I've just finished reading through the generally well-written Wikipedia article on membrane potential: http://en.wikipedia.org/wiki/Membrane_potential I found almost everything to be clear, but one question is nagging at me. Why do changes in membrane permeability directly affect membrane potential? The Wikipedia article says: I can understand why membrane permeabilities affect the equilibrium potential, but why should they affect the membrane potential instantaneously? Isn't the potential totally determined by the concentrations of charge inside and outside the membrane? Changing permeabilities by opening ion channels should shift the equilibrium potential and cause a gradual change in membrane potential in the direction of equilibrium, but they should not change the membrane potential immediately. Am I misunderstanding something? Thanks.