Answer:
Option 2, SA node
Explanation:
The heartbeat begins when the sinoatrial node (SA node) in the posterior visceral pericardial wall of the right atrium generates an electrical impulse to propagate throughout the rest of the heart's conduction system. The SA node signals the cardiac cells to depolarize, leading to contraction of both atria (top chambers of the heart) to push the blood into the ventricles (bottom chambers of the heart).
In order, once through the SA node, the impulse then travels into the atrioventricular node (AV node), bundle of His, the bundle branch, and finally to the Purkinje fibers. The impulse reaching the Purkinje fibers is what signals the heart to forcefully eject blood out of the ventricles into the circulation towards the lungs and the systemic circulation to peripheral tissues.
The SA node initiates the heartbeat, hence why it is sometimes referred to as the pacemaker of the heart, option 2 is the answer here.
