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Lecture 5

Week 5 - Sept 26-Oct1 - Chapter 5 - PSYCH 3M03

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Aadil Merali Juma

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LECTURE 5 PSYCH 3M03 Chapter 5: Thirst, Hunger and Appetite September 26, 2013 1. Thirst and Drinking  A major motivator, which rises to the top of priorities when an individual is deprived of water  A clear example of “homeostasis”  Homeostatic drive; work to restore equilibrium  Two physiological processes o Extracellular – water (1/3 of total) in blood, CSF, other body cavities  Quickly lost (bleed, menstruation, perspiration) o Cellular – water 2/3 of total) inside cells  Extracellular Thirst  Hypovolemic Thirst  Blood volume decreases  Blood pressure increases  Triggered by blood loss, perspiration, diarrhea, heavy menstrual bleeding  Required replacement of sodium and waterπ  Baroreceptors in kidneys o Renin  Angiotensin  Aldosterone  Conserve Na+ o Renin – peptide hormone in the kidneys sets off extracellular thirst o Angiotensin – peptide produced in the blood in response to renin  Causes vasoconstriction, release of vasopressin (anti-diuretic hormone) in posterior pituitary and may stimulate thirst via actions in brain o Aldosterone – produced in adrenal cortices in response to angiotensin; signals for the conservation of Na+ in kidneys  Cellular Thirst o Osmotic, osmometric or Sodium Thirst o Induced by excess salt consumption or severe thirst o Increased extracellular Na+ o Osmotic force draws H2O from cells  Salt and water bind together – high affinity  salt draws water out of cells rd o Osmoreceptors around hypothalamus – regions by 3 ventricle  Concentrated saline applied around 3 ventricle induces drinking  Distilled water applied around 3 ventricle causes drinking cessation o Body has not adapted mechanisms for superabundance of sodium; we don’t expel sodium as quickly as we intake it  Superabundance of salt in our food – more than ever in evolutionary history o Subfornical Organ – outside of the BBB; contains osmoreceptors; responds to angiotensin II  Neurons in subfornical region project into the nucleus medianus in the hypothalamus (POA) which is involved in thirst  Prandial Drinking o Occurs with food intake in many species o There is an anticipated need fo2 H O and electrolyte balance  When meal with sodium is eaten, homeostasis will be affected; takes time for food to gte through digestive tracts and upset homeostasis at the cellular level  Animals tend to drink the right amount of water at a meal to balance sodium intake before sodium has affected homeostasis  Cessation of Drinking o Occurs before fluids are rebalanced  Tendency to stop drinking before homeostasis has been reached  Species-specific mechanisms (rats vs. dogs)  Satiety derives from receptors in mouth, esophagus, and stomach, as well as the swallowing reflex  Postulated that it turns off physical thirst, but doesn’t weigh body down with water 2. Hunger and Satiety  Obesity vs. Starvation o Famine more common than abundance in evolutionary history o Humans are adapted to environments where starvation can be imminent  Evolved to shortages in food, not overabudnace (as shown by obesity o Famine  migration, aggression, unusual foods 1 LECTURE 5 PSYCH 3M03  Feeding and Fasting – 2 phases in pancreas with hormones acting on each that directly affects drive states o Feeding (Absorptive) Phase  Insulin from pancreas  Energy stored  Parasympathetic o Fasting Phase  Glucagon from pancreas  Energy released from stores  Sympathetic  Stomach Distension o Stomach pangs with subjective reports of strong hunger  Ghrelin – peptide resulting from empty stomach; signals to hypothalamus  Secreted by epithelial cells lining empty stomach and from pancreas  Rises in blood during fasting  Signals hunger and stimulates feeding  Increases fat mass o Full stomach associated with less appetite o Walter Canon – described sympathetic NS and investigated the mechanism of hunger  Student swallowed a balloon; filled it with air or released air to observe stomach and hunger  Lipostatic Factors o Hypothetical homeostatic mechanism o Food deprived animals eat to compensate o Overfed animals reduce subsequent feeding o Long term regulation, not short term o “Lipo-sensors” eluded science for decades, but note recent findings concerning leptin (below) o Set point in weight (obviously not fixed) – individuals strive to reach homeostasis (theory)  Exp/ Rat – decrease body weight t 80% below normal by underfeeding it for weeks, then put in environment where they can eat freely  they will eat until they reach normal body weight  Exp/ Rat – force feed then to obesity; then put in free feeding environment  will under eat for weeks until back to normal weight  Caused scientists to seek “lipo receptors” o Refer to Leptin – hormone that regulates homeostasis (for weight)  Glucostatic Factors o Glucose = “currency” of energy throughout the body and brain o Decreased glucose in cells  hunger  Diabetes  Type I – paradoxically; high blood sugar levels, ravenously hungry  Insulin brings glucose into the cells o Intracellular glucose is important (insulin) o Hunger/feeding correlate with blood sugar drop o Glucose receptors  Glucose receptors have not been found in the brain – controversy because brain cells respond to glucose  Liver receptors are critical  Glucose in hepatic portal vein influences appetite  Liver information reaches hypothalamus via the vagus nerve (wandering nerve; bilateral; ascending)  Vagus nerve relays information from many organs in the thoracic to the hypothalamus  Manipulate blood sugar levels in hepatic portal vein (blood toward liver) – blood sugar level correlates with hunger o Short term regulation o Hook up rats to glucose monitors – found that when blood sugar dropped, animals would seek food seeking behaviour  Brain physiology o Lesion of the Ventromedial Hypothalamus  overeating o Lesion the Lateral Hypothalamus  failure to eat o These are not “centers” for feeding and satiety o VHM Lesions and Obesity – “finicky” eating behaviour; VMH animals often compared to overweight humans; obese humans will eat more if there is less
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