Chapter 13- Homeostasis

Question 1

Why is homeostasis important from a neuroscience perspective?

Answer 1

Alterations in the internal environment (homeostasis) affects motivation- the psychological process that induces or sustains a specific behavior. If the internal state doesn’t match the regulated or intended state, it produces a drive to restore balance (like drinking water if dehydrated).

Question 2

Negative feedback

Answer 2

When moving away from a desired value (the set point) results in a compensatory action. Once the desired value is restored, the response/compensatory action is turned off (hence the “negative”). The mechanisms regulating temperature, body fluids, and metabolism are all examples of negative feedback, There is generally a set zone rather than a set point to prevent the system from going on and off too frequently.

Question 3

Homeostasis

Answer 3

Maintaining the set point for a certain variable, like blood glucose, body fat, or salt levels, through negative feedback. The information is compared to the set point, and a response might be necessary to restore it.

Question 4

Redundancies

Answer 4

Having multiple mechanisms for maintaining homeostasis. For example, the body has multiple responses to both heat and cold to change the body temperature.

Question 5

Endotherms vs ectotherms

Answer 5

While endotherms make heat within our own bodies through metabolism and muscular activity, ectotherms get their heat from the environment. Ectotherms must therefore stay near sources of warm.

Question 6

What is behavioral homeostasis?

Answer 6

Organisms can use behavioral measures to help them acquire more heat, water, or food in order to achieve and maintain homeostasis. Ectotherms and endotherms can regulate temperature by changing exposure of the body surface (by huddling or extending limbs), changing external insulation (using clothing or nests), or changing surroundings (moving into the sun or shade).

Question 7

Thermoregulation- how does the body generate heat?

Answer 7

Increased thyroid activity (increases metabolism of brown fat), constriction of cutaneous blood vessels, and shivering of muscles.

Question 8

Thermoregulation- how does the body decrease body temperature?

Answer 8

Accelerated respiration, perspiration, and dilation of cutaneous vessels, which decreases body temperature.

Question 9

How does the thermoregulatory system work?

Answer 9

Receptors in the skin, body core, and hypothalamus detect temperature and transmit that information to 3 neural regions- the spinal cord, brainstem, and hypothalamus. If the body temperature moves outside of the set zone, each of these neural regions can initiate physiological and behavioral responses to return it to the set zone.

Question 10

Osmolality

Answer 10

Osmolality is the concentration of a solute in a solution. The concentration of salt in the extracellular fluid of mammals is about .9%.

Question 11

Isotonic

Answer 11

A solution with .9% concentration of salt

Question 12

Hypotonic

Answer 12

A solution with less salt- has a salt concentration lower than .9%. Cells will gain water if placed in a less salty solution.

Question 13

Hypertonic

Answer 13

A solution with more salt, has a salt concentration higher than .9%. Cells will lose water if placed in a more salty solution.

Question 14

Body fluid compartments (2)

Answer 14

1. The intracellular compartment contains the water within our cells 2. Extracellular compartments contains the interstitial (between cells) fluid and blood plasma.

Question 15

Diffusion

Answer 15

Molecules of one substance will passively spread among molecules of another substance until a uniform concentration is achieved. Cell membranes are selective with their permeability, and will therefore only allow certain molecules to diffuse in or out.

Question 16

Osmosis

Answer 16

Osmosis is the movement of water molecules that occurs to make the concentration of two solutions equal, when a semipermeable membrane separates solutions of different concentrations of solute. If the membrane is only permeable to water, water will cross the membrane to make the solution on both sides of the membrane equally concentrated.

Question 17

Osmotic pressure

Answer 17

The physical force that pushes or pulls water across the membrane.

Question 18

Extracellular fluid

Answer 18

The movement of water can damage a cell if excessive, so the extracellular fluid serves as a buffer- a reservoir of isotonic fluid that provides and accepts water molecules so the cell can maintain internal conditions.

Question 19

What triggers osmotic thirst?

Answer 19

We lose volume of extracellular fluid due to normal physiological processes like respiration, perspiration, and urination. At the same time, concentration of the extracellular fluid increases, since the same amount of salt remains. This triggers osmotic thirst- we want to drink water to return the extracellular fluid to an isotonic state. Triggers vasopressin release to reduce urination.

