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Action Potentials

Biology: Cambridge International A Level, topic 15.1.5

Resting Potential

When a neurone has not been stimulated, it is at resting state.

Potential difference

Potential difference

  • At resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.
  • This is because there are more positive ions outside the cell than inside.
  • The difference in charge is called a potential difference.
Sodium-potassium pumps

Sodium-potassium pumps

  • The resting potential is maintained by sodium-potassium pumps in the neurone membrane.
  • Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.
  • This leads to a build-up of positive ions outside the cell.
Potassium ion channels

Potassium ion channels

  • There are potassium ion channels in the neurone membrane. This means it is permeable to K+ ions.
  • When K+ ions are transported into neurones, they can diffuse back out.
  • The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.
Resting potential

Resting potential

  • Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane.
  • This potential difference is called the resting potential.
  • The neurone is said to be polarised.
  • Resting potential is about −70mV.

Depolarisation of the Neurone Cell Membrane

When a resting neurone is stimulated, its membrane experiences a change in potential difference. This change is called depolarisation. The steps are:

Stimulation

Stimulation

  • Na+ ion channels in the cell membrane open when a neurone is stimulated.
  • Na+ ions flood into the neurone.
  • The potential difference across the membrane changes to become more positive inside the neurone.
Depolarisation

Depolarisation

  • If the potential difference increases above the threshold value (about −55mV) then the membrane will become depolarised.
  • More sodium channels open and there is a sharp increase in potential difference to about +30mV.
All-or-nothing

All-or-nothing

  • Depolarisation is an all-or-nothing response.
  • If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same.
  • If the stimulus is stronger, action potentials will be produced more frequently but their size will not increase.
Repolarisation

Repolarisation

  • After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.
  • K+ ions are transported out of the neurone and the potential difference becomes more negative.
  • This is called repolarisation.
Hyperpolarisation

Hyperpolarisation

  • There is a short period after repolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.
  • This is called hyperpolarisation.
  • Hyperpolarisation prevents the neurone from being restimulated instantly. This is called the refractory period.
Resting potential

Resting potential

  • After the refractory period, the potassium ion channels close and the membrane returns to its resting potential.
  • The process where a neurone is depolarised and returns to resting potential is called an action potential.

Action Potential

The stages involved in the depolarisation of a neurone membrane are called an action potential. Action potentials move along the neurone in a wave.

Sodium ions

Sodium ions

  • When an action potential is generated, there are more Na+ ions inside the neurone than outside.
  • Some of these Na+ ions diffuse sideways along the neurone axon.
Sodium ion channels

Sodium ion channels

  • The presence of Na+ ions creates a change in potential difference further along the neurone membrane.
  • If this reaches the threshold value, sodium ion channels at this part of the membrane open.
  • Na+ ions diffuse into the neurone.
  • This part of the neurone now becomes depolarised.
Wave of depolarisation

Wave of depolarisation

  • Na+ diffuse all along the neurone in this way.
  • Depolarisation takes place along the neurone membrane. This creates a wave of depolarisation.
Refractory period

Refractory period

  • The period of hyperpolarisation in an action potential is called the refractory period.
  • The ion channels are recovering during the refractory period. This means an action potential cannot be stimulated again instantly.
  • This ensures that the wave of depolarisation travels in one direction.

Test yourself on Action Potentials

  • Which of the following statements about resting potential are true?

  • How many Na+ ions are actively transported out of the neurone for every two K+ ions that are transported in.

  • Stages in Depolarisation of Neurone Cell Membrane

  • What is the name of the period after repolarisation, where the potential difference becomes slightly more negative than the resting potential?

  • Which of the following statements about depolarisation are true?

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