What is the outcome of Na diffusion into the cell?

When sodium ions (Na^+) diffuse into a cell, the primary outcome is depolarization. This process occurs during the action potential phase of neuronal signaling. Under resting conditions, the inside of the cell is negatively charged compared to the outside, primarily due to the distribution of ions. When sodium channels open and sodium rushes into the cell, the interior becomes more positive. This influx of positive charge decreases the membrane potential, moving it closer to zero, which is characterized as depolarization.

Depolarization is crucial for the generation and propagation of action potentials. If the depolarization reaches a certain threshold, it can trigger a full action potential, which is essential for nerve signal transmission and muscle contraction.

In contrast, repolarization refers to the process of returning the cell membrane potential back to its resting state after depolarization, while resting potential maintenance focuses on the stable, baseline state of the cell before any action potential occurs. Ion equilibrium relates to a state where there are equal concentrations of ions across the membrane, which is not the case during active depolarization events. Therefore, the correct understanding of the effect of sodium diffusion into the cell clearly identifies depolarization as the key outcome.