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Bipolar Stimulation

2d model with activation map and streamlines. 2d model of cat sciatic nerve. Rings show outline of myelinated fibers: red indicates threshold crossing. Streamlines show current flow.

Despite decades of research in electrical stimulation of the nervous system, little is known about activation at the electrode-tissue interface. Specifically, which axons are activated under a given stimulation protocol? We chose to study this question with a 3-dimensional model of cat sciatic nerve generated from stained histological slices. This model takes into account both the geometry and biophysics of each individual neuron in a single peripheral nerve fascicle. A major advantage of the computational model, created with BioPSE and Matlab, is that it allows interactive, on-line manipulation of stimulation parameters and electrode position within the nerve, along with visualization of results. The resulting scalar, vector and tensor fields provide detailed insight into neural activation as a function of cell properties, electrode position and stimulation waveform.

Threshold Detection. The primary purpose of the model is to determine which axons are activated as a function of stimulus paradigm. Since the solver only determines voltages within the volume (Fig a), further analysis is necessary to determine axon threshold crossings. First, the current density vector field is computed (Fig b). Then the maximum current density vector in the axial direction is determined for each axon. Finally, this current vector is normalized for fiber cross-sectional area and superimposed on each axon in a collapsed 2D image (Fig C).

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