A mechanoreceptor , also called mechanoceptor , is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system .
96-439: The homeostat is one of the first devices capable of adapting itself to the environment; it exhibited behaviours such as habituation, reinforcement and learning through its ability to maintain homeostasis in a changing environment. It was built by William Ross Ashby in 1948 at Barnwood House Hospital . After a few technical hiccups with short-circuits causing burn-outs, the homeostat was finally completed on 16 March 1948. It
192-494: A constant and sufficient supply of the micronutrient while simultaneously avoiding excess levels. If an insufficient amount of copper is ingested for a short period of time, copper stores in the liver will be depleted. Should this depletion continue, a copper health deficiency condition may develop. If too much copper is ingested, an excess condition can result. Both of these conditions, deficiency and excess, can lead to tissue injury and disease. However, due to homeostatic regulation,
288-403: A control center. The control center sets the maintenance range—the acceptable upper and lower limits—for the particular variable, such as temperature. The control center responds to the signal by determining an appropriate response and sending signals to an effector , which can be one or more muscles, an organ, or a gland . When the signal is received and acted on, negative feedback is provided to
384-428: A generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (nerve impulses) are triggered at the first node of Ranvier of the sensory neuron. Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in
480-417: A great many other homeostatic mechanisms, encompassing many aspects of human physiology , control other entities in the body. Where the levels of variables are higher or lower than those needed, they are often prefixed with hyper- and hypo- , respectively such as hyperthermia and hypothermia or hypertension and hypotension . If an entity is homeostatically controlled it does not imply that its value
576-403: A high plasma pH stimulates the kidneys to secrete hydrogen ions into the blood and to excrete bicarbonate into the urine. The hydrogen ions combine with the excess bicarbonate ions in the plasma, once again forming an excess of carbonic acid which can be exhaled, as carbon dioxide, in the lungs, keeping the plasma bicarbonate ion concentration, the partial pressure of carbon dioxide and, therefore,
672-400: A hormone released into the blood by the kidney in response to high PTH levels in the blood, the duodenum and jejunum . Parathyroid hormone (in high concentrations in the blood) causes bone resorption , releasing calcium into the plasma. This is a very rapid action which can correct a threatening hypocalcemia within minutes. High PTH concentrations cause the excretion of phosphate ions via
768-400: A lowering of the plasma sodium concentration, or to a fall in the arterial blood pressure, the juxtaglomerular cells release renin into the blood. Renin is an enzyme which cleaves a decapeptide (a short protein chain, 10 amino acids long) from a plasma α-2-globulin called angiotensinogen . This decapeptide is known as angiotensin I . It has no known biological activity. However, when
864-435: A range of blood pressure values by vasoconstriction and vasodilation of the arteries. High pressure receptors called baroreceptors in the walls of the aortic arch and carotid sinus (at the beginning of the internal carotid artery ) monitor the arterial blood pressure . Rising pressure is detected when the walls of the arteries stretch due to an increase in blood volume . This causes heart muscle cells to secrete
960-517: A simulation on Turing's Automatic Computing Engine (ACE) instead of building a special machine. The first published account of the homeostat appeared under the title of "Design for a Brain" in the December 1948 issue of 'Electronic Engineering', where he speculates about a perfected homeostat that could eventually play chess "with a subtlety and depth of strategy beyond that of the man who designed it." In 1949 Time described it as "the closest thing to
1056-742: A stimulus, it begins to fire impulses or action potentials at an elevated frequency (the stronger the stimulus, the higher the frequency). The cell, however, will soon "adapt" to a constant or static stimulus, and the pulses will subside to a normal rate. Receptors that adapt quickly (i.e., quickly return to a normal pulse rate) are referred to as "phasic". Those receptors that are slow to return to their normal firing rate are called tonic . Phasic mechanoreceptors are useful in sensing such things as texture or vibrations, whereas tonic receptors are useful for temperature and proprioception among others. Cutaneous mechanoreceptors with small, accurate receptive fields are found in areas needing accurate taction (e.g.
