A physical interpretation of the ionic fluxes in excitable membranes

Abstract

A mechanism is proposed for the ionic fluxes in excitable membranes based on recent studies of monolayer permeation and of interfacial ionic transport. The excitable membrane is assumed to be a homogeneous lipid bilayer with anionic groups on the inner and outer surfaces and nonselectively adsorbed cations at these sites. The bound ions are in high concentration at the membrane surface and they cause the enhanced ionic fluxes when released during activity. The free ions, which are in equilibrium with the adsorbed ions, give rise to the resting ionic fluxes. Starting with the steady state ionic concentration gradients, the current due to “depolarizing electrodes” causes the concentration of potassium ions near the outer surface to increase, and it is assumed that these ions exchange with and release the bound sodium ions. Because of the increased sodium concentration at the membrane surface, there is an increased sodium ion flux across the membrane. This flux causes a release and a reverse flux of potassium ions by a similar process. The fluxes have been calculated in terms of several membrane parameters, and they can account qualitatively for the ion fluxes and permeability ratios at rest and during activity, the actions of some ions and pharmacologic agents, the effects of ion depletion, etc. Therefore, the mechanism provides a basis for explaining the ionic events at excitable membranes in terms of physical processes at the membrane level.