During Atrial Fibrillation the atria are activated by multiple AF circuits in rapid (>300 bpm) and irregular manner. This rapid activity suppresses the normal pacemaker cell of the the heart, the SA Node.
During AF the AV node is bombarded with rapid irregular impulses from the fibrillating atria. This rapid electrical activity conducts through the AV node to the Ventricles resulting in rapid and irregular activation and contraction of the Ventricles. This is why patients with AF feel a rapid irregular pulse.
Slide 3 of 3