Apparatus for multi-channel transverse epidural spinal cord stimulation uses a multi-channel pulse generator driving a plurality of electrodes mounted near the distal end of a lead. These electrodes are mounted in one or more lines, generally perpendicular to the lead axis, and have a planar surface along one surface of the lead. The lead is implanted adjacent to spinal cord dura mater with the electrodes transverse and facing the spinal cord. Pulses generated by the pulse generator for each channel are normally simultaneous, of equal amplitude and of equal duration, however, the pulse generator is arranged such that pulses for each channel can selectably alternate in time, can selectably be of unequal amplitude, or both. The changes in pulse timing and magnitude permit shifting the electrical stimulation field and the resulting paresthesia pattern after installation to accommodate improper lead placement or postoperative dislocation and to minimize unwanted motor responses.

Multichannel apparatus for epidural spinal cord stimulation

Epidural electrical stimulation (EES) of lumbosacral segments can restore a range of movements after spinal cord injury. However, the mechanisms and neural structures through which EES facilitates movement execution remain unclear. Here, we designed a computational model and performed in vivo experiments to investigate the type of fibers, neurons, and circuits recruited in response to EES. We first developed a realistic finite element computer model of rat lumbosacral segments to identify the currents generated by EES. To evaluate the impact of these currents on sensorimotor circuits, we coupled this model with an anatomically realistic axon-cable model of motoneurons, interneurons, and myelinated afferent fibers for antagonistic ankle muscles. Comparisons between computer simulations and experiments revealed the ability of the model to predict EES-evoked motor responses over multiple intensities and locations. Analysis of the recruited neural structures revealed the lack of direct influence of EES on motoneurons and interneurons. Simulations and pharmacological experiments demonstrated that EES engages spinal circuits trans-synaptically through the recruitment of myelinated afferent fibers. The model also predicted the capacity of spatially distinct EES to modulate side-specific limb movements and, to a lesser extent, extension versus flexion. These predictions were confirmed during standing and walking enabled by EES in spinal rats. These combined results provide a mechanistic framework for the design of spinal neuroprosthetic systems to improve standing and walking after neurological disorders.

https://www.jneurosci.org/content/jneuro/33/49/19326.full.pdf

A Computational Model for Epidural Electrical Stimulation of Spinal Sensorimotor Circuits

Study design: It has been previously demonstrated that sustained nonpatterned electric stimulation of the posterior lumbar spinal cord from the epidural space can induce stepping-like movements in subjects with chronic, complete spinal cord injury. In the present paper, we explore physiologically related components of electromyographic (EMG) recordings during the induced stepping-like activity. Objectives: To examine mechanisms underlying the stepping-like movements activated by electrical epidural stimulation of posterior lumbar cord structures. Materials and methods: The study is based on the assessment of epidural stimulation to control spasticity by simultaneous recordings of the electromyographic activity of quadriceps, hamstrings, tibialis anterior, and triceps surae. We examined induced muscle responses to stimulation frequencies of 2.2–50 Hz in 10 subjects classified as having a motor complete spinal cord injury (ASIA A and B). We evaluated stimulus-triggered time windows 50 ms in length from the original EMG traces. Stimulus-evoked compound muscle action potentials (CMAPs) were analyzed with reference to latency, amplitude, and shape. Results: Epidural stimulation of the posterior lumbosacral cord recruited lower limb muscles in a segmental-selective way, which was characteristic for posterior root stimulation. A 2.2 Hz stimulation elicited stimulus-coupled CMAPs of short latency which were approximately half that of phasic stretch reflex latencies for the respective muscle groups. EMG amplitudes were stimulus-strength dependent. Stimulation at 5–15 and 25–50 Hz elicited sustained tonic and rhythmic activity, respectively, and initiated lower limb extension or stepping-like movements representing different levels of muscle synergies. All EMG responses, even during burst-style phases were composed of separate stimulus-triggered CMAPs with characteristic amplitude modulations. During burst-style phases, a significant increase of CMAP latencies by about 10 ms was observed. Conclusion: The muscle activity evoked by epidural lumbar cord stimulation as described in the present study was initiated within the posterior roots. These posterior roots muscle reflex responses (PRMRRs) to 2.2 Hz stimulation were routed through monosynaptic pathways. Sustained stimulation at 5–50 Hz engaged central spinal PRMRR components. We propose that repeated volleys delivered to the lumbar cord via the posterior roots can effectively modify the central state of spinal circuits by temporarily combining them into functional units generating integrated motor behavior of sustained extension and rhythmic flexion/extension movements. This study opens the possibility for developing neuroprostheses for activation of inherent spinal networks involved in generating functional synergistic movements using a single electrode implanted in a localized and stable region.

