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Nerve impulses must travel incredibly fast to achieve the functions they do. However, I have been taught that sodium ions move down the axons by diffusion (thus causing depolarisation of the next part of the membrane etc, and this is the action potential).

However, I was under the impression that diffusion is a very slow process? How can this be the case?

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There is a large gradient created by selective permeability of Na channels and as you're probably aware the speed of diffusion is proportional to the electrochemical gradient.

Very small numbers of ions actually cross the membrane during depolarisation and subsequent AP events.

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