Conduction of a Nerve Impulse

L5

Action Potentials in Neurones

An action potential is an abrupt but short-lived reversal of the resting potential of a neurone.

It occurs at a specific point of the neurone. The graph shows the electrical changes that occur during a single action potential.

Action potentials in neurones

1. The action potential begins when a particular stimulus causes the membrane at one part of the neurone suddenly to increase its permeability to Na+ ions.

This is labelled A and happens because the stimulus has caused voltage-sensitive Na+ gates in this part of the membrane to open.

As a result, Na+ ions diffuse rapidly into the neurone along their electrochemical gradient, reducing the negativity inside that part of the neurone.

In a positive feedback loop, this causes more and more voltage-sensitive Na+ gates to open until the voltage difference across the membrane reverses.

The highest positive membrane potential is the action potential and this part of the membrane is now said to be depolarised.

2. At a certain point, labelled B the depolarisation of the membrane causes the voltage-sensitive Na+ gates to close.

3. Shortly after the voltage-sensitive Na+ gates close, the voltage-sensitive K+ gates open. This is labelled C.

The voltage-sensitive K+ gates open more slowly than the voltage-sensitive Na+ gates and allow K+ ions to flow out of the neurone, along their electrochemical gradient.

This outflow of K+ ions restores the resting potential in the neurone. In a negative feedback loop, the outward movement of K+ ions results in the closing of the voltage-sensitive K+ gates (labelled D).

There is a slight 'overshoot' in the movement of K+ ions, which causes the membrane potential to become slightly lower than its normal resting potential. This is called hyperpolarisation and is labelled region E.

4. After the resting potential has been restored, sodium-potassium pumps move Na+ ions out of the neurone and move K+ ions back into the neurone by active transport.

Activity 8.3 Nerve Impulse Propagation Question 8.11 Answer exam questions from SoWExtension Extension 8.1 about ion channels and

episodic diseases

To Do

8.11 A new action potential will only be

generated at the leading edge of the previous one;

Because the membrane behind it will be recovering/incapable of transmitting an impulse;

The membrane has to be repolarised and return to resting potential before another action potential can be generated;