Phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions from diffusing across
Channel proteins that span the bilayer can transport the ions across the membrane
Some of these channels have gates which can be open or closed
Some carrier proteins actively transport potassium ions ito the axon and sodium ions out of the axon - sodium-potassium pump
Inside of the axon is negatively charged at -65mV
This is called being polarised
Active transport of sodium out of the cell is greater than the amount of potassium coming into the cell. 3 sodium per 2 potassium
Slow diffusion too but this is cancelled out
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Action potential
A stimulus can cause a reversal of the charge by opening channel proteins to allow diffusion
If it reaches +40mV an action potential is created and the membrane is depolarised
There are voltage-gated channels that open/close depending on the charge of its environment
At resting potential sodium voltage-gated channels are closed
Energy of the stimulus causes some of the sodium voltage-gated channels to open so sodium diffuses in along their electrochemical gradient
As sodium diffuses in, more sodium channels open causeing a greater influx of sodium
Once it is at +40mV the sodium voltage-gated channels close and potassium channels open
The potassium ions are now able to diffuse out more easily causing more potassium ion channels to open repolarising the axon
This causes an overshoot where the axon becomes hyperpolarised. Here the potassium channels close and the sodium-potassium pump restarts
The resting potential of -65mV is re-established and the axon is repolarised
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Passage of an action potential on an unmyelinated
At resting potential, there is a high concentration of sodium ions outside the axon compared to inside whereas there is a high concentration of potassium on the inside compared to the outside. The axon is polarised
A stimulus causes a sudden influx of sodium ions - depolarisation
Localised electrical currents open sodium voltage-gated channels further along the axon
This causes depolarisation further along the axon
behind this new region, the older region now shuts its sodium channels stopping the influx and potassium ions diffuse out decrease its charge
This causes depolaristion to move along the membrane
The section behind the depolarised region is being repolarised and becomes hyperpolarised to prevent the backwards flow of an impulse
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Passage of an action potential on a myelinated axo
Fatty sheath of myelin acts as an electrical insulator
every 1-3 mm there is a break called the nodes of Ranvier
Action potentials can occur at these points
Localised currents happen between adjacent nodes of Ranvier
The action potential can jump from gap to gap
This is called saltatory conduction
Action potential can happen faster when myelinated as depolarisation doesn't have to happen the whole way along the neurone, only at parts
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