How Cold Is It? Cool, Cold or Painfully Cold?

[Axon36]

By the way....those signals looks to me as "Spikes"...how far I know an "Action Potential" only can be visualize by intracellular records...sorry this is may be too much of a "cranky" scrutiny on so beautiful video!.

[drskinnerve]

... actually we always call them propagated action potentials... since they sweep from the terminal along the axon and we catch them with an electrode at some distant point... so I though Spike is just a very colloquial word for Action Potential...

[Axon36]

Ok..no problem.

[Axon36]

Other point agaings the specific modality...how can be possible than a putative nociceptor can discharge with two so different thresholds on two so different modalities on the same circuit on to the spinal cord (2do order integrators on the SG)?.

[drskinnerve]

Two different thresholds... The recording for the nociceptor takes out just one sample of a heterogenous population where thresholds ranged between ~28 and 13°C. The thresholds for the specific cold receptors ranged between 37°C and 18°C.

Putatively polymodal nociceptors give the pain signal to the brain, but I think some specific cold receptors with low temperature threshold may probably as well signal painfully cold.

[Axon36]

I'm talking about mechanical vs termperature threshold on polimodal units...and how they're integrated on the same circuit on the SG.

[drskinnerve]

I've also observed that cold sensitive CM fibers often have low mechanical thresholds. I also wonder whats the reason for that.

[Axon36]

Well...I may have a couple of pieces of this puzzle at spinal cord levels. See you in Washington, Kat!

[Axon36]

Hey Kat...another nice video. However...we know that the normal skin temperature in mammals is 32.1 Celcuis. How do you explain that "paradoxical" response and the ongoing activity at 35 C. More important, the C-LTMR's (cold nociceptors) had a mechanical threshold of less than 0.07 mN (a very low one)?.

[drskinnerve]

Hey, the ongoing activity is only present in very few of the specific cold receptors in the mouseskin, maybe 10%. Given that sepcific cold receptors represent only a small amount amongst all afferent nerve endings - which are silent at this temperature- I think the major signal the specific cold receptor gives to the brain is of differential nature,i.e. a very strong increase in firing rate when the temperature suddenly drops and an abrupt off-response when the temperature turns back to warm...

[Axon36]

OK...but why at 35 C?. Assuming than that temperature is correct there is something really odd about a cold sensory neuron reacting to a drop on 2 degrees from a point where it cannot be active don't you think?. Without even mention that that signal looks like it have dynamic components to the step.

[drskinnerve]

I think for example in the tongue a threshold of ~36°C makes a lot of sense... however in the skin the large majority has a threshold below skin temperature (this is just one in ten that has ongoing activity at skin temp. of 32°C). Compare to the guinea pig cornea you see almost all specific cold receptors be active at around this temoerature.

[drskinnerve]

The mechanical threshold of the cold nociceptor is 1.4 mN, so indeed quite low...

[Axon36]

Really too high..but I'm not surprised!. The temperature usually has a really funny effect over the mechanical sensibility and the skin compliance.