Asked by: Aimee Smith, Scotland
An itch is anything that makes you want to scratch. We have itch receptors in the top two layers of our skin and they look very similar to pain receptors – essentially they are just bare nerve fibres. But itches are transmitted to the brain in a different way and whereas pain causes us to reflexively pull away from the source, an itch makes us want to get in there and give it a good old scratch.
Itching probably evolved as a way of coping with biting insects and other ectoparasites. The itch encourages us to swat mosquitoes, and pick lice and ticks off ourselves and each other. Itches can be triggered by the central nervous system as well, even if nothing is touching your skin. This is why itches can be contagious like yawns. It might have warned our ancestors to start swatting flies proactively, or encouraged mutual grooming sessions.