<prompt> The rapid depolarization in cardiac muscle is due toCaNaKMg  <response>: B Na Resting membrane potential of individual mammalian cardiac muscle cells is about -90 mV interior negative to exterior. Stimulation produces a propagated action potential that is responsible for initiating contraction. Depolarization proceeds rapidly, and an overshoot is present, as in skeletal muscle and nerve, but this is followed by a plateau before the membrane potential returns to the baseline. In mammalian hearts, depolarization lasts about 2 ms, but the plateau phase and repolarization last 200 ms or more. Repolarization is therefore not complete until the contraction is half over. With extracellular recording, the electrical events include a spike and a later wave that resemble the QRS complex and T wave of the ECG. As in other excitable tissues, changes in the external K concentration affect the resting membrane potential of cardiac muscle, whereas changes in the external Na concentration affect the magnitude of the action potential. Initial rapid depolarization and the overshoot phase 0 are due to opening of voltage-gated Na channels similar to that occurring in nerve and skeletal muscle. Initial rapid repolarization phase 1 is due to closure of Na channels. The subsequent prolonged plateau phase 2 is due to a slower but prolonged opening of voltage-gated Ca2 channels. Final repolarization phase 3 to the resting membrane potential phase 4 is due to closure of the Ca2 channels and K efflux through various types of K channels. Voltage-gated Na channel in cardiac muscle has two gates--an outer gate that opens at the start of depolarization at a membrane potential of -70 to -80 mV; and an inner gate that then closes and precludes further influx until the action potential ends Na channel inactivation. Slow Ca2 channel is activated at a membrane potential of-30 to -40 mV. There are three types of K channels that proc jce repolarization. The first produces a transient, early outward current lTO that produces an early incomplete repolarizai on. Second is inwardly rectifying, ie, at plateau potentials it allows K influx but resists K efflux, and only at lower membrane potentials does it permit K efflux. Current it produces is called lKr. The third type is a slowly activating delayed rectifying type that produces a current called IKs. The sum of IKr and IKs is a small net outward current that increases with time and produces repolarization 