Nerve Impulses

Free Nerve Impulses OCR A Level Biology revision notes – covering specification point 5.1.3 (c).

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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	Na⁺/K⁺ pump Leaky K⁺ channel	3 Na⁺ out for 2 K⁺ in K⁺ leak out
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	Na⁺/K⁺ pump Leaky K⁺ channel	3 Na⁺ out for 2 K⁺ in K⁺ leak out
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.