Transcutaneous electrical stimulation of the spinal cord: unveiling the mechanisms of action

Spinal cord injury (SCI) causes persistent sensorimotor and autonomic deficits that severely impair quality of life and impose a substantial societal burden. Although limited spontaneous recovery can occur after injury, this recovery is typically incomplete. Neuromodulation strategies have emerged as promising approaches to enhance recovery, with epidural electrical stimulation (EES) demonstrating remarkable clinical efficacy. More recently, transcutaneous spinal cord stimulation (tSCS) has gained attention as a non-invasive alternative capable of improving motor function after SCI. Despite encouraging clinical outcomes, the biological mechanisms underlying tSCS-mediated recovery remain poorly understood. In this thesis, we developed a comprehensive preclinical framework to dissect the mechanisms of action of tSCS and to identify the neural circuits mediating recovery. We engineered a scalable, wearable stimulation device optimized for rodents, integrating novel electrode materials into a modular design that ensures stable and reproducible stimulation. This interface was combined with a robotic neurorehabilitation platform enabling precise, quantitative assessment of upper-limb motor function during longitudinal rehabilitation. Using this platform, we demonstrate that a clinically inspired high-carrier-frequency stimulation protocol (ARCEX) reproduces key functional benefits observed in humans. We show that non-invasive stimulation accesses the spinal cord primarily through proprioceptive sensory afferents, which relay stimulation evoked signals to Vsx2-expressing spinal interneurons. Through electrophysiology, transcriptional profiling, viral tracing, and targeted ablations, we establish that both proprioceptive afferents and Vsx2- expressing interneurons are required for ARCEX-mediated recovery. We further demonstrate that stimulation protocols differ fundamentally in their recruitment of sensory pathways. In contrast to ARCEX, conventional stimulation paradigms preferentially engage cutaneous nociceptive afferents, increase discomfort and pain-related neuronal activation, and fail to promote long-term functional recovery. Standardized pain metrics, molecular profiling of dorsal root ganglia, and pharmacological silencing of skin afferents reveal that excessive nociceptive recruitment constrains the therapeutic window of tSCS and disrupts rehabilitation-induced recovery. Finally, we place these findings in the context of spontaneous recovery after cervical SCI. Using whole-brain mapping, transcriptomics, and circuit-specific manipulations, we identify a distributed recovery architecture centered on the parvicellular reticular nucleus (PARN) and spinal Vsx2-expressing interneurons. We show that ARCEX engages and refines this endogenous recovery network, thereby amplifying natural repair mechanisms. Together, this work establishes a mechanistic framework linking non-invasive spinal cord stimulation to circuit level plasticity and functional recov