Temperature receptors in the body core and skin transmit information about their temperatures through afferent nerves to the brainstem and, especially, the hypothalamus, where much of the integration of temperature information occurs. The sensitivity of the thermoregulatory system to core temperature enables it to adjust heat production and heat loss to resist disturbances in core temperature. Sensitivity to mean skin temperature lets the system respond appropriately to mild heat or cold exposure with little change in body core temperature, so that changes in body heat as a result of changes in environmental temperature take place almost entirely in the peripheral tissues (see Fig. 29.2). For example, the skin temperature of someone who enters a hot environment may rise and elicit sweating even if there is no change in core temperature. On the other hand, an increase in heat production within the body, as during exercise, elicits the appropriate heat-dissipating responses through a rise in core temperature.
Core temperature receptors involved in controlling thermoregulatory responses are unevenly distributed and are concentrated in the hypothalamus. In experimental mammals, temperature changes of only a few tenths of 1°C in the anterior preoptic area of the hypothalamus elicit changes in the thermoregulatory effector responses, and this area contains many neurons that increase their firing rate in response to either warming or cooling. Thermal receptors have been reported elsewhere in the core of laboratory animals, including the heart, pulmonary vessels, and spinal cord, but the thermoregulatory role of core thermal receptors outside the CNS is unknown.
Consider what happens when some disturbance—say, an increase in metabolic heat production resulting from exercise—upsets the thermal balance. Additional heat is stored in the body, and core temperature rises. The central thermoregulatory controller receives information about these changes from the thermal receptors and elicits appropriate heat-dissipating responses. Core temperature continues to rise, and these responses continue to increase until they are sufficient to dissipate heat as fast as it is being produced, restoring heat balance and preventing further increases in body temperatures. In the language of control theory, the rise in core temperature that elicits heat-dissipating responses sufficient to reestablish thermal balance during exercise is an example of a load error. A load error is characteristic of any proportional control system that is resisting the effect of some imposed disturbance or "load."
Although the disturbance in this example is exercise, the same principle applies if the disturbance is a decrease in metabolic rate or a change in the environment. However, if the disturbance is in the environment, most of the temperature change will be in the skin and shell rather than in the core,- if the disturbance produces a net loss of heat, the body will restore heat balance by decreasing heat loss and increasing heat production.
Relation of Controlling Signal to Thermal Integration and Set Point. Both sweating and skin blood flow depend on core and skin temperatures in the same way, and changes in the threshold for sweating are accompanied by similar changes in the threshold for vasodilation. We may, therefore, think of the central thermoregulatory controller as generating one thermal command signal for the control of both sweating and skin blood flow (Fig. 29.10). This signal is based on the information about core and skin temperatures that the controller receives and on the thermoregula-tory set point—the target level of core temperature, or the setting of the body's "thermostat." In the operation of the thermoregulatory system, it is a reference point that determines the thresholds of all of the thermoregulatory responses. Shivering and thermal comfort are affected by changes in the set point in the same way as sweating and
Other deep temperatures
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.