In response to stimuli from the sympathetic nervous system, the arrector pili muscles:

• A. relax, allowing hair to lie flat
• B. dilate blood vessels in the dermis
• C. contract, causing hair to stand on end
• D. secrete sweat onto the skin surface

Answer: (C)

Verified step by step guidance

1. Step 1: Begin by understanding the role of the sympathetic nervous system. It is part of the autonomic nervous system responsible for the 'fight or flight' response, which prepares the body to respond to stress or danger.
2. Step 2: Identify the arrector pili muscles. These are small, smooth muscles attached to hair follicles in the dermis layer of the skin. Their function is to control the position of the hair on the skin.
3. Step 3: Recognize the physiological response triggered by the sympathetic nervous system. When activated, it causes the arrector pili muscles to contract. This contraction pulls the hair follicle upright, resulting in the phenomenon known as 'goosebumps.'
4. Step 4: Eliminate incorrect options based on the function of the arrector pili muscles. For example, they do not relax to allow hair to lie flat, nor do they dilate blood vessels or secrete sweat. These functions are performed by other structures in the skin.
5. Step 5: Conclude that the correct response to stimuli from the sympathetic nervous system is the contraction of the arrector pili muscles, causing hair to stand on end, which is a protective mechanism and part of the body's stress response.