L-type calcium channels in nodal cells contribute to which phases?

• A. Phase 2 and Phase 3
• B. Phase 2 of non-pacemaker AP only
• C. Late phase 4 and phase 0
• D. Phase 4 only

Answer: (C)

In nodal (pacemaker) cells, the upstroke of the action potential is driven by calcium influx rather than sodium. L-type calcium channels open as the membrane depolarizes toward threshold, carrying inward Ca2+ that produces the rapid depolarization of phase 0. They also contribute to the final portion of the pacemaker potential, the late phase 4, helping the cell reach threshold for the next firing. Early phase 4 is mainly shaped by other currents like the funny current, but the L-type calcium current becomes important as the cell approaches threshold. There is no plateau phase in nodal tissue, so phases associated with a plateau or with ventricular repolarization aren’t involved here. Thus, L-type calcium channels contribute to both late phase 4 and phase 0.