The arrector pili muscles, also known as hair erector muscles,
are small muscles attached to
hair follicles in
mammals
A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
. 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).
Each is innervated by the sympathetic division of the
autonomic nervous system.
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
Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjectivity, subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute t ...
,
fear
Fear is an unpleasant emotion that arises in response to perception, perceived dangers or threats. Fear causes physiological and psychological changes. It may produce behavioral reactions such as mounting an aggressive response or fleeing the ...
etc. may stimulate the
sympathetic nervous system, and thus cause muscle contraction.
Thermal insulation
Contraction of arrector pili muscles have a principal function in the majority of mammals of providing thermal insulation.
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 larger and more intimidating as a result.
Sebum excretion
Pressure exerted by the muscle may cause
sebum
A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur ...
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.
Clinical significance
Skin conditions such as
leprosy
Leprosy, also known as Hansen's disease (HD), is a Chronic condition, long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the Peripheral nervous system, nerves, respir ...
can damage arrector pili muscles, preventing their contraction. 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".
Additional images
File:Insertion_of_sebaceous_glands_into_hair_shaft_x10.jpg, Insertion of sebaceous glands into hair shaft
File:Skin.png, Cross-section of all skin layers
Notes
References
*Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web; https://web.archive.org/web/20110903154915/http://animaldiversity.ummz.umich.edu/site/topics/mammal_anatomy/hair.html
*
{{Authority control
Hair anatomy
Muscular system
Dermatology