Your Skin Has Tiny Muscles You Can’t Flex

Photo: Tryonisos, CC BY-SA 4.0, via Wikimedia Commons; unchanged. Image source. License.

A chill passes over you, and little bumps rise along your arm. Nobody has asked you to flex anything, yet muscles are doing the work.

They are tiny muscles attached to hair follicles. Most of the time you barely notice their existence; then a patch of skin suddenly makes them visible.

Why does your body keep a mechanism for standing its hairs on end?

A muscle at the root of the bump

The arrector pili is a small smooth muscle associated with a hair follicle. When it contracts, it changes the position of the hair and produces the familiar raised appearance of goosebumps. Signals from the sympathetic nervous system help trigger the response.

This is different from consciously tightening your biceps. You can look at the result on your arm without directly commanding each of those small muscles to contract.

A much better trick with a fur coat

In a densely furred animal, raising the hairs can help retain an insulating layer of air. Human body hair gives the same movement a less impressive thermal result. We get the visible bumps without suddenly acquiring a useful winter coat.

That apparent mismatch interested researchers. Could the structures involved in the immediate response also do something less visible?

The machinery has another connection

In a 2020 study in mice, researchers investigated the relationship among the muscle, sympathetic nerves, and hair-follicle stem cells. They found that the muscle provided structural support for nerve connections near those stem cells.

Cold-related nerve activity could produce a fast response—goosebumps—and, with prolonged stimulation, influence the stem cells involved in regenerating hair. Removing the muscle disrupted the arrangement. The work linked a familiar surface effect to a deeper system regulating the follicle.

The qualification is important: these were findings in mice. They are not evidence that deliberately getting cold will restore a person’s hair.

The little bumps are real muscle work. Their underlying biology turns out to involve more than the moment when a hair stands up.