During depolarisation of the neural cell membrane,

When a neural fiber is stimulated,  its voltage-gated Na⁺ channels open. Opening of these channels allows the inflow of the Na⁺ because the axoplasm of the nerve fibers is electrically more negative than interstitial fluid and Na⁺ concentration is higher in interstitial fluid. Inflow of Na⁺ down the electrochemical gradient produces a rapid depolarization.