The correct answer is option 3: Nerve impulse. The sodium pump, also known as the sodium-potassium pump, is responsible for maintaining the concentration gradients of sodium and potassium ions across the cell membrane. This pump is crucial for the normal functioning of nerve cells and plays a key role in generating and propagating nerve impulses.

Option 1, muscle contraction, is not the correct answer because although the sodium pump indirectly contributes to muscle contraction by maintaining ion gradients, it is not specifically involved in the process itself.

Option 2, heartbeat, is not the correct answer as the sodium pump is not directly involved in the contraction of the heart muscles.

Option 4, none of the above, is not the correct answer because the sodium pump is indeed involved in nerve impulse transmission. So, the correct answer is option 3: Nerve impulse.