The coordinated contraction of the various chambers of the heart is actually controlled by the electrical system of the heart (most people don't even realize that the heart has an electrical system). Disturbances within the electrical system are the cause of all arrhythmias, and the medical specialty dealing specifically with heart rhythm disorders is known as Cardiac Electrophysiology (known as "EP" for short). In fact, Cardiac Electrophysiology was the first sub-subspecialty of cardiology that was officially recognized by the American Board of Internal Medicine -- the ABIM has offered a "Board Certification" examination in Clinical Cardiac Electrophysiology since 1992.
The electrical signal that initiates each normal heart beat arises from a small structure located at the top of the right atrium. This structure is called the "Sinus Node" or the "Sinoatrial Node" (pronounced sign-o-AAY-tree-uhl Node). The sinus node is known as the "natural pacemaker" of the heart because it is normally responsible for controlling the heart rate (it sets the pace). Thus, when someone performs some physical activity or gets frightened or angry and the organs of the body need more blood, the sinus node speeds up to make the heart beat faster. When that person relaxes, the sinus node slows down again to a normal rate (the normal heart rate is between 60 and 80 beats per minute at rest). From the sinus node, the electrical signal rapidly spreads across the right atrium and the left atrium, like waves spreading across a pond. This electrical signal causes the atria to contract. The atria are electrically insulated from the ventricles by the AV groove that runs around the outside of the heart. The AV groove is part of the fibrous skeleton of the heart that also includes the valvular structures. There is one area of the heart where the atria and the ventricles are electrically connected, and this is deep in the center of the heart. Actually, this connection comprises the second electrical structure of the heart called the Atrioventricular Node or AV Node. The AV Node acts like the "main wire" of the heart, since all the electrical signals from the atrium must pass through the AV node in order to get to the ventricles. The AV Node is connected to a bundle of special cells in the heart designed to rapidly conduct the electrical signal through the ventricles. These fibers are referred to as the Bundle of His (no, women don't have a Bundle of Hers!). The Bundle of His branches downstream into a right bundle branch (to the right ventricle) and a left bundle branch (to the left ventricle). The fibers eventually branch out to the distant ventricular tissues and are at that point referred to as Purkinje Fibers. It is the rapid conduction of the electrical signal through these special fibers that coordinates the contraction of the heart chambers, giving rise to a very efficient "wringing" action as the ventricular muscle vigorously contracts.