chapter 9

Introduction

• Temperature significantly influences behavior.
• Temperature regulation is crucial for normal behavioral processes.
• Homeostasis: Biological processes maintaining body variables within a fixed range.

Homeostasis and Allostasis

• Homeostasis: Maintaining body variables within a fixed range.
  • Examples: temperature, water, oxygen, glucose, calcium, protein, fat, acidity.
• Set point: A single value the body aims to maintain.
• Negative feedback: Processes reducing discrepancies from the set point.
• Motivated behavior largely describable as negative feedback.
• Allostasis: Adaptive anticipation of needs based on the situation.
  • Helps avoid errors, not just correct them.
• Homeostasis and allostasis aren't perfect, leading to issues like obesity, high blood pressure, or diabetes.

Controlling Body Temperature

• Temperature regulation is a high biological priority.
• Maintaining temperature requires twice the energy of all other activities combined.
• Basal metabolism: Energy used to maintain constant body temperature at rest.
• Poikilothermic (ectothermic): Body temperature matches the environment.
  • Seen in amphibians, reptiles, and most fish.
  • Lacks internal physiological temperature regulation mechanisms.
  • Temperature regulation achieved by choosing environments.
  • Not always "cold-blooded"; depends on environmental temperature.
• Homeothermic (endothermic): Internal physiological mechanisms maintain near-constant body temperature.
  • Characteristic of mammals and birds.
  • Requires energy and fuel.
  • Cooling achieved via sweating and panting.
  • Warming achieved via shivering, reduced skin blood flow, and fluffing fur for insulation.

Surviving in Extreme Cold

• Poikilothermic animals die if body temperature drops below freezing.
  • Ice crystals form in cells and blood.
• Amphibians and reptiles burrow to avoid cold.
• Some insects and fish use antifreeze compounds in blood.
• Extraordinary blood-clotting ability repairs ruptured vessels.

Advantages of Endothermy

• Mammals evolved to maintain a constant temperature of 37^{\circ}C (98^{\circ}F).
• Muscle activity benefits from warmth.
• Allows for vigorous activity.
• Higher temperatures require more energy.
• Proteins break down at higher temperatures.
• Reproductive cells require cooler temperatures.

Brain Mechanisms

• Body temperature regulation relies on the preoptic area/anterior hypothalamus (POA/AH).
• POA/AH receives input from temperature receptors throughout the body.
• Heating POA/AH causes panting or sweating; cooling causes shivering.
• Receives input from the immune system.

Fever

• Prostaglandins and histamines trigger fever, shivering, and increased metabolism.
• POA/AH (hypothalamus) controls temperature responses.
• Fever = raised set point for body temperature.
  • Benefits: slows bacterial growth, boosts immune response.
  • Risks: 39^{\circ}C (103^{\circ}F) is harmful, 41^{\circ}C (109^{\circ}F) is life-threatening.

Integration of Temperature Information by the POA/AH

• Temperature receptors in skin, brain, and other organs send info to POA/AH.
• Immune response and infection trigger release of prostaglandins and histamine, influencing POA/AH.
• POA/AH controls shivering, sweating, heart rate, blood flow, and metabolism.

Knowledge Check 9-1: Answer

• Homeostasis reacts to changes to maintain a fixed range.
• Allostasis acts in advance to prevent or minimize changes.

Thirst

• Water constitutes 70% of the mammalian body.
• Water in the body must be regulated within narrow limits.
• Sufficient fluid is needed in the circulatory system.
• Chemical concentrations in water determine the rate of all chemical reactions in the body.
• Hypothalamus modifies cortical cell responses to increase responses to signals of water availability.

Thirst Management

• Managed by drinking and excretion.
• Conservation methods:
  • Concentrated urine
  • Reduced sweating
• Vasopressin (ADH):
  • Released by posterior pituitary
  • Constricts blood vessels (raises BP)
  • Promotes water reabsorption in kidneys
  • Produces concentrated urine

Osmotic Thirst

• Two types of thirst:
  • Osmotic thirst: from eating salty foods
  • Hypovolemic thirst: from loss of fluids (bleeding/sweating)
• Fixed solute concentration is a set point (0.15 M in mammals).

Osmotic Thirst Mechanism

• Eating salty food increases sodium ions in blood and extracellular fluid.
• High solute concentration outside causes osmotic pressure, drawing water from cells.
• Neurons detect water loss and trigger osmotic thirst.

