Itching is one of life's little nuisances. Enough to irritate and distract but not enough to be classed as any kind of serious problem. Fortunately though itching is generally an acute problem, and unless you have eczema or a rash etc, then you will normally be able to alleviate this little niggling problem simply by rubbing or scratching it (consistent itching though can in some cases be a sign of a condition such as shingles or AIDS so it is not to be ignored).
The question is why? What is going on when you itch? And how does scratching help? Actually itching and scratching is something of a mystery still to modern science, and while they have conducted many studies, and have some promising leads to follow, we have yet to get a definitive answer as to why scratching alleviates and itch. We'll look at how this might work here though and do our best to follow the clues.
What Causes Itching? A Recent Study
Itching is of course the result of many different causes. In some cases this is the result of damaged tissue and excited nerves which then tingle and send feedback to the brain. However in the case of the random itch that we all experience whenever we think about it, it seems that nerves aren't really responsible for the problem. Instead the itch is taking place deep inside the central nervous system and precisely in the 'spinothalamic tract' (or STT to its friends) – a group of neurons in the spinal cord which send information to the brain regarding temperature and touch in the brain. Studies have shown that by exciting the STT neurons using chemicals such as histamine, it is possible to send the sensation of an itch to the brain. So inject histamine into the body and you'll find that those neurons will fire up – and this has been demonstrated in monkeys.
How Does Scratching Work?
In the same experiment with monkeys, scientists then scratched the leg where they had injected the histamine, and found that the neurons in the STT stopped firing. Interestingly it seems almost as though the scratching sensation 'overrides' that of the itching and causes those STT neurons to fire in a different way. In humans it may be that these itches are caused by random firings of those STT neurons, or firing in responses to small amounts of histamine, and that the scratching sensation then causes those to cool off. These findings it is thought will lead to new treatments for humans.
However scratching the same area will not prevent the STT from registering pain interestingly, so the connection is more complex than simply 'overriding' or blocking the signal. Furthermore this study has some problems as the observed effects may have been correlated with itching rather than causative.
Other Factors Causing Itching
At the same time there are other factors involved in scratching. It is not always a response to histamine for instance and sometimes it is a result of firings of nerves which can be a result of nerve damage – perhaps to do with something going on at a cellular level – or something actual physical going on. For instance in some cases you will feel an itch and scratch it only to find you had a fly crawling across your skin. Other times you may scratch your ear to find it was blocked with ear wax. These things demonstrate that sometimes we have just cause to itch – and perhaps when we itch and there's nothing there it's just that our leg hair was blowing in a certain way, or we had something too small to see irritating the area. Scratching when you feel tickling sensations has an evolutionary response as it could prevent you from being stung by a wasp or bitten by a mosquito. Thus we have developed an urge to scratch the area whenever we get a slight tickling sensation.
At the same time some itches might originate entirely in the mind and be essentially imagined. If you try to tell yourself that you can't scratch an itch no matter how much you want to (you can try this now) then it won't be more than a few minutes before you feel like you're riddled in itches you want to scratch. The brain is eventually where the signals end up, and like pain our perception of these signals can magnify or minimize their importance. Thus if you find that scratching doesn't alleviate the problem, or that you keep having itches, then distracting yourself with a fun day out or work can be one of the best ways to make the issue go away.