535—The surface compartment model (SCM): Role of surface charge in membrane permeability changes

Small shifts in the surface charge of excitable membranes precede the ion permeability changes that are the basis of the action potential. If we assume that an increase in ion permeability is due to the opening of a molecular pore, we can model this process like subunit dissociation in oligomeric proteins. (The dissociation of hemoglobin tetramers into dimers is sensitive to changes in charge and occurs when the surface charge is approximately the same as on the inner face of the squid axon membrane.) Using the relation between the hemoglobin dissociation constant and charge, together with the estimated number of pores in the axon membrane, we have derived a relation between the fraction of pores open and the surface charge density. When this latter relation is incorporated into the surface compartment model (SCM), the gating currents give rise to voltage (polarization) dependent changes in ion permeability.