Which statement best describes the action potential of pacemaker cells?

• A. Rapid depolarization with a plateau
• B. Spontaneous depolarization and repolarization
• C. Resting membrane potential around -90 mV
• D. Sustained depolarization without repolarization

Answer: (B)

Pacemaker cells are automatic: they undergo spontaneous depolarization during the resting phase (diastolic depolarization) until they reach threshold, triggering an action potential, and then they repolarize to reset. This automaticity means there isn’t a stable, very negative resting potential, and the upstroke is driven mainly by calcium influx rather than a rapid, Na+-mediated spike. After the spike, repolarization returns the membrane toward a negative value, preparing for the next cycle.

That’s why spontaneous depolarization and repolarization best describe their action potential. The other descriptions don’t fit: a rapid depolarization with a plateau reflects contractile cells with a plateau phase; a resting potential around -90 mV implies a true resting state like ventricular cells; sustained depolarization without repolarization would mean no resetting, which doesn’t occur in pacemaker activity.