Comparison of activation and selectivity in dorsal and ventral epidural spinal cord stimulation in rats: a computational modeling study

Epidural electrical stimulation (EES) enhances motor function recovery after spinal cord injury (SCI) by modulating distinct spinal pathways through dorsal epidural electrical stimulation (dEES) and ventral epidural electrical stimulation (vEES). The characteristics between dEES and vEES remain insufficiently explored. To address this, a rat spinal computational model was developed, integrating finite element analysis and nerve fiber modeling to simulate the effects of dEES and vEES. The potential distribution generated by EES was coupled with Aα-sensory and α-motor fibers to compute thresholds, saturation amplitudes, and selectivity indices across stimulation modes. The analysis showed that dEES exhibited lower thresholds and saturation amplitudes, while vEES achieved higher muscle selectivity. Multipolar stimulation dispersed currents across multiple spinal segments, reducing target muscle selectivity and increasing thresholds and saturation amplitudes compared to monopolar stimulation. Although stimulation frequency had little effect on selectivity in both dEES and vEES, higher frequencies in dEES reduced the stimulation intensity required to achieve maximum selectivity. These findings reveal key differences in activation characteristics between dEES and vEES and highlight their potential roles in neuromodulation. Most importantly, we provided a fiber-level explanation for these differences in rats and supplemented our findings with a comparative analysis of previous studies. These insights lay a foundation for future rodent experiments aimed at optimizing epidural stimulation strategies.