What is the structure that begins heartbeat?

The sinoatrial node (SA node), also known as the sinus node, is a small, specialized group of cells located in the right atrium of the heart. It is the natural pacemaker of the heart and is responsible for initiating the electrical impulses that cause the heart to contract. The SA node generates electrical impulses that spread throughout the heart, causing the atria to contract and pump blood into the ventricles. The electrical signals from the SA node travel through the atrioventricular node (AV node), which delays them slightly to allow the atria to fill completely before the ventricles contract. The electrical signals then pass through the bundle of His, a network of fibers, and into the left and right bundle branches, which carry the signals to the left and right ventricles, causing them to contract and pump blood out of the heart.