Stimulation of the posterior tibial nerve is commonly used in the measurement of somatosensory evoked potential (SEP). To improve the efficiency of stimulation, the potential field and current density distributions under the surface electrodes were modeled and simulated. In our model, three layers were assumed: (1) the air environment, (2) electrode and paste (3) human body (skin and soft tissues). The mirror method was used to analyze the potential field of point charge. Integration of the field and the area of the stimulus gave the potential field of one surface electric pole. The potential field distribution of the bipolar stimulator was obtained by superimposition of two unipolar fields. Finally, the current density distribution was calculated by Laplace equation. The analytical solution of the potential field was found and the numerical solution of the current density distribution calculated. The potential field and current density distributions were simulated by 2-D plot. From the model and simulation, the potential and current density distributions were not found to be uniform under transcutaneous stimulation electrode and the maximum current density is located under the poles. We recommend that bipolar stimulator should be applied axially along the stimulated nerve course.
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