuous pressure states.They also act as thermoreceptors that respond for an extended period; in case of deep burn, there will be no pain as these receptors will be burned off.
In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on free nerve endings.
Types of Thermoreceptors: Free Nerve Endings
There are thermoreceptors that are located in the dermis, skeletal muscles, liver, and hypothalamus that are activated by different temperatures. These thermoreceptors, which have free nerve endings, include only two types of thermoreceptors that signal innocuous warmth and cooling respectively in our skin. The warm receptors show a maximum sensitivity at ~ 45°C, signal temperatures between 30 and 45°C, and cannot unambiguously signal temperatures higher than 45°C; they are unmyelinated. The cold receptors have their maximum sensitivity at ~ 27°C, signal temperatures above 17°C, and some consist of lightly-myelinated fibers, while others are unmyelinated. Our sense of temperature comes from the comparison of the signals from the warm and cold receptors. Thermoreceptors are poor indicators of absolute temperature, but are very sensitive to changes in skin temperature.
The Thermoreceptor Pathway
The thermoreceptor pathway in the brain runs from the spinal cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is too intense can be perceived as pain because temperature sensations are conducted along the same pathways that carry pain sensations.
Key Points
• Thermoreceptors can include: Krause end bulbs, which detect cold and are defined by capsules; Ruffini endings, which detect warmth and are defined by enlarged dendritic endings; and warm and cold receptors present on free nerve endings which can detect a range of temperature.
• The cold receptors present on free nerve endings, that can be either lightly-myelinated or unmyelinated, have a maximum sensitivity at ~ 27°C and will signal temperatures above 17°C.
• The warm receptors present on free nerve endings are unmyelinated fibers that have a maximum senstivity of ~45°C and will signal temperature above 30°C.
Key Terms
• thermoreceptor: a nerve cell that is sensitive to changes in temperature
• somatosensory: of or pertaining to the perception of sensory stimuli produced by the skin or internal organs
• epineurium: the connective tissue framework and sheath of a nerve which bind together the nerve bundles, each of which has its own special sheath, or perineurium | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.05%3A_Somatosensation_-_Thermoreception.txt |
Learning Objectives
• Explain the interaction of taste and odor
Tastes and Odors
Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty, and umami. The first four tastes need little explanation. The identification of umami as a fundamental taste occurred fairly recently. It was identified in 1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods. The adaptive value of being able to distinguish umami is that savory substances tend to be high in protein.
All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. If a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow us to sense about 10,000 different odors. Compare that to mice, for example, which have about 1,300 olfactory receptor types and, therefore, probably sense many more odors.
The senses of smell and taste combine at the back of the throat. When you taste something before you smell it, the smell lingers internally up to the nose causing you to smell it. Both smell and taste use chemoreceptors, which essentially means they are both sensing the chemical environment. This chemoreception in regards to taste, occurs via the presence of specialized taste receptors within the mouth that are referred to as taste cells and are bundled together to form taste buds. These taste buds, located in papillae which are found across the tongue, are specific for the five modalities: salt, sweet, sour, bitter and umami. These receptors are activated when their specific stimulus (i.e. sweet or salt molecules) is present and signals to the brain.
In addition to the activation of the taste receptors, there are similar receptors within the nose that coordinates with activation of the taste receptors. When you eat something, you can tell the difference between sweet and bitter. It is the sense of smell that is used to distinguish the difference. Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A person’s perception of flavor is reduced if he or she has congested nasal passages.
Key Points
• Humans can taste sweet, sour, bitter, salty, and umami; umami is the savoriness of certain foods that are commonly high in protein.
• Odors come from molecules in the air that stimulate receptors in the nose; if an organism does not have a receptor for that particular odor molecule, for that organism, the odor has no smell.
• The senses of smell and taste are directly related because they both use the same types of receptors.
• If one’s sense of smell is not functional, then the sense of taste will also not function because of the relationship of the receptors.
Key Terms
• umami: one of the five basic tastes, the savory taste of foods such as seaweed, cured fish, aged cheeses and meats
• olfactory: concerning the sense of smell
• receptor: a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.06%3A_Taste_and_Smell_-_Tastes_and_Odors.txt |
Learning Objectives
• Describe the process by which tastes and odors are sensed
Reception and Transduction
Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity. The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm2 in humans. Recall that sensory cells are neurons. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds (dogs selectively bred for their sense of smell) have about 4 billion.
Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium; extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins. It is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia. The receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.
Taste and Smell
Detecting a taste (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (singular: papilla). There are several structurally-distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances; they contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds; they also have receptors for pressure and temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior margin of the tongue.
In humans, there are five primary tastes; each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus ( tastant ). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCl) provides the sodium ions (Na+) that enter the taste neurons, exciting them directly. Sour tastants are acids which belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel which increases hydrogen ion (H+) concentrations in the taste neurons; thus, depolarizing them. Sweet, bitter, and umami tastants require a G-protein-coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them.
Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing.
Key Points
• Odorants are received by receptors in the nose, which send signals to the olfactory bulb of the brain to create an appropriate response; humans have about 12 million receptors.
• Taste results when molecules are dissolved in fluid and reach the gustatory receptors on the tongue; the signals are sent to the brain to determine which flavor (bitter, sour, sweet, salty, umami ) is being consumed.
• Taste buds are found on the tongue and contain clusters of gustatory receptors on bumps called papillae; fungiform papillae each contain one to eight taste buds; they also have receptors for pressure and temperature.
• The ability to smell and taste declines with age.
Key Terms
• tastant: any substance that stimulates the sense of taste
• papilla: a nipple-like anatomical structure
• odorant: any substance that has a distinctive smell, especially one added to something (such as household gas) for safety purposes | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.07%3A_Taste_and_Smell_-_Reception_and_Transduction.txt |
Learning Objectives
• Describe the relationship of amplitude and frequency of a sound wave to attributes of sound
Sound
Auditory stimuli are sound waves, which are mechanical pressure waves that move through a medium, such as air or water. There are no sound waves in a vacuum since there are no air molecules for the waves to move through. The speed of sound waves differs based on altitude, temperature, and medium. At sea level and a temperature of 20º C (68º F), sound waves travel in the air at about 343 meters per second.
As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude. Frequency is the number of waves per unit of time; in sound, it is heard as pitch. High-frequency (≥15.000Hz) sounds are higher-pitched (short wavelength) than low-frequency (long wavelengths; ≤100Hz) sounds. Frequency is measured in cycles per second. For sound, the most-commonly used unit is hertz (Hz), or cycles per second. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Women are typically better at hearing high frequencies, but everyone’s ability to hear high frequencies decreases with age. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz; bats, 100,000 Hz; dolphins, 150,000 Hz; and the American shad (Alosa sapidissima), a fish, can hear 180,000 Hz. Those frequencies above the human range are called ultrasound.
Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume. The sound waves of louder sounds have greater amplitude than those of softer sounds. For sound, volume is measured in decibels (dB). The softest sound that a human can hear is the zero point. Humans speak normally at 60 decibels.
Key Points
• Sound waves are mechanical pressure waves that must travel through a medium and cannot exist in a vacuum.
• There are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude.
• Frequency is the number of waves per unit of time and is heard as pitch; high-frequency sounds are high-pitched, and low-frequency sounds are low-pitched.
• Most humans can perceive sounds with frequencies between 30 and 20,000 Hz; other animals, such as dolphins, can detect sounds at far higher frequencies.
• Amplitude, the dimension of a wave from peak to trough, is heard as volume; louder sounds have greater amplitudes than those of softer sounds.
Key Terms
• frequency: characterized as a periodic vibration that is audible; property of sound that most determines pitch and is measured in hertz
• amplitude: measure of a wave from its highest point to its lowest point; heard as volume
• ultrasound: sound frequencies above the human detectable ceiling of approximately 20,000 Hz
36.09: Hearing and Vestibular Sensation - Reception of Sound
Learning Objectives
• Explain how animals sense sound
Reception of Sound
In order to hear a sound, the auditory system must accomplish three basic tasks. First, it must deliver the acoustic stimulus to the receptors; second, it must convert the stimulus from pressure changes into electrical signals; and third, it must process these electrical signals so that they can efficiently indicate the qualities of the sound source, such as frequency (pitch), amplitude (loudness, volume), and location.
The human ear can be divided into three functional segments:
• the outer ear: collects sound energy from the environment and sends it to the eardrum
• the middle ear: transduces the mechanical pressure signals from the ear drum into electrical signals
• the inner ear: interprets the electrical signals from the middle ear using hair cells
In mammals, sound waves are collected by the external, cartilaginous outer part of the ear called the pinna. They then travel through the auditory canal, causing vibration of the thin diaphragm called the tympanum, or ear drum, the innermost part of the outer ear. Interior to the tympanum is the middle ear, which holds three small bones called the ossicles (“little bones”), that transfer energy from the moving tympanum to the inner ear. The three ossicles are the malleus (also known as the hammer), the incus (the anvil), and stapes (the stirrup). The three ossicles are unique to mammals; each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach the tympanum. If we did not have the malleus and the incus, then the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. The ear ossicles are homologous to bones in a fish mouth; the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle ear to transmit vibrations to it.
Key Points
• The human ear can be divided into three functional segments: the outer ear, the middle ear, and the inner ear.
• Sound waves are collected by the pinna, travel through the auditory canal, and cause vibration of the tympanum (ear drum).
• The three ossicles of the middle ear ( malleus, incus, and stapes ) transfer energy from the vibrating ear drum to the inner ear.
• The incus connects the malleus to the stapes, which allows vibrations to reach the inner ear.
Key Terms
• malleus: small hammer-shaped bone of the middle ear
• incus: small anvil-shaped bone in the middle ear; connects the malleus to the stapes
• stapes: small stirrup-shaped bone of the middle ear
• pinna: the visible, cartilaginous part of the ear that resides outside of the head and collects sound waves
• tympanum: innermost part of the outer ear; the eardrum | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.08%3A_Hearing_and_Vestibular_Sensation_-_Sound.txt |
Learning Objectives
• Identify the structures of the vestibular system that respond to gravity
Vestibular Information
The stimuli associated with the vestibular system are linear acceleration (gravity) and angular acceleration/deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. Gravity is detected through head position, while angular acceleration and deceleration are expressed through turning or tilting of the head.
The vestibular system has some similarities with the auditory system. It utilizes hair cells just like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear, all of which help to maintain balance: the utricle, the saccule, and three semicircular canals. Together, they make up what is known as the vestibular labyrinth. The utricle and saccule are most responsive to acceleration in a straight line, such as gravity. The roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule lie below a gelatinous layer, with their stereocilia (singular: stereocilium) projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals, similar to tiny rocks. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates specific neurons that signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch of the vestibulocochlear cranial nerve that deals with balance.
The fluid-filled semicircular canals are tubular loops set at oblique angles, arranged in three spatial planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a gelatinous cap, the cupula, where they monitor angular acceleration and deceleration from rotation. They would be stimulated by driving your car around a corner, turning your head, or falling forward. One canal lies horizontally, while the other two lie at about 45 degree angles to the horizontal axis. When the brain processes input from all three canals together, it can detect angular acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. Upon cessation of acceleration or deceleration, the movement of the fluid within the canals slows or stops. For example, imagine holding a glass of water. When moving forward, water may splash backwards onto the hand; when motion has stopped, water may splash forward onto the fingers. While in motion, the water settles in the glass and does not splash. Note that the canals are not sensitive to velocity itself, but to changes in velocity. In this way, moving forward at 60 mph with your eyes closed would not give the sensation of movement, but suddenly accelerating or braking would stimulate the receptors.
Higher Processing
Hair cells from the utricle, saccule, and semicircular canals also communicate through bipolar neurons to the cochlear nucleus in the medulla. Cochlear neurons send descending projections to the spinal cord and ascending projections to the pons, thalamus, and cerebellum. Connections to the cerebellum are important for coordinated movements. There are also projections to the temporal cortex, which account for feelings of dizziness; projections to autonomic nervous system areas in the brainstem, which account for motion sickness; and projections to the primary somatosensory cortex, which monitors subjective measurements of the external world and self-movement. People with lesions in the vestibular area of the somatosensory cortex see vertical objects in the world as being tilted. Finally, the vestibular signals project to certain optic muscles to coordinate eye and head movements.
Key Points
• The vestibular system uses hair cells, as does the auditory system, but it excites them in different ways.
• There are five vestibular receptor organs in the inner ear (the vestibular labyrinth): the utricle, the saccule, and three semicircular canals; the utricle and saccule respond to acceleration in a straight line, such as gravity.
• The bending of the stereocilia stimulates specific neurons that signal to the brain that the head is tilted, allowing the maintenance of balance.
• The fluid-filled semicircular canals are tubular loops set at oblique angle, arranged in three spatial planes; the base of each canal contains a cluster of hair cells that monitor angular acceleration and deceleration from rotation.
• Neuronal projections to the temporal cortex account for feelings of dizziness; projections to autonomic nervous system areas in the brainstem account for motion sickness; and projections to the primary somatosensory cortex monitor subjective measurements of the external world and self-movement.
Key Terms
• vestibulocochlear: of or pertaining to the vestibular and cochlear nerves
• vestibular system: the sensory system in mammals that contributes to movement, sense of balance, and spatial orientation
• stereocilium: any of many nonmotile cellular structures resembling long microvilli; those of the inner ear are responsible for auditory transduction | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.10%3A_Hearing_and_Vestibular_Sensation_-_The_Vestibular_System.txt |
Learning Objectives
• Describe the anatomy that enables equilibrium and balance
Equilibrium
Along with audition, the inner ear is responsible for encoding information about equilibrium, or the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia —senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum. Together, these components make up the vestibular system.
