What muscle is responsible for causing hair to stand on end?

The arrector pili muscle is responsible for causing hair to stand on end, a phenomenon commonly known as piloerection. This small, smooth muscle is attached to hair follicles and, when stimulated by cold temperatures or emotional reactions such as fear, it contracts. This contraction pulls the hair follicle upright, which not only makes the hair stand on end but also causes the skin to form small bumps, often referred to as "goosebumps."

The other muscles listed have different functions and do not play a role in the piloerection response. The deltoid muscle is primarily involved in shoulder movement, the trapezius muscle aids in moving and stabilizing the shoulder blades, and the serratus anterior muscle plays a crucial role in the movement of the scapula. Understanding the specific roles of these muscles helps clarify why the arrector pili muscle is unique in its function related to hair follicles.