Epidural electrical stimulation of posterior structures of the human lumbosacral cord: 2. quantitative analysis by computer modeling

Abstract

Objectives: Analysis of the computed recruitment order of an ensemble of ventral and dorsal root ®bers should enlighten the relation between the position of a bipolar electrode and the observed order of muscle twitches. Material and methods: Thresholds of selected spinal root ®bers are investigated in a two step procedure. First the electric ®eld generated by the electrodes is computed with the Finite Element Method. In the second step the calculated voltage pro®le along each target neuron is used as input data for a cable model. For every electrode position the electrical excitability is analyzed for 12 large diameter ventral and dorsal root ®bers of the second and fourth lumbar and ®rst sacral segment. The predictions of the neural responses of any target ®ber are based on the activating function concept and on the more accurate computer simulations of the electrical behavior of all nodes and internodes in the vicinity of the electrode.

Epidural electric stimulation of posterior structures of the human lumbar spinal cord: 1. muscle twitches – a functional method to define the site of stimulation

“…Quantitative analysis by computer modeling of the epidural electrical stimulation of posterior structures of the human lumbar cord 27 demonstrate actual measurements of relation between stimulus strength, site of stimulation, size and the order of muscle twitches. Thus we have used a functional criteria, in contrast to computer modeling, which is based on computation of electrophysiological parameters of muscle twitches and of dimensions of anatomical structures, electrical properties, electrical ®eld and conducting axons.…”

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

“…Computer simulation of the epidural electrode setup as used in the present study showed that the largest lumbosacral posterior root fibers entering the cord caudally to the level of the cathode could still be activated by a maximum stimulation voltage of 10 V at a distance of up to 2 cm. 33 Depending on the relative rostrocaudal position of the cathode with respect to a given posterior root fiber, we suggest two different sites of the initiation of afferent volleys in the posterior roots. For cathode positions at upper lumbar cord levels, lumbar posterior root fibers entering the cord in the vicinity of the cathode will have lowest thresholds.…”

“…Geometric and electric factors result in lowthreshold sites for electrical stimulation where posterior root fibers enter the spinal cord. 18,[31][32][33][34][35] At these sites, posterior root fibers entering the cord caudal to the cathode can have lower thresholds than longitudinal fibers passing the level of the cathode. The high electric conductivity of the cerebrospinal fluid and the longitudinal orientation of the white matter allow the current to spread in a rostrocaudal direction.…”

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

“…Monopolar stimulation was carried out by selecting one of the epidural electrodes as cathode and the active area of the implanted pulse generator case ("c") as anode. The pulse generator delivered capacitively decoupled monophasic rectangular constant-voltage pulses of 210-s width, each followed by an exponentially decaying phase to adjust charge balance for each stimulus and avoid delivery of direct current (Rattay et al 2000). Programmable stimulation frequencies and intensities were 2-130 Hz and 0 -10.5 V.…”