Which structure in the heart's electrical conduction system delays impulses at the junction of atria and ventricles?

• A. SA node
• B. Atrioventricular (AV) node
• C. Purkinje fibers
• D. Bundle of His

Answer: (B)

The crucial function here is the delay of the electrical impulse at the AV node, which sits at the junction between the atria and ventricles. This pause lets the atria finish contracting and the ventricles fill with blood before they contract. The delay occurs because the AV node has slower conduction properties (smaller fibers and fewer gap junctions), creating a brief pause of about a tenth of a second. After this pause, the impulse travels rapidly through the Bundle of His and Purkinje fibers to trigger ventricular contraction. The SA node starts the impulse, but does not delay it; Purkinje fibers and the Bundle of His mainly speed conduction to the ventricles rather than creating the delay.