Final answer:
Ion concentrations in nerve cells are significantly affected after a few hundred action potentials due to the small movement of ions with each firing and active transport methods that replenish these ions.
Step-by-step explanation:
Ion concentrations are first significantly affected after a few hundred action potentials. This is because each individual action potential only causes small fractions of Na+ and K+ ions to move across the nerve cell membrane, which is not enough to seriously deplete the cell's ion concentrations. The nerve cell can fire many times in rapid succession without significant depletion due to active transport mechanisms such as the sodium-potassium pump that replenish these ions to maintain the necessary concentration gradients.