How Itching and Nerves Are Curiously Related

POSTED ON 10/22/2016 IN Research BY Christopher Centeno

referred pain

Nerves work in mysterious ways. One of those is a phenomenon called "referred pain."  This property of nerves is a topic that I've addressed on the blog—for example, that chopping off the knee isn't going to help when the pain is coming from somewhere else. I've said many times that “where it hurts may not be where the damage is located.” Another form of referred pain involves itching. The freaky part about this is that you can fool the brain so that scratching one arm relieves an itch on the other!

The Body Is Amazing

The body never ceases to amaze me. How it all works is frankly not a done deal. While most patients believe that we know 99% of what we need to about how the body functions, it's more like 60%. While it may be disturbing as a patient to understand at a visceral level that 40% of how your body works is invisible to your doctor, that's reality. This morning's study is a case in point.

How Itching Works

You may recall from high school that itching is caused by a local immune response that involves cells dumping chemicals that lead to the itch. What they never mentioned is that all of this is orchestrated by nerves. No nerves, no itch. Messed up nerves, big itch.

The “Mirror Scratching” Study

The purpose of the new research was to determine if scratching the left arm (the nonitching arm) instead of the right arm (the itching arm) reduced the severity of the real itch in the right arm when subjects believed, via a mirror illusion, that their right arm was the one being scratched. A second experiment was done in which using real-time video, subjects only visually perceived scratching on one arm or both arms or perceived no scratching at all. In both experiments, when the subjects believed the left arm (the nonitching arm) was their right arm (the itching arm), scratching the nonitching arm lessened the severity of the itch in the itching arm.

You might say, Well, it was a trick, a mirror illusion; it was all in their head. However, that's the point. Itching is a nerve-mediated phenomenon. What you brain believes to have happened, as far as your body's nerves are concerned, did happen.

Take, for example, a dog scratching. Have you ever noticed how easy it is to convince the dog that it's own leg is the one scratching an itch when in fact it's you? You scratch his torso, but the dog's leg moves as if by reflex. As far as his brain is concerned, he's the one doing the scratching.

The upshot? This study shows just how little we know about the body. It reminds me of an imaginary training study I found while in residency, where subjects just thought about training their finger strength without moving their finger, and sure enough, their strength increased. Here, we see the complex interplay of how the brain is involved in scratching an itch. Since an itch is mostly a nerve phenomenon, tricking the brain into satisfying the itch isn't hard. In fact, this study may have only "scratched the surface" of our knowledge of how the brain and nerves play into our perceptions of the world.        

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