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University of Toronto Scarborough
Janelle Leboutillier

PSYB64 – Chapter 9: Temperature Regulation, Thirst, and Hunger Homeostasis  Homeostasis is the ability to adjust physiological processes to maintain a steady internal balance or equilibrium  To achieve homeostasis, regulatory systems actively defend certain set points for variables such as temperature, fluid levels, and weight  Deviations from the body’s ideal values are rapidly assessed by the nervous system which makes appropriate internal adjustments and motivates behavior designed to regain the ideal state o Motivation activates and directs behavior. The nervous system activates behavior by generating tension and discomfort in the form of drive states such as thirst or hunger and those drive states disappear when the needs are met Regulation of Body Temperature  Temperature regulations includes: set point, mechanisms for detecting a deviation from set point, and internal behavioral elements designed to regain set point  No matter where they live, animals must maintain an internal temperature that is ideal for the normal activity of their bodies’ cells Adaptations to Temperature  Mammals and birds are referred to as endotherms because of their ability to maintain body temperature through internal metabolic activity  Amphibians, reptiles and fish are referred to as ectotherms because they rely on external factors to maintain ideal body temperature  Maintenance of body temperature is influenced by an animal’s surface-to-volume ratio o The larger the overall volume of the body, the more heat is produced by the metabolic activity o Smaller animals have more surface area relative to the overall body volume than larger animals do so small animals must use much more energy to maintain a constant body temperature  In cold climates, surface area and heat loss are reduced in animals that have compact bodies and short legs, tails and ears  In warm climates, animals have greater surface area in the form of slim bodies and long appendages Behavioral Responses to Heat and Cold  Ectotherms are more dependent on behavioral devices because they don’t share the endotherms’ ability to use internal mechanisms for temperature regulation  Body position can be adjusted in response to changes in temperature  Animals can change weight, color and composition of their fur in response to seasonal changes in temperature  Humans use dark heavy clothing to maintain and absorb heat and light colors to dissipate and reflect heat Endothermic Responses to Heat and Cold  Humans hold a set point temperature of 98.6degrees F (37 degrees C)  When the internal temperature drops below this point, we shiver which results from muscle twitches which produces heat at the cost of high expenditure of energy o Blood vessels constrict and blood is sent away from the surface of the skin where heat loss is greatest (can lead to Raynaud’s disease)  If the cold conditions continue, thyroid gland increases the release of thyroid hormone which increases the overall metabolic activity to warm the body  In human infants and small animals, the sympathetic nervous system responds to cold by stimulating greater metabolic activity in brown fat cells which are located in the torso (close to vital organs)  Very warm temperatures produce their own set of responses o Perspiration cools the skin through evaporation o Blood vessels near the surface of the skin dilate in hot environments allowing more heat loss to external environments leading to the red faces in warm temperatures Deviations in Human Core Temperature  Disturbances in the body’s ability to maintain the normal core temperature set point can result in hot flashes are characterized by sweating, flushing, heart palpitations, and subjective feeling of being very warm  Fevers due to illness result when chemical byproducts of bacteria of viruses (pyrogens), enter the brain causing it to increase the core temperature set point so the body temperature will rise gradually until the new set point it reached  A fever can have beneficial effects when some of the disease-causing organisms cannot tolerate high temperatures and they die  Prior to the discovery of antibiotics, patients of syphilis were deliberately infected with malaria to induce fever because of the potential benefits  A heat stroke, hyperthermia, occurs when the body’s normal compensations cannot keep core temperature within normal limits. It is life threatening condition o Heat stroke often results from engaging in strenuous physical activity or wearing heavy clothing in hot environments which limit the body’s ability to get rid of excess heat o It interacts with immune system functioning  Low core temperatures are also life threatening (hypothermia) o Occurs when the core body temperature drops and causes uncontrollable shivering, slurred speech, pain, and discomfort o The pupils dilate, behavior resembles drunkenness and consciousness is gradually lost o Deliberately producing hypothermia has become a common method of reducing brain damage following cardiac arrest or open heart surgery Brain Mechanisms for Temperature Regulation  Lower levels such as the spinal cord do not respond to heat or cold until an animal’s core temperature is as much as 2-3 degrees away from the set point  Higher levels of hierarchy act as much more precise thermostats; hypothalamus initiates compensation whenever core temperature deviates as little as 0.01 degrees from the ideal set point  The preoptic area (POA) coordinates incoming info from thermoreceptors with structures that trigger appropriate responses to higher core temperatures  The posteriors hypothalamus is responsible for initiating responses to cooler core temperatures  The hypothalamus is sensitive to the body’s core temperature as reflected by thermoreceptors within the hypothalamus itself  POA contains 3 types of neurons: warm-sensitive, cold-sensitive, and temperature-insensitive o Warm-sensitive neurons increase their firing rates and inhibit cold-sensitive neurons as core temperature increases o As temperature drops, the firing rates of warm-sensitive neurons decrease, reducing their inhibition of cold- sensitive neurons which respond by increasing their firing rates o Temperature-insensitive neurons retain a steady rate of responding under all temperature conditions  Experiments show that rats can be taught to press a bar to obtain a brief puff of cool air and the rate at which the rat presses the bar corresponds to changes in either skin temperature or hypothalamic temperature o Skin temperature mirrors changes in room temperature while hypothalamus temperature can be manipulated by bathing the hypothalamus with warm or cool water o When either the skin temperature or hypothalamic temperature is raised, the rat will press more frequently to obtain cool puffs of air but cooling the hypothalamus alone will reduce or suppress bar pressing even when