How does a stimulus produce an action potential?

The cell begins at a resting potential of -85mV with a high concentration of potassium (K+) inside the cell. The stimulus must be strong enough to produce the all or nothing response which triggers the opening of the voltage-gated sodium (Na+) channels. This allows Na+ ions to flood into the cell. This is known as depolarization and the cell potential difference is now at +30mV. The next stage is repolarization in which the Na+ channels close and the K+ channels open. Due to the laws of diffusion, K+ moves from a higher concentration to a lower concentration along a concentration gradient, K+ moves out of the cell. This leaves the cell potential difference at below -85mV. Finally the resting conditions are re-established with the closing of Na+ and K+ channels. The sodium-potassium exchange pump moves Na+ out and K+ in resulting in the potential difference returning to -85mV.