C fibers

Meaning: C fibers are small-diameter, unmyelinated axons of peripheral sensory neurons. Many are nociceptors with free nerve endings that respond to strong mechanical, thermal or chemical stimuli, so they signal tissue damage. Without myelin, impulses cannot jump between nodes of Ranvier, so conduction is slow. They carry dull, aching, burning, poorly localized pain that arrives after a delay and lingers (second pain). Their signals enter the dorsal horn of the spinal cord and ascend toward the thalamus.

On the MCAT: A passage describes touching a hot object: a quick, sharp, well-localized pain, then seconds later a throbbing burn. The first is carried by thinly myelinated A-delta fibers, the second by C fibers. Questions may ask why conduction speeds differ (myelination and axon diameter) or which fiber type a lesion or blocker would affect. Gate control theory is a related use: large myelinated touch fibers (A-beta) can inhibit pain transmission, which helps explain why rubbing an injury eases pain, while C fiber input tends to open the gate.

Common confusion: Students treat the two pain fibers as interchangeable or assume C means fast. Speed rises with myelination and diameter, so A-beta is faster than A-delta, which is faster than C. Sharp, fast pain is A-delta; dull, slow, lingering pain is C.