Goose bumps appear due to an involuntary nervous system response called the pilomotor reflex. This primitive response occurs when hair follicles are stimulated by specific nerves in the skin, which triggers contraction of tiny muscle cells called arrector pili at the base of the hair follicles. Contraction of these muscle cells slightly elevates the hair follicle and causes the hair to stand up from the skin surface. Goose bumps typically occur in response to chills or thrills -- meaning cold temperatures, or strong emotional or psychological stimuli.
Your body works hard to maintain a relatively constant temperature. In cool conditions, activation of nerves from the sympathetic branch of the involuntary nervous system cause constriction of blood vessels in the skin, which conserves body heat. Sympathetic nerve stimulation of the skin also frequently triggers the pilomotor reflex. In furry mammals, this reflex also helps conserve heat by trapping warm air around the skin surface. However, the reflex is not an effective heat-conservation mechanism in people since humans lack dense body hair. Thus, the pilomotor reflex is a rudimentary nervous system remnant from humans' animal ancestors.
An area of your brain called the hypothalamus regulates your target body temperature, much like the thermostat in your home. Fever occurs when the hypothalamus increases your body's target temperature to an above-normal level. This triggers responses akin to what occurs in cool environmental temperatures, including stimulation of sympathetic nerves to your skin. Thus, goosebumps can occur if you have a fever -- along with chills and shivering.
That Was Scary
Among other functions, the sympathetic nervous system stimulates your fight-or-flight response. This is the almost instantaneous, adrenaline-driven reaction that kicks in whenever you encounter a dangerous or scary situation. Thus, fear is frequently accompanied by goose bumps -- the derivation of the idiom, making your hair stand up. In some instances, the perceived threat might not be real or physically imminent. So you might get goose bumps while watching a scary movie, reading a horror fiction novel, or even pondering a terrifying thought.
Riding Emotional Waves
Strong, intense emotions often trigger a wave of goosebumps. Sexual arousal also brings on goose bumps in some people. Again, the origin is believed to be involuntary electrical signaling from the sympathetic nervous system. Almost any strong emotion can potentially stimulate the pilomotor reflex, including love, joy, admiration, anger, disgust and sadness. Goose bumps typically occur in the moment, but can also develop when recalling emotional memories. The development of goose bumps while listening to music or even thinking of certain songs is thought to relate to the music provoking intense feelings or emotional memories.
Goose bumps that seem to occur for no apparent reason might be due to faulty firing of the sympathetic nervous system -- although this is uncommon. A condition autonomic dysreflexia, for example, occurs in people with spinal cord injuries. This disorder causes sudden episodes of extremely high blood pressure, typically accompanied by headaches, sweating, facial flushing, anxiety and goose bumps. Another rare disorder called pudendal neuropathy can cause goosebumps on the skin of the buttocks and genital region, along with pelvic pain. Various other uncommon medical conditions affecting the nervous system or adrenal glands -- which produce adrenaline and other hormones -- might also cause unexplained goose bumps.
See your doctor as soon as possible if you frequently experience goose bumps for no obvious reason.
Reviewed and revised by: Tina M. St. John, M.D.