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The arrector pili muscle, also known as the hair erector muscle, is a small muscle that is connected to each hair follicle in the skin of mammals. This tiny muscle plays an important role in the skin’s reaction to cold or emotional arousal.
When the arrector pili muscle contracts, it causes the hair to stand up, commonly known as “goose bumps”. This reaction originally had an important function in our hairier ancestors and in other mammals, where the raising of the hair served to improve insulation and thus protect the body from the cold. In animals with thick fur, this response can also help to make the animal look larger and more threatening when threatened.
In humans, however, this effect is largely symbolic, as our body hair is relatively sparse compared to many other mammals. Nevertheless, the ability to get “goosebumps” is a remnant from the time when our ancestors had much more body hair. In addition to responding to cold or emotional stress, activation of the hair erector muscles can also be part of normal hair follicle regeneration and occur during certain skin conditions or reactions.
