Thearrector pili muscles, also known ashair erector muscles,[1] are small muscles attached tohair follicles inmammals. Contraction of these muscles causes the hairs to stand on end,[2] known colloquially asgoose bumps (piloerection).[3]
Each arrector pili is composed of a bundle ofsmooth muscle fibres which attach to several follicles (a follicular unit).[4] Each is innervated by the sympathetic division of theautonomic 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]
The contraction of the muscle is involuntary. Stresses such ascold,fear etc. may stimulate thesympathetic nervous system, and thus cause muscle contraction.[4]
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.
Many animals experience contraction of the arrector pili muscle in response to a perceived threat. This helps the animal seem larger and more intimidating as a result.[7]
Skin conditions such asleprosy can damage arrector pili muscles, preventing their contraction.[9] Inducing contraction of the arrector pili muscles via anα1-adrenergic receptor agonist has been shown to reduce hair shedding as a result oftraction alopecia[10]
^Journal of the American Academy of DermatologyVolume 51, Issue 2, August 2004, Pages 217-222 The contribution of the arrector pili muscle and sebaceous glands to the follicular unit structure☆ Enrique Poblet, Francisco Ortega.https://doi.org/10.1016/j.jaad.2004.01.054
