Resting Potential

  • Concentration of Na+ outside axon membrane is high (relative to inside)
  • Concentration of K+ inside axon membrane is high (relative to outside)
  • Overall concentration of positive ions is however greater on the outside, making this positive compared to the inside.
  • Axon membrane is polarised

Arrival of an action potential

    • A stimulus causes a sudden influx of Na+ ions
    • A reversal of charge on the axon membrane in that action potential and the membrane become depolarised
    • Localised electrical circuits established by the influx of Na+ causes the opening of sodium voltage-gated channels to open slightly further along the axon
    • Resulting influx of Na+ causes the region to become depolarised
    • Behind the newer region of depolarisation, the sodium voltage gated channels close and the potassium ones open
    • K+ begin to move from the axon along electrochemical gradient

Repolarisation

    • Action potential (depolarisation) is propagated in the same way further along the axon
    • Outward movement of the K+ions has continued to the extent that the axon membrane behind the action potential has return to its original charged state (where the positive outside and negative inside)
    • It has repolarised
    • Repolarisation of the axon allows Na+ ions to be actively transported out
    • This returns the axon to its resting potential

 

Passage of an action potential along a myelinated Axon

  • Myelin forms a fatty sheath around the axon which acts as an electrical insulator (thus preventing an action potential from forming)
  • At intervals (1-3mm) there are nodes of Ranvier (breaks in the myelin insulation)
  • The localised circuits therefore arise between adjacent nodes of Ranvier and the action potential is able to ‘leap’ between the nodes through saltatory conduction
  • An action potential passes along a myelinated neurone faster due to this fashion