Biological Psychology 7edition
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Chapter 13: Homeostasis: Active Regulation of the
Internal Environment (Hunger & Feeding
Introduction: A Love-Hate Relationship with Food
Homeostasis Maintains a Consistent Internal Environment: The Example of
• Homeostatic systems share several key features
1) A homeostatic system involves regulation of the temperature, water balance, or food
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
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-
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
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
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
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
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
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
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
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