Cardiac Muscle Contraction (in order)

1 Rapid depolarization occurs when fast‐opening Na + channels in the sarcolemma open and allow an influx of Na + ions into the cardiac muscle cell. The Na + channels rapidly close. 2 A plateau phase occurs during which Ca 2+ enters the cytosol of the muscle cell. Ca 2+ enters from the sarcoplasmic reticulum (endoplasmic reticulum) within the cell and also from outside the cell through slow‐opening Ca 2+channels in the sarcolemma. the Ca 2+ channels open, K + channels, which normally leak small amounts of K + out of the cell, become more impermeable to K + leakage. 3 Repolarization occurs as K + channels open and K + diffuses out of the cell. At the same time, Ca 2+ channels close. These events restore the membrane to its original polarization, except that the positions of K + and Na + on each side of the sarcolemma are reversed. 4 A refractory period follows, during which concentration of K + and Na + are actively restored to their appropriate sides of the sarcolemma by Na +/K + pumps. The muscle cell cannot contract again until Na + and K + are restored to their resting potential states.