On the Sodium Concentration Diffusion with Three-Dimensional Extracellular Stimulation

We deal with the transmembrane sodium diffusion in a nerve. We study amathematical model of a nerve fibre in response to an imposed extracellular stimulus. The presentedmodel is constituted by a diffusion-drift vectorial equation in a bidomain, that is, two parabolic equations defined in each of the intraand extra-regions. This system of partial differential equations can be understood as a reduced three-dimensional Poisson-Nernst-Planck model of the sodium concentration. The representation of the membrane includes a jump boundary condition describing the mechanisms involved in the excitation-contraction couple. Our first novelty comes from this general dynamical boundary condition. The second one is the three-dimensional behaviour of the extracellular stimulus. An analytical solution to the mathematical model is proposed depending on the morphology of the excitation.