Chapter 12Chapter 12

Ingestive BehaviorIngestive Behavior

IntroductionIntroduction

““The constancy of the internal milieu is a necessary The constancy of the internal milieu is a necessary component for a free life.” – Claude Bernardcomponent for a free life.” – Claude Bernard

Animals have evolved from single cell organisms that live in Animals have evolved from single cell organisms that live in the ocean. In order to “carry” this environment with us (i.e. the ocean. In order to “carry” this environment with us (i.e. water and solutes), our body and its cells must regulate water and solutes), our body and its cells must regulate their fluid balancetheir fluid balance

This regulation is part of what is called This regulation is part of what is called homeostasishomeostasis – – process by which the body’s substances and characteristics process by which the body’s substances and characteristics (such as temp and glucose level) are maintained at their (such as temp and glucose level) are maintained at their optimal leveloptimal level

Mammals maintain homeostatic control of our body’s fluid Mammals maintain homeostatic control of our body’s fluid and energy through our and energy through our ingestive behavioringestive behavior – intake of food, – intake of food, water, and minerals such as sodiumwater, and minerals such as sodium

Physiological regulatory Physiological regulatory mechanismsmechanisms

A physiological regulatory mechanism is one that maintains the A physiological regulatory mechanism is one that maintains the constancy of some internal characteristic of the organism in the constancy of some internal characteristic of the organism in the face of external variabilityface of external variability

e.g. keeping body temp constant despite changes in ambient tempe.g. keeping body temp constant despite changes in ambient temp Four essential features:Four essential features:

System variableSystem variable – the characteristic to be regulated; e.g. temp – the characteristic to be regulated; e.g. temp Set pointSet point – the optimal value of the system variable; e.g. 78 – the optimal value of the system variable; e.g. 78°° DetectorDetector – monitors the value of the system variable; e.g. thermostat – monitors the value of the system variable; e.g. thermostat Correctional mechanismCorrectional mechanism – restores the system variable to its set point; – restores the system variable to its set point;

e.g. AC or heatere.g. AC or heater Negative feedbackNegative feedback – a process whereby the effect produced by an – a process whereby the effect produced by an

action serves to diminish or terminate that action; when AC or action serves to diminish or terminate that action; when AC or heater is turned on and successfully changes temp back to set heater is turned on and successfully changes temp back to set point, the detector senses this and turns AC/heater offpoint, the detector senses this and turns AC/heater off

Satiation mechanismSatiation mechanism – a brain mechanism that causes cessation – a brain mechanism that causes cessation of hunger or thirst, produced by adequate and available supplies of hunger or thirst, produced by adequate and available supplies of nutrients or waterof nutrients or water

DrinkingDrinking

Fluid balanceFluid balance Body contains 4 major fluid compartments:Body contains 4 major fluid compartments:

1 of intracellular fluid – 2/3 of body’s water1 of intracellular fluid – 2/3 of body’s water 3 of extracellular fluid (intravascular fluid – blood plasma, cerebrospinal 3 of extracellular fluid (intravascular fluid – blood plasma, cerebrospinal

fluid, interstitial fluid – b/t cells)fluid, interstitial fluid – b/t cells) Intracellular and intravascular fluid must be kept in tight regulationIntracellular and intravascular fluid must be kept in tight regulation Intracellular fluid controlled by conc. of solutes in interstitial fluid Intracellular fluid controlled by conc. of solutes in interstitial fluid

(normally isotonic, or same osmotic pressure, by process of (normally isotonic, or same osmotic pressure, by process of diffusion)diffusion)

If ISF loses water (If ISF loses water (hypertonichypertonic), water will be pulled out of the cells), water will be pulled out of the cells If ISF gains water (If ISF gains water (hypotonichypotonic), water will move into the cells), water will move into the cells

Blood plasma volume must be regulated in order to pump blood Blood plasma volume must be regulated in order to pump blood effectivelyeffectively

If blood volume too low (If blood volume too low (hypovolemiahypovolemia), lead to heart failure), lead to heart failure For 2 different types of regulation, need 2 types of receptors: one For 2 different types of regulation, need 2 types of receptors: one

measuring blood volume, the other measuring cell volumemeasuring blood volume, the other measuring cell volume

