Regarding conduction velocity in peripheral nerve fibers, which pairing of fiber type and approximate velocity is correct?

• A A-delta fiber (fast sharp pain), 0.5 to 2 m/s
• B Alpha motor axon, 3 to 15 m/s
• C C fiber (postganglionic sympathetic, slow pain), 70 to 120 m/s
• D Ia afferent (muscle spindle primary), 70 to 120 m/s ✓

Explanation

Conduction velocity scales with axon diameter and myelination. Large myelinated Ia afferents and alpha motor axons conduct at 70 to 120 m/s, the fastest in the body. Small myelinated D-delta fibers conduct at roughly 5 to 30 m/s, and unmyelinated C fibers at 0.5 to 2 m/s. Options B, C, and A all mismatch the fiber class with a velocity belonging to another class, which is the classic trap in this topic.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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