Appliance of science: Why does it feel good to scratch an itch?

Lots of things can cause us to itch but there is one unanimous response to the sensation — scratching. There is nothing quite so satisfying as scratching an itch, but what causes the itch in the first place, and why does scratching bring such relief?

What exactly is an itch?

An itch is really any uncomfortable sensation on the skin that causes a desire to scratch. A more medical term for an itch ispruritus.

The average person is estimated to scratch an itch over 90 times a day.

What causes an itch?

There are many things that cause us to itch, from insect bites to skin irritations, infections, or sometimes even diseases. Triggers are often divided into chemical, mechanical or thermal stimulants, but they all effectively do the same thing; they trigger specific nerve receptors (called pruriceptors) in our skin, sending signals to the brain, resulting in an itch-scratch response. There is still very little known about the exact mechanism of this pathway or what part of the brain triggers the response. Luckily pruriceptors are only found on the skin — can you imagine if we felt an itching sensation from our internal organs too?

How does scratching help?

Scratching can help in a number of ways. Firstly, scratching an itch can brush off any potential threat, such as a biting insect. Scratching also causes dilation of blood vessels in the area, flushing away toxins.

Scratching also appears to create a distraction, although the result is often temporary. When we scratch, we stimulate pain and touch receptors in the same area as the pruriceptors, causing low-level pain signals to be released and these override the itching sensation. Effectively, these signals cause the brain to release serotonin, a pain relieving neurotransmitter.

The relief from scratching is temporary, often leading to an itch-scratch cycle that can cause tearing of the skin and possible infection.

Why do we itch at all?

Although the sensation can range from mildly irritating to debilitating, the itch sensation is ultimately thought to be a safety mechanism. It can alert us to a possible threat, such as an insect about to bite or brushing against a poisonous plant.

Some extremes

Itching can be annoying, but spare a thought for but for some people with

delusory parasitosis , who believe that their bodies are infected with invisible bugs, mites, worms, or similar. They are constantly scratching the imagined itches from infections that just aren’t there.

Then there is phantom itching that is often reported by amputee victims, where they feel an itch in the limb that has been removed. While scientists work towards cures for this issue, they are finding some initial success with the simple use of mirrors, tricking the brain into thinking that the amputated limb is being scratched.

Many people find that their skin itches more as they age; this is known as Alloknesis. This type of chronic itch is usually triggered by light pressure on the skin. Recent studies suggest that as we age, we lose pressure-sensing cells in our skin and this can make us more sensitive to touch-related itching.