The nerve impulse

Sodium ions actively transport out of axon via sodium-potassium pump.

Potassium ions actively transport into axon via sodium-potassium pump.

Active transport of Sodium > Active transport of Potassium. 3 Sodium ions out for every 2 potassium ions in.

This makes the inside of the axon more negative. As both ions have a positive charge, there are more ions in the surrounding tissue fluid, causing the outside to be more positive than the inside.

Sodium ions start to diffuse back in naturally as potassium ions start to diffuse out.

However, most of the gates on the channels that let sodium back in are closed whilst most of the gates on the channels that letpotassium out are open.

Axon membrane is 100 times more permeable to potassium. This increases the potential difference between the negative inside and the positive outside of the membrane.

There is an electrical gradient. As potassium ions diffuse out of the axon, this makes the outside more positive. The potassium ions are positively charged and are so attracted to the negative state inside the axon. They are repelled by the positive outsideand are prevented from moving out of the axon.

Equilibrium is established when the electrical and chemical gradients are balanced and there is no net movement of ions.

Resting potential, sodium and potassium voltage-gates are closed, apart from permanently open potassium gate

Stimulus; sodium channels open; sodium ions diffuse into axon along electrochemical gradient; being positively charged they cause a reversal in the potential difference along the membrane

More sodium ions diffuse in; more sodium channels open; cause influx of sodium ions by diffusion

Action potential established around +40mV; Sodium voltage gates close; Potassium voltage gates open

As electrical gradient is reversed, potassium ions move out, more channels open, more potassium opens move out, causing replarisation

Potassium ions cause hyperpolarisartion, more negative than normal, potassium gates close, Sodium-Potassium pumps out sodium, pumps in potassium. Resting potential of -65mV established. Repolarised. 

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