CHAPTER 6
Motivation and the Regulation of Internal
States
Motivation and Homeostasis
Hunger: A Complex Drive
Obesity
Anorexia and Bulimia
Motivation
and Homeostasis
•
Motivation, which literally means “to set in motion,” refers to the set of factors
that initiate, sustain, and direct behaviors.
•
An instinct is a complex behavior that is automatic and unlearned, and occurs in all the
members of a species.
–
Examples: migration and maternal behavior.
Motivation
and Homeostasis
•
According to drive theory, the body maintains a condition of homeostasis, in which any particular
system is in balance or equilibrium.
–
Any departure from homeostasis, such as depletion or nutrients or a drop in temperature,
produces an aroused condition or drive, which impels the individual to engage in appropriate action such as eating, drinking,
or seeking warmth.
•
Incentive theory recognizes that people are motivated by external stimuli, not just internal
needs.
•
Arousal theory says that people behave in ways that keep them at their preferred level of
arousal.
Motivation
and Homeostasis
•
To sustain life a number of conditions, such as body temperature, fluid levels, and energy
reserves, must be held within a fairly narrow range, which requires a control system.
•
Control systems have a set point, which is the point of equilibrium the system returns to.
–
All animals have to maintain internal temperature within certain limits in order to survive,
and they operate more effectively within an even narrower range; this is their set point.
Motivation
and Homeostasis
•
The “thermostat” is located in the preoptic area of the hypothalamus, which contains
separate warmth-sensitive and cold-sensitive cells.
–
Some of these neurons respond directly to the temperature of the blood flowing through the
area.
–
Other neurons receive input from temperature
receptors in other parts of the body, including the skin.
Motivation
and Homeostasis
•
Osmotic thirst occurs when the fluid content decreases inside the cells.
–
This happens when the blood becomes more concentrated than usual, usually because the individual
has not taken in enough water to compensate for food intake.
–
As a result, water is drawn from the cells into the bloodstream by osmotic pressure.
•
Hypovolemic thirst occurs when the blood volume drops due to a loss of extracellular water.
–
This can be due to sweating, vomiting, and diarrhea.
Motivation
and Homeostasis
•
The reduced water content of cells that contributes to osmotic thirst is detected primarily
in areas bordering the third ventricle, particularly in the OVLT (organum vasculosum lamina terminalis).
•
The OLVT communicates the water deficit to the median preoptic nucleus of the hypothalamus,
which initiates drinking.
•
The reduced blood volume in the heart that accompanies volemia is signaled by the vagus to
the NST (nucleus of the solitary tract) in the medulla.
•
Hypovolemia is detected by the receptors located where
the large viens enter the atrium of the heart – These receptors respond to stretching of the vascular alls
by the volume of blood passing through
•
From there the signal goes to the median preoptic area of the hypothalamus.
Motivation
and Homeostasis
•
Lowered blood volume is also detected by receptors in the kidneys, which trigger release
of the hormone renin.
•
Renin increases production of the hormone angiotensin II.
•
Angiotensin II circulating in the blood stream informs the brain of the drop in blood volume.
•
It stimulates the SFO (subfornical organ), a structure bordering the third ventricle and
one of the areas that is unprotected by the blood-brain barrier.
Hunger: A Complex Drive
•
The simplest form of dietary selection involves distinguishing between foods that are safe
and nutritious and those that are either useless or dangerous.
•
In humans, all taste experience is a result of just five taste sensations: sour, sweet, bitter,
salty, and the more recently discovered umami.
–
Umami is often described as “meaty” or “savory.”
•
Taste receptors are located on taste buds, which in turn are found on the surface of papillae
•
Papillae are small bumps on the tongue and elsewhere in the mouth.
•
Taste neurons travel through the thalamus to the insula, the primary gustatory (taste) area
in the frontal lobes.
•
Sensory-specific satiety means that the more a particular food an individual eats, the less
appealing the food becomes.
•
Sensory-specific satiety is the brain’s way of encouraging you to vary your food choices,
which is necessary for a balanced diet.
•
Sensory-specific satiety takes place in the NST.
•
Learned taste aversion is the avoidance of foods associated with illness or poor nutrition.
•
Learned taste preference is a preference not for the nutrient itself, but for the flavor
of a food that contains a needed nutrient.
•
Digestion begins in the mouth, where food is ground fine and mixed with saliva.
•
Saliva provides lubrication and contains an enzyme that starts the breakdown of food.
