What term describes a muscle located along the side of hair follicles?

The term that describes a muscle located along the side of hair follicles is the arrector pili muscle. This small, smooth muscle is responsible for causing the hair to stand upright, a process often referred to as "goosebumps." The arrector pili muscle responds to stimuli such as cold temperatures or emotional responses, contracting and pulling on the hair follicle, which results in the hair standing on end.

The dermal papilla is a structure located at the base of the hair follicle that contains blood vessels and nerves, providing nutrients to the hair root. While it plays a crucial role in hair growth, it is not a muscle. The bulb refers to the expanded base of the hair follicle where growth occurs, and the follicle itself is the structure housing the hair, but neither is associated with the muscular action of hair standing up. Thus, the correct answer identifies the specific muscle that interacts with the hair follicle.