Virtual prototyping of non-invasive spinal cord electrical stimulation targeting upper limb motor function

Area of Science:

• Neuroscience
• Biomedical Engineering
• Computational Biology

Background:

• Transcutaneous spinal cord stimulation (tSCS) shows promise for restoring motor function after paralysis.
• Existing electrophysiological data suggest mechanistic differences between cervical and lumbar tSCS, and across various cervical tSCS protocols.
• Clarifying these mechanisms is crucial for optimizing tSCS efficacy.

Purpose of the Study:

• To develop and validate a multi-scale computational model of tSCS.
• To investigate tSCS-induced volume conduction, axonal recruitment, and synaptic transmission.
• To elucidate discrepancies in electrophysiological findings between different tSCS paradigms.

Main Methods:

• Developed a whole-body computational model simulating tSCS effects.
• Simulated 24 cervical and 4 lumbar tSCS paradigms.
• Integrated computational simulations with electrophysiological recordings in 14 able-bodied individuals.

Main Results:

• Somatosensory afferents consistently showed lower stimulation thresholds than motor efferents across all simulated paradigms.
• Region-specific synaptic transmission differences may explain cervical vs. lumbar tSCS discrepancies.
• Cervical tSCS paradigms, particularly those using anodes on clavicles or iliac crests, engaged peripheral nerves alongside spinal roots, an effect amplified by multiphasic waveforms.

Conclusions:

• The developed model accurately represents tSCS-induced neural recruitment.
• Computational and experimental findings clarify electrophysiological differences observed in tSCS.
• Understanding volume conduction and recruitment patterns is key to optimizing tSCS for therapeutic applications.