Science Questions

What makes us itch?

Chris - Well it’s things which tickle your skin but how does that itch reach consciousness? It looks like there are itch-specific classes of nerve fibres. They’re very tiny, thin calibre nerve fibres. They’re about 1-6 microns (that’s millionths of a metre across) so some of the tiniest nerve fibres in the body. If you stimulate those nerve fibres then people do detect a sensation of itch. They seem to be there specifically to convey the sensation that something is tickling you and they go up to the spinal cord and they squirt out a chemical transmitter called gastrin-receptor releasing peptide which is the transmitter which tells the spinal cord there’s an itch in a certain part of the body. The purpose of itching is in fact to protect you because things that are usually chemical irritants, physical irritants or parasites. Things like mosquitoes that might transmit an infection to you so it’s important you know where on the body there’s a problem. It draws attention to it so you can scratch and get rid of it. How does scratching work? In the same way that you punch the wall, go ouch and then rub the affected body part better this helps to gate the pain and stops things hurting so much. When you scratch an itch what you’re doing is inflicting a little bit of pain. What scientists have found is that when you trigger a bit of pain in the area where you have an itch that gates or switches off the itch sensation.

November 2008