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  1. 1. Motivation and Homeostasis The Role of the Hypothalamus
  2. 2. Motivation <ul><li>Motivation is the driving force in behavior </li></ul><ul><li>The stronger the motivation, the more likely the behavior </li></ul><ul><li>Control of behavior involves decisions among different motivated actions with different outcomes </li></ul><ul><li>Motivation for behaviors basic to survival is controlled by the hypothalamus </li></ul>
  3. 3. The Hypothalamus & Homeostasis <ul><li>Responds to departures from ideal in homeostasis </li></ul><ul><li>Humoral response </li></ul><ul><ul><li>hypothalamic neurons respond to inputs from sensors by increasing or decreasing the release of pituitary hormones into blood stream </li></ul></ul><ul><li>Visceromotor response </li></ul><ul><ul><li>hypothalamic neurons respond to inputs from sensors by adjusting the balance between the sympathetic and parasympathetic output of the ANS </li></ul></ul><ul><li>Somatic motor response </li></ul><ul><ul><li>hypothalamic neurons, especially in the lateral hypothalamus, respond to inputs from sensors by causing a somatic motor response </li></ul></ul><ul><li>Very often all three happen together or sequentially </li></ul>
  4. 4. Regulation of Feeding Behavior <ul><li>Energy balance regulates feeding </li></ul><ul><li>Hormonal and hypothalamic regulation of body fat and feeding </li></ul>
  5. 5. Feeding & Energy Balance <ul><li>Prandial state </li></ul><ul><ul><li>after eating a meal </li></ul></ul><ul><li>Postabsorptive state </li></ul><ul><ul><li>fasting times between meals </li></ul></ul><ul><li>Obesity occurs when there is consistently more intake than usage and adipose cells enlarge and increase in number </li></ul><ul><li>Starvation occurs when there is consistently less intake than usage and loss of fat tissue occurs </li></ul><ul><li>In virtually all mammals except humans and their pets, homeostatic mechanisms act to avoid both obesity and starvation </li></ul>
  6. 6. The Prandial State <ul><li>Occurs after eating a meal </li></ul><ul><li>Glucose, fatty acids, and ketones are produced for all cells; neurons only use glucose </li></ul><ul><li>Excess energy is stored, either as glycogen in the liver and skeletal muscles, or as triglycerides in adipose tissue = fat cells </li></ul><ul><li>Anabolism is the assembly of glycogen and triglycerides from what you eat </li></ul><ul><li>Because the storage capacity for glycogen is limited, excess energy is stored as fat </li></ul>
  7. 7. The Postabsorptive State <ul><li>Fasting times between meals </li></ul><ul><li>Energy is produced from glycogen and triglycerides </li></ul><ul><li>Catabolism is the process of breaking down glycogen and triglycerides and making glucose, fatty acids, and ketones </li></ul>
  8. 8. Hormonal Regulation of Feeding <ul><li>Lipostatic hypothesis </li></ul><ul><ul><li>The brain monitors the amount of body fat and acts to keep it constant </li></ul></ul><ul><ul><li>Basically true </li></ul></ul><ul><li>Adipocytes release leptin into blood </li></ul><ul><li>Neurons in the periventricular zone of hypothalamus detect a drop in leptin levels </li></ul><ul><li>Neurons in the lateral hypothalamus are then excited and drive feeding behavior </li></ul>
  9. 9. Elevated Leptin & Feeding <ul><li>From 'holiday feasting' for example </li></ul><ul><li>Leptin receptors on neurons in the arcuate nucleus are activated </li></ul><ul><li>These neurons use two peptide transmitters: </li></ul><ul><ul><li>aMSH and CART </li></ul></ul><ul><li>Response is three-part, triggered by nerve fibers leaving the arcuate nucleus </li></ul>