Linear acceleration
The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolith. The otolith contains calcium carbonate crystals, making it denser and giving it greater inertia than the macula. Therefore, gravity will cause the otolith to move separately from the macula in response to head movements. Tilting the head causes the otolith to slide over the macula in the direction of gravity. The moving otolith layer, in turn, bends the sterocilia to cause some hair cells to depolarize as others hyperpolarize. The exact tilt of the head is interpreted by the brain on the basis of the pattern of hair-cell depolarization.
Rotational movement
The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane. The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning your head from side to side when saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Key Points
• The hair cells of the utricle and saccule of the inner ear extend into the otolith, a dense viscous substance with calcium carbonate crystals.
• The otolith slides over the macula, tissue supporting the hair cells, in the direction of gravity when the head is moved due to its greater inertia, causing a pattern of hair cell depolarization interpreted by the brain as tilting.
• The three semicircular canals of the inner ear are ring-like structures with one ring oriented in the horizontal plane and the other two rings oriented at approximately 45 degrees relative to the sagittal plane.
• The ampulla, found at the base of each semicircular canal, contains hair cells that extend into the membrane that attaches to the top of the ampulla to an area called the cupula.
• A head rotation causes the fluid in the semicircular canal to move, but with a lag which produces a deflection of the cupula in the direction opposite to the head rotation which in turn causes the hair cells to depolarize.
• Using the hair cell depolarization information from all three ampullae, the direction and speed of head movements in all three dimensions can be detected by the vestibular system.
Key Terms
• stereocilium: any of many nonmotile cellular structures resembling long microvilli; those of the inner ear are responsible for auditory transduction
• equilibrium: the condition of a system in which competing influences are balanced, resulting in no net change
• otolith: a small particle, comprised mainly of calcium carbonate, found in the inner ear of vertebrates, being part of the balance sense | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.11%3A_Hearing_and_Vestibular_Sensation_-_Balance_and_Determining_Equilibrium.txt |
Light is composed of photons that make up electromagnetic waves, which are characterized by wavelength, frequency, and amplitude.
Learning Objectives
• Describe the characteristics of light
Key Points
• Light is composed of electromagnetic waves that can travel without a medium, unlike sound.
• The behavior of light can be seen in the behavior of waves and photons, the basic unit of light.
• A wavelength (which varies inversely with frequency ) manifests itself as color, while wave amplitude is perceived as luminous intensity or brightness; it is measured by the standard unit of a candela.
• Humans can see light that ranges between 380 nm and 740 nm, but cannot see light that is below the frequency of visible red light or above the frequency of visible violet light.
• Light at the red end of the visible spectrum has long wavelengths (and is lower frequency), while light at the violet end has short wavelengths (and is higher frequency).
• Light waves enter the eye as long (red), medium (green), and short (blue) waves; the color of an object is the color the object reflects.
Key Terms
• photon: the quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime
• nanometer: one billionth of a meter; used to express wavelength of light
• electromagnetic spectrum: the entire range of wavelengths of all known radiations consisting of oscillating electric and magnetic fields, including gamma rays, visible light, infrared, radio waves, and X-rays
• wavelength: the length of a single cycle of a wave, as measured by the distance between one peak or trough of a wave and the next; it corresponds to the velocity of the wave divided by its frequency
Light
As with auditory stimuli, light travels in waves. While the compression waves that compose sound must travel in a medium (consisting of a gas, a liquid, or a solid), light is composed of electromagnetic waves and needs no medium. Light can, in fact, travel in a vacuum. The behavior of light can be described in terms of the behavior of waves and the behavior of the fundamental unit of light, the photon: a packet of electromagnetic radiation. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.
Certain variables are important when discussing perception of light. A wavelength (which varies inversely with frequency) manifests itself as color. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. However, some other animals can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers. Some non-avian reptiles sense infrared light (such as heat that prey gives off).
Wave amplitude is perceived as luminous intensity or brightness. The standard unit of intensity of light is the candela, which is approximately the luminous intensity of one common candle.
Light waves travel 299,792 km per second in a vacuum and somewhat slower in various media such as air and water. Those waves arrive at the eye as long (red), medium (green), and short (blue) waves. The term “white light” is light that is perceived as white by the human eye. This effect is produced by light that stimulates the color receptors in the human eye equally. The apparent color of an object is actually the color (or colors) the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.12%3A_Vision_-_Light.txt |
Many structures in the human eye, such as the cornea and fovea, process light so it can be deciphered by rods and cones in the retina.
Learning Objectives
• Explain how eyes have evolved to benefit organisms
Key Points
• The cornea and the lens bend light to focus the image on the retina; the iris and pupil regulate the amount of light entering the eye.
• The aqueous humour maintains the convex shape of the cornea; the vitreous humour supports the lens and maintains the shape of the entire eye.
• Presbyopia occurs because the image focuses behind the retina; it is similar to hyperopia (farsightedness), which is caused by an eyeball that is too short.
• Myopia (nearsightedness) occurs when an eyeball is elongated; images in the distance appear blurry, but images nearby are clear.
• Rods are used for peripheral and nighttime vision; cones are used for daytime and color vision.
• The fovea is responsible for acute vision because it has a high density of cones.
Key Terms
• rod: a rod-shaped cell located in the outer retina of the eye that is extremely sensitive to light
• retina: the thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain
• cone: cell located near the center of the retina that is weakly photosensitive and is responsible for color vision in relatively bright light
Anatomy of the Eye
The retina, a thin layer of cells located on the inner surface of the back of the eye, consists of photoreceptive cells, which are responsible for the transduction of light into nervous impulses. However, light does not enter the retina unaltered; it must first pass through other layers that process it so that it can be interpreted by the retina.
The cornea, the front transparent layer of the eye, along with the crystalline lens, refract (bend) light to focus the image on the retina. After passing through the cornea, light passes through the aqueous humour, which connects the cornea to the lens. This clear gelatinous mass also provides the corneal epithelium with nutrients and helps maintain the convex shape of the cornea. The iris, which is visible as the colored part of the eye, is a circular muscular ring lying between the lens and the aqueous humour that regulates the amount of light entering the eye. Light passes through the center of the iris, the pupil, which actively adjusts its size to maintain a constant level of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil. In conditions of low light, the iris relaxes and the pupil enlarges.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is a transparent, convex structure located behind the cornea. On the other side of the lens is the vitreous humour, which lets light through without refraction, maintains the shape of the eye, and suspends the delicate lens. The lens focuses and re-focuses light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through to focus it sharply on the retina. With age comes the loss of the flexibility of the lens; a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. It is a deficit similar to a different type of farsightedness, hyperopia, caused by an eyeball that is too short. For both defects, images in the distance are clear, but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry, but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones. Both are named for their general appearance. Rods, strongly photosensitive, are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones, weakly photosensitive, are located near the center of the retina. They respond to bright light; their primary role is in daytime, color vision.
The fovea is the region in the center back of the eye that is responsible for acute (central) vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.13%3A_Vision_-_Anatomy_of_the_Eye.txt |
Light is tranduced in rods and cones; visual information is processed in the retina before entering the brain.
Learning Objectives
• Explain retinal processing and the process of transduction of light
Key Points
• When light hits the photoreceptor, the retinal changes shape, which activates the photopigment rhodoposin.
• Primates have full color vision because of the three- cone (trichromatic) system; color is a result of the ratio of activity of the three types of cones.
• There are three types of cones with different photopsins: S cones respond to short waves; M cones respond to medium waves; L cones respond to light to long waves.
• If light is not present, neurons are inhibited by rods and cones; once light is introduced, rods and cones are hyperpolarized, which activates the neurons.
• Activated neurons stimulate ganglion cells, which send action potentials via the optic nerve.
• Horizontal cells can create lateral inhibition, which enhances light and dark contrast in images.
Key Terms
• tonic activity: when photoreceptors become slightly active even when not stimulated by light
• rhodopsin: a light-sensitive pigment in the rod cells of the retina; it consists of an opsin protein bound to the carotenoid retinal
Transduction of Light
The rods and cones are the site of transduction of light into a neural signal. Both rods and cones contain photopigments, which are pigments that undergo a chemical change when they absorb light. In vertebrates, the main photopigment, rhodopsin, has two main parts: an opsin, which is a membrane protein (in the form of a cluster of α-helices that span the membrane); and retinal, a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus), visual receptors become hyperpolarized and are driven away from the threshold.
Trichromatic Coding
There are three types of cones (with different photopsins) that differ in the wavelength to which they are most responsive. Some cones are maximally responsive to short light waves of 420 nm; they are called S cones (“S” for “short”). Other cones (M cones, for “medium”) respond maximally to waves of 530 nm. A third group (L cones, or “long” cones) responds maximally to light of longer wavelengths at 560 nm. With only one type of cone, color vision would not be possible; a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.
The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short are:
• red (700 nm)
• orange (600 nm)
• yellow (565 nm)
• green (497 nm)
• blue (470 nm)
• indigo (450 nm)
• violet (425 nm).
Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation; in all, about 2 million distinct colors.
Retinal Processing
Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.
Photoreceptors in the retina continuously undergo tonic activity. That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at an equilibrium; while some stimuli increase firing rate from the baseline, other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones, removing the inhibition of their bipolar cells. The now-active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on changein retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more-distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.14%3A_Vision_-_Transduction_of_Light.txt |
Visual signals are processed in the brain through several different pathways.
Learning Objectives
• Describe the complexity of visual processing in the brain
Key Points
• The magnocellular pathway carries information about form, movement, depth, and differences in brightness; the parvocellular pathway carries information on color and fine detail.
• The optic chiasma allows us to coordinate information between both eyes and is produced by crossing optical information across the brain.
• Visual signals move from the visual cortex to either the parietal lobe or the temporal lobe.
• Some signals move to the thalamus, which sends the visual signals to the primary cortex.
• Visual signals can also travel from the retina to the superior colliculus, where eye movements are coordinated with auditory information.
• Visual signals can move from the retina to the suprachiasmatic nucleus (SCN), the body’s internal clock, which is involved in sleep/wake patterns and annual cycles.
Key Terms
• superior colliculus: the primary area of the brain where eye movements are coordinated and integrated with auditory information
• optic chiasma: found at the base of the brain and coordinates information from both eyes
• suprachiasmatic nucleus: cluster of cells that is considered to be the body’s internal clock, which controls our circadian (day-long) cycle
Higher Processing
The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different parts of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for “crossing”) found at the base of the brain and allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places. Its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except smell. In the thalamus, the magnocellular and parvocellular distinctions remain intact; there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (“where”) information. A second stream projects to the temporal lobe and carries both magnocellular (“where”) and parvocellular (“what”) information.
Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body’s internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep/wake patterns and annual cycles. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/36%3A_Sensory_Systems/36.15%3A_Vision_-_Visual_Processing.txt |
Learning Objectives
• Evaluate hormones and their purpose in the body
An animal’s endocrine system controls body processes through the production, secretion, and regulation of hormones. Hormones serve as chemical “messengers” that function in cellular and organ activity to maintain the body’s homeostasis. Maintaining homeostasis within the body requires the coordination of many different systems and organs. Communication between neighboring cells and between cells and tissues in distant parts of the body occurs through the release of hormones into body fluids (usually blood), which carry them to their target cells. Target cells, those having a receptor for a signal, respond to a hormone when they express a specific receptor for that hormone. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.
By releasing hormones, the endocrine system plays a role in growth, metabolism, and sexual development. Hormones also play a role in induction or suppression of cell death, activation or inhibition of the immune system, mood swings, and hunger cravings. In humans, common endocrine system diseases include thyroid disease and diabetes mellitus.
Examples of endocrine glands include the adrenal glands, which produce hormones, such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid gland, which produces thyroid hormones that regulate metabolic rates. In organisms that undergo metamorphosis, the process is controlled by the endocrine system. The transformation from tadpole to frog, for example, is complex and nuanced to adapt to specific environments and ecological circumstances.
Key Points
• Hormones serve as chemical messengers in the body and help maintain homeostasis.
• Hormones are released into bodily fluids, like blood, which carry them to target cells.
• Target cells respond to a hormone when they express a specific receptor for that hormone.
• Hormones also play a role in the regulation of cell death, the immune system, reproductive development, mood swings, and hunger cravings.
• In the adrenal gland, epinephrine and norepinephrine regulate responses to stress; in the thyroid gland, thyroid hormones regulates metabolic rates.
Key Terms
• target cell: any cell having a specific receptor for a hormone
• hormone: any substance produced by one tissue and conveyed by the bloodstream to another to affect physiological activity
• endocrine system: a control system of ductless glands that secrete hormones which circulate via the bloodstream to affect cells within specific organs
37.02: Types of Hormones - Lipid-Derived Amino Acid-Derived and Peptide Hormones
Learning Objectives
• Recognize characteristics associated with lipid-derived, amino acid-derived, and peptide hormones
Types of Hormones
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones (which includes peptides and proteins). One of the key, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.
Lipid-Derived Hormones (or Lipid-soluble Hormones)
Most lipid hormones are derived from cholesterol, so they are structurally similar to it. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols; their chemical names will end in “-ol” for alcohols or “-one” for ketones. Examples of steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water; they are carried by transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.
Amino Acid-Derived Hormones
The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan. If a hormone is amino acid-derived, its chemical name will end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.
Peptide Hormones
The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, such as growth hormones produced by the pituitary, and large glycoproteins, such as follicle-stimulating hormone produced by the pituitary.
Secreted peptides, such as insulin, are stored within vesicles in the cells which synthesize them. They are then released in response to stimuli (e.g., as high blood glucose levels in the case of insulin). Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells; therefore, their receptors are found on the surface of the target cells.
Key Points
• Most lipid hormones are steroid hormones, which are usually ketones or alcohols and are insoluble in water.
• Steroid hormones (ending in ‘-ol’ or ‘-one’) include estradiol, testosterone, aldosterone, and cortisol.
• The amino acid – derived hormones (ending in ‘-ine’) are derived from tyrosine and tryptophan and include epinephrine and norepinephrine (produced by the adrenal medulla).
• Amino acid-derived hormones also include thyroxine (produced by the thryoid gland) and melatonin (produced by the pineal gland).
• Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ).
• Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids.
Key Terms
• oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
• epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
• estrogen: any of a group of steroids (lipid-hormones) that are secreted by the ovaries and function as female sex hormones | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.01%3A_Types_of_Hormones_-_Hormone_Functions.txt |
Learning Objectives
• Explain the ways in which hormones work
Hormones
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood.
How Hormones Work
Hormones mediate changes in target cells by binding to specific hormone receptors. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on many different cells or may be limited to a small number of specialized cells. For example, thyroid hormones act on many different tissue types, stimulating metabolic activity throughout the body. Cells can have many receptors for the same hormone, but often also possess receptors for different types of hormones. The number of receptors that respond to a hormone determines the cell’s sensitivity to that hormone and the resulting cellular response. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone, allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced.
Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses. Receptor binding alters cellular activity, resulting in an increase or decrease in normal body processes. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.
Key Points
• Hormones can only affect cells that display receptors that are specific to them; cells can display receptors for many different hormones at once.
• The more receptors for a particular hormone that a cell displays, the more sensitive to that hormone it will be.
• When a cell displays more receptors in response to a hormone, this is called up-regulation, but when a cell reduces its number of receptors for a particular hormone, this is called down-regulation.
• A hormone can make changes directly to a cell by changing what genes are activated, or make changes indirectly to a cell by stimulating particular signaling pathways inside the cell that affect other processes.
Key Terms
• phytohormone: a plant hormone
• hormone: any substance produced by one tissue and conveyed by the bloodstream to another to affect physiological activity
• receptor: a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.03%3A_How_Hormones_Work_-_Introduction.txt |
Learning Objectives
• Describe how hormones alter cellular activity by binding to intracellular receptors
Intracellular Hormone Receptors
Lipid-derived (soluble) hormones such as steroid hormones diffuse across the lipid bilayer membranes of the endocrine cell. Once outside the cell, they bind to transport proteins that keep them soluble in the bloodstream. At the target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma membrane of the target cells. They then adhere to intracellular receptors residing in the cytoplasm or in the nucleus. The cell signaling pathways induced by the steroid hormones regulate specific genes within the cell’s DNA. The hormones and receptor complex act as transcription regulators by increasing or decreasing the synthesis of mRNA molecules from specific genes. This, in turn, determines the amount of corresponding protein that is synthesized from this RNA; this is known as altering gene expression. This protein can be used either to change the structure of the cell or to produce enzymes that catalyze chemical reactions. In this way, the steroid hormone regulates specific cell processes.
Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors located in the nucleus. The hormones diffuse across both the plasma membrane and the nuclear envelope, then bind to receptors in the nucleus. The hormone-receptor complex stimulates transcription of specific genes in the same way that steroid hormones do. For example, the active vitamin D metabolite, calcitriol, mediates its biological effects by binding to the vitamin D receptor (VDR), which is principally located in the nuclei of target cells. The binding of calcitriol to the VDR allows the VDR to act as a transcription factor that modulates the gene expression of transport proteins that are involved in calcium absorption in the intestine. VDR activation in the intestine, bone, kidney, and parathyroid gland cells leads to the maintenance of calcium and phosphorus levels in the blood and to the maintenance of bone content.
Key Points
• Lipid -soluble hormones are able to diffuse directly across the membranes of both the endocrine cell where they are produced and that of the target cell, as the cell membranes are made of a lipid bilayer.
• These hormones can bind to receptors that are located either in the cytoplasm of the cell or within the nucleus of the cell.
• When these hormones bind to their receptors, this signals the cell to synthesize more or less mRNA from a gene or genes, which then results in more or less protein being created from those mRNA molecules.
• The increase or decrease in protein production can alter the cell structurally or alter how and when it catalyzes chemical reactions.
Key Terms
• gene expression: the transcription and translation of a gene into messenger RNA and, thus, into a protein
• transcription: the synthesis of RNA under the direction of DNA
• steroid: a class of organic compounds having a structure of 17 carbon atoms arranged in four rings; they are lipids, and occur naturally as sterols, bile acids, adrenal and sex hormones, and some vitamins | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.04%3A_How_Hormones_Work_-_Intracellular_Hormone_Receptors.txt |
Learning Objectives
• Describe the events that occur when a hormone binds to a plasma hormone receptor
Plasma Membrane Hormone Receptors
Amino acid-derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble or fat-soluble); therefore, they cannot diffuse through the plasma membrane of cells. Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid-insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA. Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this triggers intracellular activity to carry out the specific effects associated with the hormone. In this way, nothing passes through the cell membrane; the hormone that binds at the surface remains at the surface of the cell while the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first messenger, which activates a second messenger in the cytoplasm.
One very important second messenger is cyclic adenosine monophosphate (cAMP). When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated. G proteins are proteins separate from receptors that are found in the cell membrane. When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP. When a hormone binds to the receptor, the G protein is activated by binding guanosine triphosphate, or GTP, in place of GDP. After binding, GTP is hydrolyzed by the G protein into GDP and becomes inactive.
The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases, which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation. The phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated molecules can then mediate changes in cellular processes.
The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions. In this way, a small amount of hormone can trigger the formation of a large amount of cellular product. To stop hormone activity, cAMP is deactivated by the cytoplasmic enzyme phosphodiesterase, or PDE. PDE is always present in the cell, breaking down cAMP to control hormone activity; thus, preventing overproduction of cellular products.
The specific response of a cell to a lipid-insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.
Key Points
• When a lipid (fat) insoluble hormone binds to a plasma membrane hormone receptor, this triggers specific actions inside the cell that alter the cell’s activities, such as gene expression.
• Because the first event in this sequence is the binding of the hormone to the plasma membrane receptor, the hormone is called the “first messenger”, while the molecule that is activated within the cell and carries out intracellular change is called the ” second messenger “.
• In many cases, a hormone binding to a plasma membrane receptor activates a special kind of protein called a G protein, which in turn activates an enzyme that generates cAMP, a second messenger.
• cAMP activates another group of proteins called protein kinases, which can change the structure of other molecules by adding a phosphate group to them; these activated molecules can then affect changes within the cell.
Key Terms
• second messenger: any substance used to transmit a signal within a cell, especially one which triggers a cascade of events by activating cellular components
• cyclic adenosine monophosphate: cAMP, a second messenger derived from ATP that is involved in the activation of protein kinases and regulates the effects of adrenaline
• G protein: any of a class of proteins, found in cell membranes, that pass signals between hormone receptors and effector enzymes
Contributions and Attributions
• OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44768/latest...ol11448/latest. License: CC BY: Attribution
• Principles of Biochemistry/Hormones. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Princip...istry/Hormones. License: CC BY-SA: Attribution-ShareAlike
• receptor. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/receptor. License: CC BY-SA: Attribution-ShareAlike
• phytohormone. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/phytohormone. License: CC BY-SA: Attribution-ShareAlike
• hormone. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/hormone. License: CC BY-SA: Attribution-ShareAlike
• Insulin glucose metabolism ZP. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:In...abolism_ZP.svg. License: Public Domain: No Known Copyright
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44768/latest...ol11448/latest. License: CC BY: Attribution
• Vitamin d. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Vitamin...nism_of_action. License: CC BY-SA: Attribution-ShareAlike
• steroid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/steroid. License: CC BY-SA: Attribution-ShareAlike
• transcription. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/transcription. License: CC BY-SA: Attribution-ShareAlike
• gene expression. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gene_expression. License: CC BY-SA: Attribution-ShareAlike
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• OpenStax College, How Hormones Work. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44768/latest...e_37_02_02.jpg. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.05%3A_How_Hormones_Work_-__Plasma_Membrane_Hormone_Receptors.txt |
Learning Objectives
• Explain how the actions of different hormones regulate the excretory system
Hormonal Regulation of the Excretory System
Maintaining a proper water balance in the body is important to avoid dehydration or over-hydration (hyponatremia). The water concentration of the body is monitored by osmoreceptors in the hypothalamus, which detect the concentration of electrolytes in the extracellular fluid. The concentration of electrolytes in the blood rises when there is water loss caused by excessive perspiration, inadequate water intake, or low blood volume due to blood loss. An increase in blood electrolyte levels results in a neuronal signal being sent from the osmoreceptors in hypothalamic nuclei. The anterior pituitary is composed of glandular cells that secrete protein hormones. The pituitary gland has two components: anterior and posterior. The posterior pituitary is an extension of the hypothalamus. It is composed largely of neurons that are continuous with the hypothalamus.
Antidiuretic Hormone (ADH)
The hypothalamus produces a polypeptide hormone known as antidiuretic hormone (ADH), which is transported to and released from the posterior pituitary gland. The principal action of ADH is to regulate the amount of water excreted by the kidneys. As ADH (which is also known as vasopressin) causes direct water reabsorption from the kidney tubules, salts and wastes are concentrated in what will eventually be excreted as urine. The hypothalamus controls the mechanisms of ADH secretion, either by regulating blood volume or the concentration of water in the blood. Dehydration or physiological stress can cause an increase of osmolarity above threshold levels, which, in turn, raises ADH secretion and water retention, causing an increase in blood pressure. ADH travels in the bloodstream to the kidneys where it changes the kidneys to become more permeable to water by temporarily inserting water channels, aquaporins, into the kidney tubules. Water moves out of the kidney tubules through the aquaporins, reducing urine volume. The water is reabsorbed into the capillaries, lowering blood osmolarity back toward normal. As blood osmolarity decreases, a negative feedback mechanism reduces osmoreceptor activity in the hypothalamus; ADH secretion is reduced. ADH release can be reduced by certain substances, including alcohol, which can cause increased urine production and dehydration.
Chronic underproduction of ADH or a mutation in the ADH receptor results in diabetes insipidus. If the posterior pituitary does not release enough ADH, water cannot be retained by the kidneys and is lost as urine. This causes increased thirst, but water taken in is lost again and must be continually consumed. If the condition is not severe, dehydration may not occur, but severe cases can lead to electrolyte imbalances due to dehydration.
Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone, a steroid hormone that is produced by the adrenal cortex. In contrast to ADH, which promotes the reabsorption of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from extracellular fluid of the cells in kidney tubules. Because it is produced in the cortex of the adrenal gland and affects the concentrations of minerals Na+ and K+, aldosterone is referred to as a mineralocorticoid, a corticosteroid that affects ion and water balance. Aldosterone release is stimulated by a decrease in blood sodium levels, blood volume, or blood pressure, or an increase in blood potassium levels. It also prevents the loss of Na+ from sweat, saliva, and gastric juice. The reabsorption of Na+ also results in the osmotic reabsorption of water, which alters blood volume and blood pressure.
Aldosterone production can be stimulated by low blood pressure, which triggers a sequence of chemical release. When blood pressure drops, the renin-angiotensin-aldosterone system (RAAS) is activated. Cells in the juxtaglomerular apparatus, which regulates the functions of the nephrons of the kidney, detect this and release renin. Renin, an enzyme, circulates in the blood, reacting with a plasma protein produced by the liver called angiotensinogen. When angiotensinogen is cleaved by renin, it produces angiotensin I, which is then converted into angiotensin II in the lungs. Angiotensin II functions as a hormone, causing the release of the hormone aldosterone by the adrenal cortex, resulting in increased Na+ reabsorption, water retention, and an increase in blood pressure. Angiotensin II, in addition to being a potent vasoconstrictor, also causes an increase in ADH and increased thirst, both of which help to raise blood pressure.
Key Points
• The hypothalamus monitors the amount of water in the body by sensing the concentration of electrolytes in the blood; a high concentration of electrolytes means that the level of water in the body is low.
• Antidiuretic hormone (ADH), produced by the hypothalamus and released by the posterior pituitary, causes more water to be retained by the kidneys when water levels in the body are low.
• ADH effects water retention by creating special channels for water, called aquaporins, inside the kidneys so that more water can be reabsorbed before it is excreted.
• Aldosterone, produced by the adrenal cortex, causes the retention of water in the body by increasing the levels of sodium and potassium ions in the blood, which causes the body to reabsorb more water.
• When blood pressure is low, the enzyme renin is released, which cleaves the protein angiotensinogen into angiotensin I, which is further converted into angiotensin II.
• Angiotensin II signals the adrenal cortex to release aldosterone, which then increases the retention of sodium ions, enhancing the secretion of postassium ions, resulting in water retention and an increase in blood pressure.
Key Terms
• renin: a circulating enzyme released by mammalian kidneys that converts angiotensinogen to angiotensin-I that plays a role in maintaining blood pressure
• mineralocorticoid: any of a group of steroid hormones, characterised by their similarity to aldosterone and their influence on salt and water metabolism
• electrolyte: any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes
• aquaporin: any of a class of proteins that form pores in the membrane of biological cells
• aldosterone: a mineralocorticoid hormone, secreted by the adrenal cortex, that regulates the balance of sodium and potassium in the body
• osmoreceptor: a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure
• antidiuretic hormone: a hormone secreted by the posterior pituitary gland that regulates the amount of water excreted by the kidneys | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.06%3A_Regulation_of_Body_Processes_-_Hormonal_Regulation_of_the_Excretory_System.txt |
Learning Objectives
• Explain the regulation of the male and female reproductive systems
Hormonal Regulation of the Reproductive System
Regulation of the reproductive system is a process that requires the action of hormones from the pituitary gland, the adrenal cortex, and the gonads. During puberty, in both males and females, the hypothalamus produces gonadotropin-releasing hormone (GnRH), which stimulates the production and release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. These hormones regulate the gonads (testes in males and ovaries in females); they are called gonadotropins. In both males and females, FSH stimulates gamete production and LH stimulates production of hormones by the gonads. An increase in gonad hormone levels inhibits GnRH production through a negative feedback loop.
