Final answer:
After the SA node fires, the AV node is stimulated and acts as a relay station to delay the electrical signal.
Step-by-step explanation:
After the sinoatrial (SA) node fires, the atrioventricular (AV) node is stimulated.
The SA node initiates the electrical impulse that spreads across the atria, causing them to contract. This impulse then reaches the AV node, which is located at the bottom of the right atrium near the septum.
The AV node acts as a relay station and delays the electrical signal for a brief moment, allowing the atria to complete their contraction and the blood to flow into the ventricles. After this delay, the impulse continues to spread through the conduction system of the heart, eventually reaching the ventricles and causing them to contract as well.