  10. 10. The Response <ul><li>Humoral response is through the paraventricular nucleus </li></ul><ul><ul><li>Leads to increased release of TSH and ACTH </li></ul></ul><ul><ul><li>both of which serve to raise the metabolic rate </li></ul></ul><ul><li>Visceromotor response is to activate the sympathetic division of ANS </li></ul><ul><ul><li>increases the metabolic rate, partly by raising body temperature. </li></ul></ul><ul><ul><li>occurs both directly because of activity of arcuate nucleus outputs and through the paraventricular nucleus outputs to the spinal cord </li></ul></ul><ul><li>Somatic motor response is to inhibit feeding behavior through the neurons of the lateral hypothalamic area </li></ul>
  11. 11. Lowered Leptin & Feeding <ul><li>Turns off arcuate nucleus neurons that use aMSH and CART as transmitters </li></ul><ul><li>Turns on other arcuate nucleus neurons that use NPY and AgRP as transmitters </li></ul><ul><li>These neurons have connections with the paraventricular nucleus and the lateral hypothalamic area </li></ul><ul><li>Inhibit secretion of TSH and ACTH </li></ul><ul><li>Activate the parasympathetic division of ANS </li></ul><ul><li>Stimulate feeding behavior </li></ul>
  12. 12. Key Neurotransmitters <ul><li>aMSH and CART </li></ul><ul><ul><li>depress feeding behavior </li></ul></ul><ul><ul><li>therefore called anorectic peptides </li></ul></ul><ul><li>NPY and AgRP </li></ul><ul><ul><li>stimulate feeding behavior </li></ul></ul><ul><ul><li>therefore called orexigenic peptides </li></ul></ul>
  13. 13. Role of the Lateral Hypothalamus <ul><li>Some neurons in the lateral hypothalamic area receive direct input from the arcuate nucleus neurons </li></ul><ul><ul><li>These use the peptide transmitter, MCH </li></ul></ul><ul><ul><li>MCH neurons innervate most of the cerebral cortex </li></ul></ul><ul><li>Other neurons in the lateral hypothalamic area also receive direct input from the arcuate nucleus neurons </li></ul><ul><ul><li>Use the peptide transmitter, orexin. </li></ul></ul><ul><ul><li>Orexin neurons also innervate most of the cerebral cortex </li></ul></ul><ul><li>So, when leptin levels fall, there is a major system to drive feeding behavior! </li></ul>
  14. 14. Regulation of Feeding Behavior <ul><li>Starting a meal: </li></ul><ul><li>Cephalic phase </li></ul><ul><ul><li>sight and smell of food activates the parasympathetic and enteric divisions of the ANS </li></ul></ul><ul><ul><li>saliva is secreted in the mouth and digestive juices, in the stomach </li></ul></ul><ul><li>Gastric phase </li></ul><ul><ul><li>the cephalic phase responses are intensified by chewing, swallowing, and filling the stomach with food </li></ul></ul><ul><li>Intestinal phase (or substrate phase) </li></ul><ul><ul><li>As the filling stomach begins to empty in to the intestines, satiety signals arise to stop feeding </li></ul></ul>
  15. 15. Satiety <ul><li>Ending a meal </li></ul><ul><li>Gastric distention </li></ul><ul><ul><li>mechanoreceptors in the stomach wall connect to the nucleus of the solitary tract via the vagus nerve and inhibit eating </li></ul></ul><ul><li>The gustatory nucleus is part of the nucleus of the solitary tract, which is why great food can override signals from a full stomach and lead to a totally bloated stomach </li></ul>
  16. 16. Cholecystokinin (CCK) <ul><li>Released from neurons in the enteric nervous system and other cells lining the intestines </li></ul><ul><li>Triggered in response to stimulation of intestines by fatty foods </li></ul><ul><li>Acts on sensory receptors connected to the vagus nerve and signals to stop eating </li></ul>
  17. 17. Insulin <ul><li>Pancreatic cells produce insulin necessary to transport glucose into all cells except neurons </li></ul><ul><li>Directly affects neurons in arcuate nucleus and ventromedial nucleus of the hypothalamus </li></ul><ul><ul><li>similar to leptin </li></ul></ul><ul><li>Late in the eating stage, rising insulin is a satiety signal </li></ul>
  18. 18. The Role of Dopamine <ul><li>Liking food and wanting food are mediated by two different systems </li></ul><ul><li>Wanting food is driven by the mesocorticolimbic dopamine system </li></ul><ul><li>Animals with lesions in this system still have the hedonic experience (like food), but seem unmotivated to want it </li></ul><ul><li>Cravings that lead to addiction involve the same dopamine system </li></ul>
