Why is scratching the skin only pleasurable when it is itchy?
In: Biology
There are a few reasons.
the chemicals that make us itchy are called “pruritogens”. Pruritis is the medical term for itchy. If you see -gen at the end of a word, it means “genesis” so basically “itch creation”
* Mechanically, scratching can inhibit the signals caused by these chemicals. Sort of just crowding the signal so your brain can’t interpret the itchy one.
* The act of scratching also activates your brains reward system. So if you’re itchy and inclined to scratch, your brain likes this. There is a reason we instinctively scratch an itch, just like many other animals would.
* Scratching also activates interleukins. Interleukins (IL) are proteins that are responsible for immune response. There are…lots of them but one (amongst many other factors) responsible for itching is called IL-13. The act of scratching has been found to release a decoy receptor called IL-13Rα2. The “R” conveniently means “receptor” but this decoy will take up the IL-13 proteins floating around instead of your skin cells with the actual receptors that lead to the itching signal. The decoy receptor acts to reduce that itchy feeling by soaking up all the signals from the “real” receptor. Interleukins are also linked to a lot of auto-immune (basically when your immune system attacks healthy cells, seeing them as foreign) so if you look it up things like psoriasis, rheumatoid arthritis etc. are IL driven and many new drugs have specific IL/other immune protein targets.
The reason it doesn’t feel good if we aren’t already itchy is because there is no input in place to create that reward system and there is no “itch” signal to reduce. So we just get the straight sensation of scratching ourselves for the sake of it. It’s like when you drink a gatorade after a soccer game or water when you wake up thirsty as hell in the middle of the night- it tastes way better.
Our body is saying “we need hydration/electrolytes/sugar” so it’s using pathways to incentivize drinking. If you aren’t thirsty, those switches aren’t turned on so our brain/body isn’t wasting time or energy making it a priority to drink, or in this case, scratch.
The immune aspect is why stuff like steroid creams work. These aren’t “pump you up” anabolic steroids, but something called corticosteroids (they come from the adrenal “cor”tex, a gland that lives on top our our kidneys), which actually do the opposite, The opposite of anabolic is catabolic, which is how corticosteroids are often classified. They down-regulate a lot of processes, to include immune response. Some steroid creams used over a long time can actually lead to skin thinning and other issues. If you take them by mouth for a long time, it can cause loss of muscle/bone density and diseases like Cushing disease.
So it’s pretty interesting and there are a lot of things at play. That being said, you should try and avoid any persistent scratching. Here and there it’s fine, and obviously your body is equipped to reward it but it’s more than temporary you can get skin injury/infection or other complications.
Also, fun fact that is pretty unrelated. “Adrenal” glands that sit on top of our kidneys are named…because they sit on top our kidneys. “ad” means “to” or “toward” and “renal” means kidney in Latin. So they are toward the kidney. The adrenal glands also produce epinephrine aka adrenalin. “epi” means “near” and “nephro” means “kidney”, but in Greek. You can safely assume that “adrenalin” is back to the Latin origin.
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