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AS WE improve our understanding of chronic pain, there is a related problem that has researchers scratching their heads.

The sensation of itch, which shares many nervous pathways with pain, has long been a mystery. One reason is that it can be conjured up entirely by the brain. Just thinking about something tickly, or even watching someone else scratch, can set us off. Efforts to find out why this is, and why only scratching makes it stop, have led researchers to study how itch is processed in the skin, spinal cord and brain. In recent years they have begun piecing together a more complete story of what itch is for 鈥� and what to do when an itch can no longer be scratched.

鈥�165,000 people in the US died from prescription opioid overdoses between 1999 and 2014鈥�

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Itch, like chronic pain, is a surprisingly common problem. It comes as a side-effect of opioid painkillers, the use of which has rocketed in recent years (see graph). It also comes with skin conditions such as eczema as well as nerve damage, kidney disease and allergies. An estimated one in , and current remedies are providing little relief.

The most common treatments are antihistamines, which block the part of the immune reaction that causes itching when we are bitten by a bug, for example. Not all itches are immune-related, however, and antihistamines have proven to be ineffective for most forms of chronic itch. Neuropathic itch, which stems from damage to sensory nerves in the skin, has no effective treatment.

In recent years, researchers have discovered new kinds of receptor in the skin and spinal cord that play a role in the sensation of itch. activates these receptors in the hope of designing drugs that can deactivate them.

For now, though, the single most effective way to get rid of an itch is what we do instinctively: to scratch. But why do we rake our nails across our skin, when it causes pain and, potentially, tissue damage?

鈥淐ontagious itch may have evolved as a social signal to warn that parasites are nearby鈥�

The answer lies in the way that the spinal cord is wired, says Sarah Ross, a neurobiologist at the University of Pittsburgh. Neurons that transmit long-distance messages between the body and the brain are bridged by smaller cells called interneurons. These can either be excitatory, spurring messages towards the brain, or inhibitory, stopping the messages dead. Scratching works because the body processes this sensation as more urgent than itch, so inhibitory interneurons halt the itch message and fast-track the pain signal instead.

There are several different types of interneurons in the spinal cord, though, and not all of them require pain to stifle itch. For example, Ross has found that in mice, the neurons that detect the sensation of coolness from a cold compress or menthol make direct connections with a type of interneuron that prioritises the sensation of coldness over itch.

Qiufu Ma, a researcher at Harvard Medical School, identified another group of interneurons in the spinal cord that respond to a tickling sensation on the skin. Sensory neurons that transmit this kind of itch may be among the most sensitive neurons in the peripheral nervous system, Ma says. When stronger pressure activates other mechanically sensitive neurons, the tickle is stopped in its tracks to allow the stronger sensation to pass. The fact that pressure has the same effect on the interneurons as the pain caused by scratching suggests that rubbing can be as effective as scratching, and there is no need to tear at your skin with your nails.

鈥淚n 2014, almost 2 million people in the US abused or were dependent on prescription opioids鈥�

Source: Substance Abuse and Mental 91色情片 Services Administration, National Survey on Drug Use and 91色情片, 2014

Further clues on how to tackle itch come from brain imaging studies. Many brain areas activated by pain are also active during itch, but one area that seems distinctively responsive to itch is the precuneus. This brain region is involved in visual processing and memory, says Gil Yosipovitch at the University of Miami, Florida. 鈥淲e can鈥檛 pinpoint what the precuneus does in itch, but it鈥檚 uniquely activated with itch and not pain,鈥� he says.

Yosipovitch has also shown that the act of scratching activates pleasure centres of the brain, which explains why scratching an itch feels so good. This pleasure seems to be particularly strong for people with chronic itch conditions. 鈥淭he more itch you have, the higher the pleasure you get from scratching,鈥� he says. The fact that the pleasure of scratching only activates the reward circuits more over time explains why the itch-scratch cycle is so hard to break once the problem becomes chronic.

He has also found to contagious itch 鈥� that irresistible urge to join in when someone else is scratching. Contagious itch is thought to have evolved as a social signal, much like contagious yawning. The idea is that joining in when seeing someone else scratch would give people a headstart in dislodging any parasites that might be around.

Those brain signals might be strengthened over time in people with chronic itch, suggests Yosipovitch. Whether that happens purely as a result of experience, or whether some people鈥檚 wiring predisposes them to itch or pain remains something of a chicken-and-egg question.

One important realisation, says Yosipovitch, is that there are many types of itch, each of which activates different nerve cells and brain circuits. 鈥淣ot all types of itch are equal.鈥� And while the advances in our understanding of itch haven鈥檛 yet translated into new treatments, the same psychological interventions that are working in chronic pain could help people live with itch. 鈥淪tress is one of biggest aggravating factors,鈥� says Yosipovitch.

This article appeared in print under the headline 鈥淚tch: When pain feels good鈥