Every dog and cat knows that scratching relieves an itch. But for ages, not even neuroscientists knew why.
Now, a University of Minnesota study shows that scratching turns off activity in spinal cord nerves that transmit the itching sensation to the brain. The researchers hope eventually to learn just how the inhibition works.
That in turn could lead to new ways of duplicating the benefits of scratching without the drawbacks, which would greatly benefit people with chronic itching.
"We want to know if we can get inhibition by less dangerous means than scratching," explains neuroscience professor Glenn Giesler, Jr., who led the study with graduate student Steve Davidson. "This [study] gives hope, though not certainty, that we've found the primary place where scratching works."
Scratching an ordinary itch usually does no harm. But chronic itching may respond to scratching only very temporarily or not at all. This can rob people of sleep, throw them into a depression, or cause even worse damage, Giesler says. For example, inflammation from herpes can leave a bad itch called post-herpetic pruritis in its wake.
"There is a reliable report of a woman who had post-herpetic pruritis," he recalls. At night "she scratched through her skull, right down to her brain."
More than 50 other conditions also produce chronic itching, including eczema (which affects 2.6 percent of the U.S. population, according to Giesler), AIDS, and Hodgkin's disease. Over half of patients with renal failure or liver disease also suffer from it.
In nature, itching and scratching help animals get rids of parasites, plant material, or other unwanted baggage.
"But what value could it have in AIDS patients?" asks Giesler. "Some psychiatric patients also get chronic itch. It's amazing how little research there's been on itch over the years."
And if some of you have been scratching the whole time while reading this, he also notes that itching can also be psychogenic.
Anatomy of an itch
In a common itch like a mosquito bite, cells in the skin release a chemical called histamine. Certain sensory neurons respond to the histamine and carry the "itch" message up to the spinal cord. There the message is relayed to other neurons that run to a part of the brain called the thalamus. These neurons are part of a bundle of spinal neurons called the spinothalamic tract, or STT.
From the thalamus, the "itch" message is passed to the cerebral cortex, which interprets the signals and produces the sensation of itch.
Working with primates, Giesler and his colleagues studied neurons in the STT that fired after histamine was applied to the skin of a foot. But their response was dampened, or inhibited, when the researchers subsequently scratched the itchy skin. Scratching by itself—that is, in the absence of an itch—had no such effect.
"Before, we didn't know if the inhibition occurred in the spinal cord, the thalamus, or the cortex," Giesler notes. "We've shown that it occurs in the spinal cord and lasts for a period of time similar to the relief one gets while scratching."
All STT neurons, including those that respond to itch, also respond to pain signals, Giesler says. But the itch-sensitive neurons' response to a mildly painful stimulus—capsaicin, the burning agent in hot peppers—was not dampened by scratching. Therefore, evidence points to them as the key players in the relief of itch by scratching, says Giesler.
- Davidson et al. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nature Neuroscience, 2009; 12 (5): 544 DOI: 10.1038/nn.2292
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