Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Cutaneous, Deep, and Visceral Sensation
Temperature Sensation

Two Types of Temperature receptors are known: those that respond maximally to temperatures slightly above body temperature, and those that respond maximally to temperatures slightly below body temperature. The former are referred to as warm receptors, and the latter as cold receptors. However, the adequate stimuli for both types of receptors are two distinct thermal levels, given that cold is not a form of energy.

Mapping experiments indicate that there are discrete cold- and warm-sensitive spots on the Skin. Cold spots outnumber warm spots by four to ten times. Temperature receptors are free nerve endings that respond to absolute temperature rather than to a temperature gradient across the skin. Cold receptors respond to temperatures ranging from 10 to 38°C, whereas warm receptors respond to temperatures from 30 to 45°C. The afferent fibers for cold are Aδ and C fibers, while warm afferents are C fibers (see Table 2-1). These afferent fibers transmit information to the postcentral gyrus via the lateral spinothalamic tract and thalamic radiation.

Since the Organs of temperature sensitivity are localized subepithelially, their stimulation is determined by the temperature of the subcutaneous Tissues. Cold metallic objects feel colder than wooden objects of the same temperature because metal conducts heat away from the skin more rapidly, causing greater cooling of the subcutaneous tissue. There is no adaptation at skin temperatures below 20°C and above 40°C; however, adaptation does occur between 20 and 40°C, such that the sensation elicited by a temperature change gradually fades. Above 45°C, tissue damage begins, and the temperature sensation transitions into pain. The sensation of warmth in the 43–50°C range is likely mediated by capsaicin receptors (see below).