Itchy and scratchy

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Evolution / Health / Myths

We all know the sweet relief that comes from scratching an itch. But why does scratching feel so good? And why does it make the itch worse?

When you really have to scratch that itch! Image credit: lintmachine via Flickr

Why so itchy?

An itch is simply a sensation on the skin leading to the desire to scratch. But the science of itching is far from simple – it’s been the subject of decades of research (there’s even a scientific research journal called Itch). Researchers discovered the first gene associated with itching ten years ago, but there’s still plenty we don’t understand.

Itching can have many causes: from minor insect bites and skin irritations, through to various cancers and the treatments for those cancers. For most of us, itches are just a minor annoyance, barely noticed throughout the day. But for others, itching is a chronic condition that has a serious impact on quality of life. And chronic itching can lead to continuous scratching and severe skin damage.

Scratching the itch

On the face of it, dealing with an itch is straightforward: you scratch it. And hey presto, all is well.

Scratching an itch with a violence that would cause pain elsewhere may be experienced as one of the most exquisite pleasures. – Dr. George H. Bishop (1948)

But then all too quickly, the relief fades. Only to be replaced by an even more insistent itch. Remember being told as a kid that scratching your itch would make it worse? Well, your parents were right.

The reason scratching temporarily relieves a pesky itch is that scratching causes mild pain. This means your nerves carry pain signals, rather than itch signals to your brain and for a brief moment, the itch is gone. The pain distracts from the itch. But the problem is, your brain releases the chemical serotonin in response to the pain.

Serotonin explains how you can end up scratching until you bleed. You scratch because the pain of scratching inhibits the itch. But because your body releases serotonin in response to the pain, it takes more and more vigorous scratching to suppress the itch. At the same time, the serotonin also acts to intensify the itchy feeling. It’s a vicious cycle, which is why the best thing you can do is not start scratching in the first place.

Mice who can’t produce serotonin scratch far less in response to being itchy than normal mice. But if you’re thinking blocking serotonin would be a great way to prevent itching, not so fast. We need serotonin for all sorts of roles in the body: growth, ageing and regulating moods. Getting rid of serotonin would have far more serious consequences than relieving itching.

Catching the itch

Have you started scratching yet? Just reading about scratching can be enough to make you suddenly feel itchy. Audience members in a talk about itching itched more than those listening to a talk about a more innocuous subject. And if you see someone else scratching, you’ll probably copy without even being aware of it. It’s not a sign of a lice outbreak (hopefully), you’ve just fallen victim to socially contagious itching. Like yawning, mice, monkeys and people can all ‘catch’ itching.

There’s been plenty of research into why itching may be contagious. For example, research found that contagious scratching is more common among highly neurotic people.

Research published earlier this year showed that in mice, contagious scratching appears to be a hard-wired, instinctive behaviour. If a mouse sees another mouse scratching – even if the mouse is an image on a screen – the first mouse also starts scratching within a few seconds. A chemical is released in an area of the brain responsible for regulating mouse body clocks (the suprachiasmatic nucleus), which causes the scratching. Inject this chemical into another mouse and it too will start scratching. The mouse isn’t consciously deciding to scratch; it’s an automatic response to the change in brain chemistry.

Hard-wired behaviours tend to be important ones, and there’s probably a good explanation for why we’ve evolved to ‘catch’ scratching. We’re highly sensitive to potentially dangerous things like insects touching our skin. If someone near you is scratching, it may well be a sign of something you should respond to, perhaps a swarm of mosquitoes. Better to start scratching now and avoid getting stung.

But then again, if you don’t want the itch to drive you crazy, now may be a good time to look away.

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2 Comments

    • I like your logic there Marti! Very impressive – I spent the whole time I was writing it scratching 😃

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