Effect of Non-invasive Spinal Stimulation on Self-sustained Firing Motoneuron Model: In-Silico Study Using Human Body Model

Transcutaneous spinal electrical stimulation (tSCS) is a non-invasive neuromodulation approach using a low intensity direct current. Recent developments in the technique have opened the possibility that tSCS can help restore motor function after spinal cord injury (SCI). However, the exact mechanism of action tSCS has on the spinal circuits is still unknown. Due to the complexity of experimental synthesis in a human model to delineate the mechanisms, models that link the stimulation paradigm and circuit behaviors are advantageous. Thus, this study aims to simulate the underlying changes in motor circuit firing rates in response to external stimuli induced by tSCS. Serial stimulations combining a high-fidelity finite element model with the human torso and spinal cord with a lumped motor neuron model is constructed. The parameters for both components of the model were derived from previous studies. We focused our analysis on a lumped motor neuron model that describes sustained firing behavior of the motor neuron driven primarily by persistent inward current (PIC), a signature behavior of the motor neuron after SCI. Modulation of the PIC behaviors was achieved by stimulating voltage-dependent calcium and sodium channels in the dendrite using a tSCS-induced electric field (E-field) expressed at different a spatial locations of the motor neuron in the gray matter. The PIC behaviors of spinal motor neurons in the left ventral horn were suppressed, while for the most part invariant in the right ventral horn. These initial simulations will provide a steppingstone for future examinations that incorporate additional neuronal models of inhibitory and excitatory interneurons to access the circuit-level effect of spinal stimulation.