Regulation of the Male Reproductive System
At the pituitary, GnRH stimulates the synthesis and secretion of the gonadotropins, FSH and LH. These processes are controlled by the size and frequency of GnRH pulses, as well as by feedback from androgens and estrogens. Low-frequency GnRH pulses lead to FSH release, whereas high-frequency GnRH pulses stimulate LH release. In males, FSH stimulates primary spermatocytes to undergo the first division of meiosis, to form secondary spermatocytes, leading to the maturation of sperm cells. FSH also enhances the production of androgen-binding protein by the Sertoli cells of the testes by binding to FSH receptors on their basolateral membranes. FSH production is inhibited by the hormone inhibin, which is released by the testes.
LH stimulates production of the sex hormones (androgens) by the Leydig cells of the testes. It is also called interstitial-cell-stimulating hormone. The most widely-known androgen in males is testosterone, which promotes the production of sperm and masculine characteristics. The adrenal cortex also produces small amounts of testosterone precursor, although the role of this additional hormone production is not fully understood.
Regulation of the Female Reproductive System
In females, FSH stimulates development of egg cells (or ova) in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production in the female reproductive system. LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the ovaries. Estradiol and progesterone are steroid hormones that prepare the body for pregnancy. Estradiol produces secondary sex characteristics in females, while both estradiol and progesterone regulate the menstrual cycle.
In addition to producing FSH and LH, the anterior portion of the pituitary gland also produces the hormone prolactin (PRL) in females. Prolactin stimulates the production of milk by the mammary glands, following childbirth. Prolactin levels are regulated by the hypothalamic hormones, prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH) (which is now known to be dopamine). PRH stimulates the release of prolactin, while PIH inhibits it.
The posterior pituitary releases the hormone oxytocin, which stimulates uterine contractions during childbirth. The uterine smooth muscles are not very sensitive to oxytocin until late in pregnancy, when the number of oxytocin receptors in the uterus peaks. Stretching of tissues in the uterus and cervix stimulates oxytocin release during childbirth. Contractions increase in intensity as blood levels of oxytocin rise via a positive feedback mechanism until the birth is complete. Oxytocin also stimulates the contraction of myoepithelial cells around the milk-producing mammary glands. As these cells contract, milk is forced from the secretory alveoli into milk ducts and is ejected from the breasts in a milk ejection (“let-down”) reflex. Oxytocin release is stimulated by the suckling of an infant, which triggers the synthesis of oxytocin in the hypothalamus and its release into circulation at the posterior pituitary.
Key Points
• In males, FSH stimulates the production of sperm cells by signaling them to undergo meiosis, while in females, FSH stimulates the growth of the ovum inside the follicle of the ovary.
• In males, LH stimulates the Leydig cells within the testes to produce testosterone, which encourages sperm production and leads to secondary sexual characteristics.
• In females, LH plays a crucial role in signaling ovulation, as well as stimulating the production of other hormones that will prepare the body for pregnancy.
• Other hormones involved in the female reproductive system are oxytocin, which signals the uterus to contract during childbirth, and prolactin, which stimulates milk production.
Key Terms
• gonadotropin-releasing hormone: a trophicpeptide responsible for the release of follicle stimulating hormone and lutenizing hormone from the anterior pituitary, synthesized and released from the hypothalamus
• follicle stimulating hormone: a gonadotropic glycoprotein hormone, secreted in the anterior pituitary, that stimulates the growth of ovarian follicles in female mammals, and induces spermatogenesis in male mammals
• luteinizing hormone: a hormone, produced by part of the pituitary gland, that stimulates ovulation and the development of the corpus luteum in female mammals, and the production of androgens by male mammals
• gonad: a sex organ that produces gametes; specifically, a testicle or ovary
• inhibin: a peptide hormone, secreted by the gonads, which inhibits the secretion of follicle-stimulating hormone
• prolactin: a peptide gonadotrophic hormone secreted by the pituitary gland; it stimulates growth of the mammary glands and lactation in females
• androgen: the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of masculine characteristics in vertebrates | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.07%3A_Regulation_of_Body_Processes_-_Hormonal_Regulation_of_the_Reproductive_System.txt |
Learning Objectives
• Explain how the hormones glucagon and insulin regulate blood glucose
Blood glucose levels vary widely over the course of a day as periods of food consumption alternate with periods of fasting. Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood glucose levels. Additional regulation is mediated by the thyroid hormones.
Regulation of Blood Glucose Levels: Insulin and Glucagon
Cells of the body require nutrients in order to function. These nutrients are obtained through feeding. In order to manage nutrient intake, storing excess intake, and utilizing reserves when necessary, the body uses hormones to moderate energy stores. Insulin is produced by the beta cells of the pancreas, which are stimulated to release insulin as blood glucose levels rise (for example, after a meal is consumed). Insulin lowers blood glucose levels by enhancing the rate of glucose uptake and utilization by target cells, which use glucose for ATP production. It also stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use. As insulin binds to its target cell via insulin receptors and signal transduction, it triggers the cell to incorporate glucose transport proteins into its membrane. This allows glucose to enter the cell, where it can be used as an energy source. These actions mediated by insulin cause blood glucose concentrations to fall, called a hypoglycemic, or “low sugar” effect, which inhibits further insulin release from beta cells through a negative feedback loop.
Impaired insulin function can lead to a condition called diabetes mellitus, which has many effects on the body. It can be caused by low levels of insulin production by the beta cells of the pancreas, or by reduced sensitivity of tissue cells to insulin. This prevents glucose from being absorbed by cells, causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult for the kidneys to recover all the glucose from nascent urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced; this may result in dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system. Oversecretion of insulin can cause hypoglycemia, low blood glucose levels. This causes insufficient glucose availability to cells, often leading to muscle weakness. It can sometimes cause unconsciousness or death if left untreated.
When blood glucose levels decline below normal levels, for example between meals or when glucose is utilized during exercise, the hormone glucagon is released from the pancreas. Glucagon raises blood glucose levels, eliciting what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose. This process of glucose synthesis is called gluconeogenesis. Rising blood glucose levels inhibit further glucagon release by the pancreas via a negative feedback mechanism. In this way, insulin and glucagon work together to maintain homeostatic glucose levels.
Regulation of Blood Glucose Levels: Thyroid Hormones
The basal metabolic rate, which is the amount of calories required by the body at rest, is determined by two hormones produced by the thyroid gland: thyroxine, also known as tetraiodothyronine or T4, and triiodothyronine, also known as T3. T3 and T4 release from the thyroid gland are stimulated by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary. These hormones affect nearly every cell in the body except for the adult brain, uterus, testes, blood cells, and spleen. They are transported across the plasma membrane of target cells where they bind to receptors on the mitochondria, resulting in increased ATP production. In the nucleus, T3and T4activate genes involved in energy production and glucose oxidation. This results in increased rates of metabolism and body heat production. This is known as the hormone’s calorigenic effect.
Disorders can arise from both the underproduction and overproduction of thyroid hormones. Hypothyroidism, underproduction of the thyroid hormones, can cause a low metabolic rate leading to weight gain, sensitivity to cold, and reduced mental activity, among other symptoms. In children, hypothyroidism can cause cretinism, which can lead to mental retardation and growth defects. Hyperthyroidism, the overproduction of thyroid hormones, can lead to an increased metabolic rate, which may cause weight loss, excess heat production, sweating, and an increased heart rate.
Key Points
• When blood glucose levels rise, insulin is secreted by the pancreas, lowering blood glucose by increasing its uptake in cells and stimulating the liver to convert glucose to glycogen, in which form it can be stored.
• If insulin secretion is impaired, it can result in diabetes mellitus: a disease in which blood glucose levels remain high, leading to excess glucose in the urine, increased urine output, and dehydration, among other symptoms.
• When blood glucose levels fall, glucagon is secreted by the pancreas, which increases blood glucose levels by stimulating the breakdown of glycogen into glucose and the creation of glucose from amino acids.
• The basal metabolic rate of the body is controlled by the hormones T3 and T4, produced by the thyroid gland in response to the thyroid stimulating hormone (TSH), produced by the anterior pituitary.
• T3 and T4 bind to receptors on the mitochondria, causing an increase in the production of ATP, as well as increase in the transcription of genes that help utilize glucose and produce ATP, resulting in higher metabolism of the cell.
Key Terms
• insulin: a polypeptide hormone that regulates carbohydrate metabolism
• glucagon: a hormone, produced by the pancreas, that opposes the action of insulin by stimulating the production of sugar
• glycogen: a polysaccharide that is the main form of carbohydrate storage in animals; converted to glucose as needed
• hypoglycemia: a condition in which blood glucose levels are too low
• glycogenolysis: the production of glucose-1-phosphate by splitting a glucose monomer from glycogen using inorganic phosphate
• gluconeogenesis: the metabolic process in which glucose is formed, mostly in the liver, from non-carbohydrate precursors
• thyroxine: a hormone (an iodine derivative of tyrosine), produced by the thyroid gland, that regulates cell metabolism and growth
• triiodothyronine: the most powerful thyroid hormone, affecting almost every process in the body, including body temperature, growth, and heart rate
• hypothyroidism: the disease state caused by insufficient production of thyroid hormone by the thyroid gland
• hyperthyroidism: the excessive production of hormones by the thyroid | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.08%3A_Regulation_of_Body_Processes_-_Hormonal_Regulation_of_Metabolism.txt |
Learning Objectives
• Explain how blood calcium levels are regulated by parathyroid hormone
Regulation of blood calcium concentrations is important for generation of muscle contractions and nerve impulses, which are electrically stimulated. If calcium levels get too high, membrane permeability to sodium decreases and membranes become less responsive. If calcium levels get too low, membrane permeability to sodium increases and convulsions or muscle spasms may result.
Blood calcium levels are regulated by parathyroid hormone (PTH), which is produced by the parathyroid glands. PTH is released in response to low blood calcium levels. It increases calcium levels by targeting the skeleton, the kidneys, and the intestine. In the skeleton, PTH stimulates osteoclasts, which are cells that cause bone to be reabsorbed, releasing calcium from bone into the blood. PTH also inhibits osteoblasts, cells which deposit bone, reducing calcium deposition in bone. In the intestines, PTH increases dietary calcium absorption and in the kidneys, PTH stimulates re-absorption of the calcium. While PTH acts directly on the kidneys to increase calcium re-absorption, its effects on the intestine are indirect. PTH triggers the formation of calcitriol, an active form of vitamin D, which acts on the intestines to increase absorption of dietary calcium. PTH release is inhibited by rising blood calcium levels.
Hyperparathyroidism results from an overproduction of PTH, which leads to excessive amounts of calcium being removed from bones and introduced into blood circulation. This may produce structural weakness of the bones, which can lead to deformation and fractures, plus nervous system impairment due to high blood calcium levels. Hypoparathyroidism, the underproduction of PTH, results in extremely low levels of blood calcium, which causes impaired muscle function and may result in tetany (severe sustained muscle contraction).
The hormone calcitonin, which is produced by the parafollicular (or C) cells of the thyroid, has the opposite effect on blood calcium levels as PTH. Calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and stimulating calcium excretion by the kidneys. This results in calcium being added to the bones to promote structural integrity. Calcitonin is most important in children (when it stimulates bone growth), during pregnancy (when it reduces maternal bone loss), and during prolonged starvation (because it reduces bone mass loss). In healthy, nonpregnant, unstarved adults, the role of calcitonin is unclear.
Key Points
• The parathyroid hormone (PTH), secreted by the parathyroid glands, is responsible for regulating blood calcium levels; it is released whenever blood calcium levels are low.
• PTH increases blood calcium levels by stimulating osteoclasts, which break down bone to release calcium into the blood stream.
• PTH increases blood calcium levels by increasing the amount of calcium resorbed by the kidneys before it can be excreted in the urine.
• PTH increases blood calcium levels by triggering the formation of calcitriol, which increases absorption of dietary calcium through the intestines.
• Calcitonin, a hormone produced by the thyroid, acts in opposition to PTH by inhibiting osteoclasts, stimulating osteoblasts, and increasing excretion of calcium into the urine by the kidneys.
Key Terms
• osteoblast: a mononucleate cell from which bone develops
• parathyroid hormone: a polypeptide hormone that is released by the chief cells of the parathyroid glands and is involved in raising the levels of calcium ions in the blood
• calcitonin: a hormone, secreted by parenchymal cells, that regulates calcium and phosphate metabolism
• hypoparathyroidism: deficiency of parathyroid hormone
• hyperparathyroidism: an abnormal increase in parathyroid gland activity
• calcitriol: the active metabolite 1,25-dihydroxycholecalciferol of vitamin D3 that is involved in the absorption of calcium
• osteoclast: a large multinuclear cell associated with the resorption of bone | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.09%3A_Regulation_of_Body_Processes_-_Hormonal_Control_of_Blood_Calcium_Levels.txt |
Learning Objectives
• Describe the hormonal regulation of growth
Hormonal regulation is required for the growth and replication of most cells in the body. Growth hormone (GH), produced by the anterior portion of the pituitary gland, accelerates the rate of protein synthesis, particularly in skeletal muscle and bones. Effects of growth hormone on the tissues of the body can generally be described as anabolic (building up). Like most other protein hormones, GH acts by interacting with a specific receptor on the surface of cells. Increased height during childhood is the most widely-known effect of GH. Height appears to be stimulated by at least two mechanisms: Because polypeptide hormones are not fat-soluble, they cannot penetrate cell membranes. Thus, GH exerts some of its effects by binding to receptors on target cells, where it activates a pathway that directly stimulates division and multiplication of chondrocytes of cartilage.
GH also stimulates, through another pathway, the production of insulin-like growth factor 1 (IGF-1), a hormone homologous to proinsulin. The liver, a major target organ of GH for this process, is the principal site of IGF-1 production. IGF-1 has growth-stimulating effects on a wide variety of tissues. IGFs stimulate the uptake of amino acids from the blood, allowing the formation of new proteins, particularly in skeletal muscle cells, cartilage cells, and other target cells. This is especially important after a meal, when glucose and amino acid concentration levels are high in the blood. GH levels are regulated by two hormones produced by the hypothalamus. GH release is stimulated by growth hormone-releasing hormone (GHRH) and is inhibited by growth hormone-inhibiting hormone (GHIH), also called somatostatin. IGF-1 also has stimulatory effects on osteoblast and chondrocyte activity to promote bone growth.
