Everyone knows that scratching relieves an itch–but how? Neuroscientists now say they’ve found part of the answer in a new study of macaque monkeys. Previous research has suggested that a specific part of the spinal cord – the spinothalamic tract – plays a key role. Nerve cells in this area have been shown to be more active when itchy substances are applied to the skin. The latest work … found that scratching the skin blocks activity of nerve cells in the spinothalamic tract during itchiness – preventing the spinal cord from transmitting signals from the scratched area of skin to the brain [BBC News].
The findings could eventually lead to medical treatments for humans, says lead researcher Glenn Giesler. More than 50 conditions can cause serious itching, including AIDS, Hodgkin’s disease and the side effects of chronic pain treatment…. Some terminal cancer patients even cut back on pain medication just to reduce the itch, he said. Scratching can lead to serious skin damage and infections in people with chronic itch, he said. So scientists want to find ways for such people to relieve their distress “without tearing up their skin,” he said [AP].
In the new study, which will be published in Nature Neuroscience, researchers first implanted electrodes in the spinothalmic tracts (STTs) of the monkeys, and then injected itch-producing histamines into their legs, and watched as STT neurons started to fire at an increased rate. When researchers then used a hand-held device to scratch the monkeys’ itchy legs, the STT neurons became less active, in what researchers say was a “relief” signal. But scratching didn’t always produce this response. In contrast, when researchers scratched the leg without causing an itch first, the firing rate jumped. So the nerves somehow “know” to react much differently if there’s an itch to be relieved than if there isn’t. “It’s like there’s a little brain” in the spinal cord, Giesler said [AP].
While other researchers welcomed the advance in understanding the tricky itch, expect Gil Yosipovitch stressed that scratching and itching were complex phenomena involving factors such as emotions as well as physiology. “The main open question is what happens in patients who suffer from chronic itch where scratching may actually aggravate itch perception” [BBC News].