Monday, January 12, 2015

Apple #697: Paper Cuts

Let's talk about paper cuts and pain.

(Image from Psychology Today)

There are lots of kinds of pain.  I'm not talking about major-serious pain -- grief, despair, depression, betrayal, angst, etc.  For my money, emotional or spiritual pain is the worst kind there is.  But that's not the kind of pain I'm talking about here. 

The kind of pain I mean is physical pain.  Even in this realm, there is a continuum.  There are people who suffer horrible burns over most of their body, and live.  I'm not talking about that kind of pain.  There are people who undergo surgery without anaesthesia -- though this doesn't happen as often these days as it once did.  I'm not talking about that kind of pain.  There are people who lose limbs (physical) and feel the loss of the limb for months or years afterward (physical and emotional).  I'm not talking about that kind of pain, either.

I'm talking about mundane physical pain.  Barking your shin on the corner of the coffee table.  Dropping a hammer on your toe.  Cracking your head on a cupboard.  The kinds of things that make you swear [insert your favorite curse word or phrase here] immediately and loudly and with vigor.  This is the kind of pain I'm talking about.

OK, so, now that we have the proper context established, here's the question: Daily Apple reader Jamal wants to know, why do paper cuts hurt so much?  Relatively speaking.

A paper cut.  So insignificant, you can hardly even see it in this photo. In the grand scheme of things, this is a tiny injury.  So why does it hurt as much as it does?
(Image from No Dankes. Yes Bitte!)

They're stupid.  Just stinging little slices across your finger, made by a piece of paper.  Just a tiny thing. Yet a paper cut gives you a searing flash of pain.  And it doesn't go away immediately, but the dang thing continues to give you a piercing, stinging pain for quite some time afterward.

What's up with that?

1. You get them on your fingers.

  • . . . or maybe somewhere else on your hand, but usually you get paper cuts on your fingers, and anyway, the ones on your fingers hurt more than elsewhere on your hand.

Our fingertips are among the most sensitive parts of the body.  They are so sensitive, we can detect variations in textures that are only nanometers thick.
(Photo from Creative Commons / International Science Times)

  • We have tons of pain-sensing nerve fibers called nociceptors packed into the skin of our hands & especially in our finger-skin.
      • Nociceptors (noci- is Latin for "hurt") are nerve endings that sense all kinds of pain: heat, pressure, stretching, chemical burns, etc.
      • You have these scattered throughout your entire body, including in your internal organs. But the type of pain experienced in your internal organs is called "visceral pain" (it happens in your viscera, or guts), and that is a far different sensation than the kind of pain that happens at your skin's surface.
      • Since you do a lot of things with your hands, and since you rely on them so much, you have an especially high number of nociceptors / pain alarm bells packed into the skin of your hands, and especially in your fingertips.
  • So a cut in the skin of your fingers is going to hurt a lot more than a cut to the skin of something a lot tougher, like, say, the base of your foot where you have a big fat callous. 

2. Paper cuts stay skin-deep.

  • This reason is related to the first. Since paper cuts slice the surface of your skin, they're cutting through a lot of the super-sensitive nociceptors.  They don't penetrate down through the layers of the skin into the deeper tissue, where the pain sensation system works differently, and which doesn't have the same kind of immediate-screaming-alarm bell response.

This is a rather rudimentary drawing, but it gets the point across. The red knobs are the nociceptors, and the black lines are the nerve bundles that extend down to the rest of your nervous system.  A paper cut will slice off or otherwise activate a bunch of those red knobs / nociceptors, so you're going to get a whole lot of pain alarms going off.  A deep cut will penetrate into the skin and activate far fewer nociceptors.
(Diagram from Greenwich Medical Media Ltd.)

3. Paper cuts usually don't draw blood.

  • Because paper cuts stay skin-deep, they usually don't draw blood, or don't draw very much of it.  You might think this would signify that they hurt less (doesn't more blood = worse injury?) but in fact, in the case of paper cuts, this means they hurt more.
  • Not only because of what we've just talked about, that there are a lot more pain receptors at the surface of your skin on your fingers than in the deeper tissue, but also because blood carries with it all the equipment necessary to form scabs and heal cuts.  Without the ambulance's worth of aid that blood brings to the scene, a paper cut stays open. So you feel every subsequent bump, nudge, and re-opening of the initial cut.

4. Paper is dull.

  • Maybe you've heard it said that dull knives are more dangerous than sharp knives? This is because dull knives do more damage.  Rather than making a clean cut, they tear the flesh into ragged bits.
  • Paper is not even as sharp as a dull knife.  The cut that an edge of paper makes in your skin may look to the naked eye like a clean and even slice, but microscopically, that dull edge of paper has made all kinds of tears in your skin. And your nociceptors are reacting to pretty much every single one of those tears.

Tracing paper is thin, right?  Seems like it would be pretty smooth too, yes?  Well, think again.  This microscopic photograph shows the top surface and cross-sectioned edge of a piece of tracing paper.  Imagine slicing open your skin with that.
(Photo by Dianne van der Reyden et al., Journal of the American Institute for Conservation)

For comparison, here's the edge of a new utility knife blade.
(Photo by Onycha Banton at Midwood Science Image of the Week 4-2-2012)

Here's a heavily used utility knife blade.  Even though it's pretty dinged up and dulled, it still looks a whole lot sharper than that edge of paper, doesn't it?
(Photo by Onycha Banton at Midwood Science Image of the Week 4-2-2012) 

In Sum

So when you get a paper cut, you're slicing open one of the more sensitive parts of your body with a dull edge which results in tearing, and the wound doesn't benefit from the healing properties that blood brings to the scene.  Therefore, in terms of the amount of pain response, a paper cut may be the worst kind of cut you can get.  Not the most damaging, but maybe the most painful.

(Photo from ALD Talks)

Here are a couple final tidbits for you:
  • Nociceptors are supposed to stop firing after the damage has been healed.  But they don't always stop when they're supposed to.  Nociceptors that don't stop can become  what we call chronic pain.
  • When people who have lost a limb and experience that phantom limb sensation, that's the nociceptors in the remaining tissue continuing to send back the "there's a problem" message long after the limb is gone.

Mental Floss, Why Do Paper Cuts Hurt So Much?
ABCNews, The Peculiar Pain of Paper Cuts
Indiana Public Media, A Moment of Science, Paper Cuts, Why So Painful? December 20, 2012
One Medical Group, Quirky Questions: Why do teeny little paper cuts hurt so much? March 2, 2011
Purves, D., et al., Neuroscience, 2nd edition. Sunderland (MA): Sinauer Associates, 2001, What are Nociceptors? June 11, 2014
Alan Fein, Ph.D., University of Connecticut Health Center, Nociceptors and the Perception of Pain, February 2012

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