Physiological Thermoregulation Operates Through Graded Control of Heat Production and Heat Loss Responses

Familiar inanimate control systems, such as most refrigerators and heating and air-conditioning systems, operate at only two levels: on and off. In a steam heating system, for example, when the indoor temperature falls below the desired level, the thermostat turns on the burner under the boiler, when the temperature is restored to the desired level, the thermostat turns the burner off. Rather than operating at only two levels, most physiological control systems produce a graded response according to the size of the disturbance in the regulated variable. In many instances, changes in the controlled variables are proportional to displacements of the regulated variable from some threshold value, such control systems are called proportional control systems.

The control of heat-dissipating responses is an example of a proportional control system. Figure 29.9 shows how reflex control of two heat-dissipating responses, sweating and skin blood flow, depends on body core temperature and mean skin temperature. Each response has a core temperature threshold—a temperature at which the response starts to increase—and this threshold depends on mean skin tem

Core Temperature Sweat Rate

Control of heat-dissipating responses. These graphs show the relations of back (scapular) sweat rate (left) and forearm blood flow (right) to core temperature and mean skin temperatures (sk). In these experiments, core temperature was increased by exercise. (Left: Based on data from Sawka

MN, Gonzalez RR, Drolet LL, et al. Heat exchange during upper-and lower-body exercise. J Appl Physiol 1984,57:1050-1054. Right: Modified from Wenger CB, Roberts MF, Stolwijk JAJ, et al. Forearm blood flow during body temperature transients produced by leg exercise. J Appl Physiol 1975,38:58-63.)

Control of heat-dissipating responses. These graphs show the relations of back (scapular) sweat rate (left) and forearm blood flow (right) to core temperature and mean skin temperatures (sk). In these experiments, core temperature was increased by exercise. (Left: Based on data from Sawka

MN, Gonzalez RR, Drolet LL, et al. Heat exchange during upper-and lower-body exercise. J Appl Physiol 1984,57:1050-1054. Right: Modified from Wenger CB, Roberts MF, Stolwijk JAJ, et al. Forearm blood flow during body temperature transients produced by leg exercise. J Appl Physiol 1975,38:58-63.)

perature. At any given skin temperature, the change in each response is proportional to the change in core temperature, and increasing the skin temperature lowers the threshold level of core temperature and increases the response at any given core temperature. In humans, a change of 1°C in core temperature elicits about 9 times as great a thermoregula-tory response as a 1°C change in mean skin temperature. (Besides its effect on the reflex signals, skin temperature has a local effect that modifies the response of the blood vessels and sweat glands to the reflex signal, discussed later.)

Cold stress elicits increases in metabolic heat production through shivering and nonshivering thermogenesis. Shivering is a rhythmic oscillating tremor of skeletal muscles. The primary motor center for shivering lies in the dorsomedial part of the posterior hypothalamus and is normally inhibited by signals of warmth from the preoptic area of the hypothalamus. In the cold, these inhibitory signals are withdrawn, and the primary motor center for shivering sends impulses down the brainstem and lateral columns of the spinal cord to anterior motor neurons. Although these impulses are not rhythmic, they increase muscle tone, thereby increasing metabolic rate somewhat. Once the tone exceeds a critical level, the contraction of one group of muscle fibers stretches the muscle spindles in other fiber groups in series with it, eliciting contractions from those groups of fibers via the stretch reflex, and so on, thus, the rhythmic oscillations that characterize frank shivering begin.

Shivering occurs in bursts, and the "shivering pathway" is inhibited by signals from the cerebral cortex, so that voluntary muscular activity and attention can suppress shivering. Since the limbs are part of the shell in the cold, trunk and neck muscles are preferentially recruited for shivering—the centralization of shivering—to help retain the heat produced during shivering within the body core, and the familiar experience of teeth chattering is one of the earliest signs of shivering. As with heat-dissipating responses, the control of shivering depends on both core and skin temperatures, but the details of its control are not precisely understood.

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