[♪ INTRO] Getting a back scratch is one of  the most satisfying feelings ever.

At least until you can’t quite  hit the right spot because it’s a little to the left,  no, to the right, wait, no. Then it’s just frustrating because  it feels like your itch is actually moving around your body.

And it's all  thanks to some tricks of your nerves. An itch is a not-so-delightful sensation  that, as soon as you feel it, you feel like you need to scratch, and it can be caused by  all sorts of things, like skin irritation, overactive nerves, or even something  systemic like kidney disease. And that need to scratch distinguishes  itchiness from other unpleasant sensations like pain, but the two feelings have a  lot more in common than meets the eye.

Like, these two sensations, itchiness  and pain, send messages through the same kinds of neurons to a common  destination: the spinal cord. Which threw scientists for  a loop, making them think that itchiness was actually just a subset of pain. Research suggests that  different groups of neurons are responsible for itch, which means that  itchiness and pain are separate sensations.

But they still interact with each other. Like, processing pain might actually be  why scratching an itch feels so good. See, scratching kind of hurts, because  you’re lightly damaging the skin.

And those pain signals get sent  to the spinal cord, overriding the itchiness signals. Your brain doesn’t notice  the itchiness; it detects mostly pain. So, it releases a chemical messenger,  serotonin, to help ease that pain.

Every time you scratch, your brain  is getting bathed in serotonin, which makes it feel pleasurable. But once you’ve scratched, it  might feel like the itchiness moved a liiiiitle to the left. Scientists don’t know exactly why that  is, but there are a couple possibilities.

One has to do with the distribution  of pain receptors in the skin. There are more receptors in the skin than  nerve fibers leading to the spinal cord, so many receptors have to send  their signals to one nerve. And by the time the itch signal gets to  the brain, it might not be able to tell exactly which receptors it  started in, just a general area.

So as you scratch, it may feel like the  itchiness is moving because your brain sort of guesses where it started  and sometimes guesses wrong. That may also be related to  why an itch often comes back stronger once you stop scratching it. The serotonin that makes itching  feel good spreads from the brain back down to the spinal cord,  where it’s supposed to help reduce pain.

But serotonin also stimulates the  nerves that carry itchiness information. And when those nerves are extra  stimulated, the itch might feel even itchier than before you scratched. But because of that difference in the  number of receptors and the number of nerves, that new, extra-intense  itchiness could be perceived as coming from spots nearby the original itch.

So, your scratching kicks off a  signaling cascade that ends in feeling like the itchiness is moving. And knowing how the itchiness moves could  help scientists develop new potential treatments for disorders that cause chronic  itch, which can be really debilitating. But in a lot of ways, the itch  is still a real head-scratcher.

Thanks for watching this episode of SciShow. And thanks to our patron Mikey  for asking this question! Scishow patrons can submit questions to  our inbox, and our editors pick out the best ones to answer in episodes like this one.

And this only scratches the surface  of what we have over at Patreon; if you'd like to check out what else we  have, you can go to Patreon.com/SciShow. [♪ OUTRO]