Action potentials

• Explain what happens when a neurone is NOT stimulated

  • A neurone membrane contains voltage gated sodium-potassium pumps,and potassium ion channels.
  • The resting potential across the membrane is -70mV.
  • For every 3Na+ pumped out of the sodium-potassium pump 2K+ is pumped in
  • K+is pumped out of the potassium ion channel
• Why can K+ diffuse out of the membrane

  the membrane is permeable to K+ so can diffuse out and does not require ATP
• Does the sodium-potassium pump require ATP? And what does it not allow in 


  NO

  • It does not allow Na+ in
• What happens when a neurone is stimulated?

  Sodium ion channels open and Na+ diffuse in
• What occurs when the potential difference reaches around -50mV?

  All sodium ion channels open, causing depolarisation
• What is the term for the process when the inside of the neuron becomes less negative?

  Depolarisation
• What happens when the potential difference reaches around +40mV?

  Voltage gated sodium channels close and potassium channels open
• What is the effect of potassium ions diffusing out of the neuron?

  It causes repolarisation, making the inside more negative
• What is hyperpolarisation in the context of neuron stimulation?

  Potential difference goes below -70mV
• Why do too many K+ ions diffuse out of the neuron?

  Because potassium ion channels are slow to close
• What restores the neurone to its resting potential?

  The sodium-potassium pumps
• What are the main steps that occur when a neurone is stimulated?

  1. Sodium ion channels open, Na+ diffuse in
  2. Depolarisation occurs when potential reaches -50mV
  3. All sodium channels open, more Na+ flood in
  4. At +40mV, sodium channels close, potassium channels open
  5. K+ diffuse out, causing repolarisation
  6. Hyperpolarisation occurs due to slow closing of K+ channels
  7. Sodium-potassium pumps restore resting potential
• What is meant by the all or nothing response

  • Once a threshold is reached action potential will always fire with the same charge in voltage no matter how big the stimulus is
  • If a threshold isn’t reached an action potential won’t fire
  • A bigger stimulus won’t cause a bigger action potential, it just causes them to fire more frequently