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#17330859789321152-424: A subsequent increase in hemoglobin that increases the oxygen carrying capacity. This is the mechanism whereby high altitude dwellers have higher hematocrits than sea-level residents, and also why persons with pulmonary insufficiency or right-to-left shunts in the heart (through which venous blood by-passes the lungs and goes directly into the systemic circulation) have similarly high hematocrits. Regardless of
1248-433: A synthetic brain so far designed by man". In 1952, Ashby demonstrated it at the ninth Macy conference on cybernetics . In the same year he published a description of the homeostat in his influential book Design for a brain . In total, between 1946 and 1967, he wrote 38 entries about the homeostat in his journal. In her online biography of Ashby, Jill Ashby wrote of the homeostat, that "In 1961, Ross took it with him to
1344-433: Is an integral part of the antioxidant enzyme copper-zinc superoxide dismutase, and has a role in iron homeostasis as a cofactor in ceruloplasmin. Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the respiratory center , in the brainstem where they are regulated. The partial pressure of oxygen and carbon dioxide in the arterial blood is monitored by the peripheral chemoreceptors ( PNS ) in
1440-411: Is excited by stretch of the blood vessel. There are also juxtacapillary (J) receptors , which respond to events such as pulmonary edema , pulmonary emboli , pneumonia , and barotrauma . The knee jerk is the popularly known stretch reflex (involuntary kick of the lower leg) induced by tapping the knee with a rubber-headed hammer. The hammer strikes a tendon that inserts an extensor muscle in
1536-433: Is limited in its capacity to respond to extreme temperatures. When the core temperature falls, the blood supply to the skin is reduced by intense vasoconstriction . The blood flow to the limbs (which have a large surface area) is similarly reduced and returned to the trunk via the deep veins which lie alongside the arteries (forming venae comitantes ). This acts as a counter-current exchange system that short-circuits
1632-454: Is more of a deciding factor in the monitoring of pH. However, at high altitude (above 2500 m) the monitoring of the partial pressure of oxygen takes priority, and hyperventilation keeps the oxygen level constant. With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood and excrete bicarbonate into the urine. This is important in acclimatization to high altitude . The kidneys measure
1728-436: Is necessarily absolutely steady in health. Core body temperature is, for instance, regulated by a homeostatic mechanism with temperature sensors in, amongst others, the hypothalamus of the brain . However, the set point of the regulator is regularly reset. For instance, core body temperature in humans varies during the course of the day (i.e. has a circadian rhythm ), with the lowest temperatures occurring at night, and
1824-496: Is no change in the osmolality of the ECF, and therefore no change in the ADH concentration of the plasma. However, low aldosterone levels cause a loss of sodium ions from the ECF, which could potentially cause a change in extracellular osmolality and therefore of ADH levels in the blood. High potassium concentrations in the plasma cause depolarization of the zona glomerulosa cells' membranes in
1920-438: Is that it uses multiple coils in a milliammeter & uses the needle movement to dip in a trough carrying a current, so getting a potential which goes to the grid of a valve, the anode of which provides an output current." It was the realization of what he had described in 1946 as an " Isomorphism making machine". When Alan Turing heard of Ashby's intention to build the homeostat, he wrote to Ashby to suggest that he could run
2016-407: Is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance , being kept within certain pre-set limits (homeostatic range). Other variables include the pH of extracellular fluid , the concentrations of sodium , potassium , and calcium ions , as well as the blood sugar level , and these need to be regulated despite changes in
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#17330859789322112-438: Is therefore that hydrogen ions are lost in the urine when the pH of the plasma falls. The concomitant rise in the plasma bicarbonate mops up the increased hydrogen ions (caused by the fall in plasma pH) and the resulting excess carbonic acid is disposed of in the lungs as carbon dioxide. This restores the normal ratio between bicarbonate and the partial pressure of carbon dioxide and therefore the plasma pH. The converse happens when
2208-427: The autonomic nervous system are stimulated to influence the activity of chiefly the heart and the smallest diameter arteries, called arterioles . The arterioles are the main resistance vessels in the arterial tree , and small changes in diameter cause large changes in the resistance to flow through them. When the arterial blood pressure rises the arterioles are stimulated to dilate making it easier for blood to leave
