Having an itch can be incredibly annoying but it actually serves an important function, protecting us from damage to our skin. However, scientists have long struggled to explain what actually causes the sensation – in particular why some types of touch cause an itch whereas others do not.

Now a new study in mice has shed light on what actually happens in the body when we want to scratch an itch. The research, published in Science, could lead to treatments for many thousands of people suffering from chronic itch, a disorder causing an intense desire to scratch.

A hairy problem

The itching sensation usually occurs following a light touch on the hairy skin of our bodies. This triggers us to move our hand to the source of the insult and scratch away at it. While seemingly mindless, this simple behaviour is our body’s neat way of attempting to protect us from damage to our skin from objects in the environment or nasty insects and parasites.

The protective element comes from the fact that by scratching you may disturb whatever is on your skin causing the itch – just as when a mosquito lands on your arm and the tickle causes you to scratch the site and dislodge that freeloading blood sucker. What clever bodies we have.

But not everything that brushes against your skin requires an immediate frenzy of scratching. Your clothes, for example, brush constantly against your skin. If every touch caused an itch, you would end up scratching yourself senseless. So how exactly does the body know that sensations requiring action should be perceived as itchy while the multitude of other, unimportant touches should not?

The new study is important because it has started to unravel how this process works. It reveals a specialised group of cells, a subpopulation of “inhibitory spinal interneurons”, which exist in the spine and act as a gateway between the skin and the brain. These inhibitory cells work to either allow an itch sensation to travel up to the brain or stop it in its path by inhibiting the message.