ch 16

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University of Toronto Scarborough
Janelle Leboutillier

Chapter 16 Notes  Homeostasis: the process of maintaining the internal environment of the body in a narrow physiological range.  The hypothalamus plays a key role in the regulation of body temperature, fluid balance, and energy balance  Hypothalamus regulation of homeostasis involves three components: 1. Humoral response: neurons respond to sensory signals by stimulating or inhibiting the release of pituitary hormones 2. Visceromotor response: neurons respond to sensory signals by adjusting the balance of sympathetic and parasympathetic outputs of the ANS 3. Somatic motor response: neurons (particularly in the lateral hypothalamus) respond to sensory signals by inciting an appropriate somatic motor behaviour response  Motivated behaviours to correct a situation (such as shivering when cold) are incited by the lateral hypothalamus.  The condition in which blood is filled with nutrients, such as after a meal, is called the prandial state.  The energy is stored as either glycogen (liver and skeletal muscle) or triglycerides (adipose/fat tissue)  The fasting phase between meals is called the postabsorptive phase, in which the glycogen and triglycerides are broken down. Glucose is used by all cells, but triglycerides are not used by neurons.  If the intake of energy exceeds the usage, the amount of body fat, or adiposity, increases, resulting in obesity. If vice versa, starvation occurs.  Neurons found in the periventricular zone (from ch. 15) seem to act as monitors.  Neurons that incite feeding behaviour are found in the lateral hypothalamus (i.e. feeding is a motivated behaviour)  The idea that the brain monitors the amount of body fat and acts to defend this energy store against perturbations is called the lipostatic hypothesis.  An example of this is a forcefully fed rat when given opportunity to eat freely will cut down on caloric intake to reach optimal adiposity. Vice versa occurs for starved rats.  The ob gene found in mice was hypothesized to tell the body that fat reserves are normal. The ob gene encoded the protein leptin. It regulates body mass by acting directly on neurons of the hypothalamus that decrease appetite and increase energy expenditure.  Mice with the ob/ob defective gene are fooled into thinking stores are low, hence they overeat  Tested this using parabiosis, the union of two mice physiologically and anatomically  Lesions on both sides of the hypothalamus can have large effects on feeding behaviour and adiposity  Bilateral lesion of the lateral hypothalamus caused anorexia. Anorexia in humans caused by damage to the same area is called lateral hypothalamus syndrome. Anorexia = lateral hypothalamus syndrome  Bilateral lesions of the ventromedial hypothalamus caused animals to overeat and become obese. Overeating and obesity in humans caused by damage to the same area is called ventromedial hypothalamic syndrome.  In the Case of Elevated Leptin Levels: Leptin molecules are released into circulation by adipocyte cells. They activate leptin receptors of the arcuate nucleus of the hypothalamus.  Alpha MSH and CART neurotransmitter molecules are found in the arcuate nucleus (activated by high leptin levels)  Read pg 515-516 (the effects of elevated.... section)  Basically, the high leptin activate the Alpha MSH and CART neurons in the arcuate nucleus. These then activate the paraventricular nucleus that then causes the release of ACTH and TSH hormones. Alpha MSH and CART also inhibit the lateral hypothalamic area, inhibiting feeding.  ACTH and TSH+ Paraventricular nucleus+MSH and CART-Lateral hypothalamic area (inhibits feeding)  The injection of alpha MSH and CART into the brain mimics the response to high leptin levels. Thus they are called anorectic peptides – they diminish appetite.  In the Case of Decreased Leptin Levels  Alpha MSH and CART responses are turned off  Decreased levels stimulate release of NPY (Neuropeptide Y) and AgRP, both found in the arcuate nucleus  Basically, a fall in leptin levels stimulat
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