•
Digestion proceeds in the stomach as food is mixed with the gastric juices hydrochloric acid
and pepsin.
•
The partially processed food is then released gradually so the small intestine has time to
do its job.
•
Digesting primarily occurs in the small intestines, particularly the initial 25 cm called
the duodenum.
–
Carbohydrates are metabolized into simple sugars, particularly glucose.
–
Proteins are converted to amino acids.
–
Fats are transformed into fatty acids and glycerol.
•
The area postrema is one of the places in the brain that is outside the blood-brain barrier,
so toxins can activate it to induce vomiting.
•
For a few hours following a meal our body lives off the nutrients arriving from the digestive
system; this period is called the absorptive phase.
–
The pancreas starts secreting insulin, a hormone that enables body cells to take up glucose
for energy and certain cells to store excess nutrients.
–
Some of the glucose is converted to glycogen and stored in a short-term reservoir in the
liver and muscles.
–
Any remaining glucose is converted into fats and stored in fat cells, also known as adipose
tissue.
•
Eventually the glucose level in the blood drops and the body must fall back on its energy
stores, which is why this is called the fasting phase.
–
The pancreas ceases secretion of insulin and starts secreting the hormone glucagon, which
causes the liver to transform stored glycogen back into glucose.
•
The lateral hypothalamus initiates eating and controls several aspects of feeding behavior
as well as metabolic responses.
•
The PVN (paraventricular nucleus) initiates eating, though less effectively than the lateral
hypothalamus, and regulates metabolic processes such as body temperature, fat storage, and cellular metabolism.
•
The arcuate nucleus is a vital hypothalamic structure for monitoring the body’s nutrient
condition.
•
NPY (neuropeptide Y), which is released from the PVN and lateral hypothalamus because of
signals from the arcuate nucleus, dramatically increases eating while reducing metabolism.
•
Rats injected with NPY double their rate of eating and add six fold to their weight. Their
weight is three times greater than their increase in food intake so this suggests NPY inhibits metabolism.
•
.
•
The best known of the satiety signals is CCK (cholecystokinin), a peptide hormone that is
released as food passes into the duodenum.
–
CCK detects fats and causes the gall bladder to inject bile into the duodenum, which breaks
down the fat so it can be absorbed.
•
Another appetite-suppressing peptide hormone that is released in the intestines in response
to food is PYY (peptide YY3-36).
–
PPY is carried by the blood stream to the arcuate nucleus, where it inhibits the NPY-releasing
neurons.
–
Its non-neural route to the brain means that its action is too slow to limit the current
meal; instead it decreases calorie intake by about a third over the following 12 hours.
Hunger: A Complex Drive
•
Fat cells secrete a hormone that inhibits eating called leptin.
–
The amount of leptin in the blood is proportional to body fat.
–
Leptin helps regulate meal size, but it does so in response to long-term stores of fat rather
than the nutrients contained in the meal.
Obesity
•
According to the National Health and Nutrition Examination Surveys, the adult obesity rate
in the U.S. has doubled since 1980.
•
Obesity is most important because of its health risks.
–
As overweight and obesity increase, so does the incidence of a variety of diseases, including
diabetes, heart disease, high blood pressure, stroke, and colon cancer.
–
Obesity is also linked to cognitive decline and risk of Alzheimer’s disease.
Obesity
•
BMI (body mass index) is calculated by dividing the person’s weight in kilograms by
the squared height in meters.
•
Both adoption studies and twin studies demonstrate the influence of heredity on body weight.
–
Adopted children show a moderate relationship with their biological parents’ weights
and BMIs, but little or no similarity with their adoptive parents.
Obesity
•
In the average sedentary adult, about 75% of daily energy expenditure goes into resting or
basal metabolism, the energy required to fuel the brain and other organs and to maintain body temperature.
–
The remainder is spent about equally in physical activity and in digesting food.
Obesity
•
The standard treatment for obesity, of course, is dietary restriction.
–
Dieters who exercise lose more weight than dieters who do not exercise.
•
Another option in the treatment of obesity is medication.
–
However, it has not been a particularly promising alternative.
–
Lack of effectiveness is one problem and, because the drugs manipulate metabolic and other
important systems, they often have adverse side effects.
Obesity
•
The approval of dexfenfluramine in 1996 was the first by the FDA in 20 years.
–
But just a year later, both it and the older fenfluramine were withdrawn from the market
by the manufacturer after reports they caused heart valve leakage.
