The variation in safety factor with myelinated axon diameter: experiments with low sodium perfusion.

Theoretical considerations suggest that myelinated axons of a small diameter (and therefore with a low conduction velocity) also have a low safety factor. That these axons, many of which are involved in the mediation of pain, may fail first when action potential conduction is inhibited may explain the analgesic properties of local anesthetics. We have tested this hypothesis by perfusing frog myelinated axons with a low concentration of sodium ions and monitoring the reduction in conduction velocity, and the tendency to conduction failure. It was found that axons conducting action potentials at below 10 m/s are affected to the greatest extent by low sodium perfusion, indicating that slow conducting axons do have a low safety factor and confirming the earlier theoretical prediction.