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Central chemosensitive mechanisms related to feeding


, : Central chemosensitive mechanisms related to feeding. Handb Physiol 1(6): 249-263

Energy balance in the body is maintained through adjustments of 4 variables, food intake, work, heat production, and stored energy. In maintaining a proper equilibrium, food intake has to be matched with the other 3 variables. It is essential, therefore,that the central nervous mechanisms that regulate food intake should obtain precise information about the other 3 adjustments. Ordinarily, the energy stored in the fat depots of the body does not change appreciably over long periods, and body temperature is maintained at normal levels by adjusting the heat loss to heat production. In such situations food intake (energy intake) is adjusted mainly to that metabolic activity of the body (energy utilization) which results in work and heat production. Information about both energy intake and energy utilization is provided to the central nervous mechanisms related to feeding, through various suggestive channels. During the intake of meals, signals go up to the hypothalamic centers through the afferent nerves coming from the gastrointestinal tract. Passage of food through the oropharyngeal region and the distention (fullness) of the stomach are metered. Possible chemosensitive mechanisms in the intestinal wall provide information about digestion and absorption there. Shifts of water into the alimentary tract may further supplement this information. Energy intake then suppresses further feeding in response to chemical changes produced. Increased glucose utilization in the body activates the nervous mechanisms bringing about satiety. Increased heat production in response to SDA [specific dynamic action] and changes in the amino acid and fatty acid contents of the blood also somehow trigger the satiety mechanism, although direct evidence in support of these is lacking. When these chemical changes gradually disappear, the animal is hungry again. Thus these chemosensitive mechanisms bring about between-meal regulations. Central mechanisms related to physical activity must be integrated with central mechanisms related to feeding, because energy output in the form of work is adjusted to the amount of food intake, and because extra work is done when a hungry animal searches for food. There may be other mechanisms operating on a long-term basis to keep the body weight constant over long periods. These mechanisms come into operation whenever stored energy tends to change, and they possibly linked with the fat in the depots. When body temperature tends to change, food intake has to be appropriately adjusted so that the body temperature is kept normal. This may create situations in which demands for increased energy intake may be sacrificed for prevention of hyperthermia; for example, physical exercise in a hot environment may not increase food intake to the same extent as such exercise in a cold environment. This again calls for integration of the central mechanisms regulating body temperature and those regulating food intake. Finally, it has been stressed that in spite of the requirements of food intake in response to energy expenditure, feeding is very closely correlated with water exchanges in the body. This shows integration between the central mechanisms related respectively to feed-ding and drinking. It thus appears that the central mechanisms related to feeding are influenced not only by the sensory afferents from the alimentary canal and by chemical and thermal changes produced in the body as a result of eating, but also by many other external and internal environmental changes, and thus integrate with other regulating mechanisms in maintaining the homeostasis of the body.

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