semipermeable membrane

n. Any thin tissue which forms a boundary between biological structures and through which some material can pass and other material cannot pass. Typically some ions can diffuse through the barrier while other ions and larger particles cannot cross it. Thus nerve cell membranes are semipermeable, allowing different proportions of sodium and potassium ions into the cell and creating an electric potential or electrical difference across the membrane, which creates an electrical impulse when the permeability changes in a wave like motion.