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group_3_presentation_1_-_sweating [2018/09/28 09:28] calosac [Mechanism of Sweating] |
group_3_presentation_1_-_sweating [2018/09/28 22:37] (current) premachu |
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| ====== Sweating ====== | ====== Sweating ====== | ||
| + | <box 60% width centre|> {{ :giphy_1_.gif?nolink |}}</box| Figure 1: Sweating.> | ||
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| Sweating, also known as perspiration, is the release of fluids from sweat glands or the release of water by the skin and from the lungs (Folk & Semken, 1991). The two sweat glands that are involved in human perspiration are the apocrine and eccrine glands. Apocrine glands are typically found deep in the dermis, near hair follicles. These glands are concentrated in the axilla or armpit of humans but can be found scattered throughout the entire body as well. Apocrine glands produce viscous, somewhat oily secretions in typically less copious amounts compared to eccrine glands. Eccrine glands are found in the outer region of the dermis but are numerous on the palms and soles of humans. In contrast, eccrine glands produce dilute fluid in ample amounts (Folk & Semken, 1991). | Sweating, also known as perspiration, is the release of fluids from sweat glands or the release of water by the skin and from the lungs (Folk & Semken, 1991). The two sweat glands that are involved in human perspiration are the apocrine and eccrine glands. Apocrine glands are typically found deep in the dermis, near hair follicles. These glands are concentrated in the axilla or armpit of humans but can be found scattered throughout the entire body as well. Apocrine glands produce viscous, somewhat oily secretions in typically less copious amounts compared to eccrine glands. Eccrine glands are found in the outer region of the dermis but are numerous on the palms and soles of humans. In contrast, eccrine glands produce dilute fluid in ample amounts (Folk & Semken, 1991). | ||
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| - | When the sweat gland is stimulated, the cells secrete a fluid (primary secretion) that is similar to plasma (mostly water but with a high concentration of sodium and chloride and a low concentration of potassium) but without the proteins and fatty acids that are normally found in plasma. The source of this fluid is the spaces between the cells (interstitial spaces), which get the fluid from the blood vessels (capillaries) in the dermis. This fluid travels from the coiled portion of the gland up through the straight duct that connects to the opening/pore on the skin’s surface. Activity in the straight duct depends upon the rate of sweat production or flow: | ||
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| - | - Low sweat production occurs during resting state or cool temperatures when cells in the straight duct reabsorb most of the sodium and chlorine from the fluid. This happens because there is enough time for reabsorption. So not much sweat reaches the outside. Also, the composition of this sweat is significantly different from the primary secretion as there is not much sodium and chloride, but there is more potassium. | ||
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| - | - High sweat production occurs during active state (exercise) or warm temperatures Cells in the straight portion do not have enough time to reabsorb all of sodium and chloride from the primary secretion. So, a lot of sweat makes it to the surface of the skin and the composition is similar to, but not exactly like the primary secretion. The sodium and chloride concentrations are about half as much, and potassium is about 20 percent higher. | ||
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| - | Sweat in apocrine sweat glands is produced in the same way. However, the sweat from apocrine glands also contains proteins and fatty acids, which make it thicker and give it a milkier or yellowish color. This is why underarm stains in clothing appear yellowish. Sweat itself has no odor, but when bacteria on the skin and hair metabolize the proteins and fatty acids, they produce an unpleasant odor. This is why deodorants and anti-perspirants are applied to the underarms instead of the whole body (Freudenrich, n.d.). | ||
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| + | <box width centre|> {{ :insensible_perspiration.png?nolink&300 |}}</box| Figure 2: Insensible perspiration.> | ||
| - | {{ :insensible_perspiration.png?nolink&300 |}} | ||
| ==== Thermoregulatory Sweating (Heat/Exercise) ==== | ==== Thermoregulatory Sweating (Heat/Exercise) ==== | ||
