Which node has the highest intrinsic rate and acts as the heart's natural pacemaker?

• A. AV Node
• B. Purkinje Network
• C. Bundle of His
• D. SA Node

Answer: (D)

The heart’s natural pacemaker is the tissue whose cells depolarize the fastest on their own, setting the pace for the whole heart. The sinoatrial node has the highest intrinsic firing rate, so it regularly initiates impulses fastest among the nodal tissues. Its pacemaker cells undergo spontaneous diastolic depolarization due to the funny current and other ion channel dynamics, reaching threshold sooner than the other nodes. This automatic impulse sets the heart rate and drives the conduction sequence through the AV node, His bundle, and Purkinje system. The AV node has a slower intrinsic rate and mainly acts as a backup pacemaker if the SA node fails, while the Purkinje network is primarily responsible for rapid conduction to the ventricles rather than setting the pace.