Question 20

Osmosensory neurons

Answer 20

Monitor the concentration of extracellular fluid, located in the hypothalamus, supraoptic nucleus, and the circumventricular organs.

Question 21

Circumventricular organs

Answer 21

Located in the walls of the cerebral vessels, and contain fenestrated capillaries. Fenestrated capillaries are blood vessels that lack the usual blood brain barrier and therefore allow neurons to monitor both salt concentration and hormones in the bloodstream. The OVLT and SFO are circumventricular organs that prompt a large drinking response with changes in osmolality.

Question 22

What features do osmosensory neurons have to help them perform their function?

Answer 22

These neurons are stretchy, in contrast to other cells in the body which maintain a constant size. They can swell or shrink as the concentration of the extracellular fluid changes. The change in size of the cell membrane physically opens and closes special mechanically gated ion channels in the membrane, causing changes in cell membrane potentials that track the changes in extracellular concentration. This information is conveyed to other parts of the brain to result in thirst and homeostatic responses to conserve water.

Question 23

Why is salt important for fluid balance?

Answer 23

The amount of water that we can retain is determined primarily by the amount of salt contained in the extracellular fluid. This is why slightly salty drinks, like sports drinks, can quench thirst more effectively. For example, if the extracellular compartment contains pure water, the water will enter the cells and the cells will rupture- we shed water through urination until the solution is isotonic.

Question 24

What triggers hypovolemic thirst?

Answer 24

Hypovolemic thirst is triggered by a loss of volume of the extracellular fluid. We normally lose some fluid volume throughout the day (and salt concentration increases), but sudden and dramatic losses of fluid (due to hemorrhage, vomiting, or diarrhea) creates thirst that is primarily hypovolemic in nature. This causes a decrease in blood pressure due to decreased blood volume. In this case, the concentration of extracellular fluid does not change at first, because salt and other solutes are lost along with the water.

Question 25

How is hypovolemic thirst detected?

Answer 25

The drop in extracellular volume is detected by pressure receptors (baroreceptors), which are located in major blood vessels and in the heart. The brain receives the signal from the baroreceptors and activates thirst (to replace lost water) and salt hunger (to replace the lost solutes). Replacing water without replacing solutes results in hypotonic extracellular fluid. The sympathetic nervous system can stimulate muscles in artery walls to constrict, reducing the size of the vessels to increase blood pressure and compensate for the reduced blood volume.

Question 26

Vasopressin

Answer 26

Released from the posterior pituitary gland. Vasopressin induces additional constriction of blood vessels and instructs the kidneys to reduce the flow of water to the bladder. Vasopressin release is also promoted at night due to inputs from the suprachiasmatic nucleus, which slows urine production and prevents dehydration during sleep.

Question 27

Angiotensin 2

Answer 27

The release of the enzyme renin from the kidneys results in the production of angiotensin 2, which is another water conserving hormone. It constricts the blood vessels to maintain blood pressure, and triggers the release of vasopressin and aldosterone. It also regulates behavior through actions at neural sites located in the forebrain (especially the circumventricular organs).

Question 28

What triggers the release of renin?

Answer 28

Stretching of baroreceptors in the kidneys

Question 29

Why do we stop feeling thirsty?

Answer 29

Combination of factors (redundancies)

1. Mouth and throat feel wet
2. Estimation of how much we’ve been drinking
3. Water in the stomach, but not in the extracellular component yet

Question 30

Why do we need to eat? (4)

Answer 30

1. Energy
2. Nutrients- amino acids, fatty acids, vitamins and minerals
3. Digestion
4. Anticipate future energy needs

Question 31

How is glucose stored short term?

Answer 31

For short term storage of food energy, glucose can be converted into glycogen (a more complex molecule). Glycogen can be stored in multiple locations, but mostly in the liver and skeletal muscles.

Question 32

Glycogenesis

Answer 32

Conversion of glucose into glycogen, promoted by hormone insulin

Question 33

Glucagon

Answer 33

Another pancreatic hormone that mediates glycogenolysis, which is triggered when blood concentrations of glucose drop too low.

Question 34

Glycogenolysis

Answer 34

The conversion of glycogen back into glucose, triggered when blood concentrations of glucose drop too low.

Question 35

How is glucose stored long term?

