TEMPERATURE

Mapping experiments show that there are discrete cold-sensitive and heat-sensitive spots in the skin. There are four to ten times as many cold-sensitive as heat-sensitive spots. Cold receptors respond from 10 °C to 38 °C and heat receptors from 30 °C to over 45 °C. The afferents for cold are Aδ and C fibers, whereas the afferents for heat are C fibers. Temperature has generally been regarded as closely related to touch, but new evidence indicates that in addition to ending in the postcentral gyrus, thermal fibers from the thalamus end in the ipsilateral insular cortex. It has even been suggested that this is the true primary thermal receiving area.

Three receptors involved in temperature perception have been cloned. The receptor for moderate cold is the cold- and menthol-sensitive receptor 1 (CMR 1). Two receptors respond to high, potentially noxious heat: VR1, which also responds to the pain-producing chemical capsaicin and is clearly a nociceptor; and VRL-1, a closely related receptor that does not respond to capsaicin but is probably a nociceptor as well. All three are members of the TRP family of cation channels . The receptor that responds to moderate heat (warmth receptor) could be the ATP P2X receptor because injection of ATP causes a feeling of warmth, and mice in which the P2X receptor gene has been knocked out do not show the activity in the spinal cord normally produced by mild skin warming.

Because the sense organs are located subepithelially, it is the temperature of the subcutaneous tissues that determines the responses. Cool metal objects feel colder than wooden objects of the same temperature because the metal conducts heat away from the skin more rapidly, cooling the subcutaneous tissues to a greater degree.