A balanced production of growth hormone is critical for proper development. Underproduction of GH in adults does not appear to cause any abnormalities, but in children it can result in pituitary dwarfism, in which growth is reduced. Pituitary dwarfism is characterized by symmetric body formation. In some cases, individuals are under 30 inches in height. Oversecretion of growth hormone can lead to gigantism in children, causing excessive growth. In some documented cases, individuals can reach heights of over eight feet. In adults, excessive GH can lead to acromegaly, a condition in which there is enlargement of bones in the face, hands, and feet that are still capable of growth.
Key Points
• Growth hormone binds to receptors on target cells, causing mature cartilage cells to divide, creating new cartilage tissue.
• Growth hormone stimulates the production of IGF-1, a hormone that increases the uptake of amino acids when they are at high levels in the blood, so that they can be formed into new proteins.
• Growth hormone-releasing hormone stimulates the production of GH by the anterior pituitary, while growth hormone-inhibiting hormone inhibits its production.
• When growth hormone production is abnormally low in children, it can result in pituitary dwarfism, in which individuals can be less than 30 inches tall; when growth hormone production is high in children, it can result in gigantism, in which individuals can be over 8 feet tall.
Key Terms
• growth hormone: any polypeptide hormone secreted by the pituitary gland that promotes growth and regulates the metabolism of carbohydrates, proteins, and lipids
• somatostatin: a polypeptide hormone, secreted by the pancreas, that inhibits the production of certain other hormones
• gigantism: a condition caused by an over-production of growth hormone, resulting in excessive bone growth | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.10%3A_Regulation_of_Body_Processes_-_Hormonal_Regulation_of_Growth.txt |
Learning Objectives
• Describe the role of the adrenal glands in the “fight-or-flight” response and the body’s reaction to stress
When a threat or danger is perceived, the body responds by releasing hormones that will ready it for the “fight-or-flight” response. The effects of this response are familiar to anyone who has been in a stressful situation: increased heart rate, dry mouth, and hair standing erect.
The sympathetic nervous system regulates the stress response via the hypothalamus. Stressful stimuli cause the hypothalamus to signal the adrenal medulla (which mediates short-term stress responses) via nerve impulses, and the adrenal cortex, which mediates long-term stress responses via the hormone adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary.
Short-term Stress Response
Interactions of the endocrine hormones have evolved to ensure the body’s internal environment remains stable. Stressors are stimuli that disrupt homeostasis. The sympathetic division of the vertebrate autonomic nervous system has evolved the fight-or-flight response to counter stress-induced disruptions of homeostasis. In the initial alarm phase, the sympathetic nervous system stimulates an increase in energy levels through increased blood glucose levels. This prepares the body for physical activity that may be required to respond to stress: to either fight for survival or to flee from danger.
When presented with a stressful situation, the body responds by calling for the release of hormones that provide a burst of energy. The hormones epinephrine (also known as adrenaline) and norepinephrine (also known as noradrenaline) are released by the adrenal medulla. Epinephrine and norepinephrine increase blood glucose levels by stimulating the liver and skeletal muscles to break down glycogen and by stimulating glucose release by liver cells. Additionally, these hormones increase oxygen availability to cells by increasing the heart rate and dilating the bronchioles. The hormones also prioritize body function by increasing blood supply to essential organs, such as the heart, brain, and skeletal muscles, while restricting blood flow to organs not in immediate need, such as the skin, digestive system, and kidneys. Epinephrine and norepinephrine are collectively called catecholamines.
Long-term Stress Response
Some stresses, such as illness or injury, can last for a long time. Glycogen reserves, which provide energy in the short-term response to stress, are exhausted after several hours and cannot meet long-term energy needs. If glycogen reserves were the only energy source available, neural functioning could not be maintained once the reserves became depleted due to the nervous system’s high requirement for glucose. In this situation, the body has evolved a response to counter long-term stress through the actions of the glucocorticoids, which ensure that long-term energy requirements can be met. The glucocorticoids mobilize lipid and protein reserves, stimulate gluconeogenesis, conserve glucose for use by neural tissue, and stimulate the conservation of salts and water.
Long-term stress response differs from short-term stress response. The body cannot sustain the bursts of energy mediated by epinephrine and norepinephrine for long times. Instead, other hormones come into play. In a long-term stress response, the hypothalamus triggers the release of ACTH from the anterior pituitary gland. The adrenal cortex is stimulated by ACTH to release steroid hormones called corticosteroids. Corticosteroids turn on transcription of certain genes in the nuclei of target cells. They change enzyme concentrations in the cytoplasm and affect cellular metabolism.
There are two main corticosteroids: glucocorticoids, such as cortisol, and mineralocorticoids, such as aldosterone. These hormones target the breakdown of fat into fatty acids in the adipose tissue. The fatty acids are released into the bloodstream for other tissues to use for ATP production. The glucocorticoids primarily affect glucose metabolism by stimulating glucose synthesis. Glucocorticoids also have anti-inflammatory properties through inhibition of the immune system. For example, cortisone is used as an anti-inflammatory medication; however, it cannot be used long term as it increases susceptibility to disease due to its immune-suppressing effects. Mineralocorticoids function to regulate ion and water balance of the body. The hormone aldosterone stimulates the reabsorption of water and sodium ions in the kidney, which results in increased blood pressure and volume.
Hypersecretion of glucocorticoids can cause a condition known as Cushing’s disease, characterized by a shifting of fat storage areas of the body. This can cause the accumulation of adipose tissue in the face and neck, and excessive glucose in the blood. Hyposecretion of the corticosteroids can cause Addison’s disease, which may result in bronzing of the skin, hypoglycemia, and low electrolyte levels in the blood.
Key Points
• When the body senses stress, the hypothalamus signals the adrenal medulla to release epinephrine or norepinephrine, or the anterior pituitary to release ACTH.
• In short-term stressful situations, such as when a threat is perceived, epinephrine (adrenaline) and norepinephrine (noradrenaline) are released to prepare the body for a “fight-or-flight” response.
• Epinephrine and norepinephrine act to provide a burst of energy to the body by stimulating the breakdown of glycogen into glucose, increasing the heart rate, and dilating the bronchioles.
• In long-term stress situations, such as when the body must deal with injury or illness, ACTH is released, stimulating the production of corticosteroids, which include glucocorticoids and mineralocorticoids.
• Glucocorticoids stimulate the synthesis of glucose and inhibit the immune system.
• Mineralocorticoids regulate ion and water balance of the body by stimulating the kidneys to excrete less water and sodium ions in the urine.
Key Terms
• epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
• norepinephrine: a neurotransmitter found in the locus coeruleus which is synthesized from dopamine
• corticosteroid: any of a group of steroid hormones, secreted by the adrenal cortex, that are involved in a large range of physiological systems
• mineralocorticoid: any of a group of steroid hormones, characterised by their similarity to aldosterone and their influence on salt and water metabolism
• catecholamine: any of a class of aromatic amines derived from pyrocatechol that are hormones produced by the adrenal gland
• glucocorticoid: any of a group of steroid hormones, produced by the adrenal cortex, that are involved in metabolism and have anti-inflammatory properties
• adrenocorticotropic hormone: a peptide hormone, secreted by the pituitary gland, that stimulates the secretion of other hormones | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/37%3A_The_Endocrine_System/37.11%3A_Regulation_of_Body_Processes_-_Hormonal_Regulation_of_Stress.txt |
The musculoskeletal system provides form, support, stability, and movement to the body.
Learning Objectives
• Summarize the structure and role of the musculoskeletal system
Key Points
• The skeleton, muscles, cartilage, tendons, ligaments, joints, and other connective tissues are all part of the musculoskeletal system, which work together to provide the body with support, protection, and movement.
• The bones of the skeletal system protect the body’s internal organs, support the weight of the body, and serve as the main storage system for calcium and phosphorus.
• The muscles of the muscular system keep bones in place; they assist with movement by contracting and pulling on the bones.
• To allow motion, different bones are connected by joints which are connected to other bones and muscle fibers via connective tissues such as tendons and ligaments.
• Cartilage prevents the bone ends from rubbing directly on each other.
• Malnutrition and arthritis are examples of disorders and diseases in the body that can severely impair the function of the musculoskeletal system.
Key Terms
• prosthesis: an artificial replacement for a body part, either internal or external
• arthritis: inflammation of a joint or joints causing pain and/or disability, swelling, and stiffness due to various causes, such as infection, trauma, degenerative changes, or metabolic disorders
• musculoskeletal system: an organ system made up of the muscular and skeletal systems; the system provides form, support, stability, and movement to the body
The Musculoskeletal System
The musculoskeletal system provides support to the body and gives humans (and many animal species) the ability to move. The body’s bones (the skeletal system), muscles (muscular system), cartilage, tendons, ligaments, joints, and other connective tissue that supports and binds tissues and organs together comprise the musculoskeletal system.
Most importantly, the system provides form, support, stability, and movement to the body. For example, the bones of the skeletal system protect the body’s internal organs and support the weight of the body. The skeletal portion of the system serves as the main storage depot for calcium and phosphorus. It also contains critical components of the hematopoietic system (blood cell production). The muscles of the muscular system keep bones in place; they also play a role in movement of the bones by contracting and pulling on the bones, allowing for movements as diverse as standing, walking, running, and grasping items. To allow motion, different bones are connected by joints. Within these joints, bones are connected to other bones and muscle fibers via connective tissue such as tendons and ligaments. Cartilage prevents the bone ends from rubbing directly on each other. Muscles contract (bunch up) to move the bone attached at the joint.
Unfortunately, diseases and disorders that may adversely affect the function and overall effectiveness of the system exist and can be detrimental to the body. These potentially debilitating diseases can be difficult to diagnose due to the close relation of the musculoskeletal system to other internal systems. In humans, the most common musculoskeletal diseases worldwide are caused by malnutrition. Ailments that affect the joints, such as arthritis, are also widespread. These can make movement difficult; in advanced cases, they completely impair mobility. In severe cases in which the joint has suffered extensive damage, joint replacement surgery may be needed.
Progress in the science of prosthesis design has resulted in the development of artificial joints, with joint replacement surgery in the hips and knees being the most common. Replacement joints for shoulders, elbows, and fingers are also available. Even with this progress, there is still room for improvement in the design of prostheses. The state-of-the-art prostheses have limited durability, wearing out quickly, particularly in young or active individuals. Current research is focused on the use of new materials, such as carbon fiber, that may make prostheses more durable. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.01%3A_Types_of_Skeletal_Systems_-_Functions_of_the_Musculoskeletal_System.txt |
The hydrostatic skeleton, exoskeleton, and endoskeleton support, protect, and provide movement to the bodies of different types of animals.
Learning Objectives
• Differentiate among skeletal types: hydrostatic skeleton, exoskeleton, and endoskeleton
Key Points
• In organisms with hydrostatic skeletons, the muscles contract to change the shape of the coelom, which then produces movement due to the pressure of the fluid inside the fluid-filled cavity.
• Exoskeletons are external skeletal systems that are made up of chitin and calcium carbonate.
• Organisms with an endoskeleton are supported by a hard, mineralized skeletal system that resides inside the body.
• In vertebrates, the endoskeleton system is further divided into the axial skeleton and appendicular skeleton.
Key Terms
• coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
• peristalsis: the rhythmic, wave-like contraction and relaxation of muscles which propagates in a wave down a muscular tube
• endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage
• exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda
Types of Skeletal Designs
A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. There are three different skeleton designs that provide organisms these functions: hydrostatic skeleton, exoskeleton, and endoskeleton.
Hydrostatic Skeleton
A hydrostatic skeleton is one formed by a fluid-filled compartment within the body: the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism. This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates.
Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions (peristalsis) of the skeletal muscle of the body wall hydrostatic skeleton, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.
Exoskeleton
An exoskeleton is an external, hard, encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons. This skeleton type provides defense against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods, such as crabs and lobsters, have exoskeletons that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a strong-but-flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular and does not grow as the organism grows, arthropods must periodically shed their exoskeletons.
Endoskeleton
An endoskeleton consists of hard, mineralized structures located within the soft tissue of organisms. An example of a primitive endoskeletal structure is the spicule of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues. Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton.
The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle). | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.02%3A_Types_of_Skeletal_Systems_-_Types_of_Skeletal_Systems.txt |
The axial skeleton forms the central axis of the human body and consists of the skull, vertebral column, and thoracic cage.
Learning Objectives
• Describe the bones and function of the human axial skeleton
Key Points
• The axial skeleton provides support and protection for the brain, spinal cord, and the organs in the ventral body cavity; it also provides a surface for the attachment of muscles, directs respiratory movements, and stabilizes portions of the appendicular skeleton.
• The bones of the skull are divided into cranial bones and facial bones; their main roles consist of supporting the structures of the face and protecting the brain.
• The vertebral column protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck.
• The thoracic cage’s most notable role is in breathing; however, it also protects the organs of the thoracic cavity, provides support for the shoulder girdles and upper limbs, and functions as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders.
Key Terms
• intervertebral disc: a disc between the vertebra in the spine
• ossicle: a small bone (or bony structure), especially one of the three of the middle ear
• convex: curved or bowed outward like the outside of a bowl, sphere or circle
• vertebral column: the series of vertebrae that protect the spinal cord; the spinal column
• concave: curved or bowed inward like the inner surface of a sphere or bowl
Human Axial Skeleton
The axial skeleton forms the central axis of the human body and includes the bones of the skull, the ossicles of the middle ear, the hyoid bone of the throat, the vertebral column, and the thoracic cage (ribcage). The function of the axial skeleton is to provide support and protection for the brain, spinal cord, and organs in the ventral body cavity. It also provides a surface for the attachment of muscles that move the head, neck, and trunk; performs respiratory movements; and stabilizes parts of the appendicular skeleton, which will be discussed later.
