Spatio-temporal dynamics of a myelinated nerve fiber model in response to staircase-shape extracellular electrical stimulation

It has been suggested that position- or diameter-selective recruitment of myelinated axons can be achieved by up-staircase-shape electrical nerve stimulation delivered from an extracellular electrode. However, so far, detail properties of the position- or diameter-selectivity of the stimulation have not been clarified. In this study, we investigated the spatio-temporal dynamics of myelinated axons in response to staircase-shape stimulation by numerical simulation of a compartmental model of rabbit myelinated axons. We found that the upstaircase-shape stimulation activates not only the targeted axons but also the ones close to the electrode by anodal break excitation this property would degrade the distance selectivity of the stimulation. By considering the mechanism of extracellular electrical stimulation, we developed down-staircase-shape stimulation which can achieve selective activation of targeted axons without generating anodal break excitation near the electrode. Systematic simulation revealed that the down-staircase stimulation is better than the up-staircase one in terms of distance-selectivity of axon recruitment. However, diameter-selectivity of both stimulations was not sufficient.