Depolarization/Repolarization Cycle Of Myocardiocytes

The cardiac action potential consists of two cycles, a rest phase and an active phase. The rest phase is considered polarized. The resting potential during this phase of the beat separates the ions such as sodium, potassium and calcium. The electrical cell will then generate an impulse. The ions will then cross the cell membrane causing depolarization. The movement of these cells through the sodium, potassium and calcium channels causes the contraction of the heart muscle. The depolarization and contraction together cause a wave of movement to pass through the heart muscle. The ions will then return to their regular resting state and the heart muscle relaxes. This is called repolarization. And ECG (electrocardiogram) measures the strength of these cycles. The length of time between depolarization and repolarization is long (relatively speaking.) This period of time is called the refractory period. The refractory period is about the same amount of time between the contraction and relaxation of the heart muscle. Because of this, the cardiac muscle do not tire as easily. Myocardial cells possess the property of automaticity or spontaneous depolarization. This is the direct result of a membrane which allows sodium ions to slowly enter the cell until the threshold is reached for depolarization. Calcium ions follow and extend the depolarization even further. Once calcium stops moving inward, potassium ions move out slowly to produce repolarization. The very slow repolarization of the CMC membrane is responsible for the long refractory period.