Detection of Osmotic Pressure

• The brain detects osmotic pressure from:
  • Receptors around the third ventricle
  • OVLT (organum vasculosum laminae terminalis) and subfornical organ (SFO): detect osmotic pressure and sodium content
  • Receptors in the periphery, including stomach and digestive tract

Brain Receptors for Osmotic Pressure and Blood Volume

• Subfornical organ and OVLT located near the third ventricle.

Relay of Information in Osmotic Thirst

• Receptors in OVLT, SFO, stomach relay information to:
  • Supraoptic nucleus
  • Paraventricular nucleus
• Both control vasopressin release from posterior pituitary.
• Receptors also relay information to the lateral preoptic area, controlling drinking.

Anticipatory Mechanisms in Osmotic Thirst

• Drinking often occurs immediately after a salty meal, not after osmotic pressure changes.
• OVLT receives input from digestive tract about food, water, and salt content, anticipating osmotic need.
• OVLT also receives input from the tongue.
• Liver receptors detect osmotic concentration and signal the brain to decrease thirst if low.

Consequence of Osmotic Pressure Difference

• Higher solute concentration outside the cell causes water to flow out, shrinking the cell and equalizing concentration.

Hypovolemic Thirst and Sodium-Specific Hunger

• Hypovolemic Thirst: Thirst due to low fluid volume.
  • Low blood volume -> kidneys release renin -> angiotensin I -> angiotensin II.
  • Angiotensin II constricts blood vessels, increasing blood pressure.
  • Angiotensin II stimulates neurons in subfornical organ, releasing angiotensin II as neuromodulator.
  • Neurons in third ventricle send axons to hypothalamus, releasing angiotensin II as neurotransmitter.
• Animals with osmotic thirst prefer pure water.
• Animals with hypovolemic thirst prefer slightly salty water as pure water dilutes fluids and changes osmotic pressure.
• Sodium-specific hunger: craving for salty foods to restore solute levels.
  • Adrenal glands release aldosterone when sodium is low, causing salt retention by kidneys, salivary glands, and sweat glands.

Hormonal Response to Hypovolemia

• Low blood volume causes kidneys to release renin.
• Renin converts proteins in blood to angiotensin I, then to angiotensin II.
• Angiotensin II constricts blood vessels and stimulates subfornical organ to increase drinking.

Comparison of Osmotic and Hypovolemic Thirst

Hunger

• Animals have varying eating strategies.
• Predators: large digestive systems, infrequent meals.
• Bears: constant eating.
• Small birds: eat only what's needed, maintain light weight.
• Chickadees eat enough daily to increase body weight 10%, then lose it overnight.

Digestion and Food Selection

• Digestive system breaks down food into usable molecules.
• Digestion begins in the mouth.
  • Saliva enzymes break down carbohydrates.
  • Hydrochloric acid and stomach enzymes digest proteins.
  • Small intestine enzymes digest proteins, fats, and carbohydrates.
• Digested food absorbed into the bloodstream.
• Large intestine absorbs water and minerals and lubricates remaining material for excretion.

Consumption of Dairy Products

• Most mammals lose lactase after weaning, causing gas and cramps from dairy.
• May have evolved to encourage weaning.
• Some humans retain lactase into adulthood.
• Most adults in East Asia lack the gene for adult lactase, resulting in limited dairy tolerance.

Food Selection and Behavior

• Unsubstantiated beliefs influence food selection.
  • Sugar increases hyperactivity.
  • Turkey increases tryptophan and sleepiness.
  • Fish is brain food, which may improve memory.

Short- and Long-Term Regulation of Feeding

• Oral factors (taste and chewing) motivate hunger and satiety.
• Chewing gum from 4500 B.C. discovered.
• Sham feeding experiments do not reliably produce satiety.

The Stomach and Intestines

• Stomach distention is a primary satiety signal.
• Vagus nerve (cranial nerve X) conveys stomach stretching information to the brain.
• Duodenum (small intestine) is the initial site of nutrient absorption.
  • Distention of the duodenum produces satiety.
  • Duodenum releases cholecystokinin (CCK), regulating hunger.

