The arrector pili muscles, also known as hair erector muscles,
are small muscles attached to
hair follicles in
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
. Contraction of these muscles causes the hairs to stand on end,
known colloquially as
goose bumps
Goose bumps, goosebumps or goose-pimples are the bumps on a person's skin at the base of body hairs which may involuntarily develop when a person is tickled, cold or experiencing strong emotions such as fear, euphoria or sexual arousal.
The ...
(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 subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute thermodynamic ...
,
fear
Fear is an intensely unpleasant emotion in response to perceiving or recognizing a danger or threat. Fear causes physiological changes that may produce behavioral reactions such as mounting an aggressive response or fleeing the threat. Fear ...
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
Erection of the
porcupine's long, thick hairs causes the animal to become more intimidating, scaring
predators
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
.
Sebum excretion
Pressure exerted by the muscle may cause
sebum
A sebaceous 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 in the greatest nu ...
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 long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve damag ...
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
A sebaceous 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 in the greatest num ...
into hair shaft
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine ...
File:Skin.png, Cross-section of all skin layers
The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue guarding muscles, bones, ligaments and internal organs. Human skin is similar to mos ...
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