Which sequence describes the heart's conduction pathway to ventricular contraction?

• A. SA -> AV -> His-Purkinje
• B. AV -> SA -> His-Purkinje
• C. SA -> His-Purkinje -> AV
• D. AV -> His-Purkinje -> SA

Answer: (A)

The heart’s electrical impulse starts in the sinoatrial (SA) node, the natural pacemaker, which initiates atrial contraction. From there it travels to the atrioventricular (AV) node, where a brief delay allows the ventricles to fill. The impulse then moves rapidly through the His-Purkinje system to the ventricles, producing synchronized ventricular contraction. This SA → AV → His-Purkinje sequence is the path that leads to the ventricles contracting after the atria have filled. If the impulse started at the AV node, or skipped the AV node and went directly to the His-Purkinje system, the timing and coordination needed for proper ventricular contraction would be disrupted. Placing SA after AV or in any order that puts the ventricles before AV delay would also mis-timing the contraction.