Final answer:
Nicotinic receptor channels allow both Na+ and K+ to flow through, but the influx of Na+ exceeds the efflux of K+ due to the differences in their concentration gradients and the timing of ion channel opening and closing during an action potential.
Step-by-step explanation:
The influx of Na+ exceeding the efflux of K+ in nicotinic receptor channels is due to the differences in their concentration gradients and the opening and closing of ion channels during an action potential.
The concentration of Na+ is higher outside the cell, while the concentration of K+ is higher inside the cell. When the nicotinic receptor channels open, Na+ ions rush into the cell, down their concentration gradient, resulting in Na+ influx. At the same time, some K+ ions leave the cell through the receptor channels, but the efflux of K+ is not as significant as the influx of Na+, leading to a net influx of Na+.
Furthermore, the timing of the opening and closing of Na+ and K+ channels plays a role. During the peak of an action potential, Na+ channels close while K+ channels open. This transition allows more K+ ions to leave the cell, contributing to hyperpolarization and a more negative membrane potential.