Sodium Channels in refractory period

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Main Question or Discussion Point

After an action potential has passed the localised region enters its refractory period, and during the absolute refractory period a second impulse cannot be conducted regardless of the size of the stimulation. This is because the sodium channels cannot open immediately after closing, why is that?
 

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  • #2
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I asked this before:
DocToxyn said:
The absolute refractory period is considered to be a time during which no stimulus, no matter how great, can cause an action potential to be generated. This occurs after each nerve depolarization/action potential generation process. The way it accomplishes the depolarization is via the influx of sodium ions across the cell membrane facilitated by ion channels specific to sodium. As the action potential travels down the axon, channels are opened in a wave-like fashion to facilitate the conduction or movement of the signal from the cell body to the nerve terminal. Since these action potential are driven by ion gradients across the membrane, the cell must re-establish those gradients to repeat the process. This repositioning of ions, again accomplished by channels takes time and therefore accounts for the period during which further signals cannot be sent. A crude, but familiar, example of this might be flushing the toilet. You can only do it once and then must wait for the tank to refill (and regain the potential of the stored water) before it can be done again. This period is followed by the relative refractory period during which only a stronger than normal stimulation can elicit an action potential.
You mentioned the resting state must be re-established before another wave of depolarisation can occur, but there are sufficient ions in cells present for the nerve to be depolarised many times before the supply of ions runs out. Therefore, there is no reason why sodium channels should not be able to open due to that reason.
 
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garytse86 said:
After an action potential has passed the localised region enters its refractory period, and during the absolute refractory period a second impulse cannot be conducted regardless of the size of the stimulation. This is because the sodium channels cannot open immediately after closing, why is that?
I *think* it's because the gradient has been disrupted by the last impulse. No?
 
  • #4
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garytse86 said:
I asked this before:

You mentioned the resting state must be re-established before another wave of depolarisation can occur, but there are sufficient ions in cells present for the nerve to be depolarised many times before the supply of ions runs out. Therefore, there is no reason why sodium channels should not be able to open due to that reason.
Consider the ion concentration outside the cell as well?
 
  • #5
somasimple
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but there are sufficient ions in cells present for the nerve to be depolarised many times before the supply of ions runs out.
Not really, there is few ions that are mobile. They are the fuel and ions channels are a kind of engines that work with ions.

I made an animation of a travelling action potential (without ions channels)
The refractory period exists when all ions are inside (there is another one when all ions are outside or crossing the membrane). Concentration is not playing a role since attraction and repulsion limit incoming/outgoing ions.
http://www.somasimple.com/flash_anims/atomic14.swf
 
  • #6
somasimple
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ps: red positive ions = sodium ad blue ones = potassium :redface:
 
  • #7
DocToxyn
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garytse86 said:
I asked this before:

You mentioned the resting state must be re-established before another wave of depolarisation can occur, but there are sufficient ions in cells present for the nerve to be depolarised many times before the supply of ions runs out. Therefore, there is no reason why sodium channels should not be able to open due to that reason.
You are correct that in the cell as a whole there are enough ions, but these events happen in very localized environments surrounding those channels. As pattylou points out, the ion gradient in that specific area has been disrupted, both inside and outside, therefore you need time and energy to allow structures like Na/K transporters to re-establish the proper distribution of ions.
 
  • #8
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So am I correct in saying that the absolute refractory period is due to the lack of ions in the localised environments not because of the inability of the protein channels to open immediately after they are closed?
 
  • #9
somasimple
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You're correct and the traditional electrical theory forgets the atomic forces that stop the incoming/outgoing flows. You may have a high Na concentration outside but ions can't get inside if there is already the maximum allowed by attraction/repulsion interactions.

It is why there is a plateau when you change the gradient outside (increasing).
 
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