The organized beating of the heart takes place because of the heart's sophisticated conduction system, or "electrical wiring." The electrical impulse for a heartbeat begins in the sinoatrial (SA) node, a specialized group of pacemaker cells located in the right atrium which initiate a regular, clockwork-like series of electrical impulses that ultimately lead to a heartbeat. The SA node is responsive to the physiologic needs of the body and can increase or decrease its rate of firing accordingly. The electrical impulse that originates in the SA node travels rapidly via nerve fibers through both atria to the atrioventricular (AV) node. As the impulse passes through the atria, it is dispersed to the muscle tissue in the atria and stimulates both atria to contract simultaneously and empty their contents into their respective ventricles. The AV node is like a "grand central station" of nerve fibers, and here the electrical impulse is slowed down and organized. This allows time for blood to pass from atria to the ventricles. From the AV node the impulse passes to the nerve bundle of His and Purkinjie fibers (named after the scientists who discovered them). These nerve fibers are like a system of "freeways" that rapidly carry the impulse and distribute it to both ventricles. The impulse reaches various regions of the ventricle in a specific order that leads to a choreaographed ventricular contraction, such that contraction begins at the bottom (apex) of the heart and ends at the top (base) of the heart. This results in efficient ejection of blood out of the heart.