Which structure has the greatest degree of automaticity in the heart?

The structure with the greatest degree of automaticity in the heart is the SA Node. The SA Node, or sinoatrial node, is often referred to as the heart's natural pacemaker. It is located in the right atrium and is responsible for initiating the electrical impulses that regulate the heart's rhythm. These impulses cause the atria to contract and push blood into the ventricles. The intrinsic firing rate of the SA Node is typically between 60 to 100 beats per minute in a healthy heart, making it the most influential structure in setting the overall heart rate.

While other structures in the conduction system, such as the AV Node and Purkinje cells, possess some degree of automaticity, they have lower firing rates compared to the SA Node. The AV Node can act as a secondary pacemaker if needed, usually firing at about 40 to 60 beats per minute. The Purkinje fibers and bundle branches also have the ability to generate electrical impulses, but their firing rates are even lower, typically around 20 to 40 beats per minute. This hierarchical organization ensures that the SA Node predominantly controls the heart rate under normal physiological conditions.