The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps (piloerection).
Structure
Each arrector pili is composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit). The muscle attaches to the follicular stem cell niche in the follicular bulge, splitting at their deep end to encircle the follicle.
Function
The contraction of the muscle is involuntary. Stresses such as cold, fear etc. may stimulate the sympathetic nervous system, and thus cause muscle contraction.
Sebum excretion
Pressure exerted by the muscle may cause sebum to be forced along the hair follicle towards the surface, protecting the hair.
Hair follicle stability
Arrector pili muscles also stabilise the base of the hair follicle. Inducing contraction of the arrector pili muscles via an α1-adrenergic receptor agonist has been shown to reduce hair shedding as a result of traction alopecia.
History
The term "arrector pili" comes from Latin. It translates to "hair erector".