In a sciatic nerve block with 15 mL of 1.5% mepivacaine, which nerve fiber type is blocked first?

In a sciatic nerve block with 15 mL of 1.5% mepivacaine, which nerve fiber type is blocked first?

• A. Sympathetic, proprioception, pain, motor
• B. Sympathetic, pain, proprioception, motor
• C. Motor, pain, proprioception, sympathetic
• D. Pain, proprioception, sympathetic, motor

Answer: (B)

Local anesthetics block sodium channels to stop nerve conduction, and smaller-diameter fibers are blocked before larger ones. Among the fibers in a sciatic nerve block, autonomic (pre-ganglionic sympathetic) fibers are among the smallest and are blocked first. Next are pain fibers (A-delta), then proprioception fibers (A-beta), with motor fibers (A-alpha) blocked last. So the pattern is sympathetic first, then pain, then proprioception, and finally motor, which matches the given answer.

Local anesthetics block sodium channels to stop nerve conduction, and smaller-diameter fibers are blocked before larger ones. Among the fibers in a sciatic nerve block, autonomic (pre-ganglionic sympathetic) fibers are among the smallest and are blocked first. Next are pain fibers (A-delta), then proprioception fibers (A-beta), with motor fibers (A-alpha) blocked last. So the pattern is sympathetic first, then pain, then proprioception, and finally motor, which matches the given answer.