Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract. Under some conditions almost all heart tissue is capable of starting a heartbeat, or becoming the pacemaker.
Atrial fibrillation is a type of arrthymia.
The atria quivers instead of beats. The electrical signals then arrive in the ventricles in an irregular fashion. About 15 percent of strokes occur in persons with atrial fibrillation. Aspirin, warfarin, and cardiac medications may be used to treat atrial fibrillation.