DrinkingDrinking

2 types of thirst2 types of thirst Most times, we ingest more water or solutes than needed; these Most times, we ingest more water or solutes than needed; these

are then excreted by kidneyare then excreted by kidney When levels of either water or solutes are too low, corrective When levels of either water or solutes are too low, corrective

mechanisms are activated: thirst, or salt appetite (rare for mechanisms are activated: thirst, or salt appetite (rare for modern humans)modern humans)

Our bodies lose water continuously, through sweating, breathing, Our bodies lose water continuously, through sweating, breathing, urination, defecation, and in some circumstances through urination, defecation, and in some circumstances through vomitingvomiting

Osmometric (osmotic) thirstOsmometric (osmotic) thirst – occurs when the tonicity (solute – occurs when the tonicity (solute conc.) of the ISF increases; e.g. when eat salty meal with no waterconc.) of the ISF increases; e.g. when eat salty meal with no water

OsmoreceptorsOsmoreceptors – neuron that detects changes in the solute conc. of – neuron that detects changes in the solute conc. of the ISF that surrounds itthe ISF that surrounds it

Hypertonicity of blood plasma (where salt is absorbed into) draws Hypertonicity of blood plasma (where salt is absorbed into) draws water from ISF, which then causes water to leave cells; when blood water from ISF, which then causes water to leave cells; when blood volume increases, the kidneys begin to excrete both water and solutes, volume increases, the kidneys begin to excrete both water and solutes, allowing the blood plasma volume to remain constantallowing the blood plasma volume to remain constant

DrinkingDrinking

2 types of thirst2 types of thirst Osmotic thirst (con’t)Osmotic thirst (con’t)

Osmoreceptors located in the anterior hypothalamus, one of the Osmoreceptors located in the anterior hypothalamus, one of the circumventricular organs (CVO’s)circumventricular organs (CVO’s)

OVLT (organum vasculosum of the lamina terminalis)OVLT (organum vasculosum of the lamina terminalis) – CVO located – CVO located on the blood side of the BBB, and thus substances dissolved in the on the blood side of the BBB, and thus substances dissolved in the blood are able to pass through to the ISF of this organblood are able to pass through to the ISF of this organ

Volumetric thirstVolumetric thirst Produced when blood plasma volume is lowProduced when blood plasma volume is low Leads to both thirst and salt appetiteLeads to both thirst and salt appetite 2 types of receptor systems: Renin-angiotensin system & atrial 2 types of receptor systems: Renin-angiotensin system & atrial

baroreceptorsbaroreceptors Renin-angiotensin systemRenin-angiotensin system – hypovolemia activates kidneys to release an – hypovolemia activates kidneys to release an

enzyme called enzyme called reninrenin, which then catalyzes the conversion of a blood , which then catalyzes the conversion of a blood protein called protein called angiotensinogenangiotensinogen into a hormone called into a hormone called angiotensinangiotensin (AngII) (AngII)

AngII stimulates secretion of hormones by posterior pituitary and adrenal AngII stimulates secretion of hormones by posterior pituitary and adrenal gland to conserve water and solutes, and stimulates drinking and salt gland to conserve water and solutes, and stimulates drinking and salt appetiteappetite

DrinkingDrinking

2 types of thirst2 types of thirst Volumetric thirst (con’t)Volumetric thirst (con’t)

Atrial baroreceptorsAtrial baroreceptors The atria of the heart contains stretch receptors that detect when blood The atria of the heart contains stretch receptors that detect when blood

volume is low, which then stimulates thirstvolume is low, which then stimulates thirst

Neural mechanisms of thirstNeural mechanisms of thirst Sensory info from atria is conferred to the Sensory info from atria is conferred to the nucleus of the nucleus of the

solitary tractsolitary tract (NTS) in the medulla (NTS) in the medulla AngII crosses weak BBB near CVO’s to provide thirst and salt AngII crosses weak BBB near CVO’s to provide thirst and salt

appetite signal (esp. via appetite signal (esp. via subfornical organ (SFO)subfornical organ (SFO))) Neurons in SFO project to MnPO (Neurons in SFO project to MnPO (median preoptic nucleusmedian preoptic nucleus))

EatingEating

Some facts about metabolismSome facts about metabolism Food ingestive behaviors are more complex than those of Food ingestive behaviors are more complex than those of

water balancewater balance We must obtain adequate amounts of carbohydrates, fats, We must obtain adequate amounts of carbohydrates, fats,

amino acids, vitamins, and minerals other than sodiumamino acids, vitamins, and minerals other than sodium Absorption, fasting, and the 2 nutrient reservoirsAbsorption, fasting, and the 2 nutrient reservoirs

