Cardiac contractions are the result of a well orchestrated electrical phenomenon called depolarization. Cell membranes move from their negative resting potential to a more positive threshold which ultimately stimulates them to contract.  In the myocardium there are specialized fibers that are very conductive and allow the rapid transmission of electrical impulses across the muscle, telling them to contract. In order to maximize the force of the contraction there is uniformity in the sequence. That is, the atria contract, then the ventricles contract.  This allows both sets to fill properly before ejecting the blood to its next destination. These two sections are independent, yet linked to a single impulse, (in a healthy heart,) initiated by the sinoatrial, (or sinus) node. The tissue around the valves helps to channel the impulse from the sinus node through another collection of specialized tissue, the atrioventricular node, that is situated between the two sets of chambers. This area allows slightly slower transmission of the impulse to the ventricles, allowing the atria to empty into the ventricles before they contract and force the blood to the lungs or body. This area, the A/V Node, slows the impulse down to about 1/25th of the original signal then passes it through to the atrioventricular bundle, or the bundle of His. This bundle divides itself into two distinct tracts through the ventricles, the bundle branches, and on to the Purkinje fibers, where the muscle of the ventricle is stimulated to contract from the bottom up, maximizing the force of ejection.