In the described sciatic nerve block, which fiber type is blocked last?

In the described sciatic nerve block, which fiber type is blocked last?

• A. Sympathetic
• B. Pain
• C. Proprioception
• D. Motor

Answer: (D)

Local anesthetics block nerve conduction by inhibiting voltage-gated sodium channels, and their effect depends on fiber size and myelination. Smaller fibers, especially autonomic (sympathetic) fibers, are affected first, followed by pain fibers. Proprioception fibers (large, myelinated) are affected next, and the largest, motor fibers are the most resistant and thus blocked last. In the sciatic nerve, motor fibers are the largest-diameter A-alpha fibers, so their conduction—and thus motor function—persists longest during the block. That’s why motor is the last to be blocked.

Local anesthetics block nerve conduction by inhibiting voltage-gated sodium channels, and their effect depends on fiber size and myelination. Smaller fibers, especially autonomic (sympathetic) fibers, are affected first, followed by pain fibers. Proprioception fibers (large, myelinated) are affected next, and the largest, motor fibers are the most resistant and thus blocked last. In the sciatic nerve, motor fibers are the largest-diameter A-alpha fibers, so their conduction—and thus motor function—persists longest during the block. That’s why motor is the last to be blocked.