Which sequence describes the spread of the cardiac electrical impulse through the heart?

• A. SA node → atria → AV node → bundle of His → left and right bundle branches → ventricles
• B. Ventricles → atria → AV node → SA node
• C. AV node → SA node → ventricles
• D. SA node → atria → AV node → bundle of His → left and right bundle branches → ventricles

Answer: (D)

The main idea is how the heart’s electrical signal travels in a coordinated path so the atria contract before the ventricles. The impulse starts in the sinoatrial (SA) node, the natural pacemaker in the right atrium. From there it spreads across the atrial muscle, causing atrial contraction and pushing blood into the ventricles. The signal then reaches the atrioventricular (AV) node, where there is a brief delay to allow the ventricles to fill. After this pause, the impulse travels down the bundle of His, which splits into the left and right bundle branches and carries the signal toward the ventricles. Finally, it reaches the ventricular conduction system (including Purkinje fibers) to trigger ventricular contraction.

This sequence—SA node → atria → AV node → bundle of His → left and right bundle branches → ventricles—best captures the orderly progression needed for synchronized heartbeats. The other options break the timing (ventricles before atria), skip the AV node delay, or place components in the wrong order, which would disrupt coordinated contraction.