The Cardiac Cycle

1. The impulse originates in the SA node (1).
2. It spreads through the atrial muscles along three bands of tissue known as the internodal tracts (2), causing atrial contraction.
3. it reaches the AV node (3) where it is momentarily slowed before passing on to the bundle of His (4).
4. The impulse descends through the bundle of His and down the right and left bundle branches (5A and 5B).
5. Reaching the terminal Purkinie fibers (6) the impulse stimulates the ventricular myocardial cells at the Purkinje-myocardial junction.
6. Ventricular contraction then occurs.

The Cardiac Cycle

When the Purkinje-myocardial cells are stimulated there is a discharge of electrical forces stored within the myocardial cells. This electrical process is called depolarization; it results in ventricular contraction. After depolarization the muscle cells recover and re-store electrical energy. This recovery process is called repolarization. Under normal circumstances the next impulse from the SA node arrives when repolarization is complete; activation then occurs again. The combined periods of stimulation (depolarization) and recovery (repolarization) constitute the cardiac cycle.+

+The discharge and the storage of electrical forces within the myocardial cells during depolarization and repolarization is associated with a chemical process involving the exchange of sodium and potassium ions across the cell membrane.