Stimulation of the Human Lumbar Spinal Cord With Implanted and Surface Electrodes: A Computer Simulation Study

Abstract

Human lumbar spinal cord networks controlling stepping and standing can be activated through posterior root stimulation using implanted electrodes. A new stimulation method utilizing surface electrodes has been shown to excite lumbar posterior root fibers similarly as with implants, an unexpected finding considering the distance to these target neurons. In the present study we apply computer modeling to compare the depolarization of posterior root fibers by both stimulation techniques. We further examine the potential for additional direct activation of motoneurons within the anterior roots. Using an implant, action potentials are initiated in the posterior root fibers at their entry into the spinal cord or along the longitudinal portions of the fiber trajectories, depending on the cathode position. For transcutaneous stimulation low threshold sites of the same fibers are identified at their exits from the spinal canal in addition to their spinal cord entries. In these exit regions anterior root fibers can also be activated. The simulation results provide a biophysical explanation for the electrophysiological findings of lower limb muscle responses induced by posterior root stimulation. Efficient excitation of afferent spinal cord structures with a simple noninvasive method can become a promising modality in the rehabilitation of people with motor disorders.

Periodic modulation of repetitively elicited monosynaptic reflexes of the human lumbosacral spinal cord

“…Capogrosso et al 2013). They also closely resemble the lower-limb responses evoked by transcutaneous posterior root stimulation (Hofstoetter et al 2008(Hofstoetter et al , 2014Ladenbauer et al 2010;Minassian et al 2007bMinassian et al , 2011. Physiologically, PRM reflexes elicited by single stimuli share similarities with the H reflex (Maertens de Noordhout et al 1988), such as attenuation by sustained tendon vibration (Minassian et al 2007b(Minassian et al , 2011 and motor task-specific amplitude modulation (Courtine et al 2007;Hofstoetter et al 2008;Minassian et al 2007b).…”

Transspinal constant-current long-lasting stimulation: a new method to induce cortical and corticospinal plasticity

“…Therefore, dorsal column fibers along with their collaterals could have been excited only when stimulation was delivered at high intensities, although this is unlikely (Danner et al 2014). Based on differences between indirect (spinal stimulation) and direct (F wave) latencies, scan measurements of the distance between the dura and intervertebral foramina, and simulation studies, transspinal stimulation excites the nerve roots near their exit from the spinal column, near the emergence of the axons from the anterior horn cells, or close to the entry point of the dorsal root fibers (Danner et al 2011(Danner et al , 2014Ladenbauer et al 2010;Mills and Murray 1986;Struijk et al 1993). Although the exact excitation site cannot be determined from the present experiments, impulses following transcutaneous stimulation of the spinal cord traveled caudally and rostrally, affecting both descending motor inputs and ascending afferent inputs through synaptic and nonsynaptic actions changing motor cortex output at its origin site.…”

Comparison of spinal cord stimulation profiles from intra- and extradural electrode arrangements by finite element modelling

“…Moreover, with improved current density distributions, it may be possible to augment the progress made to date in the use of SCS to treat patients with spinal cord injury. Ladenbauer et al [20] have carried out finite element modelling of the stimulation process for this purpose, evaluating epidural and surface-electrode approaches to such therapies. Our ISCS approach expands this range of potential treatment options.…”