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 ...

in mammals. 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 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 ...

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 internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...

. 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 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 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 t ...

, 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, Erethiz ...

's long, thick hairs causes the animal to become more intimidating, scaring predators.

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 d ...

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 number ...

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