When someone brushes a hand across your skin, it’s like a breeze blowing through a forest of countless small hairs. Nerves that surround your hair follicles detect that contact, and very far away in your brain, other cells fire. Some of the neurons responding to light contact might make you shiver and give you goose bumps. Some might tell you to move away. Or they might tell you to move closer.
Scientists who study the sense of touch have explored which cells bear these messages, and they have made an intriguing discovery: Follicle cells triggered by hair movements release the neurotransmitters histamine and serotonin, chemical messengers linked to biological phenomena as varied as inflammation, muscle contraction and mood changes. The observation, reported in October in the journal Science Advances, lays the groundwork for tracing how gentle touch makes us feel the way it does.
Studying hair follicles is challenging, because they begin to decay soon after being removed from the body, said Claire Higgins, a bioengineering professor at Imperial College London and an author of the study. So she and her colleagues went to a hair transplant clinic. There, they were able to look at freshly harvested follicles, which they gently prodded with a very small rod to simulate touch.
The scientists knew from work done by other groups that the neurons in the skin surrounding hair follicles are capable of sensing movement.
“When you brush your hair, you feel it because the sensory neurons are directly being stimulated,” Dr. Higgins said.
But they were curious whether the cells of the follicle itself — the tube from which a hair sprouts — could be contributing to some of the feelings associated with more gentle touch. Not all of the follicle cells had movement sensors, but some did. The researchers identified these and watched them carefully 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.
Just because these neurotransmitters are associated with mood in the brain does not mean that they are linked to emotion elsewhere in the body, Dr. Higgins said. They are messengers, and the nature of the message they carry depends on which cells they are stimulating.
But she points to 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.
Were the neurotransmitters being released by follicle cells in this study stimulating those nerves specifically? No one knows, but Dr. Higgins hopes future work will illuminate the identity of the cells the neurotransmitters target. She is curious how increasing levels of serotonin or histamine in the skin might change what happens in the brain, at the other end of the transmission. In the tiny 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.