Final answer:
During the plateau phase, the slow Ca²+ channels open, allowing Ca²+ ions to enter the cell, while few K+ channels are open, limiting the exit of K+ ions.
Step-by-step explanation:
During the plateau phase of the cardiac action potential, there are significant changes to ion channels and ion movement. Specifically, the slow Ca²+ channels open, allowing the entry of Ca²+ ions into the cell. At the same time, few K+ channels are open, limiting the exit of K+ ions from the cell. This combination of Ca²+ entry and limited K+ exit leads to a relatively slow decline in membrane potential, resulting in the plateau phase of the action potential.