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Bio-Psych: Homeostasis (Food & Energy Regulation).doc

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PSYC 280
Neil Watson

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Biological Psychology 7edition Website-sourced Study Questions Chapter 13: Homeostasis: Active Regulation of the Internal Environment (Hunger & Feeding systems) Introduction: A Love-Hate Relationship with Food Homeostasis Maintains a Consistent Internal Environment: The Example of Thermoregulation • Homeostatic systems share several key features Study questions: 1) A homeostatic system involves regulation of the temperature, water balance, or food intake systems. 2) A modern classification of animals reflects the source from which they derive warmth. Endotherms actively generate warmth internally in order to regulate their body temperature, whereas ectotherms gain their heat from the environment. Animals in both classes actively regulate their temperature, but endotherms do so through metabolism, whereas ectotherms do so behaviorally (such as by sitting on a hot rock). 3) One advantage of endothermy is greater independence from environmental conditions. By producing a constant internal temperature, endothermy allows an animal to forage in a wider variety of environments. Also, in order to produce the chemical reactions necessary to produce heat, endothermy has allowed animals a greater capacity for oxygen utilization. Ectotherms are typically capable of only short bursts of high-intensity activity. 4) The top level of control in a negative feedback control system is a comparator. In the case of body temperature regulation, this works like a thermostat: it contains a reference value, or “set point,” and it compares the present temperature of the body to the desired temperature. The system operates by means of negative feedback from the body—that is, when the desired temperature is reached, feedback information acts to shut the system off, and it remains off until the temperature deviates from the desired level. In mammals, this optimal level is usually in the range of 36 to 38°C. 5) Mammals can increase heat generation in response to a drop in body temperature in two ways, one involving a specialized type of tissue, and the other involving a specialized type of behavior. a. In the first case, the sympathetic nervous system signals brown fat, which is full of mitochondria, to metabolize molecules to produce heat. b. In the second case, the nervous system activates muscle fibers to cause heat- producing shivers. 6) Birds incubate eggs by keeping them in contact with a special, highly vascularized area of the skin known as the brood patch. A rat mother’s own thermoregulation compensates 1 for the needs of her pups, which are born without fur and cannot regulate their own body temperatures until they are old enough to stray from the nest. 7) Early experiments with local heating and cooling (via implanted wires) implicated a particular brain structure, the hypothalamus, in the control of temperature. Subsequent work demonstrated that neurons located in subregions of this structure, namely the preoptic area (POA) and the anterior hypothalamus, change their rate of firing depending on body temperature, suggesting that this is the location of the “thermostat.” 8) Recent lesion experiments have suggested that in mammals, different brain sites are responsible for two main kinds of thermoregulation. a. Behavioral thermoregulation (e.g., locomotion) is abolished by lesions of the lateral hypothalamus, whereas b. physiological (e.g., autonomic) thermoregulation is abolished by lesions of the preoptic area. Food and Energy Regulation Nutrient Regulation Helps Prepare for Future Needs • Most of our food is used to provide us with energy • We can store energy for future needs Study questions: 1) The regulation of food intake is considerably more complicated than the regulation of fluid balance. This is probably the case because our food must provide us not only with energy but also with nutrients, which are defined as chemicals that are not used as sources of energy but are required for the effective functioning of the body, such as growth and maintenance. 2) Basal metabolism can be calculated using a mathematical expression: kcal/day = 70 × weight 0.7. This is known as the Kleiber equation. The Kleiber equation does not hold true during food deprivation. Under these circumstances, basal metabolism decreases. This phenomenon raises difficulties for people who are following a(n) diet to try to lose weight. On the plus side, food deprivation is the only known way that animals can be made to live longer. 3) The most important sugar used by the body is glucose, which is classified as a simple carbohydrate. Although most of the body can also derive energy from more complex molecules, such as ketones, which are produced when fat is metabolized, the brain is heavily dependent on glucose for its functioning. In order to maintain reserves of glucose, it is stored by the liver in a more complex form, called glycogen, which can be converted back into glucose when needed. 4) The process of shuttling glucose in and out of storage is controlled by two hormones of the pancreas: insulin, which promotes the conversion of glucose to glycogen, and glucagon, which promotes the breakdown of glycogen to glucose. 5) Long-term energy storage is accomplished by depositing fat into adipose tissue. Fat may either be eaten or made in the body from glucose plus other nutrients. Fat may be broken down into fatty acids to supply energy to most of the body, or into glucose for use by the brain. 2 Insulin Is Crucial for the Regulation of Body Metabolism • Despite their importance, neither insulin nor glucose is the sole signal for hunger or satiety Study questions: 1) Glucose gets into cells via glucose transporters that span the cell membrane; these molecules require insulin in order to function, except in the brain. a. The cephalic phase—the brain stimulates the pancreas to secrete insulin. b. The digestive phase—the presence of food in the digestive tract stimulates the release of gut hormones, which stimulate the pancreas to secrete insulin. c. The absorptive phase—glucose levels of the blood increase as food is absorbed, and this increase is detected by glucodetectors in the liver, which signal the brainstem via the vagus nerve. The brainstem then signals the pancreas to secrete insulin. 2) A lack of insulin causes the disease diabetes mellitus. In the Type I form of the disease, the pancreas stops producing insulin. Due to this lack, only the brain can make use of glucose, and the rest of the body must rely on fatty acids. Glucose also cannot be stored in the absence of insulin, so some of it is passed in the urine. Type II diabetes is due to decreased production of or sensitivity to insulin and is treated by reducing glucose intake. 3) Animals whose insulin levels have been lowered become hungry and eat large meals. Moderate levels of insulin result in the consumption of normal meals. These two results support the idea that insulin levels signal satiety to the brain. Rats treated with large doses of insulin eat large meals; presumably all of their glucose is stored, the brain has detected this situation, and the animal feels hungry. The Hypothalamus Coordinates Multiple Systems That Control Hunger • Multiple peripheral signals are integrated by a hypothalamic appetite network • Second-order hypothalamic neurons integrate appetite signals • Other systems also play a role in hunger and satiety Study questions: 1) Initial discoveries led to the dual-center theory of eating, which proposed that two brain centers, acting in opposition, control the intake of food. According to this theory, the lateral hypothalamus stimulates feeding behavior, while the ventromedial hypothalamus inhibits feeding behavior. In humans, fMRI has revealed that glucose intake after fasting affects the activity of the hypothalamus. 2) Animals with bilateral lesions of the VMH eat voraciously and soon beco
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