Dominic Stone
The arrector pili muscles, also known as hair erector muscles, are tiny smooth muscles attached to hair follicles in mammals. When contracted, these muscles cause the hair to stand on end, forming goosebumps on the skin. The arrector pili muscles are innervated by the autonomic nervous system and their contraction is involuntary, often occurring in response to stimuli such as cold or fear. This contraction serves a vital function in thermoregulation by providing insulation to the body. Additionally, the arrector pili muscles play a role in the secretion of sebum, contributing to the health of the hair and skin.
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What You'll Learn
- The arrector pili muscle is a tiny smooth muscle attached to hair follicles
- Contraction of the muscle causes hair to stand on end, forming 'goosebumps'
- The muscle is innervated by the autonomic nervous system
- Contraction aids the expulsion of the sebaceous gland's contents
- The arrector pili muscle may play a role in hair loss
The arrector pili muscle is a tiny smooth muscle attached to hair follicles
The arrector pili muscle, also known as the hair erector muscle, is a tiny smooth muscle attached to hair follicles in mammals. The muscle is composed of a bundle of smooth muscle fibres, which attach to several hair follicles, forming a follicular unit. The arrector pili muscle is innervated by the sympathetic division of the autonomic nervous system and its contraction is involuntary.
When the arrector pili muscle contracts, it pulls the hair into a vertical position, causing the hair to stand erect and creating small hillocks on the skin known as "goosebumps". This phenomenon is also called piloerection and is often caused by cold, fear, or other forms of stress. The contraction of these muscles in response to a perceived threat can also make an animal seem larger and more intimidating.
In addition to their role in piloerection, arrector pili muscles also stabilise the base of the hair follicle and play a role in thermoregulation. They help trap warm air between the erect hairs, aiding the body in retaining heat. The arrector pili muscle is also believed to be involved in the secretion of sebum from the sebaceous glands onto the skin surface. This relationship between the arrector pili muscle and the sebaceous gland may further protect the hair by forcing sebum along the hair follicle towards the surface.
Recent studies have also explored the potential role of the arrector pili muscle in hair loss. It has been observed that significant decreases in muscle volume and extensive fat infiltration around the residual APM of hair follicles are associated with androgenic alopecia (AGA). However, the exact relationship between APM degeneration and fat infiltration with follicle miniaturisation and hair loss remains unclear.
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Contraction of the muscle causes hair to stand on end, forming 'goosebumps'
The arrector pili muscles, also known as hair erector muscles, are tiny muscles attached to hair follicles in mammals. They are composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit). When the arrector pili muscle contracts, it pulls the hair into a vertical position, forming "goosebumps" or "goose flesh". This phenomenon is involuntary and is caused by various factors, including cold, fear, and other emotional disturbances.
The contraction of the arrector pili muscle has a significant function in providing thermal insulation for mammals. When the hair stands erect, air becomes trapped between the hairs, helping the animal retain heat. This process is known as piloerection and is an evolutionarily conserved response to emotional disturbances, as Charles Darwin noted in 1872.
In addition to their role in thermoregulation, arrector pili muscles also contribute to the stability of the hair follicle. They attach to the follicular stem cell niche in the follicular bulge, providing support and aiding in the expulsion of the sebaceous gland's contents.
Recent studies have also explored the potential connection between arrector pili muscle degeneration and fat infiltration in hair follicles, particularly in patients with androgenic alopecia (AGA). While the relationship is not yet fully understood, it has been observed that AGA patients exhibit decreased muscle volume and extensive fat infiltration around the residual APM of hair follicles.
Furthermore, the arrector pili muscle is believed to play a role in wound healing and the development of androgenetic alopecia, the most common cause of human hair loss. By understanding the structure and function of the arrector pili muscle, researchers can gain insights into various hair disorders and develop effective treatments, such as follicular unit transplantation.
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The muscle is innervated by the autonomic nervous system
The arrector pili muscles, also known as hair erector muscles, are tiny muscles attached to hair follicles in mammals. They are composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit). When the arrector pili muscle contracts, it pulls the hair into a vertical position, causing the hair to stand on end, which is known colloquially as "goosebumps" or "goose flesh".
