Heart conduction starts at the ____________ which initiates each cardiac cycle through spontaneous depolarization.

The correct choice is the SA node, which stands for the sinoatrial node. It is often referred to as the heart's natural pacemaker because it is responsible for initiating and regulating the heart's rhythmic contractions. The SA node generates electrical impulses that spread through the atria, causing them to contract and push blood into the ventricles. This process of spontaneous depolarization is crucial for maintaining a consistent heart rate and ensuring proper blood circulation throughout the body.

The SA node's location, being in the right atrium near the entrance of the superior vena cava, enables it to effectively control the heart's rhythm. From the SA node, the electrical signals pass to the atrioventricular (AV) node, which then conducts the impulse to the ventricles through the Bundle of His and the Purkinje fibers, but it is the SA node that is specifically responsible for initiating each cardiac cycle.