2304-411: The autonomic nervous system to a wide variety of effector organs, whose activity is consequently changed to reverse the error in the blood pressure. One of the effector organs is the heart whose rate is stimulated to rise ( tachycardia ) when the arterial blood pressure falls, or to slow down ( bradycardia ) when the pressure rises above the set point. Thus the heart rate (for which there is no sensor in
2400-433: The carotid artery and aortic arch . A change in the partial pressure of carbon dioxide is detected as altered pH in the cerebrospinal fluid by central chemoreceptors ( CNS ) in the medulla oblongata of the brainstem . Information from these sets of sensors is sent to the respiratory center which activates the effector organs – the diaphragm and other muscles of respiration . An increased level of carbon dioxide in
2496-400: The central nervous system play a homeostatic role in the balance of neuronal activity between excitation and inhibition. Inhibitory neurons using GABA , make compensating changes in the neuronal networks preventing runaway levels of excitation. An imbalance between excitation and inhibition is seen to be implicated in a number of neuropsychiatric disorders . The neuroendocrine system is
2592-432: The central nervous system . Type II and Type III mechanoreceptors in particular are believed to be linked to one's sense of proprioception . Other mechanoreceptors than cutaneous ones include the hair cells , which are sensory receptors in the vestibular system of the inner ear , where they contribute to the auditory system and equilibrioception . Baroreceptors are a type of mechanoreceptor sensory neuron that
2688-448: The hypothalamus detects a hypertonic extracellular environment, it causes the secretion of an antidiuretic hormone (ADH) called vasopressin which acts on the effector organ, which in this case is the kidney . The effect of vasopressin on the kidney tubules is to reabsorb water from the distal convoluted tubules and collecting ducts , thus preventing aggravation of the water loss via the urine. The hypothalamus simultaneously stimulates
2784-408: The renal tubular fluid after it has already undergone a certain amount of modification in the proximal convoluted tubule and loop of Henle . These cells also respond to rate of blood flow through the juxtaglomerular apparatus, which, under normal circumstances, is directly proportional to the arterial blood pressure , making this tissue an ancillary arterial blood pressure sensor. In response to
2880-416: The zona glomerulosa of the adrenal glands has an effect on particularly the epithelial cells of the distal convoluted tubules and collecting ducts of the kidneys. Here it causes the reabsorption of sodium ions from the renal tubular fluid , in exchange for potassium ions which are secreted from the blood plasma into the tubular fluid to exit the body via the urine. The reabsorption of sodium ions from
2976-524: The Biological Computing Laboratory, University of Illinois and left it there when he retired in 1970. Apparently, it was damaged soon after that when the area where it was being kept was flooded." Homeostasis In biology , homeostasis ( British also homoeostasis ; / h ɒ m i oʊ ˈ s t eɪ s ɪ s , - m i ə -/ ) is the state of steady internal physical and chemical conditions maintained by living systems . This
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3072-399: The alpha cells into the blood. This inhibits the uptake of glucose from the blood by the liver, fats cells, and muscle. Instead the liver is strongly stimulated to manufacture glucose from glycogen (through glycogenolysis ) and from non-carbohydrate sources (such as lactate and de-aminated amino acids ) using a process known as gluconeogenesis . The glucose thus produced is discharged into
3168-436: The arteries, thus deflating them, and bringing the blood pressure down, back to normal. At the same time, the heart is stimulated via cholinergic parasympathetic nerves to beat more slowly (called bradycardia ), ensuring that the inflow of blood into the arteries is reduced, thus adding to the reduction in pressure, and correcting the original error. Low pressure in the arteries, causes the opposite reflex of constriction of
3264-412: The arterioles, and a speeding up of the heart rate (called tachycardia ). If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the adrenal medulla , via "preganglionic" sympathetic nerves , to secrete epinephrine (adrenaline) into the blood. This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in
3360-464: The blood circulates through the lungs a pulmonary capillary endothelial enzyme called angiotensin-converting enzyme (ACE) cleaves a further two amino acids from angiotensin I to form an octapeptide known as angiotensin II . Angiotensin II is a hormone which acts on the adrenal cortex , causing the release into the blood of the steroid hormone , aldosterone . Angiotensin II also acts on the smooth muscle in
3456-399: The blood correcting the detected error ( hypoglycemia ). The glycogen stored in muscles remains in the muscles, and is only broken down, during exercise, to glucose-6-phosphate and thence to pyruvate to be fed into the citric acid cycle or turned into lactate . It is only the lactate and the waste products of the citric acid cycle that are returned to the blood. The liver can take up only
