Final answer:
The arrector pili muscle is the appendage attached to hair follicles that contracts causing goose bumps, usually in response to cold or emotional stimuli.
Step-by-step explanation:
The appendage of the hair follicle that contracts to cause goose bumps is known as the arrector pili muscle. This tiny smooth muscle is found in the dermis layer of the skin. It attaches to the base of a hair follicle and when it contracts, the hair stands up, resulting in the characteristic bumps on the skin surface known as goose bumps. The arrector pili are activated in response to cold temperatures or emotional stimuli as a part of the body's effort to preserve warmth or as a reaction to stress, respectively. These contractions are an automatic response that comes from the sympathetic nervous system.
The appendage of the hair follicle that causes goose bumps when it contracts is called the arrector pili muscle. These tiny muscles are located in the dermis of the skin and are connected to hair follicles. When they contract, they pull on the hair follicles, causing the hairs to stand up straight and creating the appearance of goose bumps.