Final answer:
The arrector pili muscle, located in the dermis, is responsible for causing hair to stand up and create goose bumps when it contracts in response to stimuli like cold or fear.
Step-by-step explanation:
The portion of the hair follicle responsible for causing the hair to stand up and create goose bumps is the arrector pili muscle. These tiny muscles are located in the dermis and are activated in response to external stimuli, such as cold or fear. When these muscles contract, they pull on the hair follicles, causing the hairs to stand up straight. This reaction is not only a human response but is also seen in mammals, and it serves to conserve body heat by trapping more air under the hairs, which insulates the skin.
Goosebumps are caused by the contraction of tiny muscles called arrector pili in the dermis of the skin. These muscles are responsible for causing the hair to stand up. When the body is cold or afraid, the arrector pili muscles contract, pulling on the hair follicles and making the hair shafts stand upright.