Our bodies need food for “building blocks” (i.e. to construct Our bodies need food for “building blocks” (i.e. to construct and maintain our organs and muscles) and “fuel”and maintain our organs and muscles) and “fuel”

Fuel comes from food we have consumed that travels through Fuel comes from food we have consumed that travels through the digestive tract, but those nutrients must be able to be the digestive tract, but those nutrients must be able to be stored for when the gut is emptystored for when the gut is empty

2 types of reservoirs: short-term (carbs) and long-term (fats)2 types of reservoirs: short-term (carbs) and long-term (fats)

Nutrient reservoirsNutrient reservoirs

Short-termShort-term Located in cells of liver and musclesLocated in cells of liver and muscles Cells filled with complex, insoluble carb called Cells filled with complex, insoluble carb called glycogenglycogen Cells in the liver convert glucose (obtained from diet) into Cells in the liver convert glucose (obtained from diet) into

glycogen and store it; this storage is stimulated by the glycogen and store it; this storage is stimulated by the presence of presence of insulininsulin, a peptide hormone produced by the , a peptide hormone produced by the pancreaspancreas

When glucose enters the body, some is stored as glycogen and When glucose enters the body, some is stored as glycogen and some is used as fuelsome is used as fuel

When there are low levels of glucose in the blood, the When there are low levels of glucose in the blood, the pancreas begins to secrete pancreas begins to secrete glucagonglucagon, which stimulates the , which stimulates the conversion of glycogen back into glucoseconversion of glycogen back into glucose

This reservoir primarily serves to fuel the CNSThis reservoir primarily serves to fuel the CNS

Nutrient reservoirsNutrient reservoirs

Long-termLong-term Adipose tissue – filed with Adipose tissue – filed with triglyceridestriglycerides (complex molecules that (complex molecules that

contain contain glycerolglycerol, a soluble carb, combined with 3 , a soluble carb, combined with 3 fatty acidsfatty acids)) Found beneath the skin and in various locations in the abdominal Found beneath the skin and in various locations in the abdominal

cavitycavity Cells can expand in sizeCells can expand in size Reservoir for rest of body besides brainReservoir for rest of body besides brain What keeps us alive when we are fasting; when body starts to What keeps us alive when we are fasting; when body starts to

use carb reservoir, fat cells start converting triglycerides into fuel use carb reservoir, fat cells start converting triglycerides into fuel that cells can usethat cells can use

Fatty acids can be metabolized by cells in all of the body except Fatty acids can be metabolized by cells in all of the body except the brain; glycerol can be converted to glucose in the liver for use the brain; glycerol can be converted to glucose in the liver for use in the brainin the brain

So, why does brain get all the glucose?So, why does brain get all the glucose? Insulin must be present at a cell in order for it to take up glucose into Insulin must be present at a cell in order for it to take up glucose into

itit However, neurons and glia do However, neurons and glia do notnot require insulin to take up glucose require insulin to take up glucose

MetabolismMetabolism

Fasting phaseFasting phase – the phase of metabolism during which – the phase of metabolism during which nutrients are not available to from the digestive system; nutrients are not available to from the digestive system; glucose, amino acids, and fatty acids are derived from glucose, amino acids, and fatty acids are derived from glycogen, protein, and adipose tissue during this phaseglycogen, protein, and adipose tissue during this phase

Absorptive phaseAbsorptive phase – the phase of metabolism during which – the phase of metabolism during which nutrients are absorbed from the digestive system; glucose, nutrients are absorbed from the digestive system; glucose, and amino acids constitute the principle source of energy and amino acids constitute the principle source of energy for cells during this phase, and excess nutrients are storedfor cells during this phase, and excess nutrients are stored

What starts a meal?What starts a meal?