| Thermal sweating occurs when one is exposed to a high temperature environment or during muscular exercise. Both the high temperature environment and exercise cause an increase in body temperature. As a result, the body must have a way to reduce the temperature back to its normal level. Thus, heat-induced sweating occurs. The human body has millions of tiny eccrine sweat glands distributed across its skin which directly open to the skin surface (Cui & Schlessinger, 2015). The principle function of these eccrine glands is thermoregulation during exposure to hot environments or during physical exercise. They create a thermoregulatory organ which primarily secretes water that contains electrolytes. These glands allow humans to sweat up to 4 litres per hour in order to reduce body temperature and maintain homeostasis (Cui & Schlessinger, 2015). | Thermal sweating occurs when one is exposed to a high temperature environment or during muscular exercise. Both the high temperature environment and exercise cause an increase in body temperature. As a result, the body must have a way to reduce the temperature back to its normal level. Thus, heat-induced sweating occurs. The human body has millions of tiny eccrine sweat glands distributed across its skin which directly open to the skin surface (Cui & Schlessinger, 2015). The principle function of these eccrine glands is thermoregulation during exposure to hot environments or during physical exercise. They create a thermoregulatory organ which primarily secretes water that contains electrolytes. These glands allow humans to sweat up to 4 litres per hour in order to reduce body temperature and maintain homeostasis (Cui & Schlessinger, 2015). | ||
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| The reason why some people cannot tolerate spicy food could be because their TRPV1 receptor proteins are more sensitive to capsaicin molecules (Byrnes & Hayes, 2013). Ultimately, they are more likely to feel a burning sensation than those with less sensitive TRPV1 receptors. Another reason could be because individuals are able to build a tolerance to spicy foods/capsaicin molecules. After repeated exposure to capsaicin, individuals would become desensitized and not feel much of a burning sensation compared to those who rarely eat spicy food. | The reason why some people cannot tolerate spicy food could be because their TRPV1 receptor proteins are more sensitive to capsaicin molecules (Byrnes & Hayes, 2013). Ultimately, they are more likely to feel a burning sensation than those with less sensitive TRPV1 receptors. Another reason could be because individuals are able to build a tolerance to spicy foods/capsaicin molecules. After repeated exposure to capsaicin, individuals would become desensitized and not feel much of a burning sensation compared to those who rarely eat spicy food. | ||
| - | {{ :trpv1.png?nolink&300 |}} | + | <box width centre|> {{ :trpv1.png?nolink&300 |}} </box| Figure 3: Stimulation of gustatory sweating due to spicy food.> |
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| ====== Mechanism of Sweating ====== | ====== Mechanism of Sweating ====== | ||
| - | The primary thermoregulatory center is located within the preoptic hypothalamic regions of the brain. In humans, the neural pathways responsible for sweating are not entirely understood. However, based upon evidence from animal studies, the neural pathway from the brain to sweat gland is believed to be as follows: | + | The primary thermoregulatory center is located within the preoptic hypothalamic regions of the brain. In humans, the neural pathways responsible for sweating are not entirely understood. However, based upon evidence from animal studies and human anatomical data, the neural pathway from the brain to sweat gland is believed to be as follows: |
| + | First, efferent signals from the pre-optic hypothalamus travel via the tegmentum of the pons and the medullary raphe regions to the intermediolateral cell column of the spinal cord. In the spinal cord, neurons emerge from the ventral horn, pass through the white ramus communicans and then synapse in the sympathetic ganglia. Postganglionic non-myelinated C-fibers pass through the gray ramus communicans, combine with peripheral nerves and travel to sweat glands, with these nerve fibers wrapped around the periglandular tissue of the eccrine sweat gland. | ||
| + | The sympathetic nerves distributed to sweat glands consist of large numbers of cholinergic terminals and a few adrenergic terminals. The effect of adrenergic terminals in causing sweating is minimal given that exogenous administration of adrenergic agents will cause only minimal sweating relative to acetylcholine (primary neurotransmitter causing sweating) administration. Administration of atropine (muscarinic cholinergic receptor antagonist) greatly attenuates or eliminates sweating during a resting state or cold temperature environment, confirming the dominance of the cholinergic system and muscarinic receptors in human sweating. | ||
| + | <box width centre|> {{ :mechanism1.png?direct&300 |}}</box| Figure 4. Neural mechanism of sweating from the brain to sweat glands.> | ||