Answer 35

Lipids are deposited in the fat storing cells that form adipose tissue. Some stored fats are created directly from fats in our food, but others are synthesized in the body from surplus sugars and other nutrients. During longer periods of food deprivation, fat can be converted into glucose and a secondary form of fuel called ketones, which can similarly be utilized by the body and brain.

Question 36

Gluconeogenesis

Answer 36

Conversion of fat into glucose

Question 37

What is the signal for hunger or satiety?

Answer 37

Glucose levels (and therefore insulin levels) can be an additional appetite signal, but not the only signal. People with untreated diabetes have very high levels of circulating glucose, but they are constantly hungry. Somehow, the brain integrates glucose and insulin levels with other hypothalamic signals to decide whether to initiate eating.

Question 38

Ventromedial hypothalamus (VMH)

Answer 38

In rats, bilateral lesions of the ventromedial hypothalamus (VMH) resulted in the animals eating to excess (hyperphagia). However, it’s not as simple as the VMH being the satiety center. The rats with VMH lesions exhibited a period of rapid weight gain but then stabilized at a new, higher body weight and would eat only to the extent needed to defend the new weight- shows that VMH isn’t the sole satiety controller.

Question 39

Lateral hypothalamus (LH)

Answer 39

Rats with lesions of the lateral hypothalamus (LH) experienced a cessation of eating (aphagia). These rats lost weight, but stabilized at a lower set point- not the sole “hunger center”.

Question 40

Which hormones does the body use to warn the hypothalamus about nutrient surplus/deficiency? (4)

Answer 40

1. Leptin
2. Insulin
3. Ghrelin
4. PYY (3-36)

Question 41

Leptin

Answer 41

Leptin is a peptide hormone with receptors located throughout the brain (including the cortex and hypothalamic appetite network). The brain seems to monitor circulating leptin levels to measure and regulate the body’s energy reserves in the form of fat. Defects in leptin production or leptin sensitivity cause a false underreporting of body fat and lead to overeating, especially of high fat and sugary foods.

Question 42

Which cells release leptin?

Answer 42

Fat cells

Question 43

Ghrelin

Answer 43

Ghrelin is a powerful appetite stimulant. Circulating levels of ghrelin rise during fasting and immediately drop after a meal is eaten. Ghrelin has an important role in both seeking and consuming food.

Question 44

Which cells release ghrelin?

Answer 44

Released into the bloodstream by the endocrine cells of the stomach

Question 45

PYY (3-36)

Answer 45

PYY is an appetite suppressant. It is at a low level in the blood prior to eating, but the level rises rapidly after a meal. It appears that PYY 3-36 acts in opposition to ghrelin, providing an appetite-suppressing stimulus to the hypothalamus. Lower than average levels of PYY are associated with a tendency toward obesity.

Question 46

Which cells release PYY?

Answer 46

PYY is released into the bloodstream by the cells of the small and large intestines.

Question 47

Where do feeding hormones travel to in the brain?

Answer 47

2 sets of neurons in the arcuate nucleus in the hypothalamus

Question 48

2 sets of neurons in the hypothalamus

Answer 48

1. POMC

2. NPY

Question 49

POMC neurons

Answer 49

POMC neurons act as satiety neurons when activated, inhibiting appetite and increasing metabolism. High circulating levels of leptin activate the appetite-suppressing POMC neurons

Question 50

NPY neurons

Answer 50

NPY neurons (which produce neuropeptide Y) act as hunger neurons, stimulating appetite directly and also inhibiting POMC neurons. High circulating levels of leptin inhibit the appetite-stimulating NPY neurons

Question 51

What is the function of ghrelin and PYY in the arcuate appetite system?

Answer 51

These hormones provide more rapid, hour to hour hunger signals from the stomach and gut. Both ghrelin and PYY act primarily on the appetite-stimulating NPY neurons of the arcuate. Ghrelin stimulates these cells to cause an increase in appetite, while PYY acts in opposition, inhibiting the same NPY cells to reduce appetite.

Question 52

Orexigenic neurons

Answer 52

Located mainly in the lateral hypothalamus and act to increase appetite and food intake.

Question 53

Anorexigenic neurons

Answer 53

Located in the paraventricular nucleus (PVN) act to decrease appetite and food intake.