The illustration explains the moving impulse in a sequence:

1. The first diagram indicates the cell body of the neuron and the axon. This diagram is captioned ‘At rest’.
2. The second diagram indicates the opening of the gated sodium ions at the leading edge of the impulse. It is indicated by black color and two arrows parallel to each other indicate the direction of sodium ions’ flow. Below it, the text reads ‘Action potential’. An enlarged diagram of a small section of the opening of the gated sodium ions is indicated on the top. The description below the diagram reads ‘At the leading edge of the impulse, gated sodium channels open. Na+ ions flow into the cell, reversing the potential between the cell membrane and its surroundings. This rapidly moving reversal of charge is called an action potential.’
3. The third diagram indicates the movement of the action potential further forward and the opening of the gated potassium channel is indicated. An enlarged diagram of a small section of the opening of the gates potassium ions is indicated below. The description below the diagram reads ‘As the action potential passes, gated potassium channels open, allowing K+ ions to flow out and restoring the resting potential inside the axon.’