arrector

Arrector, in anatomy, usually refers to the arrector pili muscle, a small bundle of smooth muscle associated with each hair follicle in the skin. Together with the hair follicle, it forms part of the pilomotor system. The muscle extends from the upper dermis and attaches to the follicle’s outer root sheath near the bulge region. Its fibers are typically oriented obliquely to the follicle, allowing a brief, directed contraction that elevates the hair shaft.

The arrector pili is innervated by the sympathetic nervous system. Activation occurs in response to cold, fear, or other stimuli that trigger the fight-or-flight response. Because these muscles are smooth and under autonomic control, their action is involuntary. In humans, piloerection is relatively subtle and has limited insulating value, whereas in many other mammals it can help trap a layer of air or make the animal appear larger to rivals or predators.

Distribution and terminology vary somewhat by species and region of the body. The term arrector derives from Latin for “one who raises,” and the muscle is also known as the pilomotor muscle. It is typically present in most mammals, though density and functional significance differ, reflecting evolutionary adaptations to thermoregulation and communication.