The Skull
The bones of the skull support the structures of the face and protect the brain. The skull consists of 22 bones, which are divided into two categories: cranial bones and facial bones. The cranial bones are eight bones that form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head and neck. The eight cranial bones include the frontal bone, two parietal bones, two temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone.
Fourteen facial bones form the face, provide cavities for the sense organs (eyes, mouth, and nose), protect the entrances to the digestive and respiratory tracts, and serve as attachment points for facial muscles. The 14 facial bones are the nasal bones, maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones, inferior nasal conchae, and mandible.
The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The auditory ossicles consist of six bones: two malleus bones, two incus bones, and two stapes, one of each on each side. These bones are unique to mammals.
The hyoid bone lies below the mandible in the front of the neck. It acts as a movable base for the tongue and is connected to muscles of the jaw, larynx, and tongue. The mandible articulates with the base of the skull, controling the opening to the airway and gut. In animals with teeth, the mandible brings the surfaces of the teeth in contact with the maxillary teeth.
The Vertebral Column
The vertebral column, or spinal column, surrounds and protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column is comprised of 26 bones: the 24 vertebrae, the sacrum, and the coccyx bones. In the adult, the sacrum is typically composed of five vertebrae that fuse into one. We begin life with approximately 33 vertebrae, but as we grow, several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae.
Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the spinal cord. The names of the spinal curves correspond to the region of the spine in which they occur. The thoracic and sacral curves are concave, while the cervical and lumbar curves are convex. The arched curvature of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring.
Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine, acting as a cushion to absorb shocks from movements, such as walking and running. Intervertebral discs also act as ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age, becoming less elastic. This loss of elasticity diminishes its ability to absorb shocks.
The Thoracic Cage
The thoracic cage, also known as the ribcage, is the skeleton of the chest. It consists of the ribs, sternum, thoracic vertebrae, and costal cartilages. The thoracic cage encloses and protects the organs of the thoracic cavity, including the heart and lungs. It also provides support for the shoulder girdles and upper limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders. Changes in the volume of the thorax enable breathing.
The sternum, or breastbone, is a long, flat bone located at the anterior of the chest. It is formed from three bones that fuse in the adult. The ribs are 12 pairs of long, curved bones that attach to the thoracic vertebrae and curve toward the front of the body, forming the ribcage. Costal cartilages connect the anterior ends of the ribs to the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.03%3A_Types_of_Skeletal_Systems_-_Human_Axial_Skeleton.txt |
The appendicular skeleton supports the attachment and functions of the upper and lower limbs of the human body.
Learning Objectives
• Describe the bones and functions of the human appendicular skeleton
Key Points
• The human appendicular skeleton is composed of the bones of the upper limbs, the lower limbs, the pectoral girdle, and the pelvic girdle.
• The pectoral girdle acts as the point of attachment of the upper limbs to the body.
• The upper limb consists of the arm, the forearm, and the wrist and hand.
• The pelvic girdle is responsible for bearing the weight of the body and is responsible for locomotion; it is also responsible for attaching the lower limbs to the body.
• The lower limbs, including the thighs, legs, and feet, support the entire weight of the body and absorb the resulting forces from locomotion.
Key Terms
• axial skeleton: the bones of the head and trunk of an organism
• appendicular: of or pertaining to a limb or appendage
• clavicle: the collar bone; the prominent bone at the top of the chest between the shoulder and the neck
• scapula: either of the two large, flat, bones forming the back of the shoulder
• articulate: to form a joint or connect by joints
Human Appendicular Skeleton
The human appendicular skeleton is composed of the bones of the upper limbs (which function to grasp and manipulate objects) and the lower limbs (which permit locomotion). It also includes the pectoral (or shoulder) girdle and the pelvic girdle, which attach the upper and lower limbs to the body, respectively.
The Pectoral Girdle
The pectoral girdle bones, providing the points of attachment of the upper limbs to the axial skeleton, consists of the clavicle (or collarbone) in the anterior, as well as the scapula (or shoulder blades) in the posterior. The clavicles, S-shaped bones that position the arms on the body, lie horizontally across the front of the thorax (chest) just above the first rib.
The scapulae are flat, triangular bones that are located at the back of the pectoral girdle. They support the muscles crossing the shoulder joint. The spine runs across the back of the scapula; it is a good example of a bony protrusion that facilitates a broad area of attachment for muscles to bone.
The Upper Limbs
The upper limbs contain 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius), and the wrist and hand. The humerus is the largest and longest bone of the upper limb and the only bone of the arm. It articulates (joins) with the scapula at the shoulder and with the forearm at the elbow. The forearm, extending from the elbow to the wrist, consists of two bones: the ulna and the radius. The radius, located along the lateral (thumb) side of the forearm, articulates with the humerus at the elbow. The ulna, located on the medial aspect (pinky-finger side) of the forearm, is longer than the radius. It articulates with the humerus at the elbow. The radius and ulna also articulate with the carpal bones and with each other, which in vertebrates enables a variable degree of rotation of the carpus with respect to the long axis of the limb. The hand includes the eight bones of the carpus (wrist), the five bones of the metacarpus (palm), and the 14 bones of the phalanges (digits). Each digit consists of three phalanges, except for the thumb, which, when present, has only two.
The Pelvic Girdle
The pelvic girdle attaches to the lower limbs of the axial skeleton and is responsible for bearing the weight of the body and for locomotion. It is securely attached to the axial skeleton by strong ligaments. It also has deep sockets with robust ligaments to securely attach the femur to the body. The pelvic girdle is further strengthened by two large hip bones. In adults, the hip bones are formed by the fusion of three pairs of bones: the ilium, ischium, and pubis. The pelvis joins together in the anterior of the body the pubic symphysis joint and with the bones of the sacrum at the posterior of the body.
The Lower Limbs
The lower limbs consists of the thigh, the leg, and the foot. The bones of the lower limb are the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot). The bones of the lower limbs are thicker and stronger than the bones of the upper limbs because of the need to support the entire weight of the body along with the resulting forces from locomotion.
The femur, or thighbone, is the longest, heaviest, and strongest bone in the body. The femur and pelvis form the hip joint at the proximal end. At the distal end, the femur, tibia, and patella form the knee joint. The patella, or kneecap, is a triangular bone that lies anterior to the knee joint; it is embedded in the tendon of the femoral extensors (quadriceps). It improves knee extension by reducing friction. The tibia, or shinbone, is a large bone of the leg that is located directly below the knee. The tibia articulates with the femur at its proximal end, with the fibula and the tarsal bones at its distal end. As the second largest bone in the human body it is responsible for transmitting the weight of the body from the femur to the foot. The fibula, or calf bone, parallels and articulates with the tibia. It is not weight-bearing, but acts as a site for muscle attachment while forming the lateral part of the ankle joint.
The tarsals are the seven bones of the ankle, which transmits the weight of the body from the tibia and the fibula to the foot. The metatarsals are the five bones of the foot, while the phalanges are the 14 bones of the toes. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.04%3A_Types_of_Skeletal_Systems_-_Human_Appendicular_Skeleton.txt |
Learning Objectives
• Distinguish between compact and spongy bone tissues
Bone Tissue
Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue. Osteocytes, the living cells of bone tissue, form the mineral matrix of bones. There are two types of bone tissue: compact and spongy.
Compact Bone Tissue
Compact bone (or cortical bone), forming the hard external layer of all bones, surrounds the medullary cavity (innermost part or bone marrow). It provides protection and strength to bones. Compact bone tissue consists of units called osteons or Haversian systems. Osteons are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi which transport blood. They are aligned parallel to the long axis of the bone. Each osteon consists of lamellae, layers of compact matrix that surround a central canal (the Haversian or osteonic canal), which contains the bone’s blood vessels and nerve fibers. Osteons in compact bone tissue are aligned in the same direction along lines of stress, helping the bone resist bending or fracturing. Therefore, compact bone tissue is prominent in areas of bone at which stresses are applied in only a few directions.
Spongy Bone Tissue
Compact bone tissue forms the outer layer of all bones while spongy or cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons. Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Red bone marrow is found between the trabuculae. Blood vessels within this tissue deliver nutrients to osteocytes and remove waste. The red bone marrow of the femur and the interior of other large bones, such as the ileum, forms blood cells.
Spongy bone reduces the density of bone, allowing the ends of long bones to compress as the result of stresses applied to the bone. Spongy bone is prominent in areas of bones that are not heavily stressed or where stresses arrive from many directions. The epiphysis of a bone, such as the neck of the femur, is subject to stress from many directions. Imagine laying a heavy-framed picture flat on the floor. You could hold up one side of the picture with a toothpick if the toothpick were perpendicular to the floor and the picture. Now, drill a hole and stick the toothpick into the wall to hang up the picture. In this case, the function of the toothpick is to transmit the downward pressure of the picture to the wall. The force on the picture is straight down to the floor, but the force on the toothpick is both the picture wire pulling down and the bottom of the hole in the wall pushing up. The toothpick will break off right at the wall.
The neck of the femur is horizontal like the toothpick in the wall. The weight of the body pushes it down near the joint, but the vertical diaphysis of the femur pushes it up at the other end. The neck of the femur must be strong enough to transfer the downward force of the body weight horizontally to the vertical shaft of the femur.
Key Points
• Compact bone is the hard external layer of all bones that protects, strengthens, and surrounds the medullary cavity filled with marrow.
• Cylindrical structures, called osteons, are aligned along lines of the greatest stress to the bone in order to resist bending or fracturing.
• Spongy or cancellous bone tissue consists of trabeculae that are arranged as rods or plates with red bone marrow in between.
• Spongy bone is prominent in regions where the bone is less dense and at the ends of long bones where the bone has to be more compressible due to stresses that arrive from many directions.
Key Terms
• trabecula: a small mineralized spicule that forms a network in spongy bone
• epiphysis: the rounded end of any long bone
• osteocyte: a mature bone cell involved with the maintenance of bone
• osteon: any of the central canals and surrounding bony layers found in compact bone | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.05%3A_Bone_-_Introduction.txt |
The osteoblast, osteoclast, osteocyte, and osteoprogenitor bone cells are responsible for the growing, shaping, and maintenance of bones.
Learning Objectives
• Distinguish among the four cell types in bone
Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor (or osteogenic) cells. Each cell type has a unique function and is found in different locations in bones. The osteoblast, the bone cell responsible for forming new bone, is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it. As a result, it changes in structure, becoming an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space (lacuna) surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. As is the case with osteoblasts, osteocytes lack mitotic activity. They are able to communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells: the osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity; they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. When they differentiate, they develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, while old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast, which is found on bone surfaces, is multinucleated, and originates from monocytes and macrophages (two types of white blood cells) rather than from osteogenic cells. Osteoclasts continually break down old bone while osteoblasts continually form new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant, but subtle, reshaping of bone.
Key Points
• Osteogenic cells are the only bone cells that divide.
• Osteogenic cells differentiate and develop into osteoblasts which, in turn, are responsible for forming new bones.
• Osteoblasts synthesize and secrete a collagen matrix and calcium salts.
• When the area surrounding an osteoblast calcifies, the osteoblast becomes trapped and transforms into an osteocyte, the most common and mature type of bone cell.
• Osteoclasts, the cells that break down and reabsorb bone, stem from monocytes and macrophages rather than osteogenic cells..
• There is a continual balance between osteoblasts generating new bone and osteoclasts breaking down bone.
Key Terms
• osteoclast: a large multinuclear cell associated with the resorption of bone
• osteocyte: a mature bone cell involved with the maintenance of bone
• osteoprogenitor: a stem cell that is the precursor of an osteoblast
• canaliculus: any of many small canals or ducts in bone or in some plants
• periosteum: a membrane surrounding a bone
• endosteum: a membranous vascular layer of cells which line the medullary cavity of a bone
• lacuna: a small opening; a small pit or depression; a small blank space; a gap or vacancy; a hiatus
• osteoblast: a mononucleate cell from which bone develops | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.06%3A_Bone_-_Cell_Types_in_Bones.txt |
LEARNING OBJECTIVEs
• Distinguish between intramembranous and endochondral ossification
Ossification, or osteogenesis, is the process of bone formation by osteoblasts. Ossification is distinct from the process of calcification; whereas calcification takes place during the ossification of bones, it can also occur in other tissues. Ossification begins approximately six weeks after fertilization in an embryo. Before this time, the embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage. The development of bone from fibrous membranes is called intramembranous ossification; development from hyaline cartilage is called endochondral ossification. Bone growth continues until approximately age 25. Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.
Intramembranous Ossification
Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles. Ossification begins as mesenchymal cells form a template of the future bone. They then differentiate into osteoblasts at the ossification center. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone. Connective tissue in the matrix differentiates into red bone marrow in the fetus. The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.
Endochondral Ossification
Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification.
In long bones, chondrocytes form a template of the hyaline cartilage diaphysis. Responding to complex developmental signals, the matrix begins to calcify. This calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, while osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone. Osteoclasts then break down some of the spongy bone to create a marrow, or medullary cavity, in the center of the diaphysis. Dense, irregular connective tissue forms a sheath (periosteum) around the bones. The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide.
In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify. Secondary ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert hyaline cartilage into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth plate), which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of long bones.
Key Points
• The ossification of the flat bones of the skull, the mandible, and the clavicles begins with mesenchymal cells, which then differentiate into calcium-secreting and bone matrix-secreting osteoblasts.
• Osteoids form spongy bone around blood vessels, which is later remodeled into a thin layer of compact bone.
• During enchondral ossification, the cartilage template in long bones is calcified; dying chondrocytes provide space for the development of spongy bone and the bone marrow cavity in the interior of the long bones.
• The periosteum, an irregular connective tissue around bones, aids in the attachment of tissues, tendons, and ligaments to the bone.
• Until adolescence, lengthwise long bone growth occurs in secondary ossification centers at the epiphyseal plates (growth plates) near the ends of the bones.
