During induction of local anesthesia with lidocaine around a mixed peripheral nerve, which population of nerve fibers loses function FIRST, and what property explains this?

• A Large A-alpha motor fibers, because their long nodes expose maximal channel area
• B Unmyelinated C fibers, because they lack a blood-nerve barrier entirely
• C Small myelinated B fibers, because critical length for blockade is shortest when myelin is thin and spacing of nodes is close ✓
• D All fiber types equally, because lidocaine blocks channels along the whole nerve uniformly

Explanation

Local anesthetic must bathe a critical length of node (about three successive nodes) to block propagation. Small myelinated C fibers have closely spaced nodes, so a given drug exposure covers the needed number of nodes most readily, making them the most sensitive class. Classically the sensitivity order is C greater than B greater than A-delta greater than A-alpha. The distractor B is the common misconception that thinness alone dictates sensitivity, but lack of myelin actually requires longer exposed lengths.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.