Final answer:
The rapid depolarization phase of a pacemaker cell action potential is predominantly caused by the influx of calcium ions and sodium ions when voltage-gated calcium channels and specific sodium ion channels open.
Step-by-step explanation:
The rapid depolarization phase of a pacemaker cell action potential is caused by the movement of calcium ions (Ca2+) and sodium ions (Na+). Initially, sodium ions enter the cell when specific sodium ion channels open as the membrane potential rises slowly. When the threshold is reached, voltage-gated calcium channels open, allowing calcium ions to enter the cell, which triggers a more rapid phase of depolarization until the membrane potential reaches approximately +15 mV. At this peak, the calcium channels close, and potassium channels open, leading to the efflux of potassium ions (K+) and resulting in repolarization.