Arrector pili muscle

Each arrector pili is a slender band of smooth muscle that runs obliquely from the connective tissue sheath of the hair follicle at the bulge region up to the papillary dermis. It lies on the side of the follicle that forms an obtuse angle with the skin surface, so the sebaceous gland sits in the angle between muscle and follicle.

Contraction, triggered by sympathetic adrenergic fibres in response to cold or fear, pulls the follicle upright and dimples the skin where the muscle attaches (cutis anserina, "goose flesh"). In furred mammals this traps an insulating layer of air or makes the animal look larger; in humans the thermal effect is negligible, but contraction compresses the sebaceous gland to express sebum. The muscle also anchors the bulge stem-cell niche, and sympathetic nerves wrapping it signal to the stem cells to regulate hair regeneration.