The arrector pili muscles cause hairs to stand on end when the body is chilled. These are tiny muscles in the skin that connect hair follicles to skin tissue. The muscles contract due to information from autonomic nerves located within these muscles.
The arrector pili muscles are smooth, fan-shaped muscles. When the body becomes cold, these muscles contract, making the hairs connected to them stand on end. The muscle contractions press on nearby oil glands causing the skin to form "goose bumps." Other names for these muscles include erector pili muscles and pilomotor muscles. All mammals have these types of muscles.