Which is the correct sequence of electrical activation in the cardiac conduction system starting from the SA node?

Which is the correct sequence of electrical activation in the cardiac conduction system starting from the SA node?

• A. SA node → atrial myocardium → AV node → bundle of His → right and left bundle branches → Purkinje fibers → ventricular myocardium
• B. SA node → AV node → bundle of His → Purkinje fibers → atrial myocardium → right and left bundle branches
• C. SA node → Purkinje fibers → AV node → ventricular myocardium
• D. SA node → atrial myocardium → Purkinje fibers → AV node → bundle of His

Answer: (A)

The key idea is how electrical activity travels in order through the heart to coordinate timing of contractions. The SA node starts the impulse and quickly spreads through the atrial myocardium, causing atrial depolarization and contraction. The impulse then reaches the AV node, where a brief delay allows the ventricles time to fill. From there it travels down the bundle of His, splits into the right and left bundle branches, and reaches the Purkinje fibers, which rapidly distribute the impulse to the ventricular myocardium to trigger coordinated ventricular contraction. This sequence matches the anatomical path and the functional timing: atrial activation before ventricular activation, with a deliberate AV nodal delay and fast Purkinje-mediated ventricular spread. Other sequences disrupt this order, placing Purkinje distribution before the AV node, or reversing the timing of atrial and ventricular activation, which would misalign atrial contraction with ventricular readiness.

The key idea is how electrical activity travels in order through the heart to coordinate timing of contractions. The SA node starts the impulse and quickly spreads through the atrial myocardium, causing atrial depolarization and contraction. The impulse then reaches the AV node, where a brief delay allows the ventricles time to fill. From there it travels down the bundle of His, splits into the right and left bundle branches, and reaches the Purkinje fibers, which rapidly distribute the impulse to the ventricular myocardium to trigger coordinated ventricular contraction. This sequence matches the anatomical path and the functional timing: atrial activation before ventricular activation, with a deliberate AV nodal delay and fast Purkinje-mediated ventricular spread. Other sequences disrupt this order, placing Purkinje distribution before the AV node, or reversing the timing of atrial and ventricular activation, which would misalign atrial contraction with ventricular readiness.