Researchers have been itching to clarify the intimate relationship between itch and pain for decades--for example, why scratching, which stimulates pain neurons, relieves an itch. Now they've found neurons in the central nervous system that respond specifically to itchiness. The finding shows that itch neurons are distinct from pain neurons, and it might someday spell relief for people who suffer from unremitting itches--an annoying side effect of many medical conditions.
A sensation starts its journey to the brain via peripheral nerves, which transmit signals from the rest of the body to the spinal cord. They carry three main types of sensations--pain, temperature, and touch--through well-understood neural pathways. A few years ago, researchers also found peripheral nerves that respond to itchy stimuli. The itch pathway wasn't complete, however, because they couldn't find neurons in the central nervous system--the spinal cord and brain--that fired specifically to itchy stimuli. Some researchers argued that itch-specific neurons didn't exist after all, suggesting that neurons treat itch as a type of pain.
To track down itchy messages in the central nervous system, neuroscientists David Andrew and Arthur Craig of the Barrow Neurological Institute in Phoenix, Arizona, examined a tract of neurons called lamina 1 in cats. This tract connects the spinal cord to a part of the brain called the thalamus, a gateway through which sensory input passes. The researchers first stimulated the thalamus and identified 190 individual nerve cells in the lamina 1 tract. Then they stimulated the peripheral nerves that feed into lamina 1. The researchers ruled out most of the nerves, because they responded to touch, pain, heat, or cold. They then focused on the 17 "silent" nerves and found that 10 fired in response to an itch-inducing histamine, as they describe in the January issue of Nature Neuroscience.
This research could one day help patients with conditions that sometimes cause chronic itch, such as liver disease and AIDS, says neurophysiologist Martin Schmelz of the Institute for Physiology in Erlangen, Germany. "There are many diseases where itch becomes very important," he says, "and when nothing is known about something, it's hard to treat it."
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