Scratching helps relieve an itch by blocking activity in the spinal cord nerve cells of monkeys — a finding that could lead to new treatments to alleviate persistent itching in humans.

More than 50 conditions can cause serious itching, including AIDS, Hodgkin's disease, shingles, eczema, gallbladder problems and the side-effects of chronic pain treatment.

Scratching can damage skin, and lead to infections.

Some terminal cancer patients cut back on their pain medication to ease the itching, said Glenn J. Giesler, Jr., a neuroscientist at the University of Minnesota in Minneapolis.

In people, the chemical histamine can prompt neurons to fire, sending an itch signal to the brain.

In Monday's online issue of the journal Nature Neuroscience, Giesler and his colleagues discovered which neurons were suppressed by scratching in primates.

They found scratching the skin blocks activity in part of the spinal cord called the spinothalamic tract. The spinal cord is then no longer able to transmit the signal from scratched skin to the brain.

In the experiment, researchers sedated long-tailed macaques and placed electrodes on their spinal nerves.

Next, they injected histamine into the skin of a leg, causing itching. The nerves filed electrical signals in response, which were recorded by the electrodes.

When the researchers used a hand-held device that simulated three monkey fingers to scratch the leg, the firing signal declined.

The lower signal appeared to show "relief," and indicated which neurons were targeted by scratching.

'Little brain' in spinal cord

When researchers scratched the leg without causing an itch first, however, the firing rate increased. It's as if the nerves "knew" to respond differently when there's an itch crying out for relief compared to when there isn't.

"It's like there's a little brain" in the spinal cord, Giesler told Associated Press. "We really want to understand that, because then we think we'll understand how to relieve itch."

The researchers hope to find the same neurons in people. If they can figure out how to suppress the firing using drugs or electrical pulses, it could lead to new ways of relieving itching from skin problems or other diseases.

Dr. Gil Yosipovitch of the Wake Forest University Baptist Medical Center in Winston-Salem, N.C., who didn't participate in the study, said in an email to the AP that his own work shows that particular brain circuits also play a role in how scratching quells itch.

Scratching and itching, however, also involve other factors besides physiology, such as emotions.

With files from Associated Press