A level Biology exam revision resources written by A level Examiners


The way in which neurones transmit information is as impulses of electricity passing over their membranes. The nature of the impulse is actually a wave of depolarisation passing along the membrane. Depolarisation can only occur because the membrane is normally polarised at rest. This polarisation is known as the resting potential.

The resting potential is a small electrical imbalance caused by more positive ions being found outside the neurone than inside. The magnitude of this difference is about -70mV in humans. The imbalance is caused by two processes, active transport and facilitated diffusion which occur simultaneously.

There is an active transport protein in neurones called the sodium potassium pump (Na+K+ATPase). These proteins pump three sodium ions out of the neurone and two potassium ions into the neurone for every molecule of ATP that they use.

Simultaneous to the active transport occurring there is facilitated diffusion of potassium ions out of the neurone.

These two processes cause an imbalance in the distribution of ions across the membrane. There are about 30x more sodium ions outside than in at rest. This imbalance means the inside of the neurone is negative and the outside is positive.

Figure 6 : Nerve Impulse
Figure 2 : Nerve Impulse

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