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
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- 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
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- 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
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- 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
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- 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
