ArrectorpiliMuskel

The arrector pili muscle, sometimes written as arrector pili muscle and known in Latin as musculus arrector pili, is a small bundle of smooth muscle fibers associated with hair follicles in the skin of mammals. It forms part of the pilosebaceous unit and lies in the dermis, extending from the upper dermal region to the follicle’s outer root sheath near the hair shaft.

Anatomy and arrangement: The muscle fibers run obliquely from the dermis toward the follicle, attaching to the hair follicle just above the sebaceous gland. When the muscle contracts, it pulls the hair follicle upright, causing the hair to stand and creating the appearance of goosebumps on the skin.

Innervation and function: The arrector pili muscle is under sympathetic control. Postganglionic sympathetic nerve fibers release norepinephrine, which acts on alpha-adrenergic receptors on the smooth muscle to induce contraction. In mammals, this response can help create insulation by trapping a layer of air and, in some species, enlarges the upright fur to appear larger. In humans, the effect is typically subtle and largely considered vestigial, contributing mainly to the cosmetic phenomenon of goosebumps rather than a key thermoregulatory mechanism.

Clinical notes: Variations in the function of the arrector pili muscle are generally part of normal anatomy. Neither the presence nor the strength of piloerection is commonly used in clinical diagnosis, but the muscle is routinely described in anatomical literature as part of the hair follicle apparatus.