Action Potentials

• Action potential


  A brief change in the distribution of electrical charge across the cell surface membrane, not a flow of electrons
• Action potentials


  • Caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon
  • Occur via channel proteins in the axon membrane that allow sodium ions or potassium ions to pass through
• Voltage-gated channel proteins


  Open and close depending on the electrical potential (or voltage) across the axon membrane
• Action potential

  1. Stimulus

  2. Depolarisation

  3. Repolarisation

  4. Hyperpolarisation

  5. Return to resting potential
• Stimulus

  Triggers sodium ion channels in the membrane to open, allowing sodium ions to diffuse into the neurone down an electrochemical gradient
• Stimulus reaches threshold of around -55mV


  Action potential is stimulated
• Depolarisation

  Voltage-gated sodium ion channels in the axon membrane open, sodium ions pass into the axon down the electrochemical gradient, reducing the potential difference across the axon membrane
• Depolarisation


  Triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation (positive feedback)
• Repolarisation

  Sodium ion voltage-gated channel proteins close, potassium ion voltage-gated channel proteins open, allowing potassium ions to diffuse out of the axon, returning the potential difference to normal (about -70mV) (negative feedback)
• Hyperpolarisation

  Potassium ion channels are slow to close, causing too many potassium ions to diffuse out of the neurone, making the potential difference across this section of axon membrane briefly more negative than the normal resting potential
• Returning to resting potential

  Sodium-potassium pump restores the resting potential, sodium ion channel proteins become responsive to depolarisation again
• Action potential stages


  • Stimulus
  • Depolarisation
  • Repolarisation
  • Hyperpolarisation
  • Return to resting potential
• Action potentials travel as a wave of depolarisation across the length of the neurone