Regulation of Body Temperature-Role of the Hypothalamus
Figure 73–6 shows what happens to the body “core” temperature of a nude person after a few hours’ expo-sure to dry air ranging from 30° to 160°F. The precise dimensions of this curve depend on the wind move-ment of the air, the amount of moisture in the air, and even the nature of the surroundings. In general, a nude person in dry air between 55° and 130°F is capable of maintaining a normal body core temperature some-where between 97° and 100°F.
The temperature of the body is regulated almost entirely by nervous feedback mechanisms, and almost all these operate throughtemperature-regulatingcenters located in the hypothalamus. For these feed-back mechanisms to operate, there must also be temperature detectors to determine when the body temperature becomes either too high or too low.
Role of the Anterior Hypothalamic-Preoptic Area in Thermostatic Detection of Temperature
Experiments have been performed in which minute areas in the brain of an animal have been either heated or cooled by use of athermode. This small, needle-like device is heated by electrical means or by passing hot water through it, or it is cooled by cold water. The principal areas in the brain where heat or cold from a thermode affects body temperature control are the preoptic and anterior hypothalamic nuclei of the hypothalamus.
Using the thermode, the anterior hypothalamic-preoptic area has been found to contain large numbers of heat-sensitive neurons as well as about one third as many cold-sensitive neurons. These neurons are believed to function as temperature sensors for con-trolling body temperature. The heat-sensitive neurons increase their firing rate 2- to 10-fold in response to a 10°C increase in body temperature. The cold-sensitive neurons, by contrast, increase their firing rate when the body temperature falls.
When the preoptic area is heated, the skin all over the body immediately breaks out in a profuse sweat, while the skin blood vessels over the entire body become greatly dilated. This is an immediate reaction to cause the body to lose heat, thereby helping to return the body temperature toward the normal level. In addition, any excess body heat production is inhib-ited. Therefore, it is clear that the hypothalamic-preoptic area has the capability to serve as a ther-mostatic body temperature control center.
Detection of Temperature by Receptors in the Skin and Deep Body Tissues
Although the signals generated by the temperature receptors of the hypothalamus are extremely power-ful in controlling body temperature, receptors in other parts of the body play additional roles in temperature regulation. This is especially true of temperature receptors in the skin and in a few specific deep tissues of the body.
It will be recalled from the discussion of sensory receptors that the skin is endowed with both cold and warmth receptors. There are far more cold receptors than warmth receptors—in fact, 10 times as many in many parts of the skin. Therefore, peripheral detection of temperature mainly concerns detecting cool and cold instead of warm temperatures.
When the skin is chilled over the entire body, imme-diate reflex effects are invoked and begin to increase the temperature of the body in several ways: (1) by providing a strong stimulus to cause shivering, with a resultant increase in the rate of body heat production; (2) by inhibiting the process of sweating, if this is already occurring; and (3) by promoting skin vaso-constriction to diminish loss of body heat from the skin.
Deep body temperature receptors are found mainly in the spinal cord, in the abdominal viscera, and in or around the great veins in the upper abdomen and thorax. These deep receptors function differently from the skin receptors because they are exposed to the body core temperature rather than the body surface temperature. Yet, like the skin temperature receptors, they detect mainly cold rather than warmth. It is prob-able that both the skin and the deep body receptors are concerned with preventing hypothermia—that is, preventing low body temperature.
Posterior Hypothalamus Integrates the Central and Peripheral Temperature Sensory Signals
Even though many temperature sensory signals arise in peripheral receptors, these signals contribute to body temperature control mainly through the hypo-thalamus. The area of the hypothalamus that they stimulate is located bilaterally in the posterior hypo-thalamus approximately at the level of the mammil-lary bodies. The temperature sensory signals from the anterior hypothalamic-preoptic area are also trans-mitted into this posterior hypothalamic area. Here the signals from the preoptic area and the signals from elsewhere in the body are combined and integrated to control the heat-producing and heat-conserving reac-tions of the body.
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