The ubiquitous ATP-dependent sodium-potassium pump is represented by the membrane-bound circle and two twisted arrows.) This pump removes three sodium ions (Na+) from the cytoplasm in exchange for two potassium ions (K+) from the interstitial fluid.
As in most cells, contractile cells have passive channels (cylinders) for both sodium and potassium ions. Potassium ions efflux along their electrochemical gradient as represented by the block arrow passing through the cylinder. Sodium ions diffuse into the cell along their electrochemical gradient as shown by the block arrow passing through this cylinder.
The resting membrane potential of -85 mV (RMP on action potential graph) indicates equal and opposite charge exchanges of sodium and potassium ions through their passive channels. All gated channels (sodium, calcium and potassium) are closed during the RMP phase.
Last updated:7/16/2005