Final answer:
Depolarization of the cell membrane ahead of an action potential occurs due to the movement of ions, particularly Na+ ions. This depolarization is triggered by a stimulus that makes the membrane permeable to Na+ ions. The influx of Na+ ions results in a reversal of charge across the membrane, which leads to repolarization through the opening of voltage-gated potassium channels.
Step-by-step explanation:
When traveling along an axon, the cell membrane immediately ahead of an action potential depolarizes because of the movement of ions across the membrane. Specifically, depolarization occurs when a stimulus makes the membrane permeable to Na+ ions. As a result, Na+ ions rush into the cell, causing the reversal of charge across the membrane from negative to positive. This change in the membrane potential triggers the opening of voltage-gated potassium (K+) channels, leading to repolarization.