Why paper cuts hurt so much

Source: BBC

By Jason G Goldman

Paper seems completely harmless, but anybody who has refilled a photocopier or thumbed too quickly through a book knows that this humble material harbours a deep, dark secret. Deployed properly, it can be a serious weapon: paper cuts are just the worst.

There isn’t a whole lot of scientific research effort directed at understanding the pain of paper cuts, probably because nobody would sign up for a randomised, controlled study that involved a researcher intentionally inflicting this kind of torture on study participants. But according to Dr. Hayley Goldbach, a resident physician in dermatology at UCLA, “we can use our knowledge of human anatomy to help us out here. It’s all a question of anatomy”.

It’s all to do with nerve endings. To start with, there are lots more pain receptors embedded in your fingertips than almost anywhere else in your body. Though Goldbach is quick to point out, “it would probably also hurt a lot if you got a paper cut on your face or in your genitals, if you can imagine that.” So while a paper cut on your arm, or thigh, or ankle might still be annoying, it would probably be more trivial than the intense fiery quality that finger-based paper cuts tend to have.

(Credit: iStock)

The cuts from paper aren’t deep, but they can be extremely painful (Credit: iStock)

You can actually prove this to yourself by employing a test that psychologists and neurologists use. Take a paperclip and unfold it so that both ends are pointing in the same direction. If you use it to poke yourself on your hands or face, you can probably perceive each of the clip’s two pointy ends individually. This is what’s referred to as “two point discrimination,” and because you have so many nerve endings in the skin in those parts of your body, the two points have to get really close to each other before you’re unable to tell them apart.

But now try the same thing on your back, or your legs. Chances are the two points would have to be really far apart before you’re able to tell them apart. That’s because the distribution of nerve endings there is far less dense.

The extreme pain felt when something injures your fingers is simply the result of evolution working as it should

This actually makes a good deal of evolutionary sense. “Fingertips are how we explore the world, how we do small delicate tasks,” explains Goldbach. “So it makes sense that we have a lot of nerve endings there. It’s kind of a safety mechanism.”

It’s reasonable that your brain would devote more neural real estate to continuously monitoring possible threats to your hands, since they’re the main vehicles the body has for interacting with the world. If you come into contact with something extremely hot, for example, or sharp, it’s just more likely that you would interact with it using your hands. So the extreme pain felt when something injures your fingers is simply the result of evolution working as it should, providing a little extra encouragement for you to keep those hands safe.

And then there’s the weapon itself. Do a quick Google search and you might become convinced that due to its porous nature, paper is home to a bacterial menagerie, just waiting to colonise your paper-inflicted wounds. But whether or not that’s true, the presence of bacteria and other microscopic beasties can’t explain the sensation of pain, at least not at the moment of cutting. Bacteria can lead to infections if wounds are left untreated, which themselves can be painful, but that takes a bit of time.

(Credit: iStock)

Paper edges may look straight, but they are in fact serrated, cutting through skin like a saw (Credit: iStock)

But there is something to the idea that paper is a uniquely painful weapon.

To the naked eye, it might seem as if a paper’s edge is fairly straight and smooth. But if you were to zoom in, you’d find that paper is more akin to a saw than to a blade. So when a paper cuts open your skin, it leaves behind a chaotic path of destruction rather than a smooth laceration. It rips, tears, and shreds your skin, rather than making clean slice, as a razor or knife blade would do.

And if that wasn’t enough, paper cuts are typically shallow – but not too shallow. “They’re deep enough to get past the top layer of the skin, otherwise they wouldn’t hurt. The top layer of skin has no nerve endings,” says Goldbach.

The nerves that the paper revealed when it tore apart your skin continue to be exposed to the outside world

But they don’t slice that deep into your body, which is perhaps why it’s puzzling that they should hurt so much. But it’s exactly for this reason that paper cuts are such a menace. A deeper wound would result in bleeding. The blood would clot and a scab would develop, beneath which the skin could go about healing free from the continued assault of the outside world. But the shallow wound of a paper cut doesn’t offer such protection. Unless you take care to cover it up with a bandage and perhaps some antibiotic ointment, the nerves that the paper revealed when it tore apart your skin continue to be exposed to the outside world, and that only makes them angrier.

Without the cushion of blood, pain receptors are left exposed to the elements, and unless you quickly bandage your paper cut, those neurons will keep on sending the alarm bell, warning your brain of impending disaster. That, after all, is their job.

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Categories: Biology, Psychology

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