Final answer:
Both funny channels and L-type calcium channels must open for a cardiac pacemaker cell to depolarize to threshold for an action potential.
Step-by-step explanation:
The ion channels that must open for a cardiac pacemaker cell to depolarize to threshold for an action potential are both funny channels and L-type calcium channels. Funny channels, also known as hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, allow a slow influx of sodium ions which causes the membrane potential to rise slowly. L-type calcium channels, on the other hand, allow the entry of calcium ions, further depolarizing the cell.