Without a functioning battery, the car's engine won't start, even if it's in fine condition. Without a working SA node, the heart won't beat at a normal rate, even if it's otherwise healthy. But unlike a car, which has only one battery, the heart has auxiliary pacemaker sites that can take over for a dead SA node. The problem is that they usually set a pace that's considerably slower than the master pacemaker. From the SA node, electrical impulses quickly spread through the right and left atria via muscle cells in the myocardium.
Experts disagree about whether there are also specialized conduction pathways in the atria.
It's the ventricles that are the most crucial part of the working myocardium, responsible for pumping blood to the lungs and to the rest of the body. The impulses then pass into the ventricles, where they first meet the common bundle, also known as the bundle of His. The common bundle is like a telephone cable and transmits impulses from the AV node to the rest of the ventricles. The bundle branches divide into still thinner cables called Purkinje fibers, which transmit impulses to all parts of the ventricles. After leaving the Purkinje fibers, impulses travel from muscle cell to muscle cell to all cells of the ventricular muscle layer.