Key Terms
• osteoid: an organic matrix of protein and polysaccharides, secreted by osteoblasts, that becomes bone after mineralization
• endochondral: within cartilage
• chondrocyte: a cell that makes up the tissue of cartilage
• diaphysis: the central shaft of any long bone | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.07%3A_Bone_-_Bone_Development.txt |
Long bones lengthen at the epiphyseal plate with the addition of bone tissue and increase in width by a process called appositional growth.
Learning Objectives
• Describe the processes of post-fetal bone growth and bone thickening
Key Points
• The epiphyseal plate, the area of growth composed of four zones, is where cartilage is formed on the epiphyseal side while cartilage is ossified on the diaphyseal side, thereby lengthening the bone.
• Each of the four zones has a role in the proliferation, maturation, and calcification of bone cells that are added to the diaphysis.
• The longitudinal growth of long bones continues until early adulthood at which time the chondrocytes in the epiphyseal plate stop proliferating and the epiphyseal plate transforms into the epiphyseal line as bone replaces the cartilage.
• Bones can increase in diameter even after longitudinal growth has stopped.
• Appositional growth is the process by which old bone that lines the medullary cavity is reabsorbed and new bone tissue is grown beneath the periosteum, increasing bone diameter.
Key Terms
• metaphysis: the part of a long bone that grows during development
• periosteum: a membrane surrounding a bone
• ossification: the normal process by which bone is formed
• chondrocyte: a cell that makes up the tissue of cartilage
• hypertrophy: to increase in size
• diaphysis: the central shaft of any long bone
• epiphysis: the rounded end of any long bone
• medullary: pertaining to, consisting of, or resembling, marrow or medulla
Growth of Bone
Long bones continue to lengthen (potentially throughout adolescence) through the addition of bone tissue at the epiphyseal plate. They also increase in width through appositional growth.
Lengthening of Long Bones
The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, allowing the diaphysis to grow in length. The metaphysis is the wide portion of a long bone between the epiphysis and the narrow diaphysis. It is considered a part of the growth plate: the part of the bone that grows during childhood, which, as it grows, ossifies near the diaphysis and the epiphyses.
The epiphyseal plate is composed of four zones of cells and activity.
1. The reserve zone, the region closest to the epiphyseal end of the plate, contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth; instead, they secure the epiphyseal plate to the osseous tissue of the epiphysis.
2. The proliferative zone, the next layer toward the diaphysis, contains stacks of slightly-larger chondrocytes. It continually makes new chondrocytes via mitosis.
3. The zone of maturation and hypertrophy contains chondrocytes that are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. In this zone, lipids, glycogen, and alkaline phosphatase accumulate, causing the cartilaginous matrix to calcify. The longitudinal growth of bone is a result of cellular division in the proliferative zone along with the maturation of cells in the zone of maturation and hypertrophy.
4. The zone of calcified matrix, the zone closest to the diaphysis, contains chondrocytes that are dead because the matrix around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone. The osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when osseous tissue is added to the diaphysis.
After the zone of calcified matrix, there is the zone of ossification, which is actually part of the metaphysis. Arteries from the metaphysis branch through the newly-formed trabeculae in this zone. The newly-deposited bone tissue at the top of the zone of ossification is called the primary spongiosa. The older bone at the bottom of the zone of ossification is called the secondary spongiosa.
Bones continue to grow in length until early adulthood with the rate of growth controlled by hormones. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line.
Thickening of Long Bones
While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases. This is called appositional growth. Osteoclasts, cells that work to break down bone, resorb old bone that lines the medullary cavity. At the same time, osteoblasts via intramembranous ossification, produce new bone tissue beneath the periosteum. The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis, but also increase the diameter of the medullary cavity. This process is called modeling. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.08%3A_Bone_-_Growth_of_Bone.txt |
Bone is remodeled through the continual replacement of old bone tissue, as well as repaired when fractured.
Learning Objectives
• Outline the process of bone remodeling and repair
Key Points
• Bone replacement involves the osteoclasts which break down bone and the osteoblasts which make new bone.
• Bone turnover rates differ depending on the bone and the area within the bone.
• There are four stages in the repair of a broken bone: 1) the formation of hematoma at the break, 2) the formation of a fibrocartilaginous callus, 3) the formation of a bony callus, and 4) remodeling and addition of compact bone.
• Proper bone growth and maintenance requires many vitamins (D, C, and A), minerals (calcium, phosphorous, and magnesium), and hormones ( parathyroid hormone, growth hormone, and calcitonin ).
Key Terms
• callus: the material of repair in fractures of bone which is at first soft or cartilaginous in consistency, but is ultimately converted into true bone and unites the fragments into a single piece
• spicule: a sharp, needle-like piece
• fibroblast: a cell found in connective tissue that produces fibers, such as collagen
Bone Remodeling and Repair
Bone renewal continues after birth into adulthood. Bone remodeling is the replacement of old bone tissue by new bone tissue. It involves the processes of bone deposition or bone production done by osteoblasts and bone resorption done by osteoclasts, which break down old bone. Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous, and magnesium. Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for proper bone growth and maintenance.
Bone turnover rates, the rates at which old bone is replaced by new bone, are quite high, with five to seven percent of bone mass being recycled every week. Differences in turnover rates exist in different areas of the skeleton and in different areas of a bone. For example, the bone in the head of the femur may be fully replaced every six months, whereas the bone along the shaft is altered much more slowly.
Bone remodeling allows bones to adapt to stresses by becoming thicker and stronger when subjected to stress. Bones that are not subject to normal everyday stress (for example, when a limb is in a cast) will begin to lose mass.
A fractured or broken bone undergoes repair through four stages:
1. Hematoma formation: Blood vessels in the broken bone tear and hemorrhage, resulting in the formation of clotted blood, or a hematoma, at the site of the break. The severed blood vessels at the broken ends of the bone are sealed by the clotting process. Bone cells deprived of nutrients begin to die.
2. Bone generation: Within days of the fracture, capillaries grow into the hematoma, while phagocytic cells begin to clear away the dead cells. Though fragments of the blood clot may remain, fibroblasts and osteoblasts enter the area and begin to reform bone. Fibroblasts produce collagen fibers that connect the broken bone ends, while osteoblasts start to form spongy bone. The repair tissue between the broken bone ends, the fibrocartilaginous callus, is composed of both hyaline and fibrocartilage. Some bone spicules may also appear at this point.
3. Bony callous formation: The fibrocartilaginous callus is converted into a bony callus of spongy bone. It takes about two months for the broken bone ends to be firmly joined together after the fracture. This is similar to the endochondral formation of bone when cartilage becomes ossified; osteoblasts, osteoclasts, and bone matrix are present.
4. Bone remodeling: The bony callus is then remodelled by osteoclasts and osteoblasts, with excess material on the exterior of the bone and within the medullary cavity being removed. Compact bone is added to create bone tissue that is similar to the original, unbroken bone. This remodeling can take many months; the bone may remain uneven for years. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.09%3A_Bone_-__Bone_Remodeling_and_Repair.txt |
Joints, responsible for movement and stability of the skeleton, can be classified based on structure or function.
Learning Objectives
• Differentiate among the types of skeletal joints based on structure and function
Key Points
• Fibrous joints contain fibrous connective tissue and cannot move; fibrous joints include sutures, syndesmoses, and gomphoses.
• Cartilaginous joints contain cartilage and allow very little movement; there are two types of cartilaginous joints: synchondroses and symphyses.
• Synovial joints are the only joints that have a space (a synovial cavity filled with fluid) between the adjoining bones.
• The presence of synovial fluid and an articular capsule give synovial joints the greatest range of movement among the three joint types; however they are the weakest of the joint types.
• Based on function, joints can be divided into synarthroses, amphiarthroses, and diarthroses.
• Synarthrosis joints include fibrous joints; amphiarthrosis joints include cartilaginous joints; diarthrosis joints include synovial joints.
Key Terms
• synovial fluid: a viscous, fluid found in the cavities of synovial joints whose main purpose is to reduce friction between the articular cartilage of synovial joints during movement
• diarthrosis: a joint that can move freely in various planes
• synarthrosis: immovable joint in which two bones are connected rigidly by fibrous tissue
• amphiarthrosis: slightly movable joint in which the surfaces of bones are connected by ligaments or cartilage
Classification of Joints on the Basis of Structure and Function
The point at which two or more bones meet is called a joint or articulation. Joints are responsible for movement (e.g., the movement of limbs) and stability (e.g.,the stability found in the bones of the skull). There are two ways to classify joints: on the basis of their structure or on the basis of their function.
The structural classification divides joints into fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint. The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses.
Fibrous Joints
The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones, so most fibrous joints do not move at all. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place.
Syndesmoses are joints in which the bones are connected by a band of connective tissue, allowing for more movement than in a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle. The amount of movement in these types of joints is determined by the length of the connective tissue fibers. Gomphoses occur between teeth and their sockets; the term refers to the way the tooth fits into the socket like a peg. The tooth is connected to the socket by a connective tissue called the periodontal ligament. Fibrous joints classified as synarthroses, or immovable, include: sutures, gomphoses, and synchondroses
Cartilaginous Joints
Cartilaginous joints are those in which the bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In a synchondrosis, the bones are joined by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses, hyaline cartilage covers the end of the bone, but the connection between bones occurs through fibrocartilage. Symphyses are found at the joints between vertebrae and between the pubic bones. Amphiarthroses allow only slight movement; therefore, either type of cartilaginous joint is an amphiarthrosis.
Synovial Joints
Synovial joints are the only joints that have a space between the adjoining bones. This space, referred to as the synovial (or joint) cavity, is filled with synovial fluid. Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage. The entire joint is surrounded by an articular capsule composed of connective tissue. This allows movement of the joint as well as resistance to dislocation. Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types; however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints. Since they allow for free movement, synovial joints are classified as diarthroses. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.10%3A_Joints_and_Skeletal_Movement_-_Classification_of_Joints_on_the_Basis_of_Structure_and_Function.txt |
Synovial joints allow for many types of movement including gliding, angular, rotational, and special movements.
Learning Objectives
• Differentiate among the types of movements possible at synovial joints
Key Points
• Gliding movements occur as relatively flat bone surfaces move past each other, but they produce very little movement of the bones.
• Angular movements are produced when the angle between the bones of a joint changes; they include flexion, extension, hyperextension, abduction, adduction, and circumduction.
• Rotational movement involves moving the bone around its longitudinal axis; this can be movement toward the midline of the body (medial rotation) or away from the midline of the body (lateral rotation).
• Special movements are all the other movements that cannot be classified as gliding, angular, or rotational; these movements include inversion, eversion, protraction, and retraction.
• Other special movements include elevation, depression, supination, and pronation.
Key Terms
• adduction: the movement of a bone toward the midline of the body
• abduction: moving a bone away from the midline of the body
• supination: the action of rotating the forearm so that the palm of the hand is turned up or forward
• pronation: the action of rotating the forearm so that the palm of the hand is turned down or back
Movement at Synovial Joints
The range of movement allowed by synovial joints is fairly wide. These movements can be classified as: gliding, angular, rotational, or special movement.
Gliding Movement
Gliding movements occur as relatively flat bone surfaces move past each other. They produce very little rotation or angular movement of the bones. The joints of the carpal and tarsal bones are examples of joints that produce gliding movements.
Angular Movement
Angular movements are produced by changing the angle between the bones of a joint. There are several different types of angular movements, including flexion, extension, hyperextension, abduction, adduction, and circumduction. Flexion, or bending, occurs when the angle between the bones decreases. Moving the forearm upward at the elbow or moving the wrist to move the hand toward the forearm are examples of flexion. In extension, the opposite of flexion, the angle between the bones of a joint increases. Straightening a limb after flexion is an example of extension. Extension past the normal anatomical position is referred to as hyperextension. This includes moving the neck back to look upward or bending the wrist so that the hand moves away from the forearm.
Abduction occurs when a bone moves away from the midline of the body. Examples of abduction include moving the arms or legs laterally to lift them straight out to the side. Adduction is the movement of a bone toward the midline of the body. Movement of the limbs inward after abduction is an example of adduction. Circumduction is the movement of a limb in a circular motion, as in swinging an arm around.
Rotational Movement
Rotational movement is the movement of a bone as it rotates around its longitudinal axis. Rotation can be toward the midline of the body, which is referred to as medial rotation, or away from the midline of the body, which is referred to as lateral rotation. Movement of the head from side to side is an example of rotation.
Special Movements
Some movements that cannot be classified as gliding, angular, or rotational are called special movements. Inversion involves moving the soles of the feet inward, toward the midline of the body. Eversion, the opposite of inversion, involves moving of the sole of the foot outward, away from the midline of the body. Protraction is the anterior movement of a bone in the horizontal plane. Retraction occurs as a joint moves back into position after protraction. Protraction and retraction can be seen in the movement of the mandible as the jaw is thrust outwards and then back inwards. Elevation is the movement of a bone upward, such as shrugging the shoulders, lifting the scapulae. Depression is the opposite of elevation and involves moving the bone downward, such as after the shoulders are shrugged and the scapulae return to their normal position from an elevated position. Dorsiflexion is a bending at the ankle such that the toes are lifted toward the knee. Plantarflexion is a bending at the ankle when the heel is lifted, such as when standing on the toes. Supination is the movement of the radius and ulna bones of the forearm so that the palm faces forward or up. Pronation is the opposite movement, in which the palm faces backward or down. Opposition is the movement of the thumb toward the fingers of the same hand, making it possible to grasp and hold objects. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.11%3A_Joints_and_Skeletal_Movement_-_Movement_at_Synovial_Joints.txt |
Learning Objectives
• Differentiate among the six categories of joints based on shape and structure
Types of Synovial Joints
Synovial joints are further classified into six different categories on the basis of the shape and structure of the joint. The shape of the joint affects the type of movement permitted by the joint. These joints can be described as planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints.
