Novel information on peripheral tactile mechanisms in man acquired with concentric needle electrode microneurography

Microneurography with tungsten electrodes has provided a wealth of new data on peripheral nerve fibre function in man. Yet, some lingering controversies pertaining to the technique and its results have not been resolved. In particular, the working principles of microneurography allowing single unit sampling in man are not fully understood. Additionally debated, especially during recent years, was the validity of some neurographic data which supported the long standing conventional concept that myelinated fibres are randomly distributed intraneurally. A novel approach to address these issues was provided by microneurography with concentric needle electrodes. Data obtained with the latter technique suggested that these electrodes record activity extraaxonally from single myelinated fibres in man, possibly at or close to a node of Ranvier. The mechanisms described, which allow single unit resolution in humans, might well also be valid when performing microneurography with tungsten electrodes. Other sets of data indicated that Ranvier nodes tend to occur in clusters within certain regions of a nerve fascicle. Interestingly, the nerve fibres belonging to these clustering nodes were of the same modality and tended to innervate the same skin area in the hand. The discovered nerve fibre segregation involved all the four main classes of myelinated low threshold skin afferents in the hand (RA, PC, SAI and SAII units). The fact that sensory nerve fibres with clustering nodes and of the same modality tend to run together suggests at least a partially ordered intrafascicular nerve fibre organisation. The demonstrated intraneural fibre systematisation could be of profound functional significance both under normal conditions and in disease