Mosquito bite? Poison ivy? Dry skin? Fuzzy sweater?
Everyone has an itch to scratch.
Why we and other animals itch remains something of a mystery. But now researchers at Johns Hopkins and Yale in the United States and several universities in China have found a key piece of the puzzle, identifying sensory neurons in mice that are dedicated to relaying itchy sensations from the top layers of skin to the spinal cord.
"Our study, for the first time, shows the existence of itch-specific nerves," said Xinzhong Dong, a professor of neuroscience at the Johns Hopkins University School of Medicine and the senior author of a paper about the findings in the journal Nature Neuroscience.
Scientists have debated for decades whether separate circuitry existed for itchiness or whether its signals passed through the same nerves used to transmit pain. Earlier data -- suppressing pain with morphine can cause chronic itching, for example -- indicated some overlap between the two sensations.
But the fact that evolution also produced dedicated itch nerves in mice -- and almost certainly in people as well -- suggests that itching serves an important role in survival and is not just a byproduct of the pain nerves.
"It's leading to a better understanding that itch has its own repertoire of receptors and nerves," said Dr. Gil Yosipovitch, a dermatology professor at Wake Forest not involved with the research.
In the experiments, Dr. Dong and his colleagues identified nerve cells that they knew responded to several itchy stimuli. They then genetically modified mice so that these nerve cells included proteins that bind to capsaicin, the chemical that gives chili peppers a burning sensation.
When capsaicin was rubbed on their skin, these mice did not writhe in pain but scratched, indicating that these neurons transmitted only itching, not pain.
In a second set of experiments, the scientists used a toxin to kill these nerve cells. The mice scratched less when exposed to itchy chemicals, but still responded as normal to pain.
"Pain is not being transmitted," by these cells, said Dr. Ethan A. Lerner, a Harvard professor who was not involved in the study. "It really is itch."
Notably, a chemical that set off the itchy signals in these mice was chloroquine, an effective malaria drug that many people in Africa refuse to take because it induces itching.
If people have the same itch-specific nerves as mice, the findings could lead to drugs that suppress the itchy side effect of chloroquine and alleviate chronic itchiness in skin diseases.
"Itching, for many, many people, is devastating," Dr. Lerner said.
Scientists do not yet agree on its role. Scratching, many think, removes irritants from the skin, but that explanation seems incomplete. By the time a mosquito bite is itchy, the mosquito has flown away. Some think it serves as a warning to avoid mosquitoes.
Scientists still have a lot to learn about itchiness. For one, why do people when they hear about itchiness -- or read this article, perhaps -- start feeling itchy? "When I give a seminar, people start scratching," Dr. Dong said. "That's a component of this called empathy."
This article originally appeared in The New York Times.