Nociceptors

Meaning: Nociceptors are sensory receptors, typically free nerve endings in skin, muscle, joints and organs, that respond to noxious stimuli: intense heat or cold, strong mechanical force, or chemicals released by damaged tissue. They send signals to the spinal cord and then the brain, which produces the experience of pain. Myelinated A-delta fibers conduct faster and carry sharp, localized "first pain." Unmyelinated C fibers conduct slower and carry dull, burning "second pain."

On the MCAT: Nociceptors appear in sensation and perception questions, often alongside gate-control theory. In that theory, non-pain touch input (rubbing a bumped elbow) or descending signals from the brain can reduce pain transmission in the spinal cord. A passage may ask why a pinprick gives a sharp pain followed by a lingering ache; the answer is the difference in fiber speed. Injury can also lower nociceptor thresholds, so gentle stimuli start to hurt (sensitization).

Common confusion: Nociceptors detect potential damage; they do not themselves create pain. Pain is a perception shaped by attention, emotion and context, so identical input can feel different, and pain can occur without clear tissue damage.