Which nerve fiber type is most susceptible to nerve block?

Which nerve fiber type is most susceptible to nerve block?

• A. A fibers
• B. B fibers
• C. C fibers
• D. D fibers

Answer: (B)

Blockade by local anesthetics hits the smallest, myelinated fibers first. These are the preganglionic autonomic fibers, which have a small diameter and a myelinated sheath. Their conduction relies on fewer voltage-gated sodium channels, so when the drug binds to these channels, conduction fails more readily. That makes them the most susceptible to nerve block. As a result, autonomic effects (like vasodilation) appear early, followed by blockade of pain and other sensory fibers, and finally motor fibers with larger diameters are last to be blocked.

Blockade by local anesthetics hits the smallest, myelinated fibers first. These are the preganglionic autonomic fibers, which have a small diameter and a myelinated sheath. Their conduction relies on fewer voltage-gated sodium channels, so when the drug binds to these channels, conduction fails more readily. That makes them the most susceptible to nerve block. As a result, autonomic effects (like vasodilation) appear early, followed by blockade of pain and other sensory fibers, and finally motor fibers with larger diameters are last to be blocked.