10.3 The nerve impulse

Resting potential

The movement of ions, such as NA+ and K+ accross the axon membrane is controlled in a number of ways:

The phospholipid bilayer of the axaon plasma membrande prevents sodium and potassium ions diffusing accross it.
Intrinsic proteins span the bilayer and contain ion channels. Some of these channels have gates which can open or close to allow sodium or potassium ions to move through at one time, but prevent at another. Some channels are always open
Some intrinsic proteins actively transport potassium ions into the axon and sodium ions out of the axon - sodium-potassium pump.

As a result of these controls, the inside of the axon is negatively charged compared to the outside - resting potential and ranges from 50-90mV, usually 65mV.
In this condition, the axon is polarised .
This potential difference is due to:

Sodium ions actively transported out of axon by sodium-potassium pump
Potassium ions actively transported into axon by sodium-potassium pump
Active transport of sodium is greater than potassium (3:2) so more sodium in tissue and more potassium in cytoplasm -chemical gradient.
Sodium ions begin to diffuse back into axon,potassium out of axon.
Most 'gates' for potassium ions are open, but most for sodium are closed.
This means the axon membrane is 100x more permeable to potassium ions, which therefore diffuse out of axon faster than sodium diffse in - further increaseing potential difference between -ive inside and +ive outside axon.
There is also an electical gradient. -As more potassium diffuses out of axon, this becomes more +ive, so further outward movement becomes difficult because they are attracted to the negative state of the axon, preventing them moving out.
Equillibrium established.

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A2 Biology - The nerve impulse - Unit 5