In previous chapters we examined the physiology of sensory receptors responsive to mechanical, visual and chemical stimuli, and we saw that in some cases our knowledge of the transduction processes and related cellular mechanisms is quite extensive. In this chapter we look briefly at some further senses about whose mechanisms we have rather less information.

Thermoreceptors

Sensations of warm and cold are important to ourselves and other animals in rinding a suitable environment in which to live. Conditions are suboptimal if this is too cold or too hot, and extremes of temperature are fatal. The temperature of particular objects may be important, as in the detection of warm-blooded prey by snakes or fleas, or the control of the incubation mound by the mallee fowl.

Temperature-sensitive sensory nerve fibres with endings in mammalian skin are of two types, cold fibres and warm fibres, as is shown in fig. 17.1. Both groups have a low level of activity at the body heat level of 377deg;C. Cold fibres discharge increasingly as the skin temperature falls below this, reaching a maximum in the region of 27°C; sometimes they also discharge at very high temperatures, above 50°C. Warm fibres discharge increasingly at higher temperatures, with a maximum at about 46°C and a very rapid fall-off above this (Hensel & Zotterman, 1951; Hensel & Kenshallo, 1969; Pierau & Wurster, 1981).

These fibres also show marked responses to change in temperature. Cold fibres increase their discharge rate for a few seconds on rapid cooling and decrease it on rapid warming.