The arrector pili muscle is a minuscule, specialized structure present across the skin of most mammals. This tiny muscle attaches to the hair follicle, the sheath of cells and connective tissue surrounding the hair root. Its placement allows it to influence the hair’s orientation and the surrounding skin.
Anatomical Placement and Structure
The arrector pili muscle (APM) is characterized as a small bundle of smooth muscle fibers, meaning its action is entirely involuntary. It does not respond to conscious control like the skeletal muscles that move our limbs. The muscle originates in the papillary layer of the dermis, the upper layer of the skin beneath the epidermis.
The muscle extends obliquely across the dermis before inserting into the connective tissue sheath of the hair follicle. This attachment point is found just below the sebaceous gland, which sits in the angle formed by the hair follicle and the muscle itself. The APM is absent in certain areas like the face, armpits, and external ear canal.
The Mechanism of Piloerection
The primary function of the arrector pili muscle is to cause piloerection, commonly known as getting “goosebumps.” When the muscle receives a signal, it contracts, pulling on its two attachment points simultaneously. Because the muscle is angled, contraction straightens the hair follicle, pulling the hair shaft into a vertical position on the skin’s surface.
This pulling action creates a small, visible mound on the skin where the hair exits, which is the characteristic goosebump. Simultaneously, the contraction exerts pressure on the nearby sebaceous gland, assisting in the expulsion of sebum, the skin’s natural oil. The resulting dermal deformation is a transient reflex action.
Control and Evolutionary Significance
The contraction of the arrector pili muscle is regulated by the sympathetic branch of the autonomic nervous system, the body’s involuntary control system responsible for the “fight-or-flight” response. Sympathetic nerves release a chemical signal, such as norepinephrine, which triggers the muscle fibers to contract in response to external stimuli.
These stimuli include cold temperatures, fear, or intense emotional arousal. In mammals with dense fur, raising the hair created a thicker layer of trapped air, enhancing insulation against the cold for thermoregulation. It also made the animal appear larger and more intimidating to predators or rivals, serving as a defense mechanism.
While the reflex persists in humans, it is largely considered a vestigial trait due to our sparse body hair. The insulating effect is minimal, meaning the muscle no longer serves its original thermoregulatory function effectively. However, the continued presence of the arrector pili muscle highlights an evolutionary link to our furrier ancestors.