Junctional rhythm

During junctional rhythm, the HIS bundle initiates electrical activity in the heart. This causes the atria and ventricles to be electrically activated at the same time.

In contrast, sinus rhythm initiates in the Sinus node and activates the atria then progresses into the HIS bundle, down the Purkinje fibers and up the ventrical. This causes the atria to contract before the ventricals.