Depolarization and Repolarization:

1. When cardiac cells are at rest, they are polarized, meaning no electrical activity takes place. 

2. The cell membrane of the cardiac muscle cell separates different concentrations of ions, such as sodium, potassium, and calcium. This is called the resting potential. 

3. Electrical impulses are generated by specialized cardiac cells automatically. 

4. Once an electrical cell generates an electrical impulse, this electrical impulse causes the ions to cross the cell membrane and causes the action potential, also called depolarization. 

5. The movement of ions across the cell membrane through sodium, potassium and calcium channels, is the drive that causes contraction of the cardiac cells/muscle.

6. Depolarization with corresponding contraction of myocardial muscle moves as a wave through the heart. Depolarization thus corresponds with contraction of heart. 

7. Repolarization is the return of the ions to their previous resting state, which corresponds with relaxation of the myocardial muscle. Repolarization thus corresponds with relaxation of heart. 

8. Depolarization and repolarization are electrical activities which cause muscular activity. 

9. The electrical changes in the myocardial cell during the depolarization – repolarization cycle is detected on ECG.