[Morphologic basis of conduction block]

Conduction block is electrophysiologically defined and is shown to be an important pathologic condition encountered in both central and peripheral nervous system disorders. The conduction block is encountered most frequently in peripheral nerve injuries, which are the results of compression and ischemia. It is impossible to study morphologically the particular myelinated fibers showing the conduction block in human cases, but the alterations of myelinated fibers in the experimental models have been studied. The disturbance and/or destruction of the axoglial junction, with or without subsequent paranodal and segmental demyelination, is the major cause of the conduction block and is relatively easily identified morphologically in teased fiber and electron microscopic preparations of myelinated fibers. Following paranodal and segmental demyelination, the compensatory increase of large intramembranous particles, probably corresponding to the sodium channel, may enable the continuous conduction to be successful across the demyelinated axon. The abnormality of the nodal axolemma, dysfunction and/or loss of sodium channels, is another major cause and is not easily proven by light and electron microscopic techniques. Both causes may be concomitant under certain clinical and experimental conditions. Clinical and experimental conditions characterized by the conduction block are also listed.