PhD Thesis Defense

Title: "Numerical investigation of spinal neuron facilitation with multi-electrode epidural stimulation"

Abstract: Approximately 1,275,000 people in the US have a spinal cord injury severe enough to cause some paralysis of the arms and/or legs. Epidural stimulation using implanted multi-electrode stimulating arrays over the lumbosacral spinal cord has recently shown promise in
assisting individuals with severe spinal cord injuries to stand, walk, and even facilitate voluntary movement. Both animal model and human studies have shown that sub-threshold facilitation of motor recovery gives the best results. The underlying neural mechanisms by which sub-threshold epidural stimulation leads to motor recovery are incompletely known.

This thesis uses computational methods to study the facilitation effect. A neuron is facilitated if a sub-threshold synaptic input can cause a neuronal output under the influence of a stimulating electric field. The analysis in this thesis is based on a computational model of the epidural spinal stimulation process in the rat spinal cord. This model includes a time-domain finite element simulation (using COMSOL) of the various tissues in the spinal cord with the appropriate
anisotropic and frequency-dependent complex relative permittivities. The voltages obtained from the finite element simulations were used as
the extracellular voltage in NEURON simulations.

A population of neurons were simulated under a wide variety of conditions.  These simulations highlight the effect of neuron orientation, location, and synaptic timing as key parameters which influence facilitation.

This study indicates that regions of the spinal cord that have previously been ignored may be actively involved in motor recovery. These results may also enable the design of specialized epidural electrode arrays and the design of new stimulation protocols.