the room remains very warm  Pyrogens’ target in the brain is located in the hypothalamus. The BBB is weak near the POA which allows the pyrogens to exit the blood supply and enter the tissue  Once in the POA, pyrogens stimulate release of prostaglandin E2 which inhibits the firing rate of warm-sensitive neurons Temperature Regulation in Infancy  Human infants possess much more body fat at birth than other primate infants possibly as protection from hypothermia  Premature infants must spend time in a warm incubator before leaving the hospital because their ability to regulate temperature is unusually immature Thirst: Regulation of the Body’s Fluid  Molecules that have been dissolved in a fluid are called solutes and the fluids that contain the solutes are known as the solution  If the solutes break into ions when dissolved, they are called electrolytes Intracellular and Extracellular Fluids  The body has 3 major compartments for storing water  2/3 of the water is contained within cells as ICF and the remaining 1/3 is found in the ECF which is further divided into the blood supply and the interstitial fluid surrounding the body’s cells  Cerebrospinal fluid makes up a tiny percent of the ECF  The relative concentration of total solutes is the same in the ICF and ECF regardless of different amounts of solutes present in each compartment  Two solutions with equal concentrations of solutes are called isotonic Osmosis Causes Water to Move  The balance found in the isotonic state reduces any movement of fluid into or out of the body’s cells due to osmosis (force that causes water to move from an area of low conc. of solutes to higher conc. of solutes)  In osmosis, it is the water that moves across a barrier to equalize concentrations of the solutes on either side  Hypotonic – solutions that are lower in conc. or solutes than a reference solution  Hypertonic – solutions that are relatively higher in conc. of solutes The Role of the Kidneys  Blood enters the kidneys where it is filtered through a complex system made up of nephrons  Impurities and excess water and Na are removed by the nephrons and sent to the bladder for excretion as urine  The filtered blood returns to the circulation  In addition to urination, we lose water through several other normal body processes such as steaming breath on a cold day, perspiration, evaporation through skin and defecation The Sensation of Thirst  Humans need an average of about 2.5 liters of water per day which is roughly equivalent to 8 glasses  When a drop in the body’s water supply is perceived, 2 processes are initiated o We experience the sensation of thirst and our bodies begin to conserve whatever water we still have  Cannon proposed that the sensation of a dry mouth was the critical stimulus responsible for feelings of thirst but this was wrong; Bernard provided direct evidence that when an opening is made in the esophagus (fistula), as long as that opening remained closed, all the water consumed reached the stomach but when the fistula was open, all the consumed water would escape without reaching the stomach (the mouth would get wet but the water never reached the stomach so their thirst would not be quenched)  Thirst actually occurs in two processes o Osmotic thirst – in response to cellular dehydration that results from drops in the ICF volume; more common o Hypovolemic thirst – in response to drops in blood volume; used as an emergency backup system  The combination of these 2 processes contributes to thirst Mechanisms of Osmotic Thirst  people with untreated diabetes are unable to move sugars out of the blood supply, causing the blood to become hypertonic and so the cells attempt to compensate by releasing water and a sensation of thirst develops  strong sensations of thirst and frequent urination are early warning signs of diabetes mellitus  cellular dehydration is detected by specialized osmoreceptors located in the brain o an area located around the third ventricle, organum vasculosum of the lamina terminalis (OVLT), has been implicated in the detection of cellular dehydration o OVLT is particularly well situated for a role in detecting blood solute levels because the BBB is weak in this area Mechanisms of Hypovolemic Thirst  Hypovolemic thirst results when we experience a drop in volume of interstitial fluid, blood, or both  Obvious cause of this type of thirst is the loss of blood because of internal bleeding or severe injury  Low blood volume is sensed by baroreceptors located in the heart muscles and they measure blood pressure; when the blood volume decreases, BP decreases as well  Kidneys contain blood-flow receptors that also respond to changes in blood volume so when the blood volume is low, thirst is initiated and the kidneys act to conserve the remaining fluids Hormones, Sodium, and Thirst  Osmoreceptors and baroreceptors stimulate the release of antidiuretic hormone (ADH) which is also known as vasopressin (ADH promotes water retention)  ADH has 2 major effects on the kidneys o It signals the kidneys to reduce urine production o It stimulates the kidneys to release renin into the blood which triggers the conversion of angiotensinogen to angiotensin II (angiotensin II constricts the blood vessels to maintain BP) o Angiotensin II also triggers release of aldosterone which signals the kidneys to retain Na rather than excrete it in the urine  ADH controls the retention of water and aldosterone controls the retention of Na, which is essential for the maintenance of ECF (without the Na ion, ECF would be hypotonic)  Low blood volumes stimulate specific hunger for Na o Injections of formalin can produce both excessive thirst and slat cravings in animals and once hypovolemia is artificially induced, animals will reject the hypertonic Na solution to regular water  If excretion of Na by the kidneys in inadequate, high BP and other complications will occur  High Na levels in the blood promote the release of water from cells into the circulation due to osmosis (this high blood volume increases blood pressure) The Initiation of Drinking  Angiotensin II stimulates drinking through its action on the subfornical organ (SFO)  In an experiment in which the SFO was surgically disconnected from other parts of the brain, the neurons in the SFO increased their firing rates when angiotensin II was injected into the blood o This supports the idea that the SFO can respond to circulating angiotensin II without any input from other parts of the brain  The SFO forms connections with median preoptic nucleus which cannot respond to angiotensin II  The median preoptic nucleus received input from the nucleus of the solitary tract (NTS) which in turn receives input from baroreceptors in the circulatory system and osmoreceptors in the digestive tract  The median preoptic nucleus communicates with the lateral hypothalamus (LH) which projects to the zona incerta and that sends info to m
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