Which statement best contrasts non-pacemaker and pacemaker cell action potentials?

Which statement best contrasts non-pacemaker and pacemaker cell action potentials?

• A. Non-pacemaker cells have spontaneous depolarization; pacemaker cells have a plateau.
• B. Non-pacemaker cells have rapid depolarization with a plateau; pacemaker cells have spontaneous depolarization.
• C. Non-pacemaker cells rest at -90 mV; pacemaker cells rest at -70 mV.
• D. Both cell types have identical action potential shapes.

Answer: (B)

Automaticity drives the key difference. Non-pacemaker heart cells (like ventricular and atrial myocytes) sit at a stable resting membrane potential and, when they fire, show a rapid upstroke from Na+ influx followed by a plateau caused by Ca2+ entry balancing K+ efflux, then repolarize. They do not initiate their own impulses. Pacemaker cells (SA/AV node) lack a true resting plateau and instead slowly depolarize during phase 4, a diastolic depolarization that gradually brings them to threshold so an action potential fires automatically. Their upstroke is mainly Ca2+-dependent rather than Na+-dependent. Because of this automatic, spontaneous depolarization and the absence of a pronounced plateau, the statement comparing the two accurately describes non-pacemaker cells as having rapid depolarization with a plateau, and pacemaker cells as having spontaneous depolarization.

Automaticity drives the key difference. Non-pacemaker heart cells (like ventricular and atrial myocytes) sit at a stable resting membrane potential and, when they fire, show a rapid upstroke from Na+ influx followed by a plateau caused by Ca2+ entry balancing K+ efflux, then repolarize. They do not initiate their own impulses. Pacemaker cells (SA/AV node) lack a true resting plateau and instead slowly depolarize during phase 4, a diastolic depolarization that gradually brings them to threshold so an action potential fires automatically. Their upstroke is mainly Ca2+-dependent rather than Na+-dependent. Because of this automatic, spontaneous depolarization and the absence of a pronounced plateau, the statement comparing the two accurately describes non-pacemaker cells as having rapid depolarization with a plateau, and pacemaker cells as having spontaneous depolarization.