For the normal heartbeat, depolarization stimulus originates in

The correct option is Sinoatrial (SA) node.

In a normal, healthy heart, the cardiac cycle and heartbeat are initiated by a specialized cluster of cells located in the right atrium called the Sinoatrial (SA) node. Often referred to as the heart's natural pacemaker, the SA node possesses the unique property of autorhythmicity, meaning it can spontaneously generate electrical impulses (action potentials) without external nervous system stimulation.

The step-by-step pathway of the depolarization wave through the cardiac conduction system is as follows:

1. Generation of Action Potential: The SA node spontaneously depolarizes, initiating an electrical impulse.
2. Atrial Depolarization: The stimulus spreads rapidly across the muscle cells of both the right and left atria, causing them to contract (atrial systole). This is represented by the P wave on an electrocardiogram (ECG).
3. Atrioventricular (AV) Node: The impulse reaches the AV node, where it is briefly delayed to allow the ventricles to fill completely with blood from the atria.
4. Bundle of His and Purkinje Fibers: The signal then travels down the AV bundle (Bundle of His), through the bundle branches, and into the Purkinje fibers, triggering ventricular depolarization and subsequent contraction.

Because the initial depolarization stimulus originates in the SA node during a normal heartbeat, it is the primary pacemaker of the heart, dictating the sinus rhythm.