3552-419: The blood, or a decreased level of oxygen, will result in a deeper breathing pattern and increased respiratory rate to bring the blood gases back to equilibrium. Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as apnea , which freedivers use to prolong the time they can stay underwater. The partial pressure of carbon dioxide
3648-401: The blood, the latter combines with the excess hydrogen ions in the plasma that stimulated the kidneys to perform this operation. The resulting reaction in the plasma is the formation of carbonic acid which is in equilibrium with the plasma partial pressure of carbon dioxide. This is tightly regulated to ensure that there is no excessive build-up of carbonic acid or bicarbonate. The overall effect
3744-598: The blood. This combination (high blood insulin levels and low glucagon levels) act on effector tissues, the chief of which is the liver , fat cells , and muscle cells . The liver is inhibited from producing glucose , taking it up instead, and converting it to glycogen and triglycerides . The glycogen is stored in the liver, but the triglycerides are secreted into the blood as very low-density lipoprotein (VLDL) particles which are taken up by adipose tissue , there to be stored as fats. The fat cells take up glucose through special glucose transporters ( GLUT4 ), whose numbers in
3840-499: The body (especially the heart, lungs, and brain). These reactions usually correct the low arterial blood pressure ( hypotension ) very effectively. The plasma ionized calcium (Ca ) concentration is very tightly controlled by a pair of homeostatic mechanisms. The sensor for the first one is situated in the parathyroid glands , where the chief cells sense the Ca level by means of specialized calcium receptors in their membranes. The sensors for
3936-417: The body) is not homeostatically controlled but is one of the effector responses to errors in arterial blood pressure. Another example is the rate of sweating . This is one of the effectors in the homeostatic control of body temperature, and therefore highly variable in rough proportion to the heat load that threatens to destabilize the body's core temperature, for which there is a sensor in the hypothalamus of
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4032-416: The body. For instance, the signal (be it via neurons or hormones ) from the sensor to the effector is, of necessity, highly variable in order to convey information about the direction and magnitude of the error detected by the sensor. Similarly, the effector's response needs to be highly adjustable to reverse the error – in fact it should be very nearly in proportion (but in the opposite direction) to
4128-684: The brain. Mammals regulate their core temperature using input from thermoreceptors in the hypothalamus , brain, spinal cord , internal organs , and great veins. Apart from the internal regulation of temperature, a process called allostasis can come into play that adjusts behaviour to adapt to the challenge of very hot or cold extremes (and to other challenges). These adjustments may include seeking shade and reducing activity, seeking warmer conditions and increasing activity, or huddling. Behavioral thermoregulation takes precedence over physiological thermoregulation since necessary changes can be affected more quickly and physiological thermoregulation
4224-612: The cell wall are increased as a direct effect of insulin acting on these cells. The glucose that enters the fat cells in this manner is converted into triglycerides (via the same metabolic pathways as are used by the liver) and then stored in those fat cells together with the VLDL-derived triglycerides that were made in the liver. Muscle cells also take glucose up through insulin-sensitive GLUT4 glucose channels, and convert it into muscle glycogen. A fall in blood glucose, causes insulin secretion to be stopped, and glucagon to be secreted from
4320-442: The cell, into the interstitial fluid and two potassium ions into the cell from the interstitial fluid. This creates an ionic concentration gradient which results in the reabsorption of sodium (Na ) ions from the tubular fluid into the blood, and secreting potassium (K ) ions from the blood into the urine (lumen of collecting duct). The total amount of water in the body needs to be kept in balance. Fluid balance involves keeping
4416-445: The chemical processes take place inside the cell or in the interstitial fluid bathing the cells. The best-known homeostatic mechanisms in humans and other mammals are regulators that keep the composition of the extracellular fluid (or the "internal environment") constant, especially with regard to the temperature , pH , osmolality , and the concentrations of sodium , potassium , glucose , carbon dioxide , and oxygen . However,
4512-486: The constancy of the internal environment in which the cells of the body live and survive. The term cybernetics is applied to technological control systems such as thermostats , which function as homeostatic mechanisms but are often defined much more broadly than the biological term of homeostasis. The metabolic processes of all organisms can only take place in very specific physical and chemical environments. The conditions vary with each organism, and with whether
