Action Potentials
Action potentials
- 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
Stage 1: Stimulus
- 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
Stage 2: Depolarisation
- 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
Stage 3: Repolarisation
- 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.
Stage 4: Hyperpolarisation
- 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
Stage 5: Returning to the resting potential
- 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
Action Potential Table
The five stages of an action potential: stimulus, depolarisation, repolarisation, hyperpolarisation and return to resting state
Exam Tip
Action potentials travel as a wave of depolarisation across the length of the neurone.
