Optic Nerve Microcirculation: Fluid Flow and Electro-Diffusion.

Flows of water and various organic and inorganic molecules in the central nervous system are important in a wide range of biological and medical processes, as has recently become apparent (Nicholson et al. 2017 [1]). However, the exact mechanisms that drive these flows are often not known. Here we investigate the forces and flows in a tridomain model of the central nervous system. We use the experimental system of the optic nerve, investigated by the Harvard group Orkand et al [3, 20] as a protype of the central nervous system in general. We construct a model and choose its parameters to fit the experimental data. Asymptotic analysis and numerical computation show the significant role of water in convective ion transport. The full model (including water) and the electro-diffusion model (excluding water) are compared in detail to show the main physiological consequences of the complex structure as viewed in our model. In the full model, convection due to water flow dominates inside the glial domain. This water flow in the glia contributes significantly to spatial buffering of potassium in the extracellular space. Convection in the extracellular domain does not contribute significantly to spatial buffering. Electrodiffusion is the dominant mechanism for flows confined to the extracellular domain.