Macroscopic View of Electron Transfer
The rate at which electron transfer takes place between the metal electrode and the molecules in the electrolyte medium, i.e. current injected into the tissue medium, is determined by rate constants (which may be potential dependent) that govern reactions at the surface of the electrode (includes molecular rearrangement of the reactants and crystalline reorganization at the electrode surface) and transport of the reactants to and from the site of electron transfer. A schematic representation of the processes involved in electron transfer is given in the figure.
At the time a stimulus pulse applied to an electrode, we can assume that the concentration of the reactants at the stimulating electrode is similar to that of the bulk concentration, far away from the electrode, for all potential reactants. After the application of an external potential to the electrode the electron energy levels of the electrons in the metal rise (cathodic) or fall (anodic) until electron transfer occurs at a rate equal to that at which current is supplied by the external circuit.
For a reaction to occur the reactant must be brought close enough to the electrode for electron transfer to occur by adsorption or electron tunneling. Once the reaction has occurred the reactant can diffuse away and may not be available for reaction reversal.

REFERENCE:
A. J. Bard and L. R. Faulkner, ELECTROCHEMICAL METHODS: Fundamentals and Applications, 2nd Edition, 2001, John Wiley & Sons, Inc, Hoboken, NJ 07030