/pdf/stepping-like-movements-in-humans-with-complete-spinal-cord-38jl5382wb.pdf

Stepping-like movements in humans with complete spinal cord injury induced by epidural stimulation of the lumbar cord: electromyographic study of compound muscle action potentials

The numerical modeling and experimental testing of a nerve cuff technique for selective stimulation of superficial peripheral nerve trunk regions are presented. Two basic electrode configurations ('snug' cuff monopolar and tripolar longitudinally aligned dots) have been considered. In addition, the feasibility of steering excitation into superficial nerve trunk regions using subthreshold levels of current flow from an electrode dot located on the opposite side of the nerve has been tested. The modeling objectives were to solve for the electric field that would be generated within a representative nerve trunk by each electrode configuration and to use a simple nerve cable model to predict the effectiveness of each configuration in producing localized excitation. In three acute experiments on cat sciatic nerve the objective was to characterize the effectiveness of each electrode configuration in selectively activating only the medial gastrocnemius muscle. Modeling and experimentation both suggest that longitudinally aligned tripolar dot electrodes on the surface of a nerve trunk, and bounded by a layer of insulation (such as a nerve cuff), will restrict excitation to superficial nerve trunk regions more successfully than will monopolar dot electrodes. Excitation steering will improve the spatial selectivity of monopolar and tripolar electrode configurations. >

A nerve cuff technique for selective excitation of peripheral nerve trunk regions

In epidural spinal cord stimulation it is likely not only that dorsal column fibers are activated, but also that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometric fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis are a major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers. >

/pdf/excitation-of-dorsal-root-fibers-in-spinal-cord-stimulation-1jw035fnp7.pdf

Excitation of dorsal root fibers in spinal cord stimulation: a theoretical study

This first paper, of two, describes a three-dimensional finite element computer model representing nonhomogeneous electrical conductivity of the human thorax. The spinal canal is treated in detail, taking account of various tissue types and other biological materials. Solutions are presented for the electrical fields generated within the spinal cord from bipolar epidural electrodes. Included in an Appendix is a validation study which compares experimental data in the literature, for subdural electrode arrays in a monkey, to theoretical solutions following the same method as the main work.

A Theoretical Study of Epidural Electrical Stimulation of the Spinal Cord Part I: Finite Element Analysis of Stimulus Fields

This second paper, of two, draws upon earlier finite element solutions for electrical fields generated within the spinal cord by epidural electrodes. Given those fields, a lumped network model of the myelinated nerve axon is used to predict stimulation thresholds for afferent fiber pathways in the dorsal columns and dorsal roots. Threshold predictions for dorsal column fibers of 5 and 10 , im diameters correspond closely with sensory thresholds reported experimentally. Descending fibers in the lateral corticospinal tracts are also considered. Comparisons are made to other possible targets of stimulation, and the theoretical indications are discussed in the context of clinical findings.

Barry Coburn

Papers

A Theoretical Study of Epidural Electrical Stimulation of the Spinal Cord Part I: Finite Element Analysis of Stimulus Fields

A Theoretical Study of Epidural Electrical Stimulation of the Spinal Cord - Part II: Effects on Long Myelinated Fibers