Phases 1 & 2 are absent in the action potential of a ____________ cell.

Phases 1 & 2 are absent in the action potential of a ____________ cell.

• A. Pacemaker
• B. Atrial myocyte
• C. Ventricular myocyte
• D. Purkinje cell

Answer: (A)

Pacemaker cells operate with automaticity rather than a strong contractile drive, so their action potential structure is different from working heart muscle. In contractile atrial and ventricular myocytes, the fast depolarization is followed by a brief phase 1 notch and a sustained phase 2 plateau caused by a balance of Ca2+ influx through L-type channels and K+ efflux. Pacemaker cells, like those in the SA and AV nodes, rely on a slower, Ca2+-mediated upstroke and do not exhibit a prominent plateau; they lack the fast Na+–driven phase 0 and have minimal, if any, phases 1 and 2. Instead, their resting potential drifts upward during phase 4 due to the funny current and other inward currents, producing automatic depolarization. This is why phases 1 and 2 are absent in pacemaker cell action potentials.

Pacemaker cells operate with automaticity rather than a strong contractile drive, so their action potential structure is different from working heart muscle. In contractile atrial and ventricular myocytes, the fast depolarization is followed by a brief phase 1 notch and a sustained phase 2 plateau caused by a balance of Ca2+ influx through L-type channels and K+ efflux. Pacemaker cells, like those in the SA and AV nodes, rely on a slower, Ca2+-mediated upstroke and do not exhibit a prominent plateau; they lack the fast Na+–driven phase 0 and have minimal, if any, phases 1 and 2. Instead, their resting potential drifts upward during phase 4 due to the funny current and other inward currents, producing automatic depolarization. This is why phases 1 and 2 are absent in pacemaker cell action potentials.