Physiology And Pathophysiology Of Responses To Heat And Cold

Body temperature is regulated by a feedback mechanism that matches net heat loss from the body to the rate of heat production by metabolism. This feedback mechanism is integrated by the hypothalamus, particularly the preoptic anterior area. Although the control of thermo-regulatory processes such as shivering and sweating has been attributed to various other regions of the hypothalamus, discrete localization no longer seems valid. In fact, some thermoregulation occurs even in the absence of brain centers above the medulla. Nevertheless, the most sensitive regulation occurs in the hypothalamus and may be ascribed to an "integrative center'' that may or may not have an anatomic correlate. The sensors for the feedback regulation of body heat content or temperature, called thermoreceptors, are located both in the periphery and in the central nervous system. The preoptic anterior hypothalamus contains temperature-sensitive neurons that increase firing with increasing core temperature. Other neurons respond to cold by increasing their rate of firing. In addition, the spinal cord and the abdominal organs have both heat and cold receptors. The skin also has numerous heat- and cold-responding receptors that convey information about the ambient temperature to the central nervous system.

Because the body core temperature must be maintained within critical limits, it seems that the central ther-moreceptors are the primary input to the feedback regulation of body temperature, but the peripheral receptors also appear to play an important role, particularly by allowing the system to anticipate conditions that might compromise maintenance of the body core temperature. For example, just a gust of cold air may cause one to have goose bumps and even shiver before any change in the body core temperature has occurred. Experiments show that heating the preoptic hypothala-mic area results in immediate cutaneous vasodilation and sweating; however, the critical hypothalamic temperature at which sweating begins is lower when the skin temperature is higher. Cooling the preoptic hypothalamus results in shivering and cutaneous vasoconstriction, but the temperature threshold for shivering is found to be higher when the skin temperature is lower. In other words, when the body core temperature and the environmental temperature change in the same direction, the physiologic response is enhanced. This dual regulatory system has the advantage of allowing the body to respond more rapidly to changing environmental conditions and with less variation in the body core temperature. For example, when the skin is cold, shivering and cutaneous vasoconstriction begin to occur with very little fall in the core temperature, and, when the skin is warm, vasodilation and sweating occur with little rise in the core temperature. On the other hand, when one is exercising in a cold environment, body core temperature may be elevated by the increased heat production, but sweating is reduced because of the cold skin temperatures.

As shown in Fig. 4, temperature information from the preoptic-anterior hypothalamic area and skin ther-moreceptors is conveyed to a hypothalamic integrative center. It appears that the integrated temperature afferent input is compared with a desired temperature, called the set-point, in much the same way that room temperature is compared with the temperature setting of a furnace thermostat. When the integrated temperature that is sensed falls below the temperature set-point, heat loss is reduced by vasoconstriction and inhibition of sweating, and heat production is increased by shivering. When the temperature that is sensed rises above the set-point, heat loss is increased by cutaneous vasodilation and sweating, and shivering is inhibited. Heat production may also be reduced by behavioral responses to decrease the level of motor activity, but obviously cannot be reduced below the basal metabolic rate. Based on the difference between the temperature sensed and the set-point, the integrative center coordinates the efferent responses involving cutaneous vasomotor activity, sweating, shivering, and chemical thermogenesis, as well as behavioral responses coordinated at higher levels.

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