Continuous conduction in demyelinated mammalian nerve fibres

Abstract

MYELINATED nerve fibres conduct in a saltatory fashion, with sites of inward membrane current restricted to the nodes of Ranvier¹. Loss of myelin causes long delays in the inter nodal conduction time, extreme refractoriness and conduction block². In the large rat ventral root fibres studied the slowed conduction always remained saltatory, with discrete sites of inward current, but the limited spatial resolution of the method used prevented conclusions about whether any membrane beyond that of the original nodes was excited. To find out more about the inter nodal axon membrane exposed by demyelination, we have used an improved technique of external longitudinal current analysis. A smaller electrode separation (120 µm as against 400–600 µm) has improved the spatial resolution³, and signal averaging has made possible recordings from smaller axons. We have found that in these fibres not only is internodal axon membrane electrically excitable, but that it can support continuous conduction across a demyelinated inter-node. This result provides a new basis for interpreting the effects of demyelinating disease.