Modulation of spinal circuits following phase-dependent electrical stimulation of afferent pathways

Peripheral electrical stimulation (PES) of afferent pathways is a tool commonly used to induce neural adaptations in some neural disorders such as pathological tremor or stroke. However, the neuromodulatory effects of stimulation interventions synchronized with physiological activity (closed-loop strategies) have been scarcely researched in the upper-limb. Here, the short-term spinal effects of a 20-minute stimulation of afferent pathways protocol applied with a closed-loop strategy named Selective and Adaptive Timely Stimulation (SATS) was explored. The SATS strategy was applied to the radial nerve in-phase (INP) or out-of-phase (OOP) with respect to the muscle activity of the extensor carpi radialis (ECR). The neural adaptations at the spinal cord level were assessed for the flexor carpi radialis (FCR) by measuring disynaptic Group I inhibition, Ia presynaptic inhibition, and Ib facilitation from the H-reflex, and estimation of the neural drive before, immediately after, and 30 minutes after the intervention. SATS strategy was proved to deliver synchronous stimulation with the real-time measured muscle activity with an average delay of 17±8 ms. SATS-INP induced an increase of the disynaptic Group I inhibition (77±23 % of baseline conditioned FCR H-reflex), while SATS-OOP elicited the opposite effect (125±46 %). Not all the subjects maintained the changes after 30 minutes. Additionally, no other significant specific neural adaptations were found for the rest of measurements. These results suggest that the short-term modulatory effects of phase-dependent PES occur at the specific targeted spinal pathways for the wrist muscles in healthy individuals. Overall, timely recruitment of afferent pathways with the muscle activity is a fundamental principle which should be considered in tailoring PES protocols for the specific neural circuits to be modulated.