Help! Neurones and action potential

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Amefish
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#1
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Long story short: I don't understand neurones and action potential (the details of Na+ and K+). I know the different steps - resting state, depolarisation & action potential, repolarisation, hyperpolarisation... I just don't understand what happens in each of the steps.

Can anyone explain it for me simply?
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Hajra Momoniat
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#2
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Sure happy to help

Resting potential: The membrane of the axon is said to be polarised in this stage and its permeability differs from Na+ ions to K+ ions so that at any given time there are more Na+ ions in the tissue fluid than in the axon with more K+ ions in the axon than in the tissue fluid. This is done by the process as follows: The Na-K pump actively transports the Na+ and K+ ions against their electrochemical gradient (concentration gradient) where 3 Na+ ions are pumped out of the axon and 2 K+ ions are pumped into the axon. The K+ ion channels that are located in the membrane then allow K+ ion channels to move down their concentration gradient into the tissue fluid by facilitated diffusion. This makes the tissue fluid relatively more positive than in the axon. NB: There are not many Na+ ion channels in the membrane so only few N+ ions can leave the axon but it is ok if you don't mention this. Repolarisation allows the potential difference in the axon to be around -70mv.

Depolarisation: This is where a stimulus cause the Na+ ion gated channels to open causing an influx of ions into the axon down their electrochemical gradient (concentration gradient) by facilitated diffusion. NB: The K+ ion gated channels remain close throughout the whole of the depolarisation. As more and more Na+ ions enter the axon this causes more Na+ ion gated channels to open and allowing more Na+ ions to enter. This allows the threshold to be met which allows an action potential to be created. NB: Threshold is the minimum potential difference needed to create an action potential if you would like to say If you do chemistry this definition is similar to activation energy. The threshold is around -55mv and this is known as the all-or-nothing principal. The largest action potential triggered is around 35mv. The Na+ ions then diffuse down the axon.

Repolarisation: This is where the K+ ion channels open causing an influx of K+ ions into the axon down their electrochemical gradient (concentration gradient) into the axon. This causes the potential difference in the axon to lower. NB: The Na+ ion gated channels remain close throughout the whole of repolarisation.

Refractory period and hyperpolarisation: This follows repolarisation when the K+ ion channels remain open for too long causing the potential difference to drop lower than -70mv, the potential difference at resting potential. This is known as hyperpolarisation. Refractory period amends this by keeping both Na+ and K+ ion gated channels close and using the Na-K pump to restore the potential difference to -70mv or about. NB: Refractory period is also needed for discrete impulses as the Na+ ion and K+ ion gated channels open and close at different stages and help the impulse to be unidirectional where the Na+ ions in the axon cannot go back as the axon is 'recovering'.

THESE ANSWERS ARE BASED ON THE AQA SPEC. Hope this has helped
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Amefish
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#3
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(Original post by Hajra Momoniat)
Sure happy to help

Resting potential: The membrane of the axon is said to be polarised in this stage and its permeability differs from Na+ ions to K+ ions so that at any given time there are more Na+ ions in the tissue fluid than in the axon with more K+ ions in the axon than in the tissue fluid. This is done by the process as follows: The Na-K pump actively transports the Na+ and K+ ions against their electrochemical gradient (concentration gradient) where 3 Na+ ions are pumped out of the axon and 2 K+ ions are pumped into the axon. The K+ ion channels that are located in the membrane then allow K+ ion channels to move down their concentration gradient into the tissue fluid by facilitated diffusion. This makes the tissue fluid relatively more positive than in the axon. NB: There are not many Na+ ion channels in the membrane so only few N+ ions can leave the axon but it is ok if you don't mention this. Repolarisation allows the potential difference in the axon to be around -70mv.

Depolarisation: This is where a stimulus cause the Na+ ion gated channels to open causing an influx of ions into the axon down their electrochemical gradient (concentration gradient) by facilitated diffusion. NB: The K+ ion gated channels remain close throughout the whole of the depolarisation. As more and more Na+ ions enter the axon this causes more Na+ ion gated channels to open and allowing more Na+ ions to enter. This allows the threshold to be met which allows an action potential to be created. NB: Threshold is the minimum potential difference needed to create an action potential if you would like to say If you do chemistry this definition is similar to activation energy. The threshold is around -55mv and this is known as the all-or-nothing principal. The largest action potential triggered is around 35mv. The Na+ ions then diffuse down the axon.

Repolarisation: This is where the K+ ion channels open causing an influx of K+ ions into the axon down their electrochemical gradient (concentration gradient) into the axon. This causes the potential difference in the axon to lower. NB: The Na+ ion gated channels remain close throughout the whole of repolarisation.

Refractory period and hyperpolarisation: This follows repolarisation when the K+ ion channels remain open for too long causing the potential difference to drop lower than -70mv, the potential difference at resting potential. This is known as hyperpolarisation. Refractory period amends this by keeping both Na+ and K+ ion gated channels close and using the Na-K pump to restore the potential difference to -70mv or about. NB: Refractory period is also needed for discrete impulses as the Na+ ion and K+ ion gated channels open and close at different stages and help the impulse to be unidirectional where the Na+ ions in the axon cannot go back as the axon is 'recovering'.

THESE ANSWERS ARE BASED ON THE AQA SPEC. Hope this has helped
Thank you, have you got any useful diagrams that I could use to have something visual to refer to? My teacher put one on the board, but it just confused me more because it showed the K+ channel as being closed during the resting potential...
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Hajra Momoniat
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#4
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(Original post by Amefish)
Thank you, have you got any useful diagrams that I could use to have something visual to refer to? My teacher put one on the board, but it just confused me more because it showed the K+ channel as being closed during the resting potential...
Np . I cant find any useful diagrams also but I will draw some and show it to you if that's what you prefer. Ive got some on my resource but they not all that good. If you go on to the link and then create an account and message me (my name on get revising is the same as the one on here). Then click on my name that will be in blue in your 'sent messages' and all my resources should come up. It is called organisms respond to changes in their internal and external environments AQA A2 Biology PART 4 of 9 TOPICS: Nerve impulses. sorry about the long title
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