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In the second half of the BBC radio (and podcast) program Cats and Itch; Discovery, The Curious Cases of Rutherford and Fry Episode 2 of 5 the phenomenon and origin of the "itch" and related sensations are discussed, with input from several specialists, including "Neuroscientist Prof Francis McGlone from Liverpool John Moores University and dermatologist Dr Brian Kim from the Center for the Study of Itch at Washington University. Yes, that is a real place."

Here is an unofficial transcription of a small segment starting at about 17:30:

Until quite recently, the same nerve fibers were thought to transport feelings of both pain and itch, from the skin to the brain. In 1997, a ground-breaking discovery revealed that itch has its own separate nerves responsible for that oh-so-familliar itchy-scratchy sensation. Now, if you accidentally lean on your “hot”, the sensation will travel through your fast pain fibers at 80 miles per hour; you can almost-instantly pull your hand away.

By contrast though, itchy signals crawl towards your brain at a sluggish 2 miles per hour, so once you’ve been bitten, it takes a while for the itching to begin.

Those quoted speeds equate to about 130 m/s and only 3.2 m/s, respectively.

What causes the "itch" fibers to transmit signals to the brain so slowly? Is there thought to be some evolutionary benefit to such slow conduction, considering that the speed of most signals from skin to brain are transmitted so much faster?

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    $\begingroup$ @uhoh - I am flattered! Great question btw +1 $\endgroup$
    – AliceD ♦
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    $\begingroup$ @AliceD I've orphaned your comment through clean-up, but indeed this site has great moderation; a key ingredient for a successful SE site. $\endgroup$
    – uhoh
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    $\begingroup$ Cleaning up is great. Thanks for your words. Mods often get virtually yelled at. Some appreciation to balance the scales is warmly welcomed. $\endgroup$
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What causes the "itch" fibers to transmit signals to the brain so slowly? Is there thought to be some evolutionary benefit to such slow conduction, considering that the speed of most signals from skin to brain are transmitted so much faster?

The interview most likely refers to the findings of Schmelz et al., J. Neuroscience, 1997: Specific C-receptors for itch in human skin

In essence they found tested the sensitivity of unmyelinated nerve fibers (note: myelination also increases transmission speed) towards distinct stimuli. They found that some of those fibers would selectively respond to stimuli which would be perceived as itching (such as histamines). They find that:

This slow conduction velocities attributable to small axon diameters ...

Note that since they don't need to, they also don't speculate on the physiological reason, why itching should be processed by slower fibers (and some potential explanations will be recognized quite well by readers of their article).

Please also note that the original claim, from 1997, however has been softened with the years, and there appears no strict separation been stimuli like "hot", and itching, in terms of distinct fibers and transmission speeds; e.g.: Davidson et Giesler, Trends in Neuroscience, 2011: The multiple pathways for itch and their interaction with pain

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  • $\begingroup$ Great! Thanks for the references, I'll give them a read. $\endgroup$
    – uhoh
    Commented Nov 21, 2017 at 2:17
  • $\begingroup$ OK I've looked at these references. While I can't quite say I've read them thoroughly because I don't have the background to take it all in, I've learned more about the diversity of nerve fibers, and also that in contrast to what I'd thought before, some nerve fibers do not require a myelin sheath. Thanks for your help! $\endgroup$
    – uhoh
    Commented Nov 27, 2017 at 2:22
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    $\begingroup$ From a biologist background, one consideration: a distinction between itching and other sorts of pain would be that itching usually signals to our body that we have some prolonged inflammation or stimulation, whereas our reaction to strong heat or cold should be immediate. To avoid us having to sense itching all the time (which would be uncomfortable and distracting), it would seem likely that small or temporary signals must be filtered out. By having a slow transmission this may be achievable as spreading out a signal over time can ensure that only strong and prolonged signals are registered. $\endgroup$
    – tsttst
    Commented Nov 27, 2017 at 19:29

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