Emergency Medical Technician (EMT) Intermediate Practice Exam

As electricity travels down the left and right bundle branches, which part is stimulated first?

1. Ventricles

2. Interventricular septum

3. Atria

4. Cardiac muscles

The correct answer is: Interventricular septum

When electrical impulses travel through the heart's conduction system, they follow a specific pathway to ensure coordinated contractions. As the impulse reaches the Bundle of His, it splits into the left and right bundle branches, which extend into the ventricles. The interventricular septum is the wall separating the left and right ventricles, and it is situated between the two bundle branches. The correct answer, which identifies that the interventricular septum is stimulated first, is based on the inherent structure of the conduction system. As the bundle branches are activated, they initially depolarize the interventricular septum before the rest of the ventricles. This stimulation is crucial for ensuring proper contraction mechanics, allowing the heart to pump blood efficiently. In contrast, the ventricles as a whole are depolarized after the septum, meaning their contraction follows the initial activation of the septum. The atria are stimulated prior to the valves and are part of an earlier phase in the cardiac cycle; thus, they are not involved in this specific sequence. Cardiac muscles are responsive to the electrical signals but are not specific structures in the conduction pathway like the interventricular septum. Therefore, understanding the sequence of stimulation in the heart's conduction system clarifies why