Free Nerve Impulses revision notes for OCR A Level Biology – covering specification point 5.1.3 (c).
Nerve Impulses
Neurones transmit information as electrical impulses.
A nerve impulse is a series of action potentials travelling along the plasma membrane of a neurone (such as an axon).
The table below outlines the key events of each stage in the action potential:
| Stage | Potential Difference | Active/Open Proteins | Ion movement |
|---|---|---|---|
| Resting (polarised) | -60 mV |
|
|
| Depolarisation | -60 mV → +40 mV | Voltage-gated Na+ channel | Na+ influx |
| Hyperpolarisation | -60mV → -70mV | K+ channels closing slowly | Some K+ efflux |
| Refractory Period | -70mV → -60 mV |
|
|
| Repolarisation | +40 mV → -60 mV | Voltage-gated K+ channel | K+ efflux |
Electrochemical gradients are established and maintained by carrier proteins moving specific ions to one side of the plasma membrane.
Resting Potential
When a neurone is not sending an electrical impulse, it is said to be at rest.
At rest, neurones maintain a potential difference of approximately –60 mV*. The inside of the neurone is negative compared to the outside.
*It is important to note that OCR A Level Biology specifies the resting potential at -60 mV, whilst it is more commonly cited at or around -70 mV.
Depolarisation
Depolarisation is when the potential difference of the cell is decreased.
Depolarisation is important because if a neurone depolarises sufficiently (reaches its threshold potential), then it will trigger an electrical impulse; the action potential.
Action Potential
An action potential is the series of changes in potential difference across a membrane, which occurs after a neurone is sufficiently excited (stimulated so that it reaches its threshold potential).
During an action potential, the potential difference across the plasma membrane changes from -60 mV to +40mV, then reverses temporarily beyond the resting potential (-65 mV to -70mV) in a phenomenon known as hyperpolarisation, before repolarising to the resting potential of -60 mV.*
*It is important to note that these values are true of post-synaptic neurones as they are presented in OCR A Level Biology, but may not be true of other cells, or in other courses.
All action potentials in the neurone are the same size, peaking at about +40 mV.
