Arrector muscle of hair

The arrector pili muscle (also known as the arrector muscle of hair) is a small bundle of smooth muscle fibers attached to the connective tissue sheath of each hair follicle. These tiny muscles are found in the dermis of the skin and are innervated by the sympathetic division of the autonomic nervous system.

Anatomy and Attachment

The arrector pili muscle originates from the superficial dermis (papillary layer) and inserts into the connective tissue sheath surrounding the hair follicle, below the sebaceous gland. Each muscle is oriented at an oblique angle relative to the skin surface.

Function

When the arrector pili muscle contracts — typically in response to cold temperatures, emotional stimuli (such as fear), or sympathetic nervous system activation — it causes the hair to become erect (stand upright), producing the phenomenon commonly known as "goosebumps" (cutis anserina). This contraction also compresses the sebaceous gland, aiding in the secretion of sebum onto the skin surface.

Clinical and Physiological Significance

• In many mammals, erection of the hair serves as a thermoregulatory mechanism by trapping a layer of insulating air close to the skin. In humans, this function is largely vestigial.

• The arrector pili muscles also play a role in the hair follicle stem cell niche, as they connect to the bulge region of the hair follicle, which houses epithelial stem cells important for hair cycling and wound repair.

• In certain dermatologic conditions (e.g., pilomotor seizures, keratosis pilaris), the behavior or anatomy of these muscles may be clinically relevant.

Innervation

The muscles are exclusively innervated by adrenergic sympathetic nerve fibers, with norepinephrine acting on alpha-1 adrenergic receptors to trigger contraction. Notably, the arrector pili muscles are one of the few structures in the body that receive only sympathetic innervation without parasympathetic counterpart.