The arrector pili muscles are innervated by the autonomic nervous system. This system is responsible for regulating involuntary functions in the body, such as heart rate, digestion, and respiration. The specific division of the autonomic nervous system that innervates the arrector pili muscles is the sympathetic nervous system. This system is often associated with the "fight or flight" response, which is activated in response to stressful or dangerous situations.
The sympathetic nervous system can be stimulated by various stresses, such as cold temperatures or fear. When this system is activated, it causes the arrector pili muscles to contract involuntarily. This contraction has several potential functions. Firstly, it can provide thermal insulation for the body by trapping air between the erect hairs, helping the body retain heat. This is particularly important for mammals that have a thick coat of fur, as the trapped air creates a layer of warmth around the body.
Additionally, the contraction of the arrector pili muscles can make an animal seem larger and more intimidating when it is experiencing a perceived threat. This response is common in many animals and can act as a defensive mechanism. Furthermore, the contraction of these muscles may aid in the expulsion of the sebaceous gland's contents, as the pressure exerted can force sebum along the hair follicle towards the surface, providing protection and lubrication to the hair.
The autonomic innervation of the skin, including the arrector pili muscles, is a complex process involving different subsets of adrenergic and cholinergic fibres. These fibres have been studied through confocal microscopy and histochemical analysis, revealing a specific distribution of neuropeptides. The understanding of these nerve fibres and their chemical coding is an ongoing area of research.
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Contraction aids the expulsion of the sebaceous gland's contents
The arrector pili muscles, also known as hair erector muscles, are tiny muscles attached to hair follicles in mammals. They are composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit). When the arrector pili muscle contracts, it pulls the hair into a vertical position, forming small hillocks known as goosebumps.
The pilosebaceous unit is a complex mini-organ consisting of three anatomic components: hair follicle, sebaceous gland, and arrector pili muscle. The sebaceous glands and the arrector pili muscle, as part of the pilosebaceous unit, should be viewed as important parts of this organised follicular unit structure.
The arrector pili muscle and the sebaceous gland have a close relationship within the follicular unit. The arrector pili muscle is arranged as a muscular unit structure at the periphery of each follicular unit. Then, at a lower level, the muscle divides and encircles the sebaceous gland, and penetrates between the sebaceous lobules towards the follicular attachment zone.
The close relationship between the arrector pili muscle and the sebaceous gland means that the contraction of the arrector pili muscle aids the expulsion of the sebaceous gland's contents. This is because, as the arrector pili muscle shortens during contraction, the concave portion abutting the sebaceous gland may cause sebum to be forced along the hair follicle towards the surface, protecting the hair.
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The arrector pili muscle may play a role in hair loss
The arrector pili muscle (APM) is a small band of smooth muscle that connects the hair follicle to the connective tissue of the basement membrane. When the APM contracts, it pulls the hair into a vertical position, forming small hillocks known as goosebumps. The APM is innervated by the sympathetic division of the autonomic nervous system.
The APM may play a role in hair loss. Several studies have suggested a possible connection between the APM degenerating and hair loss due to the subsequent disconnection between various hair follicle stem cell populations. In androgenetic alopecia (AGA), the most common hair loss condition, the APM undergoes fat degeneration, which is irreversible. In contrast, the APM remains intact in alopecia areata (AA) patients, which may explain why it is much easier to grow back hair for people with AA compared to people with AGA.
The potential role of the APM in hair loss disorders has been described in recent studies, and a model explaining APM changes in hair loss has been proposed. For example, the loss of the APM could result in an altered angle between the isthmus and infundibulum, causing retention of shed telogen hairs. Torkamani et al. have proposed that the APM degenerates in irreversible hair loss conditions such as advanced AGA.
Furthermore, the APM maintains sympathetic innervation to hair follicle stem cells (HFSCs). It is possible that the loss of APMs leads to the loss of sympathetic nerves, making HFSCs more difficult to activate. Researchers hope to develop small molecule drugs that can activate ADRB2 receptors as a means of regenerating hair.
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Frequently asked questions
The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals.
When the arrector pili muscle contracts, it pulls the hair into a vertical position, causing the hair to stand on end, a phenomenon known as "goosebumps".
Stresses such as cold, fear, or a perceived threat can stimulate the sympathetic nervous system, causing the involuntary contraction of the arrector pili muscle.
