Which sequence correctly describes the normal cardiac conduction pathway?

• A. SA node → AV node → atria → bundle of His → Purkinje fibers → ventricles contract.
• B. SA node → atria → bundle of His → AV node → Purkinje fibers → ventricles contract.
• C. SA node → atria → AV node → bundle of His → left and right bundle branches down interventricular septum → Purkinje fibers → ventricles contract.
• D. SA node → ventricles → AV node → Purkinje fibers.

Answer: (C)

Normal conduction starts in the sinoatrial (SA) node, which sends the impulse through the atrial muscle to make the atria contract. The impulse then reaches the atrioventricular (AV) node, where a brief delay occurs to allow the ventricles time to fill. From there, the signal travels down the bundle of His, which splits into left and right bundle branches along the interventricular septum, and finally into the Purkinje fibers to depolarize the ventricles and trigger contraction. This sequence—SA node, atrial activation, AV node with delay, bundle of His, septal bundle branches, Purkinje fibers, then ventricular contraction—best fits the normal pathway. The other sequences misplace the AV node, skip the delay, or put ventricular activation before atrial conduction, which would disrupt the timing of heartbeats.