E24 Temperature and Thermal Flow

Topic

The cutaneous thermoreceptors of the perceiver detect both an absolute temperature and a change in that temperature via thermosensitive ion channels belonging to the same family; consequently, each of these ion channels is activated only above or only below a fixed temperature threshold specific to that channel—a fixed threshold ultimately determined by the activation energy of its own internal conformational transition. When the temperature of the perceiver's tissue crosses the fixed threshold of a specific ion channel, that particular channel—and not the others—is activated within the tissue.

Heat flux, rather than the absolute temperature with which this model began, is the physical quantity these cutaneous thermoreceptors actually detect. This explains why metal at room temperature feels colder than wood at that same temperature: metal has much higher thermal conductivity than wood and, consequently, transfers more heat per unit of time from the perceiver's tissue to the metal itself. What is actually felt when touching either object is the rate of heat loss from the tissue to the object, not the object's absolute temperature. If a touched object has higher thermal conductivity, the heat flux from the perceiver's tissue to the object is greater, causing the object to feel colder than one with lower thermal conductivity at the same temperature. Meanwhile, thermal diffusion within the perceiver's skin follows the same diffusion equation established by another model in this set: just one second after contact with an object, the thermal signal's penetration depth reaches the precise level within the skin where the cutaneous thermoreceptors—introduced at the start of this model—are located. Consequently, this implies that the cutaneous thermoreceptors respond to contact with a specific delay—a delay that remarkably matches the actual reaction time required for the perceiver to withdraw their hand upon touching a hot object. While a hotter object does not in itself alter the response delay of the cutaneous thermoreceptors, it does determine the time that elapses before the perceiver reacts to the detected heat.

In contrast, cutaneous thermoreceptor adaptation explains why a hot bath feels progressively less hot over time: the receptor's firing rate drops exponentially from a high initial value—characteristic of the response to a recent temperature change—to a much lower steady state, which is proportional to the bath's constant absolute temperature. Consequently, the thermoreceptor system is far more sensitive to recent temperature changes than to a constant temperature, following the same differential threshold logic established by another model in this set regarding biological sensors. As the bath temperature remains constant, the cutaneous thermoreceptor's firing rate continues to decline toward that lower steady state. Consequently, the specific heat flux and adaptation characteristics present at the onset of this model result in a surprisingly low temperature-change detection threshold for this cutaneous thermoreceptor system; this requires the thermoreceptors to amplify even a slight variation in heat flux into a significant shift in their own membrane potential, achieved through substantial internal gain within each receptor. A snake such as the python detects warm-blooded prey using its loreal organ with a detection threshold far lower than that of humans, thanks to the even greater internal gain within that specialized organ. Thus, the higher internal gain of a given species' thermoreceptor organ results in an even lower temperature-change detection threshold for that species.

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