Goose bumps, goosebumps or goose-pimples are the bumps on a person's skin at the base of body hairs which may involuntarily develop when a person is tickled, cold or experiencing strong emotions such as fear, euphoria or sexual arousal. The formation of goose bumps in humans under stress is considered to be a vestigial reflex. Its function in other apes is to raise the body's hair, and would have made human ancestors appear larger to scare off predators or to increase the amount of air trapped in the fur to make it more insulating. The reflex of producing goose bumps is known as piloerection or the pilomotor reflex, or, more traditionally, horripilation. It occurs in many mammals; a prominent example is porcupines, which raise their quills when threatened, or sea otters when they encounter sharks or other predators. Anatomy and biology Goose bumps are created when tiny muscles at the base of each hair, known as arrector pili muscles, contract and pull the hair straight up. The reflex is started by the sympathetic nervous system, which is responsible for many fight-or-flight responses. The muscle cells connected to the hair follicle have been visualized by actin immunofluorescence. Arrector pili muscle Arrector pili muscles (APM) are smooth muscles which connect the basement membrane to the hair follicle. When these muscles contract, they increase the trapping of air on the surface of the skin and in turn, causes thermoregulation to the body. It used to be believed that each APM was connected to an individual hair follicle. More recent studies have disproved this and now explain that there can be multiple hair follicles connected to a single APM. In between the hair follicle and the APM there are lobules which form an angular shape. These lobules are sebaceous gland lobules which are supported by the APM. Hair follicle Hair follicles have four parts. There is the bulb, supra bulbar area, isthmus and infundibulum. The bulb is to be known as the part that is responsible for the growth of the rest of the hair follicle. As a response to cold In animals covered with fur or hair, the erect hairs trap air to create a layer of insulation. Goose bumps can also be a response to anger or fear: the erect hairs make the animal appear larger, in order to intimidate enemies. This can be observed in the intimidation displays of chimpanzees, some