  19. 19. Drinking <ul><li>Water, not alcohol </li></ul><ul><li>Two different drives for drinking </li></ul><ul><ul><li>use two different mechanisms </li></ul></ul><ul><li>Decrease in blood volume drives drinking </li></ul><ul><li>Increase in the concentration of solutes in the blood, blood osmolarity, drives drinking </li></ul>
  20. 20. Volumetric Thirst <ul><li>Decreased blood volume drives magnocellular neurons in the hypothalamus </li></ul><ul><ul><li>release ADH </li></ul></ul><ul><ul><li>leads to concentrated urine </li></ul></ul><ul><ul><li>humoral response </li></ul></ul><ul><li>Decreased blood volume stimulates the sympathetic division of the ANS </li></ul><ul><ul><li>helps to correct low blood pressure </li></ul></ul><ul><ul><li>visceromotor response </li></ul></ul><ul><li>Decreased blood volume stimulates neurons in the lateral hypothalamus </li></ul><ul><ul><li>drives seeking and consuming water </li></ul></ul><ul><ul><li>somatic motor response </li></ul></ul>
  21. 21. Osmometric Thirst <ul><li>Hypertonicity of the blood is sensed by vascular organ of the lamina terminalis (OVLT) </li></ul><ul><ul><li>outside the blood brain barrier just like the subfornical organ </li></ul></ul><ul><li>OVLT cells when excited, directly drive magnocellular neurons to release ADH </li></ul><ul><ul><li>humoral response </li></ul></ul><ul><li>Hypertonicity drives the OVLT to stimulate the motivation to drink water through the lateral hypothalamic area </li></ul><ul><ul><li>somatic motor response </li></ul></ul>
  22. 22. Diabetes Insipidus <ul><li>One type of diabetes includes failure to release ADH </li></ul><ul><li>Caused by loss of magnocellular neurosecretory neurons in hypothalamus, or viral infection of hypothalamus </li></ul><ul><li>ADH is not secreted, urine is very dilute </li></ul><ul><li>Osmometric thirst drives patients to drink vast amounts of water </li></ul><ul><li>Vast amounts of urine are passed </li></ul><ul><li>Patients become sorely sleep deprived - dangerous </li></ul><ul><li>Synthetic ADH can be administered nasally </li></ul>
  23. 23. Temperature Regulation <ul><li>In addition to the somatosensory thermal receptors, there are lots of neurons in the brain sensitive to temperature. </li></ul><ul><li>Most important cluster is in the anterior hypothalamus </li></ul><ul><ul><li>monitors blood temperature </li></ul></ul><ul><li>Humoral and visceromotor responses are then initiated by the medial preoptic area of the hypothalamus </li></ul><ul><li>Somatic motor responses are initiated by the lateral area of the hypothalamus </li></ul>
  24. 24. Decreasing Temperature <ul><li>A fall in temperature in the blood leads to: </li></ul><ul><li>Humoral response </li></ul><ul><ul><li>TSH is released by the anterior pituitary causing: </li></ul></ul><ul><ul><li>thyroxin release by the thyroid gland </li></ul></ul><ul><ul><li>increases cellular metabolism and produces heat </li></ul></ul><ul><li>Visceromotor response </li></ul><ul><ul><li>constricted blood vessels near the skin surface and piloerection (goose bumps) </li></ul></ul><ul><li>Somatic motor response </li></ul><ul><ul><li>involuntary shivering </li></ul></ul><ul><ul><li>voluntary action to seek warmth </li></ul></ul>
  25. 25. Increasing Temperature <ul><li>A rise in temperature in the blood leads to: </li></ul><ul><li>Reduction of TSH release </li></ul><ul><ul><li>humoral response </li></ul></ul><ul><li>Blood is shunted toward surface capillary beds </li></ul><ul><ul><li>visceromotor response </li></ul></ul><ul><li>Animal seeks shade </li></ul><ul><ul><li>voluntary somatic motor response </li></ul></ul><ul><li>Animal pants or humans sweat </li></ul><ul><ul><li>involuntary somatic motor response </li></ul></ul>