Which cells are considered non-pacemaker cells?

• A. Sinoatrial and atrioventricular nodes
• B. Atrial and ventricular contracting myocytes; Purkinje fibers
• C. Pacemaker cells
• D. Purkinje fibers only

Answer: (B)

Understanding automaticity in the heart helps distinguish pacemaker cells from non-pacemaker cells. Pacemaker cells are those that generate impulses spontaneously, setting the heart rate. The primary pacemakers are the cells in the sinoatrial node and the atrioventricular node, which show diastolic depolarization and drive the rhythm.

Non-pacemaker cells do not generate impulses on their own. They are the working myocardium and the fast-conducting pathway: the atrial and ventricular contracting myocytes, which respond to incoming impulses with contraction, and the Purkinje fibers, which rapidly propagate impulses through the ventricles but do not set the rhythm themselves. Under normal conditions, Purkinje fibers act as conduits for conduction rather than pacemakers. (They can take over if higher-order pacemakers fail, but that’s an emergency backup, not their usual role.)

So, the cells that are considered non-pacemaker are the atrial and ventricular contracting myocytes and the Purkinje fibers.