•
In June 2007, the FDA denied approval of rimonabant, which blocks the endogenous cannabinoid
receptors that are responsible for the “marijuana munchies” and produces five percent weight losses.
–
The panel was concerned by reports linking the drug to increased psychiatric problems, including
suicide.
Obesity
•
Even the two approved drugs have their problems:
•
Orlistat (Xenical) causes cramping and severe diarrhea because it blocks water absorption.
•
Sibutramine (Meridia) has been linking to a number of deaths due to cardiovascular problems.
•
Sibutramine blocks norepinephrine and serotonin reuptake and acts as an appetite suppressant.
–
Drugs that block serotonin reuptake reduce carbohydrate intake.
•
The experimental drug C75 reduces fat storage by oxidizing fatty acids.
–
At the same time, it reduces appetite by interfering with NPY production.
Anorexia and Bulimia
•
Anorexia nervosa is known as the “starving disease” because the individual restricts
food intake to maintain weight at a level so low that it is threatening to health.
•
There are two subgroups of anorexics.
–
Restrictors rely only on reducing food intake to control their weight.
–
Purgers restrict their calorie intake as well, but they also resort to purging, by vomiting
or using laxatives.
•
The anorexic individual’s unwillingness to eat does not necessarily imply lack of hunger.
–
NPY levels are high and leptin levels are low.
Anorexia and Bulimia
•
Bulimia nervosa also involves weight control, but the behavior is limited to bingeing and
purging.
•
If the bulimic restricts food intake, it is only for a few days at a time, and restricting
takes a backseat to bingeing and purging.
•
Their ghrelin levels between meals are a third higher than in controls, decrease less following
a meal.
•
In addition, PYY levels do not rise as much following a meal.
Anorexia and Bulimia
•
Because of the role serotonin has in eating and in obesity, as well as in depression, researchers
have suspected that anorexics and bulimics have lower than normal serotonin activity.
•
Bulimics do have reduced levels in their cerebrospinal fluid of the serotonin metabolic by-product
5-HIAA.
•
Besides depression, bulimics have an increased rate of anxiety, alcoholism, and other drug
abuse, and impulsive behavior, including stealing and sexual activity.
•
All these characteristics are associated with low serotonin activity.
RECAP
Prader‑Willi
syndrome, the result of a "genetic accident" on chromosome 15, results in uncontrollable eating and extreme obesity
Motivation
and Homeostasis: Motivation, the set of factors that initiate, sustain, and direct behaviors, is a concept that is imposed on behavior
Theoretical Approaches to Motivation
Instinct
theory
An instinct
is a complex behavior that is automatic and unlearned
Early theorists
identified numerous human instincts; contemporary theory gives them less importance or denies them altogether
Drive theory
Homeostasis: Any departure will produce a drive ; May not apply to all kinds
of motivation
Incentive
theory: Behavior is motivated by external stimuli
Arousal
theory: Behavior maintains the individual's preferred
level of Arousal
Brain state
theory gives more importance to the brain in drives
than the Condition of the tissues
Simple Homeostatic Drives
A control system detects departures from homeostasis and signals actions to return condition to the set point
Temperature
Regulation
Homoeothermic
animals regulate body temperature by manipulating their environment
Endothermic
animals are able to regulate body temperature internally
The preoptic
area of the hypothalamus, with its warm‑sensitive and cold‑sensitive cells, is the most important brain area in
temperature regulation
Thirst
Osmotic thirst,
due to reduced fluid content in cells, is detected primarily around the third ventricle, particularly in the OVLT
Hypovolemic
thirst is due to a loss of extracellular water and reduced blood volume
Detected by
baroreceptors in the heart, signaled to hypothalamus by vagus nerve
Detected by
baroreceptors in the kidneys, signaled to subfornical organ by angiotensin
The satiety
mechanism (which stops drinking before tissue needs are satisfied) probably involves water receptors in the stomach and liver
Hunger:
A
Complex Drive
The Role
of Taste
The primary
tastes of sweet, salty, and umami often identify nutritious foods, sour is associated with spoiled foods and bitter often indicates
toxic substances
Sensory‑Specific
Satiety:
Varying the
Choices encourages a varied diet
The more of
something you eat, the less appealing it becomes
A selection
of different foods increases appetite
Learned
Taste Aversion: Avoiding Dangerous Foods
Conditioned
aversion after getting sick helps avoid dangerous foods
May help avoid
