What are the small muscles that cause hair to stand on end called?

The small muscles that cause hair to stand on end are known as arrector pili muscles. These tiny, smooth muscles are located at the base of hair follicles in the skin. When stimulated, typically by cold temperatures or emotional responses like fear, these muscles contract, leading to the hair being pulled upright. This phenomenon is commonly referred to as "goosebumps," and it has evolutionary significance as it serves to trap air for insulation or make an animal appear larger in the face of a threat.

Other terms can refer to muscles in the body, but they do not directly describe this specific function. Erector muscles can refer to several muscle groups that help with posture and movement but are not specific to hair follicles. Skeletal muscles, on the other hand, are under voluntary control and do not contribute directly to the movement of hair follicles. Accessory muscles can refer to muscles that assist in various movements but are not specifically related to hair standing on end. Therefore, arrector pili accurately describes the muscles that create this characteristic response in the integumentary system.