Non-pacemaker action potentials arise in which cells?

• A. SA and AV nodal cells
• B. Ventricular and atrial myocytes
• C. Purkinje fibers
• D. Specialized conduction cells only

Answer: (B)

Non-pacemaker action potentials come from the working myocardium, the atrial and ventricular myocytes, which do not fire spontaneously. These cells sit at a stable resting potential and respond to a depolarizing impulse from the pacemaker tissue. Their action potential has a fast upstroke due to rapid Na+ entry, a plateau from Ca2+ entry, and a subsequent repolarization from K+ efflux. This contrasts with pacemaker cells (like those in the SA and AV nodes) that depolarize spontaneously due to If and related currents, setting the heart rhythm. Purkinje and other specialized conduction cells are part of the conduction system, not the primary source of non-pacemaker action potentials in the sense of working myocardium. So the working atrial and ventricular myocytes are where non-pacemaker action potentials arise.