Why Do We Itch?

The American poet Ogden Nash said happiness is having a scratch for every itch. But while scratching can feel pretty good, we all know what happens next. When considering scratching an itch, it may be better to follow the old Yiddish proverb. A quarrel is like an itch. The more you scratch, the more it itches. There are plenty of things that cause that irritating, unpleasant, itchy sensation. But why do we itch in the first place? The itch sensation, also known as pruritus, is not the most pleasant thing. But it evolved for a good reason. Stretched out, your skin covers about 20 square feet, the biggest organ you have. Your other organs on the inside are protected by your immune system. But skin is your first line of defense, the wall that guards the human realm, exposed to the elements, and it's got a unique way to deal with different types of threats. Where there's an itch, there's a desire to scratch, often an unconscious desire. This reflex likely prompted our outdoor-dwelling ancestors to remove dead skin or parasitic threats and other irritating things like thorns and allergens. And much like yawning, being itchy is thought to be contagious, as some of you may be noticing. We're social animals, so watching some infested tribe mate get their scratch on could have given us a head start to get rid of those parasites. So we know the purpose of itch, but what happens inside our bodies that makes it feel different from other sensations, like pressure or heat? Turns out we don't know all the pieces to the itch puzzle yet. Up until about a decade ago, scientists thought itch was just a dialed-down, less intense version of pain. When something makes contact with our bodies, nerve endings in the epidermis, the outermost layer of our skin, relay this information through electrical and chemical messages up the spine and to the brain. Different stimuli activate different nerve pathways, and cause different sensations in our brains. Light touch might feel nice, a punch to the face will probably hurt. We now know that there's special itch-specific circuitry in our nervous systems, involving its own chemicals and cells. And we share one universal response to itching. Scratching. And boy does it feel good. But why? Scratching is technically a form of pain. I mean, the name of a small wound that breaks the skin is a scratch. Scratching causes a low-level pain signal to the brain that masks and overrides the itch signal, which is why slapping, pinching, and pressing itchy regions also give you relief. Remember that the next time your mom tells you not to scratch. Your brain responds to pain by triggering the release of chemicals at the irritated site, which tunes down the painful sensation. One of those relief-providing chemicals is serotonin, a neurotransmitter. But that serotonin also makes it easier for the itch signal to be re-triggered. So the itch nerves fire again, and you get even itchier. Scientists call this constant loop of itching and scratching the itch-scratch cycle. Very clever. And it can be maddening. In extreme cases, damage to itch-sensing nerves can cause itching without an actual stimulus. This is the kind of itch that scratching can't help. People who have recovered from shingles, a virus infection that affects your nervous system, can be left with an unexplainable, never-ending itch once the rash recedes. One woman had such an itchy scalp after healing from shingles, she scratched straight through to her brain. What? So what can the rest of us do to avoid scratching and itch? Itchy ailments have been documented throughout history, and humans have found some pretty clever ways to find relief. The Greeks and Romans had mineral baths and animal fat, the Persians used silver, menthol soothed itchy skin in ancient China, and camphor, a chemical from evergreen trees historically used to make explosives, has been soothing itchy skin since the 13th century. Today we also have anesthetics, which numb the skin of all feeling, counter-irritants like capsaicin extracted from chili peppers, and antihistamines and steroid creams, just to name a few. But since itchiness can be caused by so many things, there isn't a one-size-fits-all remedy. Luckily, with so many treatments to choose from, at least you don't have to start from scratch. If you're itchy, just remember, unlike beauty, itch doesn't run only skin deep. It's a sensation that reaches from the epidermis to the brain. The origins of that itch you might be feeling right now extend way back on the evolutionary tree, and we've only scratched the surface of knowing how it all works. Stay curious. Oh, and here's a video challenge for you. Rewatch this without itching yourself uncontrollably. Hey guys, I wanted to tell you about another great channel that's part of the PBS Digital Studios family. Check out Above the Noise. It's a channel that cuts through the hype about the news so that you can understand the facts behind it and how they relate to your life. I recently helped them out on an episode about why only some countries have nuclear weapons. We'll leave a link down in the description so you can check them out.