What makes us scratch an itch? Scientists finally have the answer

Harriet Dempsey-Jones, PhD student in Clinical Neurosciences, University of Oxford

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 interneuron (here described as relay neuron) – the cell that governs when you itch.

(Credit: CostaPPPR/wikimedia)

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.

Chronic, chemical and contagious itch

The researchers found that when they bred mice to have a selective deficiency in these special cells, the mice began to display a startling amount of itching behaviour, even removing tufts of hair in their vigorous efforts. Poor little fellows. These behaviours mirror those seen in chronic itch disorder, which affects about 8.4% of the general population.

The new research also suggests that, if the results from the study also apply to humans, chronic itch disorder may be caused by a specific deficiency in these special spinal cells. This may lead to targeted treatments to help people suffering with this disorder in the future.

The researchers also revealed that while itching caused by light touch on the hairy skin was disrupted in the mice, there was no change in the way they responded to itches that caused an inflammatory response, for example one caused by a mosquito bite.

Why is this interesting? Because even though both kinds of itch feel subjectively the same to you, your body is sending specific information about the kind of itch which is occurring to your brain, via completely distinct pathways. The researchers call cells like these your “spinal brain” as they are a good example of how your nervous system can produce highly complex behaviours, without your conscious input.

The mice also showed completely normal responses to touch-induced pain. Interestingly, previous research has shown that there is a complicated relationship between chemical itch (from things like insect bites) and pain. It turns out that a painful touch or heat sensation can actually suppress the feeling of a chemical itch (not that this seems like a particularly good trade-off). It is for this reason that it feels so good to scratch at a rash – because it is the pain of scratching that actually relieves the itch. Unfortunately, the effect is all too temporary.

Surprisingly, there is some evidence to suggest it is not only occurrences on your skin that can cause itch and there may be a psychological element. Reports of contagious itching where watching others scratch can cause a person to feel an itch are widespread. Indeed, a recent study showed that visual and auditory scratch-related stimuli during a lecture caused a significant increase in scratching behaviour in the audience.

In the light of this finding, while the phenomenon of itching and the mechanisms which act to initiate or prevent it are a topic worth endless discussion, I think it best to stop here as I imagine you are starting to feel a little itchy yourself.

This article was originally published on The Conversation. Read the original article.