Potential Distribution and Nerve Fiber Responses in Transcutaneous Lumbosacral Spinal Cord Stimulation
Transcutaneous electrical spinal cord stimulation is a non-invasive method to stimulate afferent structures connected to the human spinal cord. Here, computer simulations are presented that aim at shedding light on why distant skin electrodes selectively activate specific groups of afferent fibers localized in the spinal canal and whether other neural structures are concomitantly stimulated.
The simulation was conducted in two steps: i) A finite element model of the human trunk was applied to calculate the electric potential generated by electrodes placed over the paravertebral skin and the abdomen. ii) The electric potential evaluated along the trajectories of target neural structures was used as the input for nerve fiber models and to calculate activating functions. Due to the electrophysiological findings, the responses of large diameter myelinated fibers in the posterior root (PR), anterior root (AR) and posterior column (Pcol) of the lumbar spinal cord were simulated.
The activating functions revealed sites of strong depolarization at the entrance of the PR fibers into the spinal cord and at the entrances/exits of the PR and AR fibers into/from the spinal canal. The nerve fiber model confirmed that action potentials were initiated at these low threshold sites. No such ‘hot-spots’ were found for the Pcol fibers. Activation thresholds for themost preferentially located fibers of each class were 14.1 V, 22.6 V and 45.4 V for the PRs, ARs and Pcols, respectively.
‘Hot-spots’ for extrecellular stimulation appear at axon bends and at transitions through media with different conductivities. PRs are the preferential targets, while direct coactivation of Pcol fibers is improbable.
KeywordsSpinal cord stimulation posterior root-muscle reflexes computer simulation activating function
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