The arrector pili muscles, also known as hair erector muscles,[1] are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end,[2] known colloquially as goose bumps (piloerection).[3]
Structure
Each arrector pili is composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit).[4] Each is innervated by the sympathetic division of the autonomic nervous system.[4] The muscle attaches to the follicular stem cell niche in the follicular bulge,[3][4][5] splitting at their deep end to encircle the follicle.[6]
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.[4]
Thermal insulation
Contraction of arrector pili muscles have a principal function in the majority of mammals of providing thermal insulation.[4] Air becomes trapped between the erect hairs, helping the animal retain heat.
Self defence
Many animals experience contraction of the arrector pili muscle in response to a perceived threat. This helps the animal seem more larger and more intimidating as a result.[7]
Sebum excretion
Pressure exerted by the muscle may cause sebum to be forced along the hair follicle towards the surface, protecting the hair.[8]
Hair follicle stability
Arrector pili muscles also stabilise the base of the hair follicle.[5][6]
Clinical significance
Skin conditions such as leprosy can damage arrector pili muscles, preventing their contraction.[9] 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[10]
History
The term "arrector pili" comes from Latin. It translates to "hair erector".[1]
Additional images
Insertion of sebaceous glands into hair shaft
Cross-section of all skin layers
Notes
References
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