Which structure in the conduction pathway is responsible for slowing the electrical impulse between atria and ventricles?

• A. Bachmann's bundle
• B. Purkinje fibers
• C. Sinoatrial node
• D. Atrioventricular node

Answer: (D)

The key idea is that the heart slows the electrical signal to time the beat between atrial contraction and ventricular contraction. The structure that does this is the atrioventricular node. When the impulse travels from the atria toward the ventricles, the AV node acts like a bottleneck: its tissue conducts more slowly because the cells are smaller and have fewer gap junctions, which reduces conduction velocity and prolongs the delay. This pause gives the atria time to finish contracting and filling the ventricles before the signal moves on to the His-Purkinje system to activate the ventricles.

In contrast, other parts of the pathway don’t slow the impulse. Bachmann's bundle mainly helps rapidly conduct the impulse across the atria to synchronize atrial contraction. Purkinje fibers are designed for fast, widespread ventricular activation. The sinoatrial node initiates the impulse, not delay it.