Thermal afferents in the control of body temperature

Abstract

In most mammalian species internal body temperature is maintained within narrow limits despite large variations in ambient temperature. This phenomenon is called homeothermy and is usually considered to be the result of mechanisms controlling body temperature itselL Alternative models have suggested body heat content (Houdas et al., 1978) or the rate of heat flow (Webb, 1970; Webb et al., 1978) as controlled variables, rendering the relative stability of internal temperature a side effect of heat content or heat flow regulation, respectively. However, available evidence does not lend much support to these suggestions (for detailed discussion, see Cabanac, 1975; Hensel, 1981) but favors the view that body temperature is the controlled variable of the thermoregulatory system. According to this concept the temperature of the body is transduced by sensory elements into afferent temperature signals which are compared to some sort of reference. Any difference between temperature signals and reference creates a central drive initiating an appropriate response of heat producing or heat dissipating mechanisms which tends to limit that difference (Hammel, 1968). The neurally mediated control system of body temperature is complemented by local feedback loops: e.g. the rates of skin blood flow and sweating do not depend solely on the central drive but are subject additionally to local responses to temperature. This leads to the question of what temperature or temperatures in the body represent the regulated variable. Under a variety of thermal loads, a highly stable core temperature contrasts with variable surface or even shell temperatures (Aschoff and Wever, 1958): skin temperature in unclothed man (Kitzing et al., 1972) and, to a lesser extent, in animals (Hallwachs, 1960) is essentially dependent on air temperature. This is often thought to indicate that body core temperature is the regulated variable. However, the most stable temperature in a temperature controlled system is not necessarily the controlled variable (Mitchell et al., 1972). An alternative concept rests on the view that the temperature of any body region which contains thermosensitive structures generating afferent temperature signals, is part of the regulated variable. Body temperature as the regulated variable is thought to be represented by an integrated signal the fractions of which reflect the distribution of thermosensitive structures throughout the body and the relationship between temperature and signal generation at any site. The relative stability of core temperature in standard conditions of thermal stress is then merely a consequence of the fact that a large share of all signal sources resides in the body core but does not preclude skin temperature from becoming, in a particular set of circumstances, a significant component of the regulated variable (WaRes, 1962; Dmi'el and Robertshaw, 1983). A central drive to increase heat production or heat loss limits, but does not eliminate, the difference between the values of the regulated variable and the reference. This is to say, any central drive originates from and is proportional to the difference between the two. As a consequence of proportional control, a constant body temperature under a sustained thermal load requires a constant difference between the regulated variable and the reference. This generally accepted concept (Hammel, 1968; Hardy, 1965; Werner, 1980) is