Extracellular potentials of myelinated and demyelinated human motor nerve fibres

The extracellular potentials of myelinated and demyelinated human motor nerve fibres in an unbounded volume conductor are studied. Using our previous double-cable models of normal and demyelinated human fibres, the spatial and temporal intracellular potentials are calculated in the cases of point polarization and adaptation of the fibres. The intracellular potentials are then used as input to a line source model that allows to calculate the corresponding spatial and temporal extracellular potentials at various radial distances in the surrounding volume conductor. Four fibre demyelinations (termed as internodal focal\systematic and paranodal focal\systematic demyelinations, respectively) are studied. In all investigated cases, the radial decline of the peak-to-peak amplitude of the extracellular potential depends on the radial distance of the field point and increases with the increase of the distance. The results are consistent with the interpretation that the considerably different spatial and temporal distributions of the extracellular potentials depend not only on the cable properties of the fibres, but on the methods of fibre stimulation. In the case of fibre adaptation, the temporal extracellular potentials in the normal and demyelinated cases correspond well with electromyograms (EMGs) from healthy subjects and patients with demyelinated disorders as reported in the literature. Simulation results indicate that the models used are rather promising tools in studying the main properties of compound action potentials in patients with demyelinated disorders which up till now have not been sufficiently well understood.