by Dominika Dabrowski
The body keeps its core temperature constant at about 37 C by physiological adjustments controlled by the hypothalamus (Thermostat Center) where there are neurons sensitive to changes in skin and blood temperatures. The temperature-regulating centers are found in the Preoptic Area (the anterior portion of the hypothalamus). This area receives input from temperature receptors in the skin and mucous membranes (Peripheral Thermoreceptors) and from internal structures (Central Thermoreceptors), which include the hypothalamus itself. The temperature sensory signals from the from the preoptic area and those form the periphery are combined in the posterior hypothalamus to control the heat producing and conserving reactions of the body. The hypothalamic thermostat works in conjunction with other hypothalamic, autonomic and higher nervous thermoregulatory centers to keep the core temperature constant. Some of these thermoregulatory responses are involuntary, mediated by the autonomic nervous system, some are neurohormonal and others are semi-voluntary or voluntary behavioral responses.
RESPONSES TO COLD: Standing outside in underwear in a January snow storm drops your skin temperature quickly. This stimulates skin cold receptors (increase in their activity) and cools the blood flowing into the skin. These signals are received by both the hypothalamic thermostat and higher cortical centers. The thermostat is also activated by the change in blood temperature. It initiates responses that promote heat gain and inhibits centers that promote heat loss. The activation of Sympathetic Centers results in several responses including 1) Norepinephrine release from sympathetic fibers constricts skin vessels. 2) Brown fat (found in infants and some animals) oxidation increases causing thermogenesis. 3) Piloerection, occurs which traps air close to skin. 4) Epinephrine secretion from adrenal medulla increases thermogenesis. A Shivering Center in the hypothalamus is also activated which activates the Brainstem Motor Centers to initiate involuntary contraction of skeletal muscles causing shivering, which generates heat. Cold also activates some compensatory behavioral responses including huddling, voluntary physical activity (hand rubbing, pacing), sheltering next to a heat source and wearing warm clothing. Voluntary or semivoluntary behaviors in response to cold are activated by the higher brain centers, mainly the cortex and limbic system. When the environmental temperature decreases gradually (ex. summer to fall), the hypothalamus releases Thyrotropin Releasing Hormone which activates the anterior pituitary gland to release Thyroid Stimulating Hormone (TSH). TSH induces the thyroid gland to liberate large amounts of thyroid hormone (T3 and T4) into the blood. Thyroid hormone increases metabolic rate, which increases the amount of body heat production. As the body gets warmer, the hypothalamic sensors detect the warmth and diminish the heat producing and heat loss prevention responses.
RESPONSES TO HEAT: When the body is exposed to heat (sun, fire, too much clothing), body temperature rises. Skin warmth receptors and blood convey these changes to the hypothalamic thermostat. The thermostat inhibits the adrenergic activity of the sympathetic nervous system, which control vasoconstriction and metabolic rate, thus causing cutaneous vasodilation and reducing BMR. This causes an increase in heat loss via the skin and a decrease in heat production in the core. If the heat is sufficiently intense, the cholinergic sympathetic fibers, which innervate sweat glands release ACh, stimulating sweat. Sweating is the most effective involuntary heat fighting response in man. Behavioral responses to heat, such as lethargy, resting or lying down with limbs spread out, decreases heat production and increases heat loss. Wearing loose and light clothing, fanning and drinking cold drinks also helps with heat loss.
REFERENCES: Guyton, A.C. 1996. Body Temperature, Temperature Regualation and Fever, 911-922. Textbook of Medical Physiology, W.B. Saunders Company, Philadelphia.
Kapit, W. Et al. 1987. The Physiology Coloring Book, 101, 134. Harper Collins Publishing, Philadelphia