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Why are “itch signals” transmitted so slowly from the skin to the human brain?

Why are “itch signals” transmitted so slowly from the skin to the human brain?


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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 about17:30:

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?


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