4608-411: The distal convoluted tubules and collecting ducts is impermeable to water in the absence of antidiuretic hormone (ADH) in the blood. ADH is part of the control of fluid balance . Its levels in the blood vary with the osmolality of the plasma, which is measured in the hypothalamus of the brain. Aldosterone's action on the kidney tubules prevents sodium loss to the extracellular fluid (ECF). So there
4704-413: The earlier reactions are insufficient to correct the hypothermia . When core temperature rises are detected by thermoreceptors , the sweat glands in the skin are stimulated via cholinergic sympathetic nerves to secrete sweat onto the skin, which, when it evaporates, cools the skin and the blood flowing through it. Panting is an alternative effector in many vertebrates, which cools the body also by
4800-480: The environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life. Homeostasis is brought about by a natural resistance to change when already in optimal conditions, and equilibrium is maintained by many regulatory mechanisms; it is thought to be the central motivation for all organic action. All homeostatic control mechanisms have at least three interdependent components for
4896-544: The error that is threatening the internal environment. For instance, arterial blood pressure in mammals is homeostatically controlled and measured by stretch receptors in the walls of the aortic arch and carotid sinuses at the beginnings of the internal carotid arteries . The sensors send messages via sensory nerves to the medulla oblongata of the brain indicating whether the blood pressure has fallen or risen, and by how much. The medulla oblongata then distributes messages along motor or efferent nerves belonging to
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#17330859789324992-444: The evaporation of water, but this time from the mucous membranes of the throat and mouth. Blood sugar levels are regulated within fairly narrow limits. In mammals, the primary sensors for this are the beta cells of the pancreatic islets . The beta cells respond to a rise in the blood sugar level by secreting insulin into the blood and simultaneously inhibiting their neighboring alpha cells from secreting glucagon into
5088-414: The excess water in the body. Urinary water loss, when the body water homeostat is intact, is a compensatory water loss, correcting any water excess in the body. However, since the kidneys cannot generate water, the thirst reflex is the all-important second effector mechanism of the body water homeostat, correcting any water deficit in the body. The plasma pH can be altered by respiratory changes in
5184-424: The fact that water losses from the body, (through unavoidable water loss through the skin which is not entirely waterproof and therefore always slightly moist, water vapor in the exhaled air , sweating , vomiting , normal feces and especially diarrhea ) are all hypotonic , meaning that they are less salty than the body fluids (compare, for instance, the taste of saliva with that of tears. The latter has almost
5280-439: The fine-tuning adjustment of body homeostasis. The word homeostasis ( / ˌ h oʊ m i oʊ ˈ s t eɪ s ɪ s / ) uses combining forms of homeo- and -stasis , Neo-Latin from Greek : ὅμοιος homoios , "similar" and στάσις stasis , "standing still", yielding the idea of "staying the same". The concept of the regulation of the internal environment was described by French physiologist Claude Bernard in 1849, and
5376-579: The fingertips). In the fingertips and lips, innervation density of slowly adapting type I and rapidly adapting type I mechanoreceptors are greatly increased. These two types of mechanoreceptors have small discrete receptive fields and are thought to underlie most low-threshold use of the fingers in assessing texture, surface slip, and flutter. Mechanoreceptors found in areas of the body with less tactile acuity tend to have larger receptive fields . Lamellar corpuscles , or Pacinian corpuscles or Vater-Pacini corpuscle, are deformation or pressure receptors located in
5472-400: The fluid volume stabilized, and also keeping the levels of electrolytes in the extracellular fluid stable. Fluid balance is maintained by the process of osmoregulation and by behavior. Osmotic pressure is detected by osmoreceptors in the median preoptic nucleus in the hypothalamus . Measurement of the plasma osmolality to give an indication of the water content of the body, relies on
5568-438: The front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle, which activates stretch receptors within the muscle called muscle spindles . Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called intrafusal muscle fibers . Stretching an intrafusal fiber initiates a volley of impulses in the sensory neuron (a I-a neuron) attached to it. The impulses travel along
5664-567: The gut. Another example are the most well-characterised endocannabinoids like anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG), whose synthesis occurs through the action of a series of intracellular enzymes activated in response to a rise in intracellular calcium levels to introduce homeostasis and prevention of tumor development through putative protective mechanisms that prevent cell growth and migration by activation of CB1 and/or CB2 and adjoining receptors . The homeostatic mechanism which controls
