The arrector pili is a bundle of smooth muscle cells attached to each hair follicle. This muscle is positioned so that the hair within the follicle stands on end when the muscle contracts. If a person is emotionally upset or very cold, nerve impulses may stimulate the arrector pili muscles to contract, causing gooseflesh or goose bumps. Each hair follicle also has one or more sebaceous glands associatedmycontentbreak with it.