AQA A2 Biology BIOL5 - 17th June 2015

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At resting potential, most Potassium ion channels are open and most Sodium ion channels are closed

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The sodium-potassium pump actively transports sodium ions out of the axon and actively transports potassium ions into the axon.

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3 sodium ions are actively transported out of the axon for every 2 potassium ions that are actively transported in

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There are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and more potassium ions in the cytoplasn than in the tissue fluid, so there is a chemical gradient

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The inside of an axon is negatively charged in relation to the outside

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The resting potential is around 65 mV and in this condition, the axon is said to be polarised

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Because most potassium ion channels are open and most sodium ion channels aren't, the axon membrane is more permeable to potassium ions.

1.

At resting potential, the sodium ion channels are mostly closed

2.

A stimulus excites the neurone cell membrane and causes the sodium ion channels to diffuse into the axon through these channels

3.

As the sodium ions diffuse into the axon, more sodium ion channels open, causing a greater influx of sodium ions by diffusion (the membrane becomes more permeable to sodium ions)

4.

Once the potential difference reaches around +40 mV, the sodium ion channels close and the potassium ion channels begin to open

5.

From bullet points 2-4, that process is known as depolarisation

6.

Potassium ions diffuse out of axon, causing repolarisation of the axon

7.

There is an overshoot; too many potassium ions diffuse out of axon because the potassium ion channels are too slow to close. The potential difference becomes more negative than resting potential (-70mV). This is called Hyperpolarisation

8.

The Potassium ion channels close and the sodium potassium pump re-establishes the resting potential by pumping sodium out and potassium in.