Question

Akao

Answer 1

Temperature regulation is an important homeostatic activity which is primarily controlled by the hypothalamus. If we consider the dangerous effects of temperature extremes on the body, a center designed for regulating this variable is of obvious importance. Electrical stimulation of the anterior hypothalamus, particularly the supraoptic area, triggers a thermolytic response, That is, those activities which cause the body temperature to drop are set into operation. Conversely, stimulation of the posterior hypothalamus, particularly the tuberal area, triggers a thermogenic response, reflected both in increased heat conservation and production. Thermolytic responses include cutaneous vasodilation in order to increase heat loss by radiation, sweating to increase heat loss by evaporation, and panting in animals like the dog. Thermogenic responses include cutaneous vasoconstriction to prevent heat loss by radiation, shivering to produce heat by increased muscular activity, cessation of sweating to reduce heat loss by evaporation, and an increase in the production and release of thyroxine in order to increase the metabolic rate. Thermoreceptors in the hypothalamus are sensitive to very small changes in the temperature of circulating blood. Because blood temperature varies closely with changes in core temperature, the hypothalamus is continually kept informed of changes in the overall temperature of the body. Subsequently it can activate appropriate thermolytic or thermogenic activities in order to restore body temperature to normal. The hypothalamus also receives input from cutaneous thermoreceptors which keep it informed of changes in the environmental temperature. Consequently the hypothalamus is continually informed of both external and internal temperature changes and is well equipped through neural activation of appropriate effectors to prevent temperature fluctuations by regulating body temperature within very narrow limits.

The hypothalamus is well equipped to respond to changes in the total amount of body water. A poorly localized area of the hypothalamus called the "thirst center" is stimulated by a dry mouth as well as body dehydration, Projections from the thirst center to the thalamus and then to the conscious cortex inform us of the need for water. This triggers the sensation of thirst and initiates the conscious desire for water. The hypothalamus also takes subconscious steps to correct dehydration.

Osmoreceptors in the supraoptic nuclei respond to dehydration (typically associated with increased osmolality in the circulating blood) by increasing the production and release of antidiuretic hormone (ADH). This hormone is produced in the supraoptic nucleus (SON) and transported via the axons of the hypothalamohypophyseal tract to the posterior pituitary lobe for temporary storage and ultimate release into the circulation. Once released, ADH promotes an increase in total body water by facilitating water reabsorption in the kidneys so that more is returned to the blood and less is lost in the urine. ADH operates by increasing the water permeability of the distal tubules and collecting ducts of the nephrons. This causes water to be osmotically reabsorbed from the less osmotic glomerular filtrate to the more osmotic extracellular fluid of the kidney medulla and renal blood supply.