Social and environmental factorsSocial and environmental factors Often we eat out of habit or because of some stimuli present in Often we eat out of habit or because of some stimuli present in

our env’t (e.g. clock, smell food)our env’t (e.g. clock, smell food) Meal schedule very important: rarely adjust times of meals, but Meal schedule very important: rarely adjust times of meals, but

can adjust can adjust sizesize of meals of meals If we have eaten recently or if a previous meal was large, we If we have eaten recently or if a previous meal was large, we

tend to eat a smaller mealtend to eat a smaller meal However, due to other social factors, such as parental cues However, due to other social factors, such as parental cues

(“finish your plate”) or peer influence, satiety signals can be (“finish your plate”) or peer influence, satiety signals can be ignoredignored

DeCastro and DeCastro (1989) found that the amount of food DeCastro and DeCastro (1989) found that the amount of food eaten was directly proportional to the amount of other people eaten was directly proportional to the amount of other people who were present during a mealwho were present during a meal

What starts a meal?What starts a meal?

Physiological hunger signalsPhysiological hunger signals The amount of food that we eat is inversely related to the The amount of food that we eat is inversely related to the

amount of nutrients left over from previous mealsamount of nutrients left over from previous meals Fall in glucose level (hypoglycemia) is a potent stimulus for Fall in glucose level (hypoglycemia) is a potent stimulus for

hungerhunger GlucoprivationGlucoprivation – a dramatic fall in the level of glucose available to – a dramatic fall in the level of glucose available to

cellscells Hunger can also be caused by Hunger can also be caused by lipoprivationlipoprivation ( fall in level of ( fall in level of

fatty acids available to cells)fatty acids available to cells) 2 sets of detectors for these metabolic fuels: one set located in 2 sets of detectors for these metabolic fuels: one set located in

the brain (sensitive to glucoprivation) and the other in the liver the brain (sensitive to glucoprivation) and the other in the liver (sensitive to both glucoprivation and lipoprivation)(sensitive to both glucoprivation and lipoprivation)

What stops a meal?What stops a meal?

2 types of satiety signals: short-term info from 2 types of satiety signals: short-term info from gastrointestinal tract, long-term from adipose tissuegastrointestinal tract, long-term from adipose tissue

Head factorsHead factors Receptors located in in head (e.g. eyes, nose, tongue, and Receptors located in in head (e.g. eyes, nose, tongue, and

throat) provide info about appearance, odor, taste, texture, throat) provide info about appearance, odor, taste, texture, and temp of foodand temp of food

Most effects involve learning: taste and odor of foods can serve Most effects involve learning: taste and odor of foods can serve as stimuli that permit animals to learn about the caloric as stimuli that permit animals to learn about the caloric density of foods (e.g. sweet taste = glucose, fuel)density of foods (e.g. sweet taste = glucose, fuel)

Rats can learn to eat less of a food with a particular flavor Rats can learn to eat less of a food with a particular flavor when the eating of that food was paired with caloric infusionwhen the eating of that food was paired with caloric infusion

What stops a meal?What stops a meal? Gastric factorsGastric factors

Stomach not necessary for feelings of hunger?Stomach not necessary for feelings of hunger? Not completely true: when stomach is empty, a peptide called Not completely true: when stomach is empty, a peptide called

ghrelinghrelin is secreted which activates a hunger signal is secreted which activates a hunger signal The stomach also contains receptors that can detect the The stomach also contains receptors that can detect the

presence of nutrientspresence of nutrients Intestinal factorsIntestinal factors

Afferent axons from the Afferent axons from the duodenumduodenum (first portion of small (first portion of small intestine) are sensitive to the presence of glucose, amino intestine) are sensitive to the presence of glucose, amino acids, and fatty acidsacids, and fatty acids

Entry of food into the duodenum suppresses food intake in Entry of food into the duodenum suppresses food intake in rats; rats fitted with a rats; rats fitted with a gastric fistulagastric fistula ( a tube that drains ( a tube that drains contents out of the stomach) continue to consume food (this contents out of the stomach) continue to consume food (this method is called method is called sham feedingsham feeding))

The duodenum controls the normal rate of stomach empyting The duodenum controls the normal rate of stomach empyting by secreting a peptide called by secreting a peptide called cholecystokinin (CCK)cholecystokinin (CCK), which also , which also serves a a satiety signal in the brainserves a a satiety signal in the brain

What stops a meal?What stops a meal? Liver factorsLiver factors

The liver is the first organ to “learn” that food is being received The liver is the first organ to “learn” that food is being received by the intestines; when it does so it sends a satiety signal to by the intestines; when it does so it sends a satiety signal to the brainthe brain

