Dominic Stone
The pilomotor muscle, also known as the arrector pili muscle, is a small muscle attached to hair follicles in mammals. The contraction of these muscles causes hair to stand on end, resulting in goose bumps. This process is known as the pilomotor reflex, which can be triggered by various stimuli such as cold temperatures, fear, or sexual activity. The primary function of the pilomotor reflex is to provide thermal insulation by trapping air between the erect hairs, helping the body retain heat.
| Characteristics | Values |
|---|---|
| Definition | Moving or tending to cause movement of the hairs of the skin |
| Other names | Arrector pili muscles, hair erector muscles |
| Composition | Bundle of smooth muscle fibres |
| Function | Contraction of the muscles causes the hairs to stand on end, known as goose bumps (piloerection) |
| Innervation | Sympathetic division of the autonomic nervous system |
| Attachment | Follicular stem cell niche in the follicular bulge of the hair follicle |
| Contraction | Involuntary; can be stimulated by cold, fear, and other stressors |
| Role | Provides thermal insulation in mammals by trapping air between erect hairs |
| Imaging | Can be imaged along with sweat glands to quantify nerve fibre densities |
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What You'll Learn
The pilomotor reflex
In addition to being a response to fear or stress, the pilomotor reflex also serves a functional purpose. When the hairs are erect, air becomes trapped between them, providing thermal insulation and helping the animal retain heat. This mechanism is particularly important for mammals that rely on their fur for warmth.
In summary, the pilomotor reflex is an involuntary response that leads to the erection of hairs on the skin due to the contraction of arrector pili muscles. This reflex is observed across different animal species and serves both communicative and functional purposes, contributing to intimidation displays and thermal insulation.
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Arrector pili muscles
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). Each muscle bundle attaches to the bulge region of the follicle and extends to its superior attachment in the upper dermis.
The arrector pili muscles are innervated by the sympathetic division of the autonomic nervous system. When these muscles contract, they pull the hair into a vertical position, causing the hair to stand on end, known colloquially as goose bumps (piloerection). The contraction of these muscles is involuntary and can be stimulated by various stresses such as cold, fear, or a perceived threat. This response may help an animal seem larger and more intimidating, while also serving the primary function of providing thermal insulation by trapping air between the erect hairs to retain heat.
In addition to their role in piloerection, arrector pili muscles also stabilise the base of the hair follicle and are related to the sebaceous gland at the hair follicle. The contraction of these muscles aids in the expulsion of the gland's contents, which can help protect the hair. Skin conditions such as leprosy can damage arrector pili muscles, preventing their contraction.
The term "arrector pili" comes from Latin and refers to the function of these muscles in elevating the hair. The arrector pili muscles are absent from the hairs of the face, axilla, pubis, eyelashes, eyebrows, nostrils, and external auditory meatus.
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The contraction of pilomotor muscles
The pilomotor muscles, more specifically called arrector pili muscles, are small muscles attached to hair follicles in mammals. When these muscles contract, they cause the hairs to stand upright. Each arrector pili muscle is composed of a bundle of smooth muscle fibres that attach to and encircle several hair follicles, forming a follicular unit. The contraction of these muscles is involuntary and can be stimulated by various factors.
One common factor triggering pilomotor muscle contraction is a drop in temperature. This is particularly observed in mammals, where the contraction of these muscles serves a crucial function in providing thermal insulation. As the hairs stand erect, air becomes trapped between them, helping the animal retain body heat. This adaptive mechanism is essential for survival in cold environments.
Another factor that can induce pilomotor muscle contraction is a response to a perceived threat or stress. This response is often associated with the "fight or flight" reaction, where the body prepares for potential danger. By causing the hairs to stand on end, the animal appears larger and more intimidating to potential predators. Additionally, the contraction of these muscles can exert pressure on the hair follicles, forcing sebum towards the surface and providing protection to the hair.
The pilomotor reflex is not limited to threatening situations; it is also observed during sexual activity in both men and women, although the effect is less noticeable on smaller male breasts. This reflex can be imaged, and nerve fibre densities can be quantified to study its function further.
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The function of pilomotor muscles
The pilomotor reflex is a process that causes the hairs on the skin to stand up, often in response to mild tactile stimuli, local cooling, or stress. This reflex is caused by the contraction of small, smooth arrector pili muscles, which attach to hair follicles. These muscles are supplied by autonomic nerves, specifically postganglionic sympathetic nerve fibers. When these nerves are stimulated, the arrector pili muscles contract, pulling on the hair follicles and causing the hairs to stand up. This process is often accompanied by the release of sebum, which lubricates the hair and can make it appear more intimidating or larger, particularly in mammals with thicker coats.
The pilomotor reflex can be observed in humans as goose bumps, which are a result of the contraction of the arrector pili muscles and the subsequent raising of the hair. This reflex can be triggered by various stimuli, including cold temperatures, fear, or sexual activity, and is less noticeable on smaller areas of hair, such as male breasts.
In addition to providing a visual response to stimuli, the pilomotor reflex also serves a functional purpose, particularly in hairier mammals. When the arrector pili muscles contract, they create a layer of insulation by trapping air between the erect hairs. This helps the animal retain heat and can be crucial for survival in cold environments.
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Imaging pilomotor muscles
Quantitative pilomotor axon-reflex testing is a novel approach to understanding pilomotor function. This involves stimulating piloerection through methods such as iontophoresis of phenylephrine or the application of a vasodilating agent like nitroprusside. High-resolution photographs or laser-Doppler imaging is then used to capture the piloerection response, allowing for the quantification of pilomotor function.
One example of this imaging technique is described in a study by Freeman, where silicone impressions were taken of the skin during piloerection. These impressions were analysed using image analysis software to quantify the number and area of erect hair follicles. This method provides a non-invasive way to study pilomotor function and can be applied to research in various fields, including neurology and dermatology.
Furthermore, imaging pilomotor muscles can aid in evaluating the effectiveness of treatments for hair-related conditions. For example, inducing contraction of the arrector pili muscles using an α1-adrenergic receptor agonist has been shown to reduce hair shedding associated with traction alopecia. Visual assessment of pilomotor muscle contraction can, therefore, be a valuable tool in the development and evaluation of treatments for hair loss and other dermatological conditions.
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Frequently asked questions
Pilomotor refers to the movement of hairs on the skin, also known as the pilomotor reflex.
The pilomotor reflex is the process by which the arrector pili muscles contract, causing hairs to stand on end, resulting in goose bumps.
The pilomotor reflex can be caused by a mild application of a tactile stimulus, local cooling, or stress such as cold or fear.
The pilomotor reflex serves to provide thermal insulation in mammals by trapping air between the erect hairs, helping the body retain heat.
