Scientists have barely scratched the surface when it comes to explaining what makes an itch, itch. The itching pathways are like a big nerve loop connecting the skin, spinal cord and brain and the itch can start anywhere and be felt somewhere else, making it a bit hard to scratch if you can't find the source of the itch! Sometimes a vicious cycle of itching and scratching can get out of control and only treating a problem far removed from the skin can stop it. Some liver diseases can cause itching so severe that sufferers will consider transplants to make it stop!
It used to be thought that the sensation of itching was caused by low level stimulation of the nerves responsible for feeling pain. Like pain, the feeling of itchiness is delayed slightly from the event that causes the pain or itch. But both sensations lead to different responses. Pain leads to a reflex action that removes us from the source of injury while an itch causes us to scratch. Interestingly, scratching too can be a reflex reaction that needs no input from the brain.
Then in 1997 some researchers identified a family of tiny slow-conducting nerves with broad tentacles in the skin that respond to itch alone. More recently, brain scans were used to identify which parts of the brain are activated during an itch. No particular area appears to be involved, instead the areas involved in sensation, planning and initiating movement, and the bits involved in feeling pleasure and pain were all stimulated. So itching is a big web of sensation, action and emotion.
But why do we scratch an itch? It could be that the slight pain acts as a counter irritation that distracts the brain from the itch. Sometimes, however, scratching makes it worse, as if the upper layers of the skin are being damaged and release an unknown molecule that causes more itching.
While itching can be caused by lots of conditions such as eczema and jaundice, what about all those itchs with no obvious cause? Well, a lot of the time it is psychological - how many time have you itched while reading this article? Weird itching tends to be more common in those with drier skin such as old folk and is made worse by irritants (like lavender bath salts and woolen clothes). If you're not old or ill with some serious disease or infected with lice, its probably just all in your mind. Try telling yourself that next time you have an itch that wants to be scratched - see how long your will-power can hold out!
But why did itching evolve in humans and other animals in the
first place? I guess that by itching an irritation we keep ourselves
free of skin parasites and knock off any bug that might want to
bite us - like when a mosquito lands on your skin and tickles
you with its little legs, and you can squash it before it starts
sucking your blood. The minor skin damage caused by slight scratching
can also free up any particles stuck in the skin that shouldn't
be there. An unanswered question is why is it so fun to scratch
an itch?
