A Computational Model of the Interaction Between Residual Cortico-Spinal Inputs and Spinal Cord Stimulation After Paralysis

Epidural spinal cord stimulation (SCS) improves voluntary motor control after spinal cord injury. It has been hypothesized that SCS increases the excitability of spinal motoneurons thus amplifying residual supra-spinal inputs that enable volitional movement. Understanding these mechanisms would be critical to optimizing SCS for clinical use. However, no studies to date have examined how SCS interacts with residual voluntary commands. To simulate this interaction, we implemented a realistic computational model of spinal motoneurons. Our model included inputs both from proprioceptive afferents that were recruited by SCS and residual excitatory cortico-spinal axons. We simulated subthreshold or suprathreshold SCS pulses as synchronous synaptic inputs and subthreshold cortico-spinal inputs as naturally stochastic. Suprathreshold SCS completely dominated motoneurons responses, uninfluenced by cortico-spinal inputs. In contrast, cortico-spinal inputs amplified close-to-threshold postsynaptic potentials induced by SCS. However, this led to firing rates that were too low to generate movement. Therefore, our results suggest that other mechanisms in addition to excitatory supra-spinal inputs may be required to explain experimental results of SCS.