What causes an action potential?

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  • Created on: 27-03-13 10:19
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WHAT CAUSES AN ACTION POTENTIAL?
An action potential is caused by changes in the permeability of the cell membrane to NA and K, due to the opening and closing of
voltage-dependent Na and K channels. At the resting potential, these channels are blocked. Changes in the voltage across the membrane cause
the channels to open.
DEPOLARISATION
Depolarisation occurs once a neurone is stimulated ­ the change in the potential difference across the membrane causes a change in
shape of the NA gate, opening some of the voltage-dependent sodium ion channels.
As sodium flows in, depolarisation increases triggering more gates to open once a certain potential difference threshold is reached.
This is POSITIVE FEEDBACK.
There is a higher concentration of sodium outside the axon therefore they flow inwards through the channel, causing a build-up of
positive charges inside.
This reverses the polarity of the membrane ­ it has now reached +40mv.
REPOLARISATION
The voltage-dependent Na channels spontaneously close and Na permeability of the membrane returns to its usually very low level.
Voltage dependent K channels open due to the depolarisation of the membrane ­ the potassium ions move out of the axon, down the
electrochemical gradient.
As potassium ions flow out of the cell, the inside of the cell once again becomes more negative than the outside. This is repolarising
the membrane.
RESTORING THE RESTING POTENTIAL
The membrane is now highly permeable to potassium ions, and more ions move out than occurs at resting potential, making the
potential difference more negative.
This is hyperpolarisation of the membrane.
The resting potential is re-established by closing the voltage-dependent K channels and potassium ions diffuse into the axon.
If lots of action potentials occur in the neurone, the sodium ion concentration inside the cell rises rapidly ­ the sodium-potassium pumps
start to function, restoring the original ion concentrations.

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