Physics and neuroscience, a fruitful fusion. A computational study on electrodiffusion in the periaxonal space
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The myelin around an axon greatly increases conduction velocity and was once thought of as an inert insulating layer. New research shows that the periaxonal space is highly conductive, suggesting a rapid flow of charges inside this space during an action potential. The propagation of such an action potential in an axon relies on the flow of ions in or out of the cell and because of the small volume of the periaxonal space, the concentration changes there are significant. We present the physics governing this ionic flow and show an equivalence between the Nernst-Planck formalism and cable theory. The simulation environment NEURON specializes in applying cable theory to model neurons and we tested how NEURON handles ion accumulation and diffusion. NEURON does not take longitudinal conductive flux into account when modelling ionic accumulation, but the effect this has when modelling an action potential is most likely negligible. A double cable model was set up in NEURON, with the second cable corresponding to the periaxonal space. The rise in potassium concentration in the periaxonal space was high (50\% to 100\%) and resulted in less gradual re-polarization of the nodal transaxonal potential. This points to the necessary presence of potassium clearing mechanisms in the adaxonal membrane and a triple cable model showed that transadaxonal potential allows for potassium to flow into the myelin, suggesting activity dependent communication between the axon and its myelin.