| + | In addition to the central neural drive, sweating can also be initiated by an axon reflex. Exogenous administration of acetylcholine not only directly stimulates muscarinic cholinergic receptors on sweat glands, but also activates an axon reflex via stimulation of axonal nicotinic cholinergic receptors. The neural impulse due to the activated axon terminal travels through nerve terminals, resulting in the release of acetylcholine. Acetylcholine released from cholinergic nerves is rapidly hydrolyzed by acetylcholinesterase. Thus, acetylcholinesterase is capable of modulating sweat rate during low to moderate sweating activity. | ||
| - | First, efferent signals from the pre-optic hypothalamus travel via the tegmentum of the pons and the medullary raphe regions to the intermediolateral cell column of the spinal cord. In the spinal cord, neurons emerge from the ventral horn, pass through the white ramus communicans and then synapse in the sympathetic ganglia. Postganglionic non-myelinated C-fibers pass through the gray ramus communicans, combine with peripheral nerves and travel to sweat glands, with these nerve fibers wrapped around the periglandular tissue of the eccrine sweat gland. The sympathetic nerves distributed to sweat glands consist of large numbers of cholinergic terminals and a few adrenergic terminals. The effect of adrenergic terminals in causing sweating is minimal given that exogenous administration of adrenergic agents will cause only minimal sweating relative to acetylcholine (primary neurotransmitter causing sweating) administration. | + | <box width centre|> {{ :mech2.png?direct&300 |}}</box| Figure 5. Release of acetylcholine at the level of the sweat gland.> |
| + | The neurotransmitter(s) responsible for active cutaneous vasodilation has yet to be fully understood. However, neuropeptides such as calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), and substance P as well as nitric oxide (NO) have been implicated. All these neurotransmitters except substance P have been shown to increase sweat secretion. Even though acetylcholine is the primary neurotransmitter responsible for sweat secretion, enhanced sweating due to local administration of VIP, CGRP, or NO suggest that these peptides may contribute to the overall modulation of sweating during a thermal challenge as well. | ||
| + | When the eccrine sweat gland is stimulated, the cells secrete a fluid (primary secretion) via a family of membrane water channel proteins localized to the apical membrane of multiple secretory glands called Aquaporins (AQPs) (Crandall & Shibasaki, 2010). The primary secretion forms due to water with a high concentration of sodium and chloride ions and a low concentration of potassium ions being drawn from plasma in the interstitial space, which extract the fluid from the capillaries in the dermis. The proteins and fatty acids in the plasma remain in the interstitial space and don’t contribute to the formation of sweat. The primary secretion travels from the coiled portion of the gland up through the straight duct that connects to the opening/pore on the skin’s surface (AQP5). Activity in the straight duct depends upon the rate of sweat production or flow: | ||
| - | <box width centre|> {{ :mechanism1.png?direct&300 |}}</box| Figure#. Mechanism of sweating picture at skin surface.> | + | Low sweat production occurs during a resting state or in a cool temperature environment and results in a slight stimulation of the sweat gland. This allows for cells in the interstitial space to reabsorb most of the sodium and chloride ions from the fluid in the straight duct. As a result, a low volume of sweat that is concentrated in K+ and less concentrated in Na+ and Cl- gets secreted. |
| - | In addition to the central neural drive, sweating can also be initiated by an axon reflex. Exogenous administration of acetylcholine not only directly stimulates muscarinic receptors on sweat glands, but also activates an axon reflex via stimulation of axonal nicotinic receptors. Acetylcholine released from cholinergic nerves is rapidly hydrolyzed by acetylcholinesterase. Acetylcholinesterase is capable of modulating sweat rate during low to moderate sweating activity but its effectiveness is greatly reduced when sweat rate is substantially increased. | + | High sweat production occurs during an active state (exercise) or in a warm temperature environment and results in a strong stimulation of the sweat gland. As a result, cells in the interstitial space do not have enough time to reabsorb all the sodium and chloride from the primary secretion in the straight duct. Therefore, a high volume of sweat that is concentrated in Na+ and Cl- and less concentrated in K+ gets secreted. |
| - | <box width centre|> {{ :mech2.png?direct&300 |}}</box| Figure#. Release of acetylcholine at the level of the postsynaptic neuron.> | + | <box width centre|> {{ :1.gif?nolink&300 |}} </box| Figure 6. Movement of water and ions during the production of sweat.> |
