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Skin thermoreceptors detect absolute temperature and temperature changes through thermosensitive ion channels of the TRP (Transient Receptor Potential) family. TRPV1 channels are activated above 43°C (also by capsaicin), TRPV2 above 52°C, TRPM8 below 28°C (also by menthol). Each channel has a fixed temperature threshold given by the activation energy of the conformational transition.

The heat flux q = -k·T/ is the physical quantity detected, not the absolute temperature. This explains why metal at room temperature is perceived as colder than wood even though both are at the same temperature: metal has greater conductivity k, transferring more heat per unit of time from the skin to the object. What is felt is the rate of heat loss.

Thermal diffusion in the skin follows T/t = ·²T/x², with _skin 9×10 m²/s. The penetration depth in 1 second is _T 2(t) 0.6 mm, where the cutaneous thermoreceptors are (at 0.21 mm depth). This means that thermoreceptors respond with a delay of t (/2)²/ 0.11 s, which coincides with the reaction time when touching a hot object.

The thermoreceptor adaptation r(t) = r·exp(-t/) + r_ explains why a hot bath is perceived as progressively less hot: the firing rate drops exponentially from the initial value r (high response to change) towards a steady state r_ proportional to the absolute temperature. The system is more sensitive to changes than to constant temperatures.

The temperature change detection threshold is surprisingly low: humans detect T 0.02°C under optimal conditions (slow change over large area). This requires thermoreceptors to amplify small variations in heat flux into significant changes in membrane potential, an internal gain of 50100 imp/s·°C. Snakes such as the python detect warm-blooded prey at T 0.003°C with their loreal organ.

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