Since the interstitial water of the central nervous system is in narrow channels between cells that have surface charge, electroosmotic flow of water should accompany current flow. Current density is high close to an excitatory postsynaptic membrane and the electroosmotic effect must be significant. Simultaneity relationships between action potentials in pre- and postsynaptic cells affect postsynaptic membrane current density and modulate the effect in a way which should produce "learning."