How does electrical stimulation of the heart coordinate the overall cardiac cycle?

Millisecond timing: The 2 atria act as reservoirs for returning blood; the 2 ventricles are the "pumpers". The 4 chambers must be coordinated with millisecond timing to maximize the amount of blood pumped (cardiac output). This is done by a network of specialized cells which are found in tracts or bundles that conduct a micro-electrical current to time motion. A gross analogy: think of the distributor in your car.