Structure X is a thermoreceptor in the skin

Steps:

• Recall that skin contains sensory receptors for various stimuli, including temperature.
• Identify structure X as a thermoreceptor based on its location in the dermis or epidermis.
• Match the function to detecting temperature changes, which aligns with thermoreceptor role.
• Eliminate other options by comparing to functions of different skin structures.

Why B is correct:

• Thermoreceptors are specialized nerve endings that detect temperature changes and send signals to the brain, per sensory physiology definitions.

Why the others are wrong:

• A: Capillary constriction is regulated by smooth muscle in vessel walls, not a specific structure like X.
• C: Impulses from the central nervous system are received by motor neurons, not sensory structures in skin.
• D: Sweat gland stimulation occurs via sympathetic nerves, not a detection structure like X.

Final answer: B