Multiple thermoregulatory effectors with independent central controls

This review first considers how mammalian body temperature regulation evolved, and how the brain's responses to thermoregulatory challenges are likely to be organised differently from the way an engineer would design them. This is because thermoregulatory effector mechanisms would have evolved one at a time, with each being superimposed on pre-existing mechanisms. There may be no functional need for the final ensemble of control loops to be coordinated by neural cross-connections: appropriate thermal thresholds would solve the problem sufficiently. Investigations first into thermoregulatory behaviours and later into unconscious thermoregulatory mechanisms (autonomic and shivering) have led investigators to the realisation that multiple control loops exist in the brain, with each effector system apparently regulated by its own central temperature sensors. This theme is developed with reference to data on four temperature-regulated neural outflows that have been studied on anaesthetized rats under standard conditions in the authors' laboratory. Direct comparisons were made between the behaviour of sympathetic nerves supplying the tail vasculature, vessels in the proximal hairy skin, interscapular brown adipose tissue (BAT) and fusimotor fibres to hind limb muscle. All four outflows were activated by cooling the skin, and all were silenced by neuronal inhibition in the medullary raphé. Their thermal thresholds were quite different, however, as were their relative responsiveness to core temperature. This was ranked as: tail > back skin > BAT > fusimotor. These and other data indicate that the four thermoeffector outflows are driven by separate neural pathways, each regulated by independent brain temperature sensors.