Discovering Behavioral Neuroscience: An Introduction to Biological Psychology
Detailed notes on Chapter 9 Homeostasis, Motivation and Reward.
Freberg, L. (2018). Discovering Behavioral Neuroscience: An introduction to Biological Psychology. Cengage Learning.
W8 CH 9 - Homeostasis, Motivation and Reward
Homeostasis and Motivation
Homeostasis - A physiological state of equilibrium or balance.
Set point - A value that is defended by regulatory systems, such as core temperature or a
particular body weight.
- Set points for many of our physiological systems work much like the setting on the
thermostat of your home. Deviations from the set point initiate behaviors (turning the
heat or air conditioning on) that return the system to homeostasis or equilibrium.
Motivation - The process of activating and directing behavior.
Regulating Body Temperature
- Temperature regulation involves a set point, mechanisms for detecting deviations, and
internal and behavioral elements to regain it.
- Animals must maintain an internal temperature that is ideal for the normal activity of
their cells -> If temperatures are too low, ice crystals form within cells and damage the
cell membrane. In high temperatures, the proteins necessary for carrying out cell
functions become unstable.
Adaptations Maintain Temperature
Two solutions have evolved to help animals maintain an optimum body temperature in a varying
environment:
- Mammals and birds are referred to as endotherms (endon is the Greek word for
"within'') due to their ability to maintain body temperature through internal metabolic
activity.
- Amphibians, reptiles, and fish are referred to as ectotherms (ektos is the Greek word
for "outside") because they rely on external factors, such as basking in the sunlight or
retreating to the shade below a rock, to maintain ideal body temperature.
- 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.
- The higher an animal's surface-to-volume ratio, the harder it must work to maintain core
temperature. The amount of heat loss is a function of the body surface area, and body
volume determines the amount of heat generated by metabolic activity.
- Because smaller animals have larger surface-to-volume ratios, maintaining core
temperature is harder for them than for larger animals such as humans.
- Rats have larger surface-to-volume ratios than humans or elephants. Consequently, rats
must work much harder than humans or elephants to maintain core temperature.
Endothermic Responses to Heat and Cold
- Endotherms defend a temperature set point of 37°C (98.6°F). When internal
temperature drops below this set point, they shiver, producing heat but at a high
, expenditure of energy. Blood vessels constrict, keeping most of the blood away from the
skin, leading to Raynaud's disease.
- The thyroid gland increases the release of thyroid hormone, which warms the body.
Deficits in thyroid activity are often diagnosed on the basis of lower-than-normal body
temperature.
- In human infants and small animals, the sympathetic nervous system responds to cold
by stimulating greater metabolic activity in brown fat cells, located in the torso. These fat
cells appear brown due to large numbers of mitochondria, the organelles responsible for
energy production.
Deviations in Human Core Temperature
- Hot flashes are experienced by nearly 80% of women in the months or years surrounding
menopause, caused by changes in sympathetic nervous system activation due to reduced
estrogen levels. Frequent hot flashes can affect a woman's quality of life, particularly
since they are correlated with poor sleep.
- Fever - A carefully controlled increase in the body's
thermal set point that is often helpful in ridding the
body of disease causing organisms.
- Hyperthermia - A life-threatening condition in which
core body temperature increases beyond normal limits
in an uncontrolled manner.
- Serotonin syndrome - A life-threatening condition
characterized by hyperthermia and muscular rigidity
caused by excess serotonin activity due to use of
therapeutic or recreational drugs.
- Hypothermia - A potentially fatal core body
temperature below 31°C/87.8°F.
Brain Mechanisms for Temperature Regulation
- Preoptic area (POA) - A part of the hypothalamus
involved in a number of regulatory functions, including temperature control.
- The preoptic area (POA) of the hypothalamus, along with adjacent areas of the anterior
hypothalamus and septal area, manages appropriate responses to higher core
temperatures such as panting, sweating, and the dilation of blood vessels. The posterior
hypothalamus is responsible for initiating responses to cooler core temperatures such as
shivering and blood vessel constriction.
- The POA contains three types of neurons that contribute to the temperature set point:
warm-sensitive, cold-sensitive, and temperature-insensitive:
- Warm-sensitive neurons: make up about 30 percent of the POA; have receptors
embedded in their membranes that respond directly to changes in the
temperature of the brain and blood in their vicinity. Maintain a background level
of activity that increases sharply when core temperatures surpass 37°C.
- Cold-sensitive neurons: make up about 5 percent of the POA; receive input from
thermoreceptors in the skin and spinal cord, however, they do not have any
special membrane receptors for sensing coldness in the nearby brain and blood.
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