Action potentials

• How do signals travel in the brain?

  • via action potentials
  • electrically or chemically
• Resting potential 


  • -65 mV
  • the inside is more negative than the outside
• rising phase


  • rapid depolarisation of the membrane
  • voltage gated sodium ion channels open, which allows for sodium ions to move back into the axon from the extracellular fluid
  • rises until +40 mV
• Overshoot 


  • the inside of the neurone is more positively charged than the outside
  • +40 mV - all or nothing principle
• Falling phase 


  • rapid repolarisation until the inside inside of the membrane is more negative than the resting potential
  • voltage gated sodium ion channels close and voltage gated potassium ion channels open
• Undershoot


  • gradual restoration of the resting potential
• generation of action potential

  • stimuli causes the membrane to be stretched
  • this opens voltage gated sodium ion channels
  • due to the concentration gradients, sodium ions move out of the cell
  • if this generator potential achieve the critical level (threshold), the membrane will generate an action potential
• Generation of multiple action potentials 


  • absolute refractory period - period in which it is impossible to initiate another action potential
  • relative refractory period - the amount of current required to depolarise the neurone to action potential is elevated above normal
• Propagation of the action potential 


  • action potentials can propagate at different speeds - 0.1 m/sec to 100m/sec
  • the thicker the axon, the more ions can move down the axon, which means sodium channels can be opened earlier, causing action potentials to propagate faster
  • myelinated axons are faster as the action potentials jump form node of Ranvier - saltatory conduction
• Optogenetics


  • introduces foreign genes into neurone that express membrane ion channels that open in response to light
  • developed to overcome the limitation of having to use mirco-electrodes to inject electrical current