Which peripheral nerve fiber class conducts action potentials at the greatest velocity?

• A A-beta fibers
• B A-delta fibers
• C C fibers
• D A-alpha fibers ✓

Explanation

A-alpha fibers are the largest (12 to 20 micrometers) and heavily myelinated, conducting at 70 to 120 m/s. They carry proprioception from muscle spindles and motor output to extrafusal muscle. The best distractor is the A-delta fiber: it is myelinated but small (2 to 5 micrometers), so it conducts at only 5 to 30 m/s and carries fast pain and temperature. Conduction velocity rises with both diameter and myelination.

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

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