Action Potentials (OCR A Level Biology): Revision Note

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