Extracellular ion transport

It is a misconception to think that extracellular electric currents of biologic origin are carried by all of the ions in the extracellular medium. It is established that, for certain reasonable boundary conditions, only those ion species that are transported across the plasma membranes of the biological system, or that chemically derive from or contribute to these species, can contribute to the extracellular electric current at steady state. In the absence of convection, the extracellular current will be carried largely by diffusion of the transported ion species, at steady state. Extracellular electric potential gradients are shown to arise in a secondary manner, as a result of the electroneutrality condition. The effect of non-turbulent convection is included in the derivations and discussion.