| - | The neurotransmitter(s) responsible for active cutaneous vasodilation has yet to be fully understood. However, neuropeptides such as calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), and substance P as well as nitric oxide (NO) have been implicated. All of these neurotransmitters except substance P have been shown to increase sweat secretion. Even though acetylcholine is the primary neurotransmitter responsible for sweat secretion, enhanced sweating due to local administration of VIP, CGRP, or NO suggest that these peptides may contribute to the overall modulation of sweating during a thermal challenge. | + | Sweat in apocrine sweat glands is produced in the same way. However, the sweat from apocrine glands also contains the proteins and fatty acids from plasma, which makes the secretion thicker and gives it a milkier or yellowish color. This explains why underarm stains in clothing appear yellowish. Sweat itself has no odor, but when bacteria on the skin and hair metabolize the proteins and fatty acids found in sweat from apocrine sweat glands, they produce an unpleasant odor (Freudenrich, 2010). |
| - | Aquaporins (AQPs) are a family of membrane water channel proteins. AQP5 has been localized to the apical membrane of multiple secretory glands in humans, including lacrimal glands, salivary glands and submucosal glands of airways. These glands facilitate the secretion of large amounts of fluid (Crandall & Shibasaki, 2010). | ||
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| - | {{ :ach.png?nolink&300 |}} | + | <box 35% width centre|> {{ :ach.png?nolink&300 |}} </box| Figure 7: Structure of acetylcholine hormone.> |
| ====== Sweating in Animals ====== | ====== Sweating in Animals ====== | ||
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| Cats and dogs sweat in similar ways. The main source of sweating for dogs is panting and having their tongues stick out. By dogs panting, the water evaporates from their tongues, nasal passages and lungs (Goldbaum, 2016). Ultimately, this lowers their overall body temperature. Another way dogs sweat is through their paws. Likewise, cats also sweat through their paws. Although both these animals have sweat glands in their feet, there is a much larger surface area for panting which makes it more efficient (Goldbaum, 2016). | Cats and dogs sweat in similar ways. The main source of sweating for dogs is panting and having their tongues stick out. By dogs panting, the water evaporates from their tongues, nasal passages and lungs (Goldbaum, 2016). Ultimately, this lowers their overall body temperature. Another way dogs sweat is through their paws. Likewise, cats also sweat through their paws. Although both these animals have sweat glands in their feet, there is a much larger surface area for panting which makes it more efficient (Goldbaum, 2016). | ||
| - | {{ :cat.jpg?nolink&300 |}} | + | <box 35% width centre|> {{ :cat-paw-reading.jpg?nolink&300 |}} </box| Figure 8: Cat paws that contain sweat glands.> |
| - | ==== Horses ==== | + | |
| - | The sweat of horses contain a chemical called latherin. This chemical is a natural detergent and functions to lather the body (Vance et al., 2013). According to researcher Malcolm Kennedy of the University of Gaslow, latherin's original function was found to be protein in saliva that helps with the breakdown of fibrous foods (Vance et al., 2013). However, eventually this chemical evolved to be expired during sweat. When this type of sweat is secreted in a horse, it spreads across the pelt to lower it's body temperature (Vance et al., 2013). | + | |
| - | {{ :horse.jpg?nolink&300 |}} | + | ==== Horses ==== |
| + | The sweat of horses contains a chemical called latherin. This chemical is a natural detergent and functions to lather the body (Vance et al., 2013). According to researcher Malcolm Kennedy of the University of Gaslow, latherin's original function was found to be protein in saliva that helps with the breakdown of fibrous foods (Vance et al., 2013). Eventually, this chemical evolved to be expired during sweat. When this type of sweat is secreted in a horse, it spreads across the pelt to lower its body temperature (Vance et al., 2013). | ||
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| + | <box 35% width centre|> {{ :horse_sweat.jpg?nolink&300 |}} </box| Figure 9: Horse sweat which contains latherin (white substance).> | ||
| ==== Hippos ==== | ==== Hippos ==== | ||
| The sweat of hippos is a red oily substance. This substance prevents damage caused by the sun and helps cool them off (Seeker, 2016). | The sweat of hippos is a red oily substance. This substance prevents damage caused by the sun and helps cool them off (Seeker, 2016). | ||
| - | {{ :hippo-face-web620.jpg?nolink&300 |}} | + | <box 35% width centre|> {{ :hippo_sweat.jpg?nolink&300 |}} </box| Figure 10: The red colour of hippos indicate their sweat.> |
| ====== Treatments for Excessive Sweating ====== | ====== Treatments for Excessive Sweating ====== | ||
