What is the site of the principal delay in conduction from the SA node to the ventricular myocardium?

• A. AVN
• B. SAN
• C. His bundle
• D. Purkinje system

Answer: (A)

The main slowing of the impulse from the atria to the ventricles happens at the AV node. This nodal tissue sits at the junction between the atria and ventricles and conducts mainly via calcium-dependent action potentials, with relatively few gap junctions, which makes its conduction velocity slower. That built-in delay, typically around 100 milliseconds, gives the atria time to contract and ventricles time to fill before they are activated. The sinoatrial node is the natural pacemaker, while the His-Purkinje system conducts very rapidly to coordinate ventricular contraction, so they do not provide this principal delay. Autonomic tone and certain drugs can modify AV nodal conduction, affecting the delay and AV conduction safety.