Final answer:
The membrane potential approaches EK when K+ permeability is high, and ENa when Na+ permeability is high, depending on which ion channels are open.
Step-by-step explanation:
When the K+ permeability is much higher than the Na+ permeability, the membrane potential approaches EK (the equilibrium potential for potassium), and when the Na+ permeability is much higher than the K+ permeability, the membrane potential approaches ENa (the equilibrium potential for sodium).
The resting membrane potential is primarily determined by the permeability of the membrane to K+ ions due to the K+ channels that are frequently open. When K+ leaves the cell following its concentration gradient, a negative charge is left behind, contributing to the resting membrane potential. Conversely, during an action potential, the temporary increase in Na+ permeability due to open Na+ channels leads to a massive influx of Na+ ions, causing the membrane potential to approach ENa and creating a positive spike known as the peak action potential.