Glucose, Insulin, and Glucagon

• Glucose: Main digestion product and energy source.
  • Primary fuel for the brain.
• Insulin and glucagon regulate glucose flow into cells.
• Excess glucose enters the liver and fat cells.
• Insulin (pancreatic hormone) enables glucose to enter cells.
  • After a meal, blood glucose and insulin levels fall, glucose enters cells slowly, and hunger increases.
  • Pancreas releases glucagon, which stimulates the liver to convert glycogen to glucose.
• If insulin levels stay high:
  • Blood glucose drops, and hunger increases.
  • Food is rapidly deposited as fat and glycogen, causing weight gain.
  • Typical preparation for winter in some animals; humans eat more in autumn.

Diabetes and Appetite

• Type 1:
  • Low insulin, high blood sugar.
  • Increased eating but weight loss (glucose not used).
• Type 2:
  • Most common, often starts in middle age.
  • Linked to obesity and inactivity.
  • Insulin present, but cells resist it.

Leptin

• Long-term hunger regulation is managed by monitoring fat supplies.
• Fat cells produce leptin, signaling the brain to increase or decrease eating.
• Low leptin increases hunger.
• High leptin reduces eating and increases physical/immune system activity.
• Puberty is triggered by a minimum level of leptin during adolescence.

Brain Mechanisms

• The arcuate nucleus receives information from all parts of the body regarding hunger.
• The arcuate nucleus (hypothalamus) contains neurons sensitive to hunger and satiety signals.

The Arcuate Nucleus and Paraventricular Hypothalamus

• Ghrelin (neurotransmitter) released in the brain increases appetite and triggers stomach contractions via actions on the hypothalamus.
• Nicotine stimulates satiety neurons in the arcuate nucleus, decreasing appetite.

The Arcuate Nucleus

• Satiety-sensitive cells of the arcuate nucleus receive short-term and long-term satiety signals.
  • Intestine distention triggers CCK release.
  • Blood glucose stimulates satiety cells.
  • Body fat releases leptin.
• NPY and AgRP block the satiety action of the paraventricular nucleus and provoke overeating.

The Feeding Circuit in the Hypothalamus

• Complex interplay of signals including ghrelin, leptin, insulin, and CCK influence hunger and satiety.
• These signals act on two kinds of neurons in the arcuate nucleus and lateral nucleus of the hypothalamus.
• Output affects other brain areas, influencing feeding behavior.

Paraventricular Hypothalamus

• The arcuate nucleus sends output to the paraventricular nucleus of the hypothalamus.
• The paraventricular nucleus inhibits the lateral hypothalamus, which is important for eating.
• Satiety-sensitive cells of the arcuate nucleus send an excitatory message to the paraventricular nucleus, causing the release of melanocortins.
• Cells in the lateral hypothalamus release orexin, increasing animals’ persistence in seeking food.

Melanocortin

• Melanocortin is a chemical important for limiting food intake.
• Deficiencies in this receptor lead to overeating.
• Input from hunger cells of the arcuate nucleus inhibits the paraventricular nucleus, which inhibits the lateral hypothalamus.

The Lateral Hypothalamus

• Feeding-related functions of the lateral hypothalamus:
  • Controls insulin secretion
  • Alters taste responsiveness
• Stimulation of the lateral hypothalamus increases the drive to eat, while damage causes aversion to food.

Dopamine's Role in the Lateral Hypothalamus

• Many axons containing dopamine pass through the lateral hypothalamus.
• Axon functions:
  • Affect taste sensation and salivation.
  • Increase cortical cell response to taste, smell, and sight of food.
  • Increase pituitary gland’s secretion of hormones, increasing insulin secretion.
  • Control digestive secretions

Medial Areas of the Hypothalamus

• Output from the ventromedial hypothalamus (VMH) inhibits feeding.
• Damage to this nucleus leads to overeating and weight gain.
• Rats eat normal-sized meals but eat more frequently.
• Increased stomach motility; stomach empties faster than normal.

Study Questions

• How does temperature regulation influence behavior?
• What are homeostasis and allostasis, and how do they differ?
• Why is maintaining a high, stable body temperature worth the energy cost?
• Which brain areas control body temperature?
• Why is a moderate fever helpful during infection?
• What’s the difference between osmotic and hypovolemic thirst, and how does the brain detect each?
• What is sodium-specific hunger, and when does it occur?
• How do genetics influence the ability to digest dairy?
• What factors affect our food choices?
• What physiological signals control short-term and long-term hunger and fullness?
• Which brain regions are involved in controlling eating behavior?
• What are common eating disorders, and how do they affect regulation of hunger and body weight?