The sinoatrial node (often abbreviated SA node; also commonly called the sinus node and less commonly the sinuatrial node) is the normal natural pacemaker of the heart and is responsible for the initiation of the cardiac cycle (heartbeat). It spontaneously generates an electrical impulse, which after conducting throughout the heart, causes the heart to contract. Although the electrical impulses are generated spontaneously, the rate of the impulses (and therefore the heart rate) is set by the nerves innervating the sinoatrial node. The sinoatrial node is located in the myocardial wall near where the sinus venarum joins the right atrium (upper chamber); hence sino- + atrial.