Selective microstimulation of central nervous system neurons

The goal of this study was to identify stimulus parameters and electrode ge ometries that were effective in selectively stimulating targeted neuronal p opulations within the central nervous system (CNS). Cable models of neurons that included an axon, initial segment, soma, and branching dendritic tree , with geometries and membrane dynamics derived from mammalian motoneurons, were used to study excitation with extracellular electrodes. The models re produced a wide range of experimentally documented excitation patterns incl uding current-distance and strength-duration relationships. Evaluation of d ifferent stimulus paradigms was performed using populations of fifty cells and fifty fibers of passage randomly positioned about an extracellular elec trode(s). Monophasic cathodic or anodic stimuli enabled selective stimulati on of fibers over cells or cells over fibers, respectively. However, when a symmetrical charge-balancing stimulus phase was incorporated, selectivity was greatly diminished. An anodic first, cathodic second asymmetrical bipha sic stimulus enabled selective stimulation of fibers, while a cathodic firs t, anodic second asymmetrical biphasic stimulus enabled selective stimulati on of cells. These novel waveforms provided enhanced selectivity while pres erving charge balancing as is required to minimize the risk of electrode co rrosion and tissue injury. Furthermore, the models developed in this study can predict the effectiveness of electrode geometries and stimulus paramete rs for selective activation of specific neuronal populations, and in turn r epresent useful tools for the design of electrodes and stimulus waveforms f or use in CNS neural prosthetic devices. (C) 2000 Biomedical Engineering So ciety. [S0090-6964(00)00X03-1].