If you have an itch, you scratch it. Scratch and itch; they go together like peas and carrots and everyone—humans, apes, dogs and cats—knows it. What we didn't understand for a very long time was the physiological connection between the two—why a good scratch relieves a bad itch.
A study by a group of neuroscientists at the University of Minnesota recently explained the itch-scratch link.* The group hypothesized that the relief mechanism doesn't take place along the nerves of itchy skin, as had been thought, but deep in the central nervous system, in the same area that the itches themselves are communicated. Previous studies showed that neurons in the spinothalamic tract (STT)—a sensory pathway originating in the spinal cord that transmits information about pain, temperature and touch to the thalamus—were activated with the application of itch-inducing chemicals, and these are the neurons that send itch sensations to the brain.
In the new study, the UM researchers implanted recording electrodes in the spinothalmic tracts (STT) of macaques monkeys (the STT is at the base of the spinal cord; most STT neurons respond to pain and some to both pain and itch). Then researchers injected itch-inducing histamines into the monkeys' legs and watched as the STT neurons fired. They then scratched the monkey's itchy legs with a device that mimicked the feel of monkey fingers, and the firing rate of the STT neurons dropped rapidly.
The sudden drop, the researchers said, is the neurological equivalent of the relief you feel after a good scratch, indicating that itching and relief sensations are both rooted in the spinal cord and relief from an itch comes from inhibiting—via scratching—the STT neurons. Scratching basically tells all those tattle-tale neurons who are whining to the brain about an itch to just shut up already.
Of course, the itch and the scratch still hold plenty of mystery. When the team scratched the monkey's legs without first inducing an itch, the STT neurons fired in a normal response to stimuli, but the scratching didn't slow the firing.
Scratching also had no effect on neurons' response to an application of capsaicin, the spicy component in hot peppers. The STT neurons, it appears, react differently to the sensation of a scratch depending on whether an itch exists, and the nerve-dampening effect of scratching only works when the neurons are firing because of an itch, not pain. Somehow, the neurons know the difference. Itching isn't all physiological, either; it can be caused by emotional and psychological factors and can even be picked up as a "contagious itch" (a study showed that itching can be induced purely by visual stimuli: watching other people scratch).
Once all that is sussed out, though, the UM team's discovery could lead to ways of duplicating the end results and benefits of scratching (quiet, polite STT neurons) without its drawbacks. That's great news for people with the sorts of chronic itching associated with AIDS, Hodgkin's disease and the side effects of some pain medications. Chronic itching, of course, leads to plenty of scratching, which can lead to skin damage, infections and worse (remember the New Yorker article with the woman who scratched right through to her brain?)
* Davidson et al. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nature Neuroscience, 2009; 12 (5): 544