[♩INTRO].

If you’ve ever been bitten by a mosquito, a bed bug, or a spider, you probably know the sensation of an itch really well. Your immediate reaction is probably to scratch the area and hope the itch goes away.

And for a second, it does. There’s that sweet, sweet feeling of relief and satisfaction. Then, the itch comes back with a vengeance.

So you scratch harder, and then it gets itchy again, and… you get the idea. Creatively, scientists call this the itch-scratch cycle. And to understand it, it helps to know how scratching and itching work in the first place.

For a while, researchers thought that an itch was just a mild kind of pain caused by irritated skin cells, but that’s not true. Today, we actually know of a few itch-specific receptors that make sure itch signals and only itch signals get sent to the brain. For example, there’s an important one in mice called GRPR.

But even if the two sensations aren’t the same thing, there is considerable overlap in the nerve pathways that transmit them. And this is why scratching an itch is just so satisfying. The sharp sensation of your nails on your skin stimulates certain pain receptors, and that sends pain signals to your brain that overwhelm the itch ones.

Essentially, scratching distracts your brain for a second. But the story doesn’t stop there. See, once those pain signals come in, your brain responds by releasing the neurotransmitter serotonin to dull them.

But that also has a downside. Serotonin is probably best known as a happiness-related chemical, but it actually does various things throughout the body. And unfortunately, when it comes to your skin, it can end up intensifying an itch.

In mice, this happens because serotonin over-activates those itch-specific GRPR neurons. And since these neurons play such a big role in making mice feel itchy in the first place, that increases the intensity of the sensation. Many researchers are pretty confident this is at least part of how things work in humans, too.

So if you scratch that itch again, it ultimately just triggers your brain to release more serotonin, which keeps the cycle going. Scientists speculate this could happen because GRPR and serotonin receptors are spatially close to each other. And that may cause them to interfere with each other’s functions.

But even though the reason for serotonin’s role is unclear right now, researchers have shown that disrupting this “conversation” between serotonin and GRPR can stop the itching almost entirely. And if we can do that safely, it could be a really valuable treatment option for people who suffer from chronic itch conditions. Of course, like a lot of things in science, there is as chance that there’s even more going on, because there are other proteins and receptors involved in itching, too.

For now, though, scientists will just have to keep researching. And while they do, you might want to try resisting the urge to scratch… no matter how relieving it is in the short-term. Thanks for watching this episode of SciShow, and special thanks to Tim and Karen Ford and Marcus, who all asked this question!

If you want to submit a question to be answered or vote on the one we answer next, you can go to patreon.com/scishow. [♩OUTRO].