Axonal Excitability and lntermittent Conduction Block in Demyelinated Axons

Abstract

Healthy myelinated axons conduct securely because of a high density of Na? channels at the node of Ranvier, but conduction in demyelinated axons is precarious. C onduction block occurs when the driving current at the blocking node is insufficient to reach threshold far action potential generation. Conduction block can be stable, detectable by routine nerve conduction studies with supramaximal stimuli delivered at 1 Hz, or intermittent, due to either hyperpolarization at the blocking node or a reduction in the driving Na+ current (or both). There are a number of physiological mechanisms that will cause partially demyelinated axons to stop conducting, e.g., changes in membrane potential (both depolarizing and hyperpolarizing), ischaemia and its release, and fluctuations in temperature. As a result, a patient's deficit will fluctuate if a sufficient number of axons start or stop conducting. The detection of intermittent conduction block requires protocols additional to the routine test with supramaximal stimuli at 1 Hz to a motor nerve of a resting patient. A protocol that should detect intermittent conduction block as well as stable conduction block is presented.