  Complex fluids flow in complex ways in complex structures. Transport of water
and various organic and inorganic molecules in the central nervous system are
important in a wide range of biological and medical processes [C. Nicholson,
and S. Hrab\v{e}tov\'a, Biophysical Journal, 113(10), 2133(2017)]. However, the
exact driving mechanisms are often not known. In this paper, we investigate
flows induced by action potentials in an optic nerve as a prototype of the
central nervous system (CNS). Different from traditional fluid dynamics
problems, flows in biological tissues such as the CNS are coupled with ion
transport. It is driven by osmosis created by concentration gradient of ionic
solutions, which in term influence the transport of ions. Our mathematical
model is based on the known structural and biophysical properties of the
experimental system used by the Harvard group Orkand et al [R.K. Orkand, J.G.
Nicholls, S.W. Kuffler, Journal of Neurophysiology, 29(4), 788(1966)].
Asymptotic analysis and numerical computation show the significant role of
water in convective ion transport. The full model (including water) and the
electrodiffusion model (excluding water) are compared in detail to reveal an
interesting interplay between water and ion transport. In the full model,
convection due to water flow dominates inside the glial domain. This water flow
in the glia contributes significantly to the 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.
