The arrector pili muscles, also known as hair erector muscles,
are small muscles attached to
hair follicles
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
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
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
fibres which attach to several follicles (a follicular unit).
Each is innervated by the sympathetic division of the
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
.
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
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...
, 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
Erection of the
porcupine
Porcupines are large rodents with coats of sharp spines, or quills, that protect them against predation. The term covers two families of animals: the Old World porcupines of family Hystricidae, and the New World porcupines of family, Erethizont ...
's long, thick hairs causes the animal to become more intimidating, scaring
predators.
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
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
.
Clinical significance
Skin conditions such as
leprosy can damage arrector pili muscles, preventing their contraction.
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