5760-412: The highest in the afternoons. Other normal temperature variations include those related to the menstrual cycle . The temperature regulator's set point is reset during infections to produce a fever. Organisms are capable of adjusting somewhat to varied conditions such as temperature changes or oxygen levels at altitude, by a process of acclimatisation . Homeostasis does not govern every activity in
5856-400: The hormone atrial natriuretic peptide (ANP) into the blood. This acts on the kidneys to inhibit the secretion of renin and aldosterone causing the release of sodium, and accompanying water into the urine, thereby reducing the blood volume. This information is then conveyed, via afferent nerve fibers , to the solitary nucleus in the medulla oblongata . From here motor nerves belonging to
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#17330859789325952-591: The human body is capable of balancing a wide range of copper intakes for the needs of healthy individuals. Many aspects of copper homeostasis are known at the molecular level. Copper's essentiality is due to its ability to act as an electron donor or acceptor as its oxidation state fluxes between Cu ( cuprous ) and Cu ( cupric ). As a component of about a dozen cuproenzymes, copper is involved in key redox (i.e., oxidation-reduction) reactions in essential metabolic processes such as mitochondrial respiration, synthesis of melanin , and cross-linking of collagen . Copper
6048-424: The lactate, and, by the process of energy-consuming gluconeogenesis , convert it back to glucose. Controlling iron levels in the body is a critically important part of many aspects of human health and disease. In humans iron is both necessary to the body and potentially harmful. Copper is absorbed, transported, distributed, stored, and excreted in the body according to complex homeostatic processes which ensure
6144-452: The mechanism by which the hypothalamus maintains homeostasis, regulating metabolism , reproduction, eating and drinking behaviour, energy utilization, osmolarity and blood pressure. Mechanoreceptor Cutaneous mechanoreceptors respond to mechanical stimuli that result from physical interaction, including pressure and vibration. They are located in the skin, like other cutaneous receptors . They are all innervated by Aβ fibers , except
6240-400: The mechanorecepting free nerve endings , which are innervated by Aδ fibers . Cutaneous mechanoreceptors can be categorized by what kind of sensation they perceive, by the rate of adaptation, and by morphology. Furthermore, each has a different receptive field . Cutaneous mechanoreceptors can also be separated into categories based on their rates of adaptation. When a mechanoreceptor receives
6336-469: The nearby thirst center causing an almost irresistible (if the hypertonicity is severe enough) urge to drink water. The cessation of urine flow prevents the hypovolemia and hypertonicity from getting worse; the drinking of water corrects the defect. Hypo-osmolality results in very low plasma ADH levels. This results in the inhibition of water reabsorption from the kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting rid of
6432-527: The neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron. The optimal sensitivity of a lamellar corpuscle is 250 Hz, the frequency range generated upon finger tips by textures made of features smaller than 200 micrometres . There are four types of mechanoreceptors embedded in ligaments . As all these types of mechanoreceptors are myelinated , they can rapidly transmit sensory information regarding joint positions to
6528-445: The normal state. At the cellular level, effectors include nuclear receptors that bring about changes in gene expression through up-regulation or down-regulation and act in negative feedback mechanisms. An example of this is in the control of bile acids in the liver . Some centers, such as the renin–angiotensin system , control more than one variable. When the receptor senses a stimulus, it reacts by sending action potentials to
6624-435: The one hand, and calcitonin on the other can very rapidly correct any impending error in the plasma ionized calcium level by either removing calcium from the blood and depositing it in the skeleton, or by removing calcium from it. The skeleton acts as an extremely large calcium store (about 1 kg) compared with the plasma calcium store (about 180 mg). Longer term regulation occurs through calcium absorption or loss from
6720-427: The outer layer of the adrenal cortex . This causes the release of aldosterone into the blood. Aldosterone acts primarily on the distal convoluted tubules and collecting ducts of the kidneys, stimulating the excretion of potassium ions into the urine. It does so, however, by activating the basolateral Na /K pumps of the tubular epithelial cells. These sodium/potassium exchangers pump three sodium ions out of
