Which receptors are activated by painful stimuli?

• A. Nociceptors
• B. Target cells
• C. Nonsteroid hormones
• D. Steroid hormones

Answer: (A)

Pain begins when nociceptors—specialized free nerve endings of small-diameter sensory neurons—are stimulated by tissue damage or a threat of damage. These receptors respond to mechanical, thermal, and chemical stimuli in tissues like skin, joints, and organs. When activated, they convert the stimulus into electrical signals that travel along peripheral nerves (often via A-delta and C fibers) to the spinal cord and brain, producing the experience of pain. This direct role in detecting harmful or potentially harmful stimuli is what makes nociceptors the receptors activated by painful stimuli.

In contrast, target cells are cells that respond to hormones, and nonsteroid or steroid hormones act through receptors on those target cells—these are chemical signaling mechanisms, not direct detectors of tissue damage.