non‑nutritious foods
Learned
Taste Preferences: Selecting Nutritious Foods
Conditioned
to taste of food when the food makes the individual feel better
Role in human
behavior is unclear
Regulating
Food Intake
The Digestive
Process
1. In the mouth,
saliva provides lubrication and an enzyme that begins digestion
2. The stomach
mixes food with hydrochloric acid and pepsin
3. Toxic substances
are detected in the stomach and the area postrema
4. Most digestion
occurs in the small intestine, especially the duodenum
5. Carbohydrates
are converted to simple sugars, proteins to amino acids, and fats to fatty acids
and glycerol
6. Breakdown
products are transported to the liver via the hepatic portal vein
During The
Absorptive Phase
1. Blood sugar levels rise
2. Autonomic activity shifts from sympathetic to parasympathetic
3. Insulin secretion increases, enabling cells to take up glucose
4. Excess glucose is converted to glycogen and stored in the liver or converted to fat and stored in adipose tissue (fat cells)
During The
Fasting Phase
1. Blood sugar levels drop
2. Autonomic activity shifts from parasympathetic to sympathetic
3. Insulin secretion stops, glucagon secretion begins
4. Stored fat is broken down to fatty acids and glycerol
5. Muscle proteins are broken down into amino acids (emergency situations)
Signals
that Start a Meal
1. Glucoprivic
hunger is triggered by low blood glucose level
2. Lipoprivic
hunger results from low fatty acid in the blood
3. Information
travels via the vagus nerve to the medulla and the paraventricular nucleus
of the hypothalamus
4. The PVN
releases neuropeptide Y (NPY), which stimulates eating
Signals
that End a Meal
1. Stretch receptors in the. stomach send a signal via the vagus to the PVN
2. Cholecystokinin released in the duodenum signals the PVN via the vagus
3. Nutritional content of the food, detected in the liver, affects meal size
Long ‑Term
Controls
1) The hormone
leptin is secreted by fat cells
Low leptin triggers NPY release in the PVN
High leptin reduces NPY release
2) Insulin
level also signals fat level and regulates NPY release
Eating Disorders
Obesity increases
health risks, including susceptibility to several diseases,
as the body
mass index (BMI) exceeds approximately 26
The Myths
of Obesity:
Research has
not indicated that obesity is due
to inadequate
impulse control, maladaptive eating style, or learned overindulgence
The Contribution
of Heredity
Correlations
are high among relatives, low between children and their adoptive parents
Mice with the
oblob genes do not produce leptin; dbldb mice are insensitive to it
Heredity influences
meal size and frequency, energy intake, activity, metabolic rate, and nutrient proportions in the diet
Obesity
and Reduced Metabolism
1. Basal metabolism
rate (BMR) is one explanation for weight gain
2. Body defends
weight by shifting BMR, but defends against loss more than gain
3. Defense
aggressiveness varies among individuals
4. Set point
may shift higher after prolonged weight gain
5. Set
point may be altered by dieting and lead to more weight gain over time
Treating
Obesity
1. Dietary
restriction is typically effective only if combined with exercise
2. Medication
Currently approved
medications suppress appetite by inhibiting norepinephrine and serotonin reuptake
Serotonin reuptake
inhibiters work only in carbohydrate cravers (who experience mood improvement from carbohydrates, which increase serotonin
levels)
Leptin administration
may be effective for some individuals
The experimental
fatty acid synthase inhibitor C75 reduced food intake 90% in mice
Anorexia
and Bulimia
Anorexia involves restricting food intake to a point that is threatening
to health
Purgers binge
on food then purge by vomiting or using laxatives
Restrictors
restrict food intake to reduce calories
Bulimia
nervosa involves bingeing and purging
Environmental
and Genetic Contributions
Western culture
encourages unreasonable slimness in women
Twin studies
and the comorbidity of neurotransmitter‑based disorders in relatives indicate a genetic contribution
The Role
of Serotonin
Bulimics have
reduced serotonin activity, and antidepressants that increase serotonin activity reduce binge eating
Anorexics commonly
have reduced serotonin activity while they are underweight, but this may be due to starvation. The heart is a particular
risk and death by heart attack may occur
Purgers
vs. Restrictors
Purgers are
impulsive, socially outgoing, emotionally responsive, sexually active, typical of low serotonin activity
Purgers do
not show the same increases in serotonin as restrictors after recovery
Cryproheptadine
reduces serotonin and aids restrictors, but impairs purgers' treatment
Purging anorexics
are more like bulimics in personality and physiology
