Figure 4.

(A) Potassium current on the lateral membrane I_K,Lat is always outward and rapidly changes during an action potential because extracellular potassium [K]_e and reversal potential E_K,e are invariant along the membrane facing bulk unrestricted volumes. (B) Extracellular potassium in the cleft [K]_cl (red curve) is elevated relative to invariant bulk extracellular potassium [K]_e = 20 mM (dashed black trace) between action potentials. (C) The cleft potassium equilibrium potential E_K,cl (blue curve) is elevated relative to bulk (dashed black trace). Between action potentials, E_K,cl decreases early after repolarization and begins to rise as the lateral membrane begins to depolarize. (D) Cleft potassium current I_K,cl (blue curve) is inward early between action potentials, becomes transiently outward as the cell begins to depolarize, becomes strongly outward during the early phase of the action potential, and becomes strongly inward for a prolonged period of time during repolarization.