Conduction-blocking concentrations of anesthetics increase with nerve axon diameter: studies with alcohol, lidocaine and tetrodotoxin on single myelinated fibers.

The equilibrium blocking concentrations of benzyl alcohol, lidocaine and tetrodotoxin just sufficient to block nerve impulse conduction were determined on myelinated single fibers of the bullfrog. For all three anesthetics it was found that the fastest conducting fibers (45 m/sec; about 18 micron diameter) required about 4 times higher blocking concentrations than the slowest fibers (8 m/sec; about 3 micron diameter). The drugs did not affect the frog sciatic nerve length constant (2 mm), using tetrodotoxin to block the action potential. In agreement with Uehara's single fiber study using urethane (Uehara, Y.:Jap. J. Physiol. 10: 267-274, 1960), it is concluded that smaller myelinated fibers are more sensitive to anesthetic blockade.