When someone runs a hand over your skin, it feels like a breeze flowing through a forest of countless tiny hairs. Nerves surrounding your hair follicles detect this contact and send signals to your brain. Some of the neurons responding to light contact may make you shiver and give you goose bumps. Others may tell you to move away or move closer.
Scientists studying the sense of touch made an interesting discovery: Follicle cells triggered by hair movements release the neurotransmitters histamine and serotonin, which are chemical messengers linked to various biological phenomena such as inflammation, muscle contraction, and mood changes. This observation was reported in October in the journal Science Advances and provides a foundation for understanding how gentle touch affects our sensations.
Hair follicle studies are challenging because the follicle cells begin to decay soon after being removed from the body, explained Claire Higgins, a bioengineering professor at Imperial College London and co-author of the study. To address this challenge, she and her colleagues went to a hair transplant clinic where they were able to examine freshly harvested follicles, which they gently prodded with a small rod to simulate touch.
The scientists knew that neurons in the skin surrounding hair follicles are capable of sensing movement from work done by other research groups.
“When you brush your hair, you feel it because the sensory neurons are directly being stimulated,” Dr. Higgins said.
The researchers were curious whether the cells of the follicle itself could contribute to feelings associated with gentle touch. Not all follicle cells had movement sensors, but some did. The researchers identified these cells and observed them closely as the rod touched them.
“We found that when we stimulated our hair follicle cells, they actually released mood-regulating neurotransmitters serotonin and histamine,” Dr. Higgins said.
Blocking the receptors for these neurotransmitters on nearby neurons meant that they no longer fired when the hair was stroked, confirming the link between the follicle cells and the neurons’ response.
Dr. Higgins explained that just because these neurotransmitters are associated with mood in the brain, it does not mean that they are linked to emotion elsewhere in the body. They are messengers, and the nature of the message they carry depends on which cells they are stimulating.
Dr. Higgins cites research by Francis McGlone, a neuroscientist at Liverpool John Moores University in England, who has studied the rewarding feelings we get from touch. He and his colleagues have identified nerves in the skin that respond to gentle touch, generating that warm glow we get from human contact.
The neurotransmitters being released by follicle cells in this study may be stimulating those nerves specifically, but no one knows for sure. Dr. Higgins hopes that future work will illuminate the identity of the cells the neurotransmitters target. She is curious about how increasing levels of serotonin or histamine in the skin might change brain function. In the small sheath of cells containing each hair, there may be answers to questions about something as fundamental as human connection.
“The follicle never ceases to amaze me,” she said.
