The arrector pili is the small smooth muscle bundle attached to each hair follicle, responsible for piloerection (goosebumps). Activated by cold or emotional stimuli via sympathetic adrenergic nerves, it pulls the hair follicle erect. In humans it serves minimal thermoregulatory function but remains as an evolutionary remnant. The arrector pili also provides mechanical support to the hair follicle stem cell niche.
| Origin | Papillary dermis (superficial dermis) via a fibrous anchor; the small smooth muscle bundle attaches to the basement membrane zone of the papillary dermis |
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| Insertion | Hair follicle outer root sheath, inserting at the junction of the infundibulum and isthmus portions of the follicle (approximately at the level of the sebaceous gland duct) |
| Nerve Supply | Sympathetic adrenergic fibres (alpha-1 adrenoreceptors) — no parasympathetic supply; the arrector pili is unique among smooth muscle in being under pure sympathetic control |
| Blood Supply | Cutaneous branches of segmental arteries; superficial vascular plexus of the dermis |
| Actions | Contraction pulls the hair follicle erect (goosebumps, piloerection, horripilation) by shortening the obtuse angle of the follicle to the skin surface; traps an insulating air layer against the skin for thermoregulation in furred animals (vestigial in humans); visually communicates arousal, fear, or threat response |
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The arrector pili is relevant as the target of keratosis pilaris (rough skin from blocked follicles at the arrector pili insertion), as a landmark for hair follicle anatomy in hair transplantation, and in cutaneous autonomic dysreflexia (goosebumps below the level of spinal cord injury indicating sympathetic dysreflexia). Piloerection below a spinal cord injury level during autonomic dysreflexia is a cardinal clinical sign. In dermatology, the bulge region of the hair follicle at the arrector pili insertion contains the hair follicle stem cells used in wound healing research.
Autonomic dysreflexia from a bladder, bowel, or skin stimulus below a T6 or higher spinal cord injury triggers massive sympathetic outflow below the lesion, producing hypertension, bradycardia, headache, and visible piloerection (goosebumps) and sweating below the injury level; identifying and removing the triggering stimulus and nifedipine administration for hypertension are the emergency management steps.
