Learning Objective:

Describe the normal pathway of cardiac impulse conduction from the SA node through the atria, AV node, His–Purkinje system, and ventricles, and explain how electrical coupling via gap junctions ensures coordinated excitation.

Cardiac cells are tightly interconnected through gap junctions, allowing ions and electrical impulses to travel directly between adjacent cells. As a result, depolarization of one cell quickly spreads to its neighbors, ensuring synchronized contraction.

The SA node serves as the primary pacemaker because it has the highest intrinsic firing rate. Its spontaneous depolarization initiates each heartbeat and sets the rhythm for the entire cardiac cycle.

Activity

Normal Conduction Pathway

1. Sinoatrial (SA) Node

• Located in the right atrium near the superior vena cava
• Initiates the action potential that begins each heartbeat
• Spreads depolarization across both atria through atrial myocytes and interatrial pathways

2. Atrial Conduction

• Electrical activity spreads through the atrial muscle fibers
• Leads to atrial contraction, pushing blood into the ventricles

3. Atrioventricular (AV) Node

• Located at the base of the right atrium
• Provides the AV nodal delay (~0.1 sec)
• Ensures ventricles fill before they contract
• Slow conduction due to small fibers and low gap junction density

4. Bundle of His

• The only normal electrical connection between atria and ventricles
• Conducts rapidly to the interventricular septum

5. Right and Left Bundle Branches

• Conduct impulses down the interventricular septum
• Spread the depolarization toward the apex of the heart

6. Purkinje Fiber Network

• Distributes depolarization throughout the ventricular myocardium
• Fastest conduction system (2–4 m/s)
• Ensures synchronous ventricular contraction from apex → base

7. Ventricular Myocyte Activation

• Electrical impulse spreads cell-to-cell through gap junctions
• Produces coordinated ventricular contraction and effective ejection of blood

Activity