6816-454: The oxygen content rather than the partial pressure of oxygen in the arterial blood. When the oxygen content of the blood is chronically low, oxygen-sensitive cells secrete erythropoietin (EPO) into the blood. The effector tissue is the red bone marrow which produces red blood cells (RBCs, also called erythrocytes ). The increase in RBCs leads to an increased hematocrit in the blood, and
6912-425: The pH. Respiratory compensation a mechanism of the respiratory center , adjusts the partial pressure of carbon dioxide by changing the rate and depth of breathing, to bring the pH back to normal. The partial pressure of carbon dioxide also determines the concentration of carbonic acid, and the bicarbonate buffer system can also come into play. Renal compensation can help the bicarbonate buffer system. The sensor for
7008-515: The partial pressure of carbon dioxide; or altered by metabolic changes in the carbonic acid to bicarbonate ion ratio. The bicarbonate buffer system regulates the ratio of carbonic acid to bicarbonate to be equal to 1:20, at which ratio the blood pH is 7.4 (as explained in the Henderson–Hasselbalch equation ). A change in the plasma pH gives an acid–base imbalance . In acid–base homeostasis there are two mechanisms that can help regulate
7104-529: The partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in anemia , but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, vitamin B12 and folic acid , EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal. The brain can regulate blood flow over
7200-446: The plasma bicarbonate concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. The metabolism of these cells produces carbon dioxide, which is rapidly converted to hydrogen and bicarbonate through the action of carbonic anhydrase . When the ECF pH falls (becoming more acidic) the renal tubular cells excrete hydrogen ions into
7296-417: The plasma pH, constant. Cerebrospinal fluid (CSF) allows for regulation of the distribution of substances between cells of the brain, and neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope . Inhibitory neurons in
7392-415: The plasma sodium concentration is rather more complex than most of the other homeostatic mechanisms described on this page. The sensor is situated in the juxtaglomerular apparatus of kidneys, which senses the plasma sodium concentration in a surprisingly indirect manner. Instead of measuring it directly in the blood flowing past the juxtaglomerular cells , these cells respond to the sodium concentration in
7488-482: The presynaptic terminal to bind to the CB1 receptor for modulation of neurotransmitter release to obtain homeostasis. The polyunsaturated fatty acids (PUFAs) are lipid derivatives of omega-3 (docosahexaenoic acid, DHA , and eicosapentaenoic acid, EPA ) or of omega-6 (arachidonic acid, ARA ) are synthesized from membrane phospholipids and used as a precursor for endocannabinoids (ECs) mediate significant effects in
7584-510: The receptor that stops the need for further signaling. The cannabinoid receptor type 1 (CB1), located at the presynaptic neuron , is a receptor that can stop stressful neurotransmitter release to the postsynaptic neuron; it is activated by endocannabinoids (ECs) such as anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG) via a retrograde signaling process in which these compounds are synthesized by and released from postsynaptic neurons, and travel back to
7680-436: The release of renin from the juxtaglomerular apparatus is halted, ceasing the production of angiotensin II, and its consequent aldosterone-release into the blood. The kidneys respond by excreting sodium ions into the urine, thereby normalizing the plasma sodium ion concentration. The low angiotensin II levels in the blood lower the arterial blood pressure as an inevitable concomitant response. The reabsorption of sodium ions from
7776-422: The renal tubular fluid halts further sodium ion losses from the body, and therefore preventing the worsening of hyponatremia . The hyponatremia can only be corrected by the consumption of salt in the diet. However, it is not certain whether a "salt hunger" can be initiated by hyponatremia, or by what mechanism this might come about. When the plasma sodium ion concentration is higher than normal ( hypernatremia ),
7872-484: The role of the cutaneous mechanoreceptors for feedback in fine motor control . Single action potentials from Meissner's corpuscle , Pacinian corpuscle and Ruffini ending afferents are directly linked to muscle activation, whereas Merkel cell-neurite complex activation does not trigger muscle activity. Insect and arthropod mechanoreceptors include: Mechanoreceptors are also present in plant cells where they play an important role in normal growth, development and
7968-440: The same salt content as the extracellular fluid, whereas the former is hypotonic with respect to the plasma. Saliva does not taste salty, whereas tears are decidedly salty). Nearly all normal and abnormal losses of body water therefore cause the extracellular fluid to become hypertonic . Conversely, excessive fluid intake dilutes the extracellular fluid causing the hypothalamus to register hypotonic hyponatremia conditions. When
