Primary afferent input from cutaneous thermoreceptors.

Many receptors of the skin and mucous membranes in several animal species are thermosensitive. To distinguish among these receptors, specific thermoreceptors have been defined as being insensitive to mechanical stimulation. Specific cold receptors often have a bimodal temperature sensitivity with maximal firing occurring between 25 and 30 C and above 45 C. Specific warm receptors are monomodal with maximal discharge rates occurring between 40 and 50 C. In addition, cold receptors demonstrate burst patterns of activity that may also encode thermal information at temperatures below 30 C. Both warm and cold receptors are markedly sensitive to temperature transients. The sensitivity to warm and cold transients differs not only in direction and intensity of temperature change, but also in the temperature of maximal sensitivity. Cold receptors have specialized terminals and are innervated by A delta fibers whereas warm receptors have free nerve endings and innervated by C fibers. The relationships of these structures to function are unknown.