5.3.4 Action Potentials

Question 1

What are action potentials?

Answer 1

Unlike a normal electric current, an action potential is not a flow of electrons but instead occurs via a brief change in the distribution of electrical charge across the cell surface membrane
Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon
There are channel proteins in the axon membrane that allow sodium ions or potassium ions to pass through
These are known as voltage-gated channel proteins. They open and close depending on the electrical potential (or voltage) across the axon membrane
They are closed when the axon membrane is at its resting potential
Several different things occur during an action potential: stimulus, depolarisation, repolarisation, hyperpolarisation and the return to resting potential

Question 2

Describe Stage 1: Stimulus.

Answer 2

A stimulus triggers sodium ion channels in the membrane to open allowing sodium ions to diffuse into the neurone down an electrochemical gradient
The stimulus can either be an electrical impulse from another neurone or a chemical change to the membrane of the neurone
When a large enough stimulus is detected by a neurone, the resting potential can be converted into an action potential
The potential difference across the membrane must reach a threshold of around -55mV to trigger depolarisation

Question 3

Describe Stage 2: Depolarisation.

Answer 3

When the threshold (around -55mV) is reached, an action potential is stimulated and the following steps occur:
Voltage-gated sodium ion channels in the axon membrane open
Sodium ions pass into the axon down the electrochemical gradient (there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
The movement of sodium ions reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – a process known as depolarisation
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
This is an example of positive feedback
The action potential that is generated will reach a potential of around +30mV

Question 4

Describe Stage 3: Repolarisation.

Answer 4

Very shortly (about 1 ms) after the potential difference has reached +30mV, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon
Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
This returns the potential difference to normal (about -70mV) – a process known as repolarisation
This is an example of negative feedback.

Question 5

Describe Stage 4: Hyperpolarisation.

Answer 5

Potassium ion channels are slow to close and as a result, too many potassium ions diffuse out of the neurone causing a short period of hyperpolarisation
This means that the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential

Question 6

Describe Stage 5: Returning to the resting potential.

Answer 6

Once the potassium ion voltage-gated channel proteins are closed the sodium-potassium pump restores the resting potential
The sodium ion channel proteins in this section of membrane become responsive to depolarisation again

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