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Scientists have identified what may be a dedicated sensory organ responsible for triggering the urge to scratch: fine vellus hairs, commonly known as "peach fuzz." The findings, based on experiments in mice, suggest these tiny hairs play a key role in detecting itch-inducing stimuli, Science Alert reports.
In mice, vellus hairs are most densely concentrated behind the ears and on the hind legs, where the skin is more exposed. Humans are covered almost entirely with similar fine hairs, particularly on exposed areas of the body.
Vellus hairs are already known to help regulate body temperature by trapping heat when raised and wicking away sweat in hot conditions. Researchers now believe they also contribute to sensing certain types of touch, especially those that trigger an irresistible itch.
Scientists at the University of Michigan noted that mechanically induced itch—caused by movement of fine hairs—has received relatively little attention. Previous studies showed that gently vibrating vellus hairs on the chin, unlike similar stimulation on the forehead, face, or arms, produces a strong itching sensation in healthy people.
To investigate the mechanism, researchers conducted experiments in mice and found that each vellus hair is connected to a specific class of fast-conducting sensory neurons capable of transmitting itch signals in response to even slight hair movement. In mice with chronic skin inflammation, these neurons became highly active, causing persistent itching.
When the researchers disabled the genes responsible for activating these sensory neurons, the mice no longer perceived the inflammation as an itch.
The findings, published in the journal Neuron, suggest that these specialized hairs and their associated nerve endings are essential for trichoknesis—itch triggered by contact with hair—and may play a central role in chronic itch disorders.
Earlier research also challenged long-standing medical understanding of how human hair grows, suggesting textbook descriptions of the process have been incomplete.