Planar Joints
Planar joints have bones with articulating surfaces that are flat or slightly curved. These joints allow for gliding movements; therefore, the joints are sometimes referred to as gliding joints. The range of motion is limited and does not involve rotation. Planar joints are found in the carpal bones in the hand and the tarsal bones of the foot, as well as between vertebrae.
Hinge Joints
In hinge joints, the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone. In this way, one bone moves while the other remains stationary, similar to the hinge of a door. The elbow is an example of a hinge joint. The knee is sometimes classified as a modified hinge joint.
Pivot Joints
Pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone. This structure allows rotational movement, as the rounded bone moves around its own axis. An example of a pivot joint is the joint of the first and second vertebrae of the neck that allows the head to move back and forth. The joint of the wrist that allows the palm of the hand to be turned up and down is also a pivot joint.
Condyloid Joints
Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone. This is also sometimes called an ellipsoidal joint. This type of joint allows angular movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and up and down.
Saddle Joints
Each bone in a saddle joint resembles a saddle, with concave and convex portions that fit together. Saddle joints allow angular movements similar to condyloid joints, but with a greater range of motion. An example of a saddle joint is the thumb joint, which can move back and forth and up and down; it can move more freely than the wrist or fingers.
Ball-and-Socket Joints
Ball-and-socket joints possess a rounded, ball-like end of one bone fitting into a cup-like socket of another bone. This organization allows the greatest range of motion, as all movement types are possible in all directions. Examples of ball-and-socket joints are the shoulder and hip joints.
The Role of Rheumatologists
Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders, osteoporosis, and autoimmune diseases such as ankylosing spondylitis and rheumatoid arthritis.
Rheumatoid arthritis (RA) is an inflammatory disorder that primarily affects the synovial joints of the hands, feet, and cervical spine. Affected joints become swollen, stiff, and painful. Although it is known that RA is an autoimmune disease in which the body’s immune system mistakenly attacks healthy tissue, the cause of RA remains unknown. Immune cells from the blood enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining. Breakdown of cartilage results in bones rubbing against each other, causing pain. RA is more common in women than men; the age of onset is usually 40–50 years of age.
Rheumatologists diagnose RA on the basis of symptoms (joint inflammation and pain), X-ray and MRI imaging, and blood tests. Arthrography, a type of medical imaging of joints, uses a contrast agent, such as a dye, that is opaque to X-rays. This allows the soft tissue structures of joints, such as cartilage, tendons, and ligaments, to be visualized. An arthrogram differs from a regular X-ray by showing the surface of soft tissues lining the joint in addition to joint bones. An arthrogram allows early degenerative changes in joint cartilage to be detected before bones become affected.
There is currently no cure for RA; however, rheumatologists have a number of treatment options available. Early stages can be treated by resting the affected joints, using a cane or joint splints, to minimize inflammation. When inflammation has decreased, exercise can be used to strengthen the muscles that surround the joint in order to maintain joint flexibility. If joint damage is more extensive, medications can be used to relieve pain and decrease inflammation. Anti-inflammatory drugs such as aspirin, topical pain relievers, and corticosteroid injections may be used. Surgery may be required in cases in which joint damage is severe.
Key Points
• Planar joints have bones with articulating surfaces that are flat or slightly curved, allowing for limited movement; pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone to allow rotational movement.
• Hinge joints act like the hinge of a door; the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone; one bone remains stationary.
• Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone to allow angular movement along two axes.
• Saddle joints include concave and convex portions that fit together and allow angular movement; ball-and-socket joints include a rounded, ball-like end of one bone fitting into a cup-like socket of another bone which allows the greatest range of motion.
• Rheumatologists diagnose and treat joint disorders, which include rheumatoid arthritis and osteoporosis.
• Immune cells enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining, which breaks down cartilage, resulting in bones rubbing against each other, causing pain.
Key Terms
• condyloid joint: consists of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone
• ball-and-socket joint: consists of a rounded, ball-like end of one bone fitting into a cup-like socket of another bone, allowing the first segment to move around an indefinite number of axes which have one common center
• rheumatoid arthritis: chronic, progressive disease in which the immune system attacks the joints; characterized by pain, inflammation and swelling of the joints, stiffness, weakness, loss of mobility, and deformity | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.12%3A_Joints_and_Skeletal_Movement_-_Types_of_Synovial_Joints.txt |
The most common bone and joint disorder are types of arthritis.
Learning Objectives
• Describe the causes and treatments for gout, rheumatoid arthritis, and osteoarthritis
Key Points
• Arthritis is a common disorder of synovial joints that involves inflammation of the joint; there are a few major subtypes of this disorder.
• The most common type of arthritis is osteoarthritis, which is associated with “wear and tear” of cartilage.
• Gout is a form of arthritis resulting from the deposit of uric acid crystals within a body joint.
• Rheumatoid arthritis is an autoimmune disease in which the joint capsule and synovial membrane become inflamed.
Key Terms
• synovial joints: The most common type of joint in the body, which includes a joint cavity.
• arthritis: A joint disorder that involves inflammation in one or more joints.
Synovial joints are the most common type of joint in the body. A key structural characteristic for a synovial joint that is not seen at fibrous or cartilaginous joints is the presence of a joint cavity.
Arthritis is a common disorder of synovial joints that involves inflammation of the joint. This often results in significant joint pain, along with swelling, stiffness, and reduced joint mobility. There are more than 100 different forms of arthritis. Arthritis may arise from aging, damage to the articular cartilage, autoimmune diseases, bacterial or viral infections, or unknown (probably genetic ) causes.
Arthritis is the most common cause of disability in the USA. More than 20 million individuals with arthritis have severe limitations in function on a daily basis.
Osteoarthritis
The most common type of arthritis is osteoarthritis, which is associated with aging and “wear and tear” of the articular cartilage. Risk factors that may lead to osteoarthritis later in life include injury to a joint; jobs that involve physical labor; sports with running, twisting, or throwing actions; and being overweight.
Osteoarthritis of the Finger Joints
The formation of hard nobs at the middle finger joints (known as Bouchard’s nodes) and at the farther away finger joint (known as Heberden’s node) are a common feature of Osteoarthritis in the hands.
Osteoarthritis begins in the cartilage and eventually causes the two opposing bones to erode into each other. Osteoarthritis typically affects the weight-bearing joints, such as the back, knee and hip. There is no cure for osteoarthritis, but several treatments (surgery, lifestyle changes, medications) can help alleviate the pain.
Gout
Gout is a form of arthritis that results from the deposit of uric acid crystals within a body joint. Usually only one or a few joints are affected, such as the big toe, knee, or ankle. The attack may only last a few days, but could return to the same or another joint. Gout occurs when the body makes too much uric acid or the kidneys do not properly excrete it. A diet with excessive fructose has been implicated in raising the chances of a susceptible individual developing gout.
Rheumatoid Arthritis
Other forms of arthritis are associated with various autoimmune diseases, bacterial infections of the joint, or unknown genetic causes. Autoimmune diseases like rheumatoid arthritis produce arthritis because the immune system of the body attacks the body joints.
In rheumatoid arthritis, the joint capsule and synovial membrane become inflamed. As the disease progresses, the articular cartilage is severely damaged or destroyed, resulting in joint deformation, loss of movement, and severe disability. The most commonly involved joints are the hands, feet, and cervical spine, with corresponding joints on both sides of the body usually affected, though not always to the same extent.
Rheumatoid Arthritis: A untreated hand affected by rheumatoid arthritis. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.13%3A_Joints_and_Skeletal_Movement_-_Bone_and_Joint_Disorders.txt |
The muscular system controls numerous functions, which is possible with the significant differentiation of muscle tissue morphology and ability.
Learning Objectives
• Describe the three types of muscle tissue
Key Points
• The muscular system is responsible for functions such as maintenance of posture, locomotion, and control of various circulatory systems.
• Muscle tissue can be divided functionally (voluntarily or involuntarily controlled) and morphologically ( striated or non-striated).
• These classifications describe three distinct muscle types: skeletal, cardiac and smooth. Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated, and smooth muscle is involuntary and non-striated.
Key Terms
• myofibril: A fiber made up of several myofilaments that facilitates the generation of tension in a myocyte.
• myofilament: A filament composed of either multiple myosin or actin proteins that slide over each other to generate tension.
• myosin: A motor protein which forms myofilaments that interact with actin filaments to generate tension.
• actin: A protein which forms myofilaments that interact with myosin filaments to generate tension.
• striated: The striped appearance of certain muscle types in which myofibrils are aligned to produce a constant directional tension.
• voluntary: A muscle movement under conscious control (e.g. deciding to move the forearm).
• involuntary: A muscle movement not under conscious control (e.g. the beating of the heart).
• myocyte: A muscle cell.
The Musculoskeletal System
The muscular system is made up of muscle tissue and is responsible for functions such as maintenance of posture, locomotion and control of various circulatory systems. This includes the beating of the heart and the movement of food through the digestive system. The muscular system is closely associated with the skeletal system in facilitating movement. Both voluntary and involuntary muscular system functions are controlled by the nervous system.
Muscle is a highly-specialized soft tissue that produces tension which results in the generation of force. Muscle cells, or myocytes, contain myofibrils comprised of actin and myosin myofilaments which slide past each other producing tension that changes the shape of the myocyte. Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force.
Muscle tissue can be classified functionally as voluntary or involuntary and morphologically as striated or non-striated. Voluntary refers to whether the muscle is under conscious control, while striation refers to the presence of visible banding within myocytes caused by the organization of myofibrils to produce constant tension.
Types of Muscle
The above classifications describe three forms of muscle tissue that perform a wide range of diverse functions.
Skeletal Muscle
Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement. For example, contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or to the skin, as seen in
the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei.
Cardiac Muscle Tissue
Cardiac muscle tissue is found only in the heart, where cardiac contractions pump blood throughout the body and maintain blood pressure.
As with skeletal muscle, cardiac muscle is striated; however it is not consciously controlled and so is classified as involuntary. Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs that control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.
Smooth Muscle Tissue
Smooth muscle tissue is associated with numerous organs and tissue systems, such as the digestive system and respiratory system. It plays an important role in the regulation of flow in such systems, such as aiding the movement of food through the digestive system via peristalsis.
Smooth muscle is non-striated and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.14%3A_Muscle_Contraction_and_Locomotion_-_Structure_and_Function_of_the_Muscular_System.txt |
Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
Learning Objectives
• Outline the structure of a skeletal muscle fiber
Key Points
• Muscles are composed of long bundles of myocytes or muscle fibers.
• Myocytes contain thousands of myofibrils.
• Each myofibril is composed of numerous sarcomeres, the functional contracile region of a striated muscle. Sarcomeres are composed of myofilaments of myosin and actin, which interact using the sliding filament model and cross-bridge cycle to contract.
Key Terms
• sarcoplasm: The cytoplasm of a myocyte.
• sarcoplasmic reticulum: The equivalent of the smooth endoplasmic reticulum in a myocyte.
• sarcolemma: The cell membrane of a myocyte.
• sarcomere: The functional contractile unit of the myofibril of a striated muscle.
Skeletal Muscle Fiber Structure
Myocytes, sometimes called muscle fibers, form the bulk of muscle tissue. They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation of terminology which differentiates them from generic animal cells.
Generic cell > Myocyte
Cytoplasm > Sarcoplasm
Cell membrane > Sarcolemma
Smooth endoplasmic reticulum > Sarcoplasmic reticulum
Myocyte Structure
Myocytes can be incredibly large, with diameters of up to 100 micrometers and lengths of up to 30 centimeters. The sarcoplasm is rich with glycogen and myoglobin, which store the glucose and oxygen required for energy generation, and is almost completely filled with myofibrils, the long fibers composed of
myofilaments that facilitate muscle contraction.
The sarcolemma of myocytes contains numerous invaginations (pits) called transverse tubules which are usually perpendicular to the length of the myocyte. Transverse tubules play an important role in supplying the myocyte with Ca+ ions, which are key for muscle contraction.
Each myocyte contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to myocytes. These myoblasts asre located to the periphery of the myocyte and flattened so
as not to impact myocyte contraction.
Myofibril Structure
Each myocyte can contain many thousands of myofibrils. Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end. Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules. The sarcoplasmic reticulum acts as a sink of Ca+ ions, which are released upon signalling from the transverse tubules.
Sarcomeres
Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins. These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction. The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance.
Sarcomere: The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments.
Myofilaments (Thick and Thin Filaments)
Myofibrils are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments. Thin filaments are composed of actin, tropomyosin, and troponin. The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.15%3A_Muscle_Contraction_and_Locomotion_-_Skeletal_Muscle_Fibers.txt |
In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere.
Learning Objectives
• Describe the sliding filament model of muscle contraction
Key Points
• The sarcomere is the region in which sliding filament contraction occurs.
• During contraction, myosin myofilaments ratchet over actin myofilaments contracting the sarcomere.
• Within the sarcomere, key regions known as the I and H band compress and expand to facilitate this movement.
• The myofilaments themselves do not expand or contract.
Key Terms
• I-band: The area adjacent to the Z-line, where actin myofilaments are not superimposed by myosin myofilaments.
• A-band: The length of a myosin myofilament within a sarcomere.
• M-line: The line at the center of a sarcomere to which myosin myofilaments bind.
• Z-line: Neighbouring, parallel lines that define a sarcomere.
• H-band: The area adjacent to the M-line, where myosin myofilaments are not superimposed by actin myofilaments.
Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk. The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.
Sarcomere Structure
To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighbouring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin. Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin
molecules.
Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments. The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament.
The region between two neighboring, parallel I-bands is known as the A-band and contains the entire length of single myosin myofilaments. Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments. Within the H-band is the M-line, which is composed of myosin myofilaments and titin molecules crosslinked by myomesin.
Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity provides the underpinning of muscle contraction. Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere. Another protein, nebulin, is thought to perform a similar role for actin myofilaments.
Model of Contraction
The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament.
At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant.
The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.16%3A_Muscle_Contraction_and_Locomotion_-_Sliding_Filament_Model_of_Contraction.txt |
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