Electrophysiological evidence of Ranvier node clustering in human sensory nerve fascicles.

Thin concentric needle electrodes were used to explore intact median nerve fascicles in human subjects. In particular, the presence of single units, probably recorded from nodes of Ranvier, was studied in different parts of a fascicle. Single-unit activity in myelinated fibers was rarely found at numerous sites. In many other intrafascicular areas, a substantial number of single units could be discriminated in the same or nearby recording sites with the same technique. To account for the neurophysiological results, stochastic models and statistical tests were developed to test various hypotheses concerning intrafascicular nerve fiber arrangements. The acquired data suggested both an intrafascicular modality grouping of nerve fibers and a simultaneous clustering of the Ranvier nodes of these fibers within very restricted areas of a fascicle. It was further concluded that the yield when searching for units in different types of nerve preparations may depend upon the ultrastructure of the explored nerve segments.