Metabolic factors present in the bloodMetabolic factors present in the blood Insulin receptors in the brain may serve as a satiety signalInsulin receptors in the brain may serve as a satiety signal

Long-term satiety: signals from adipose tissueLong-term satiety: signals from adipose tissue Signals from the long-term nutrient reservoir may either Signals from the long-term nutrient reservoir may either

suppress hunger signals or augment short-term satiety signalssuppress hunger signals or augment short-term satiety signals Some variable related to body fat (as opposed to weight) may Some variable related to body fat (as opposed to weight) may

serve as the system variableserve as the system variable LeptinLeptin – hormone secreted by adipose tissue; decreases food – hormone secreted by adipose tissue; decreases food

intake and increases metabolic rateintake and increases metabolic rate Genetically obese mice (Genetically obese mice (ob mouseob mouse) cannot produce leptin, thus ) cannot produce leptin, thus

become grossly obesebecome grossly obese

Brain mechanismsBrain mechanisms

Brain stemBrain stem Ingestive behaviors are evolutionarily old; thus controlled by Ingestive behaviors are evolutionarily old; thus controlled by

“older” parts of the brain (mid- and hindbrain)“older” parts of the brain (mid- and hindbrain) DecerebrateDecerebrate animals (animals in which the brain stem has animals (animals in which the brain stem has

been severed from the forebrain) can still perform basic been severed from the forebrain) can still perform basic ingestive behaviors (e.g. chewing, swallowing) but not more ingestive behaviors (e.g. chewing, swallowing) but not more complex ingestive behaviors (e.g. foraging)complex ingestive behaviors (e.g. foraging)

HypothalamusHypothalamus Lateral hypothalamus (LH) lesions produce anorectic effects Lateral hypothalamus (LH) lesions produce anorectic effects

(stop eating)(stop eating) Ventromedial hypothalamus (VMH) lesions produce increase in Ventromedial hypothalamus (VMH) lesions produce increase in

food intake and severe weight gainfood intake and severe weight gain

Brain mechanismsBrain mechanisms

Hypothalamus (con’t)Hypothalamus (con’t) Role in hungerRole in hunger

Two populations of neurons in LH secrete hormones that stimulate Two populations of neurons in LH secrete hormones that stimulate hunger and increase metabolic rate: hunger and increase metabolic rate: melanin-concentrating melanin-concentrating hormone (MCH)hormone (MCH) and and orexinorexin

Arcuate nucleus of the hypothalamus: secretes a NT called Arcuate nucleus of the hypothalamus: secretes a NT called neuropeptide Y (NPY)neuropeptide Y (NPY) and a peptide called and a peptide called agouti-related peptide agouti-related peptide (AGRP)(AGRP); these both act on the MCH and orexin neurons of the LH to ; these both act on the MCH and orexin neurons of the LH to induce hungerinduce hunger

Also, ghrelin secreted from stomach induces hungerAlso, ghrelin secreted from stomach induces hunger Role in satietyRole in satiety

Arcuate nucleus also contains neurons that secrete both Arcuate nucleus also contains neurons that secrete both CARTCART and and α-MSHα-MSH, which serve to induce satiety, which serve to induce satiety

Eating disordersEating disorders

ObesityObesity Evolutionarily old bodies living in a modern environmentEvolutionarily old bodies living in a modern environment

i.e. our bodies still act accordingly to possible times of famine; but i.e. our bodies still act accordingly to possible times of famine; but in modern industrialized nation, this is obviously unnecessaryin modern industrialized nation, this is obviously unnecessary

Both genetic and environmental factorsBoth genetic and environmental factors Treatments include: pharmacotherapy, behavior therapy, Treatments include: pharmacotherapy, behavior therapy,

gastric surgery, combogastric surgery, combo Unfortunately very common in modern worldUnfortunately very common in modern world

Anorexia nervosa/bulimia nervosaAnorexia nervosa/bulimia nervosa Both exaggerated concern of body imageBoth exaggerated concern of body image AN is refusal to maintain above certain BMI by not eatingAN is refusal to maintain above certain BMI by not eating BN concerns cycles of binge eating and purging behaviors (e.g. BN concerns cycles of binge eating and purging behaviors (e.g.

vomiting, laxative use)vomiting, laxative use) Not as common as obesity, ~2% of populationNot as common as obesity, ~2% of population