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Scattered throughout the axon membrane are sodium channels and potassium channels.

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Ions diffuse through these channels.

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Sodium/potassium pumps capture ions and actively push them through the membrane.

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The positively charged sodium ions are repelled by the other positive charges outside the axon,

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and attracted to the negative charges inside, but are kept out by the sodium gates in the cell membrane.

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When the neuron fires, the part of the axon nearest the cell body opens its sodium gates,

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allowing the sodium ions to rush in.

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The influx of these positively charged ions depolarizes that part of the axon making it electrically positive relative to the extracellular fluid.

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This in turn causes the next section of the axon to open its gates and become depolarized,

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and the electrical impulse - called the action potential - moves down the axon like a wave.

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By this time (about 1/2 millisecond later) the sodium gates in the first section of the axon close up again.

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Other gates open to allow some of the potassium ions, repelled by the sudden positive shift,

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to flow out of the axon, therefore repolarizing the membrane.