Final answer:
During the rising phase of an action potential, there is greater Na+ flux compared to K+ due to the rapid opening of activation gates in Na+ channels, while K+ channels open more slowly and with a delay, leading to later K+ efflux for repolarization.
Step-by-step explanation:
During an action potential, both Na+ and K+ channels are activated by depolarization. However, during the rising phase of an action potential, what we see is a greater flux of Na+ ions compared to K+ ions. The reason behind this lies in the behavior of voltage-gated Na+ and K+ channels. Voltage-gated Na+ channels have two gates - an activation gate and an inactivation gate. When the membrane potential crosses -55 mV, the activation gate opens, allowing Na+ to rush into the cell, leading to a rapid depolarization. Subsequently, around the peak of this depolarization, the inactivation gate closes, preventing more Na+ from entering the cell.
On the other hand, voltage-gated K+ channels open more slowly and only after a delay. This means that during the initial phase of depolarization, the entry of Na+ ions is predominant. As a result, the membrane potential rises more rapidly due to Na+ influx. It is only after the Na+ channels begin to inactivate, and K+ channels finally open, that K+ leaves the cell, leading to repolarization. This sequence of events is crucial for the propagation of action potentials along axons.