8064-413: The second are the parafollicular cells in the thyroid gland . The parathyroid chief cells secrete parathyroid hormone (PTH) in response to a fall in the plasma ionized calcium level; the parafollicular cells of the thyroid gland secrete calcitonin in response to a rise in the plasma ionized calcium level. The effector organs of the first homeostatic mechanism are the bones , the kidney , and, via
8160-518: The sensing of their environment. Mechanoreceptors aid the Venus flytrap ( Dionaea muscipula Ellis) in capturing large prey. Mechanoreceptor proteins are ion channels whose ion flow is induced by touch. Early research showed that touch transduction in the nematode Caenorhabditis elegans was found to require a two transmembrane, amiloride -sensitive ion channel protein related to epithelial sodium channels (ENaCs). This protein, called MEC-4, forms
8256-448: The sensory axon to the spinal cord where they form several kinds of synapses : In somatosensory transduction , the afferent neurons transmit messages through synapses in the dorsal column nuclei , where second-order neurons send the signal to the thalamus and synapse with third-order neurons in the ventrobasal complex . The third-order neurons then send the signal to the somatosensory cortex . More recent work has expanded
8352-416: The skin and also in various internal organs. Each is connected to a sensory neuron. Because of its relatively large size, a single lamellar corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency can be applied to the corpuscle by stylus, and the resulting electrical activity detected by electrodes attached to the preparation. Deforming the corpuscle creates
8448-407: The tubular fluid as a result of high aldosterone levels in the blood does not, of itself, cause renal tubular water to be returned to the blood from the distal convoluted tubules or collecting ducts . This is because sodium is reabsorbed in exchange for potassium and therefore causes only a modest change in the osmotic gradient between the blood and the tubular fluid. Furthermore, the epithelium of
8544-402: The tubular fluid to leave the body via urine. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH. The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions released into the plasma. When hydrogen ions are excreted into the urine, and bicarbonate into
8640-455: The upper small intestine, increasing their capacity to absorb calcium from the gut contents into the blood. The second homeostatic mechanism, with its sensors in the thyroid gland, releases calcitonin into the blood when the blood ionized calcium rises. This hormone acts primarily on bone, causing the rapid removal of calcium from the blood and depositing it, in insoluble form, in the bones. The two homeostatic mechanisms working through PTH on
8736-407: The urine. Since phosphates combine with calcium ions to form insoluble salts (see also bone mineral ), a decrease in the level of phosphates in the blood, releases free calcium ions into the plasma ionized calcium pool. PTH has a second action on the kidneys. It stimulates the manufacture and release, by the kidneys, of calcitriol into the blood. This steroid hormone acts on the epithelial cells of
8832-417: The variable being regulated: a receptor, a control center, and an effector. The receptor is the sensing component that monitors and responds to changes in the environment, either external or internal. Receptors include thermoreceptors and mechanoreceptors . Control centers include the respiratory center and the renin-angiotensin system . An effector is the target acted on, to bring about the change back to
8928-495: The walls of the arterioles causing these small diameter vessels to constrict, thereby restricting the outflow of blood from the arterial tree, causing the arterial blood pressure to rise. This, therefore, reinforces the measures described above (under the heading of "Arterial blood pressure"), which defend the arterial blood pressure against changes, especially hypotension . The angiotensin II-stimulated aldosterone released from
9024-472: The warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather. The subcutaneous limb veins are tightly constricted, not only reducing heat loss from this source but also forcing the venous blood into the counter-current system in the depths of the limbs. The metabolic rate is increased, initially by non-shivering thermogenesis , followed by shivering thermogenesis if
9120-441: The word homeostasis was coined by Walter Bradford Cannon in 1926. In 1932, Joseph Barcroft a British physiologist, was the first to say that higher brain function required the most stable internal environment. Thus, to Barcroft homeostasis was not only organized by the brain—homeostasis served the brain. Homeostasis is an almost exclusively biological term, referring to the concepts described by Bernard and Cannon, concerning
9216-420: Was an adaptive ultrastable system, consisting of four interconnected Royal Air Force bomb control units with inputs, feedback , and magnetically driven, water-filled potentiometers . It illustrated his law of requisite variety — automatically adapting its configuration to stabilize the effects of any disturbances introduced into the system. In 1946, Ashby described the design of the units thus "Its principle
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