Improvement of Diameter Selectivity in Nerve Recruitment Using Multi-cuff Electrodes

Abstract

Since the conventional methodof electrical stimulation deliveredusing an extracellular electrode has poor selectivity in terms of nerve fiber type, it is difficult to avoid recruiting antitarget nerve fibers. For example, recruitment of motor fibers by electrical stimulation frequently causes unpleasant symptoms such as pain sensation andnumbness. Many electrical stimulation method s have been proposedto achieve nerve type- specific recruitment based on diameter dependence ; however, experimental evaluations have suggested insufficient selectivity of these methods. In this study, we evaluated diameter-selective recruitment using triple- cuff electrodes, which are composed of a single cathodic cuff electrode sandwiched between two anodic cuff electrodes with various anode-to-cathode distances, De . Using numerical simulations, we determined the stimulation-response characteristics as functions of stimulus intensity (IC) andfiber d iameter D (1-20 μmϕ), when De values were set at infinite and2-mm distances. We observedthat thinner axons were recruitedas IC in- creased and that the thresholds for cathodic excitation and anodal block were dependent on De. The dependence of threshold intensities for cathodic excitation and anodal block on various axon diameters at each De was analyzed . The results confirmedthat thin axons hadhigher threshold s andthat thresholdintensities were nonmonotonically dependent on De . When De was set to infinite distance (corresponding to a single-cuff electrode) and IC was relatively low, only thick axons (≥10 μmϕ ,e . g., Aα fiber group) were selectively recruited. By decreasing De to about 2 mm andincreasing the stimulus intensity, thicker axons were suppressedby anodal block ; in contrast, thinner axons (≤10 μmϕ) became selectively recruited. It was possible to recruit either moderately thick (5-9 μmϕ ,e . g., Aβ fiber group) or thin (2-4 μmϕ ,e . g., Aδ fiber group) axons by scrutinizing the De and IC dependence of nerve recruitment. Axon response was robust against radial position in the nerve bundle. These results suggest that electrical stimulation using a triple-cuff electrode properly adjusted for both stimulus intensity and distance between the anodic and cathodic electrodes has the potential for nerve-specific recruitment within a nerve bundle.