Any specialized free nerve ending (see receptor) that is sensitive to temperature. Mammals have several types of thermoreceptors in the skin and tongue, each specific for detecting changes within a certain temperature range. Some respond when the skin is warmed (i.e. above body temperature) and others when the skin is cooled (i.e. below body temperature). Curiously, the same receptors also respond to chemicals that are perceived as ‘hot’ or ‘cool’ when tasted, such as cayenne pepper or menthol. In many cases the response of the thermoreceptor involves the opening of TRP protein ion channels in the cell’s plasma membrane, which triggers the influx of calcium ions and the initiation of a signal to the brain via sensory neurons. Thermoreceptors are also located at various sites inside the body, including the large blood vessels, viscera, abdominal wall, spinal cord, and brain. Information from both external and internal thermoreceptors is conveyed to the body’s ‘thermostat’ in the hypothalamus, where it is integrated to enable the appropriate responses.