Piloerection

Piloerection, also known as goose bumps or gooseflesh, is the involuntary contraction of the arrector pili muscles—small smooth muscles at the base of hair follicles—causing hair to stand upright and the skin to dimple. The effect results from the pilomotor reflex, a sympathetic nervous system response.

Anatomy and mechanism: Each arrector pili muscle extends from the mid-dermis to the hair follicle. When stimulated, sympathetic postganglionic nerves release norepinephrine, activating alpha-adrenergic receptors in the muscle and causing contraction. This lifts the hair shaft and creates a visible ridge of skin. In humans, the insulating advantage is minimal because body hair is sparse, but in many other mammals it can trap an insulating layer of air and may amplify signaling or appearance.

Triggers and function: Piloerection occurs in response to cold, fear, strong emotions, or tactile stimuli. In animals it has roles in thermoregulation, camouflage, and social signaling; in humans it is largely a vestigial reflex. The onset is rapid and typically brief.

Clinical notes: Piloerection is a normal autonomic reflex that can be observed during cold exposure, emotional arousal, or certain neurological states. The term describes this pilomotor response and is used in dermatology and physiology to describe the visible outcome of arrector pili contraction.