The development of conduction block in single human axons following a focal nerve injury

Using microneurography with a conventional monopolar electrode, the action potentials of ten myelinated axons in the peripheral nerves of human subjects were followed while they developed conduction block. The action potentials had initially ( n = 6) or developed ( n = 4) a positive double-peaked morphology. The time interval between the two positive peaks represents the conduction time across the impaled internode. When the interpeak interval was < 500 μs, conduction across the site of impalement was secure, even if the conduction time was markedly prolonged. When the interval was > 600 μs, intermittent conduction failure occurred. For all units the longest interpeak interval recorded just prior to complete conduction failure was, on average, 1.12 ms (range, 0.8–1.4 ms). For five axons, there was evidence that natural activity triggered the conduction failure. Impalement of the nerve fibre by the microelectrode impairs the ability of the axon to conduct impulses across the site of injury, but impulse transmission can be secure even when the conduction time across individual internodes is prolonged to 500 μs. These findings are therefore relevant to the conduction deficits that occur in focal injuries of human axons.