Grid-based transcutaneous spinal cord stimulation: probing neuromodulatory effect in spinal flexion reflex circuits

<i>Objective.</i>Non-invasive spinal stimulation has the potential to modulate spinal excitability. This study explored the modulatory capacity of sub-motor grid-based transcutaneous spinal cord stimulation (tSCS) applied to the lumbar spinal cord in neurologically intact participants. Our objective was to examine the effect of grid spinal stimulation on polysynaptic reflex pathways involving motoneurons and interneurons likely activated by A<i>β</i>/<i>δ</i>fiber-mediated cutaneous afferents.<i>Approach.</i>Stimulation was delivered using two grid electrode montages, generating a net electric field in transverse or diagonal directions. We administered tSCS with the center of the grid aligned with the T10-T11 spinous process. Participants were seated for the 20 min stimulation duration. At 30 min after the cessation of spinal stimulation, we examined neuromodulatory effects on spinal circuit excitability in the tibialis anterior muscle by employing the classical flexion reflex paradigms. Additionally, we evaluated spinal motoneuron excitability using the<i>H</i>-reflex paradigm in the soleus muscle to explore the differential effects of tSCS on the polysynaptic versus monosynaptic reflex pathway and to test the spatial extent of the grid stimulation.<i>Main results.</i>Our findings indicated significant neuromodulatory effects on the flexion reflex, resulting in a net inhibitory effect, regardless of the grid electrode montages. Our data further indicated that the flexion reflex duration was significantly shortened only by the diagonal montage.<i>Significance.</i>Our results suggest that grid-based tSCS may specifically modulate spinal activities associated with polysynaptic flexion reflex pathways, with the potential for grid-specific targeted neuromodulation.