Thermoreceptors

A thermoreceptor is a sensory receptor that responds to temperature, primarily within the innocuous range. In the mammalian peripheral nervous system warm receptors are thought to be unmyelinated C-fibres (slow conduction velocity), while those responding to cold have thinly myelinated Aδ axons (faster conduction velocity).

A special form of thermoreceptor is found in some snakes, the viper pit organ and this specialised structure is sensitive to energy in the infrared part of the spectrum.

Location
In mammals, temperature receptors innervate various tissues including the skin, cornea and bladder. Neurons from the pre-optic and hypothalamic regions of the brain that respond to small changes in temperature have also been described, providing information on core temperature. The hypothalamus is involved in thermoregulation, the thermoreceptors allowing feed-forward responses to a predicted change in core body temperature in response to changing environmental conditions.

Structure
Thermoreceptors have been classically described as having 'free' non-specialised endings; the mechanism of activation in response to temperature changes is not completely understood. However, it is likely that proteins of the Transient Receptor Potential (TRP) family as well as some specific potassium channels (two-pore domain) are involved.

Function
Cold-sensitive thermoreceoptors give rise to the sensations of cooling, cold and freshness. In the cornea cold receptors are thought to respond with an increase in firing rate to cooling produced by evaporation of lacrimal fluid 'tears' and thereby to elicit a reflex blink.

Warm and cold receptors play a part in sensing innocuous environmetal temperature. Temperatures likely to damage an organism are sensed by sub-categories of nociceptors that may respond to noxious cold, noxious heat or more than one noxious stimulus modality (i.e they are polymodal).