Which cells are included in the heart’s pacemaker system?

• A. SA nodal cells, AV nodal cells, Purkinje cells
• B. Only SA nodal cells
• C. Only AV nodal cells
• D. Only Purkinje cells

Answer: (A)

Automaticity in specialized cardiac cells keeps the heart beating even if the main pacemaker slows or fails. The heart’s pacemaker system isn’t limited to one place; it includes the cells of the sinus node, the atrioventricular (AV) node, and the ventricular Purkinje network. The SA node has the fastest intrinsic rate and sets the normal rhythm. If the SA node slows or stops, the AV node can take over as a secondary pacemaker, producing impulses around 40–60 beats per minute. If both SA and AV nodes fail, the Purkinje fibers in the ventricles can generate impulses on their own, albeit at a slower rate, roughly 20–40 beats per minute. Because all three areas can initiate rhythmic impulses under different conditions, the heart’s pacemaker system comprises SA nodal cells, AV nodal cells, and Purkinje cells.