Pacemaker action potentials are generated in which cells?

• A. Ventricular myocytes
• B. Working myocytes
• C. SA and AV nodal cells
• D. Purkinje fibers

Answer: (C)

Automaticity in specialized nodal cells drives pacemaker activity. The sinoatrial node has the fastest intrinsic firing rate, so it sets the heart’s pace, and the atrioventricular node can take over if the SA node slows or fails. These nodal cells depolarize spontaneously during diastole due to the If current and calcium influx, producing a slow, gradual phase 4 depolarization that reaches threshold to trigger action potentials. In contrast, ventricular and other working myocytes are contractile cells with stable resting potentials and fast, plateau-type action potentials; they do not fire spontaneously under normal conditions and rely on impulses from the nodal/conduction system. Purkinje fibers mainly conduct impulses rapidly rather than generate pacemaker impulses, though they can exhibit automaticity only in abnormal situations. Hence, pacemaker action potentials originate from the sinoatrial and atrioventricular nodal cells.