The sinoatrial node is a bundle of tissue at the top of the heart near where the superior vena cava enters the right atrium. It initiates the electrical impulse that starts the cycle of the heart beat based on the nerves leading into it that control the actual heart rate, a combination of the vagus nerve and the nerves of the paralimbic system. As such, it is usually referred to as the heart's primary pacemaker even though other cells can contract independently. It was first identified in a mole heart by a British medical student, Martin Flack, who published his results in 1907.
If the sinoatrial node is permanently diabled, either by damage to the node or by disconnecting the node from the rest of the heart, the atrioventricular node will often take over as the primary pacemaker.