I itch, therefore I scratch

Before you read any further, let me warn you: today’s column may cause itching.

It’s got nothing to do with the ink it’s printed with, either, or mysterious radiation from your computer monitor. It’s simply because I’m going to write about the science of itching, and one of the peculiar things about itching is that talking about it can cause it.

You would think that such a universally experienced sensation would be well understood by now, but you’d be wrong.

Itching was in the news recently because Zhou-Feng Chen, a neuroscientist at Washington University in St. Louis, announced at the recent annual meeting of the Society for Neuroscience in Washington that he and his colleagues had discovered a new “itchy gene” in the spinal cord.

Called GRPR (for gastrin-releasing peptide receptor, of course) it causes laboratory mice to scratch “like crazy” when it’s injected under their skin.

The importance of its discovery isn’t that it is a sure-fire way to make mice itch, satisfying a revenge as that might be on the little vermin, but rather that scientists might now be able to design better drugs to alleviate itch: not the occasional itch we all get from things like mosquito bites and allergic reactions, which can usually be relieved by antihistamines, but chronic itch, which does not respond to antihistamines, can be caused by more than 50 diseases and conditions (including kidney disease, eczema, shingles and HIV), and plagues as many as one out of 10 people.

That’s great, but it still doesn’t tell us why we itch in the first place. Our lack of understanding is summed up in the way itch is defined: an unpleasant sensation that provokes the desire to scratch. Since that definition was crafted by the German physician Samuel Hafenreffer in 1660, you can see we haven’t made a great deal of progress in understanding the phenomenon.

When we itch, we scratch, which feels wonderful: but the itch-scratch-itch cycle can have horrifying results, with some people so caught up in it that they cause themselves serious injury.

Itching and scratching probably evolved as a way to protect us from insects (which can carry all kinds of nasty diseases) and plant toxins. For a long time, scientists assumed itching was just a weak form of pain. But research 20 years ago showed that the two are entirely separate sensations; and ten years ago, after research involving a lot of very brave and very patient volunteers, scientists announced they had found a type of nerve that is specific to itch.

As you might expect, it’s incredibly sensitive. A single itch nerve fiber can pick up an itchy sensation from seven and a half centimeters away. By contrast, pain fibers only pick up sensations from a millimeter away.

Itching can be triggered either chemically (by poison ivy, for instance) or mechanically (from the feel of a mosquito’s legs on your arm).

Scientists have also discovered which parts of the brain “light up” with activity when we itch: the part that tells you where the sensation occurs, the part that governs your emotional response, and the part that processes irresistible urges–in this case, to scratch.

But there are still mysteries. We don’t know why scratching relieves itching; it may interfere with the transmission of the itch signal to the brain…or not.

We don’t know why sometimes something brushing against your skin makes you itch, whereas other times it tickles. And then there’s the biggest puzzle of all: why can you make yourself itchy just by thinking about it?

A German professor once gave a lecture that included two sets of slides. The first set featured things likes fleas, lice and people scratching; the second featured baby skin, bathers, soft down. Video cameras revealed much more audience scratching in the first half of the lecture than the second.

Clemens Forster, a researcher at the University of Erlangen in Germany, puts it this way: “Itch happens in your brain, not on your skin.”

Maybe. But it’s my skin I’ve been scratching all the time I’ve been writing this column…and I suspect it’s been the same with you as you’ve read it.

Sorry about that…but remember: you were warned.

Permanent link to this article: https://edwardwillett.com/2008/11/i-itch-therefore-i-scratch/

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