2
$\begingroup$

I just read this question on Physics SE about how the body detects cold or warm surfaces. As someone who already understands how heat flux devices like thin film gauges and thermocouples work (by exploiting the known relationship between temperature gradients and electrical/thermal properties of the relevant materials), I was actually more interested in how the nerve endings in my body actually implement this.

When I touch something warm or cold, what is the nervous mechanism that causes the correct signal to be sent to my brain, causing me to perceive a hot or cold surface?

How is the "thermocouple", which tells me whether a surface is hotter or colder than my finger, implemented?

Improve this question
$\endgroup$
1
2
$\begingroup$

Sensory neurons in the skin express TRP channels such as TRPV1 channels, TRPM8, and others (see Voets et al 2004 for more information on how these channels work). Different sensory neurons express different combinations of TRP channels .

Changes in temperature shift the probability of whether these channels are open or closed. Opening or closing of ion channels changes the membrane potential of the sensory neuron, which changes the rate of action potentials which informs the rest of the nervous system about the temperature.

The TRP channels can have other ligands, as well, such as capsaicin or menthol. The activity of these ligands at temperature-sensitive receptors is why we perceive chilis or mustard to taste "hot" and menthol to feel "cold".

More generally, the nervous system typically responds to changes better than constant stimuli through adaptation. Therefore, if your hand is fairly cold and you touch something warm or hot, you will perceive a hotter stimulus than if your hand started out already warm.

There isn't really a thermocouple or temperature difference detector in the skin, just these two mechanisms: channels whose open probability is a function of temperature, and sensory adaptation which heightens perceptual sensitivity to changes in temperature.

Voets, T., Droogmans, G., Wissenbach, U., Janssens, A., Flockerzi, V., & Nilius, B. (2004). The principle of temperature-dependent gating in cold-and heat-sensitive TRP channels. Nature, 430(7001), 748-754.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Is it then correct to say that the proteins responsible for detecting temperature (the TRP channels) respond to absolute temperatures, but the nervous system itself causes us only to perceive differentials? $\endgroup$ – quant Dec 31 '20 at 1:06
  • 1
    $\begingroup$ @quant That would be a reasonable birds-eye summary, though it's not precisely true that only differentials are perceived (they are just emphasized), there are ways that the sensitivity of the channels themselves can be altered by endogenous agonists and antagonists such that absolute temperature isn't the only thing they respond to, and the channel gating itself is complex, non-linear, and voltage-dependent (the Voets reference would be a good place to start). Even seemingly simple things in biology are much more complex the more regulation you start to consider. $\endgroup$ – Bryan Krause♦ Dec 31 '20 at 1:37
  • $\begingroup$ I also found it interesting that TRP channels play a role in spices being perceived as hot or cold. I had always thought this perception was cultural/linguistic. $\endgroup$ – quant Dec 31 '20 at 3:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.