Simulation of action potentials in a one-dimensional bidomain

An approximate method of calculating action potentials in a nerve fiber within a coaxial sheath is presented. The method is of the implicit type, but it avoids any large matrix inversion and is adapted to run conveniently on a small PC/AT. It produces traces of the extracellular voltage V _out, which is not calculated by other methods, as well as the intracellular potentials V _in. For homogeneous fibers with small sheath resistance, V _out is proportional to d ²V _in/dz ², as expected. In more complex cases, V _out remains small, but its shape is hard to predict. One such example is presented in which an extracellular voltage trace of novel shape is calculated.