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When it comes to A2/A-level Biology, do you still remember what causes an action potential?
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.
References:
1. https://www.cambridge.org/core/books/basic-physiology-for-anaesthetists/nerve-action-potential-and-propagation/31B6C11A49B20CC69D3EB1E5CCCFB208
That's the end of the topic!
Drafted by Bonnie (Biology)