On our nerves: what makes us itch or feel wet?

Michael Brooks’s science column. 

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We’re all going to feel some unwanted damp on the skin over the next few weeks – welcome to autumn. But for those who feel wet due to a medical condition rather than the weather, researchers at Loughborough University have made what might just prove to be a welcome breakthrough.

It starts with a seemingly innocuous question: what makes wet stuff feel wet? By the end of this exploration, we will have encountered Joni Mitchell, patients with multiple sclerosis (MS) and a host of people suffering in ways that evoke Dante’s Inferno.

To some animal species, wetness is so critical to survival that evolution has equipped them to determine their state of external hydration: insects have humidity sensors. Human beings, however, don’t have wetness sensors on their skin, so understanding how we differentiate the sense of wetness from other sensations is a puzzle.

We have found clues in some of the tricks one can play on our species. The Loughborough researchers have shown that if you reduce the skin’s temperature using a dry cooling method, people feel as though their skin is wet. If you put something wet in contact with the skin, but at a temperature warmer than it, people don’t perceive it as wet.

So, clearly, we don’t feel wetness, we infer it. Our skin has an array of sensors for temperature and pressure, and it is a combination of these senses which tells us that something we are touching is wet. To find out what that combination might be, the Loughborough team experimented on 13 students, blocking and releasing their nerve sensitivities.

It turns out that crucial to wetness perception are nerves known as A-nerve fibres. Block the blood supply to these – using something like a blood-pressure cuff – and you become far worse at sensing wetness. Unsurprisingly, it is easier to sense cold wetness than warm wetness. The interplay of these different sensitivities enabled the researchers to create a model for the brain’s interpretation of wetness; in essence, it applies a weighting to each set of inputs in order to come to a probability-based conclusion on the body’s state.

This is more than an academic discovery because skin sensitivity is a serious medical issue. People suffering with MS frequently report an unpleasant feeling of cold wetness on their skin. It is a couple of short steps from feeling cold wetness to pain. One side effect of diabetes, for instance, can be dysaesthesia, when diabetics experience a burning or stabbing sensation on their skin, or feel the slightest touch from clothing or bedlinen as excruciating pain. In other cases, some diabetics can’t feel heat or touch sensitively enough to avoid injuring themselves.

It’s not just about the side effects of recognised diseases, though. There are various medical conditions associated with nerves sending pain signals in response to (apparently) nothing. Sufferers of central pain syndrome can report sensations such as being torn apart with hot knives, or being burned alive. No wonder it gets referred to as a Dante-type condition. Another oddity is Morgellons Disease. Joni Mitchell is perhaps the best-known sufferer of this unstoppable itching, which feels as if something is crawling under the skin. The medical orthodoxy is that the condition is indicative of a psychiatric disorder. However, if we knew more precisely what our skin’s nerve endings transmit to the brain, we might be able to help sufferers, delusional or not. 

Michael Brooks holds a PhD in quantum physics. He writes a weekly science column for the New Statesman, and his most recent book is At the Edge of Uncertainty: 11 Discoveries Taking Science by Surprise.

This article appears in the 08 October 2014 issue of the New Statesman, Grayson Perry guest edit