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Can anyone summarize the mechanism by which when an object of a given temperature is placed in contact with, say, the skin on a human fingertip, the average speed of the particles of the object is converted into nerve signals to the brain?

If you can answer that, how about the format of how the temperature is encoded in the nerve signals?

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up vote 30 down vote accepted

In the periphery (e.g. on our fingertips), our body senses external temperature through nerve terminals, expressing certain TRP channels. These are ion channels that are sensitive to temperature (note that TRP channels can be sensitive to several things, such as pH, light, and stretch) and allow entrance of cations in the cell when the temperature is higher or lower than a certain threshold.

Six TRP channels have been described as been involved in sensing different temperature ranges1,2:

  • TRPV1 is activated at >43 °C
  • TRPV2 at >52 °C
  • TRPV3 at ~> 34-38 °C
  • TRPV4 at ~> 27-35 °C
  • TRPM8 at ~< 25-28 °C
  • TRPA1 at ~< 17 °C

Not surprisingly, TRPV1 and TRPV2 are also involved in nociception (=pain perception).

The exact molecular mechanisms by which different temperatures open different TRP channels are unclear, although some biophysical models have been proposed.3

I am not sure of the exact "format" in which sensory fibers encode different temperature, but I would assume that the neuron would fire faster the more the temperature is distant from the specific threshold (hot or cold).

1 Thermosensation and pain. - J Neurobiol. 2004 Oct;61(1):3-12.
2 Sensing hot and cold with TRP channels. - Int J Hyperthermia. 2011;27(4):388-98.
3 - Thermal gating of TRP ion channels: food for thought? - Sci STKE. 2006 Mar 14;2006(326):pe12.

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How are objects of temperatures in the middle of the 18-24 °C sensed? Is it expected that further channels will be found? I really enjoyed your answer - it's particularly interesting to me to have discovered, as a by-product of following your links, how the burning effect of chilli peppers is as a result of TRPV1. – Rory M Dec 28 '11 at 23:12
@Rory: 18-24 is covered by TRPM8 – nico Dec 29 '11 at 11:00
Also, about the chillies: that is a very nice example of a "false sensation". Chillies don't really burn you, but as capsaicin is detected by the same receptors as high temperatures your brain cannot distinguish between the two. – nico Dec 29 '11 at 11:03

To complement on nico's answer (I don't have enough rep to comment), TRP channels seems to be also sensitive to increment or decrement of temperature as reported recently in a paper by Gallio et al1.

Probably that channels have incomplete adaptative behavior that make them sensitive both to increment and absolute temperature values.

[1] Gallio M, Ofstad TA, Macpherson LJ, Wang JW, Zuker CS (2011). The coding of temperature in the Drosophila brain. Cell 144: 614-624.

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