Movement of ions, such as Na+ and K+, across the axon membrane is controlled by:
- Phospholipid bilayer of the axon plasma membrane presents sodium and potassium ions alternating across it
- Channel proteins span the phospholipid bilayer where there is ion channels which pass through them. Some of these channels have gates on them which are able to open and close so that sodium or potassium ions can move through them via facilitated diffusion at any given time. Some of the channels however remain open all the time so that the sodium and potassium ions move unhindered through them by facilitated diffusion.
- Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon (using a Sodium potassium pump)
As a result of the above processes the inside of the axon is negatively charged relative to the outside.
The resting potential ranges from 50mW to 90mW with in humans averaging 65mW. So in this state it is polarised.
For the neurone to become polarised:
- Sodium ions are actively transported out of the axon by sodium potassium pumps
- Potassium ions are actively transported into the axon by sodium potassium pumps
- Active transport of sodium ions is greater than that of potassium ions.
3Na+ move out for every 2K+ that move in.
- Although both sodium and potassium have a +1 charge, the outward movement of sodium ions is greater than the inward movement of potassium ions. Ultimately there are more sodium ions in the tissue fluid that the axon thus creating an electrochemical gradient
- The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon
- However most of the potassium gates remain open whereas the sodium channels are closed
- When a stimulus reaches a threshold, the energy causes a temporary reversal of the charges either side of the axon membrane.
- If the stimulus is great enough it will cause the -65mW inside the membrane to become +40mw. This is the action potential and in this condition this part of the axon membrane is said to be depolarised.
- This depolarisation occurs as the voltage-gated channels in the axon membrane change shape and hence open or close depending on the voltage across the membrane
For the neurone to become polarised:
- At resting potential some potassium voltage-gated channels are open (mainly permanently open gates) but the sodium voltage gated channels remain closed.
- The energy of the stimulus result in some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient.
- As Sodium ions diffuse into the axon, so more sodium channels open causing an even greater influx of sodium ions by diffusion.
- Once the action potential of around +40mV has been established the voltage gates on the sodium ion channels close (and therefore preventing a further influx of sodium ions) but the potassium voltage gates begin to open
- Some of the potassium voltage gates being open the electrical gradient that was preventing further outward movement of potassium ions is now revered, resulting in more potassium ion channels to open. This starts the repolarisation of the axon.
- The outward diffusion of these potassium ions cause a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual, hyperpolarisation. The gates on the potassium ion channels now close and the activities of the sodium potassium pumps once again cause sodium ions to be pumped out and potassium ions in.
- The neuron repolarises at -65mW.