Thermoreceptors are specialized neurons designed to be sensitive to changes in temperature. These cells generally detect temperature variations within the normal range, while neurons known as nociceptors detect temperatures that could be dangerous to the body. Many organisms rely on these neurons for a variety of things, from alerting them to the fact that they are stepping into a cold river to assisting with the regulation of internal body temperature.
In the skin, thermoreceptors provide the brain with information about environmental temperature. This can be important, as it will alert the body to unusually cool or warm temperatures that might require an action, such as putting on a coat or shedding a layer of clothing to become more comfortable. These neurons are also used to provide more general information about a body's environment, such as that one area of a room is cooler than other spots.
Inside the body, these cells are part of the body's complex and interconnected series of systems that are designed to keep the body in balance. Neurons in the hypothalamus are sensitive to changes in core temperature that could require a response from the body, and there are others inside some body organs, such as the bladder. When these cells fire, they alert the brain to an imbalance in internal temperature that needs to be addressed; for example, cells sensitive to heat in the eyes alert the tear glands to produce more fluid to keep the eyes lubricated.
Some studies have shown that thermoreceptors that are sensitive to cold may play a role in taste. Many people have noticed that the same food can taste very different when it is eaten cold than when it is eaten warm or hot, and these neurons may be involved in this process. There is certainly a high concentration of cold cells around the mouth and tongue, indicating that the mouth has an increased sensitivity to cold temperatures.
If a temperature crosses the line into being dangerous, it will be picked up by nociceptors, neurons that are sensitive to pain. These neurons fire to alert the body to the fact that it is freezing or burning, and that a prompt response could be critical. For example, when someone grabs a hot pan barehanded, nociceptors fire to indicate that the body is burning, and to inform the brain that the cells of the hand have been damaged by the heat, so that the brain can fire a signal in response to get the hand to drop the pan.