A premature junctional beat (PJB) typically depolarizes which part of the heart?

A premature junctional beat (PJB) typically depolarizes which part of the heart?

• A. The atria only
• B. Both the atria and ventricles
• C. The ventricles primarily
• D. The SA node

Answer: (B)

A premature junctional beat (PJB) arises from the junctional area of the heart, which lies between the atria and the ventricles. This type of beat occurs when an ectopic impulse originates in the atrioventricular (AV) junction. As a result, the impulse spreads through both the atria and ventricles, leading to depolarization of both structures.

When a PJB occurs, the electrical impulse moves upward into the atria and downward into the ventricles simultaneously. This simultaneous depolarization results in the contractions of both the atria and ventricles. Therefore, the correct choice highlights that a PJB affects both the atrial and ventricular myocardium during the conduction process.

The other options do not accurately depict the mechanism of a PJB; for instance, focusing solely on the atria or ventricles ignores the dual depolarization characteristic of this junctional beat. Similarly, suggesting depolarization of the SA node misrepresents the origin of the beat, as the SA node is not responsible for initiating a PJB.