After the SA node fires, which structure coordinates ventricular contraction?

• A. Atrioventricular node
• B. Sinoatrial node
• C. Purkinje fibers
• D. Bundle of His

Answer: (A)

After the SA node fires, the impulse first reaches the atrioventricular (AV) node, which serves as the gatekeeper between the atria and ventricles. The AV node introduces a short delay, allowing the atria to contract and push blood into the ventricles before they start to contract. This delay is the key that coordinates the sequence.

Once the AV node passes the impulse along, it travels through the bundle of His and into the Purkinje fibers, which rapidly spread the impulse through the ventricles, producing a synchronized contraction. So the AV node’s delaying and relaying role sets the tempo for ventricular activation, making it the structure that coordinates when the ventricles contract after the SA node fires.