firing

[girlspowerss]

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when does the neuron actually fire? i understand depolarization and repolarization and everything, but when does the neuron actual fire? when does it happen? when does this all or nothing firing take place? right before the Na+ channels close?

 

[lDanny]

the sodium channels begin to open and once the stimulus is greater than the threshold stimulus, depolarization begins. So if the threshold is not reached then there will not be an AP

 

[girlspowerss]

so when does the firing occur? during depolarization and repolarization?

 

[demayette]

so when does the firing occur? during depolarization and repolarization?

I think firing occur during depolarization after sodium channels begin to open

 

[lDanny]

IIRC, once depolarization occurs that is 'firing' that's why it is all or nothing. If it takes too long to achieve the threshold. The AP might not occur. hope that helps

 

[Geekchick921]

so when does the firing occur? during depolarization and repolarization?

I typed a nice long response to you and it logged me out because I took too long to post it. Lame.

Firing occurs when the neuron is depolarized to threshold, around -50mV, and the action potential occurs. At that point, the voltage-gated sodium channels open and the action potential begins. Na+ ions rush into the cell, depolarize it completely then shoot it up to around +40mV. At that point, the sodium channels close and the potassium channels open, allowing K+ to rush out of the cell and bring it back down to RMP.

During that point is the refractory period. First is the absolute refractory period, as the charge drops down to RMP; the neuron cannot fire during this time. Once it reaches around -70mV again, it undershoots that slightly before it levels off at RMP. The undershoot is the relative refractory period. The neuron can potentially fire again at that point, but it requires a stronger stimulus to depolarize it to threshold.

Hope that helps!

 

[girlspowerss]

what about absolute refractory period?
thanks

 

[Eye Cue]

To build onto the question...

I know that Na+ is responsible for depolarization, while K+ is responsible for repolarization, and even hyperpolarization. So while at the most hyperpolarized, or most negative, state... which ion is responsible for bringing the cell back to it's 'less negative' resting potential? I think I remember reading something once that some of the extracellular K+ diffuses away so that the cell regains its resting potential, but could someone confirm this?

 

[MLT2MT2DO]

what about absolute refractory period?
thanks

The very first part of repolarization.

Good Question Eye Cue, I always assumed the normal concentration gradient just lagged in the 3 for 2 exchange until the normal resting state was achieved. Not sure if this is correct though

 

[Geekchick921]

I'm not sure, Eye Cue. Perhaps just the Na+/K+ pump?

 

[lDanny]

iirc, it is the Na K pump.

 

[Eye Cue]

The very first part of repolarization.

Good Question Eye Cue, I always assumed the normal concentration gradient just lagged in the 3 for 2 exchange until the normal resting state was achieved. Not sure if this is correct though

I'm not sure, Eye Cue. Perhaps just the Na+/K+ pump?

Well, I didn't think the Na+/K+ pump would be responsible for this specifically, because the pump functions to make the cell more negative. In this small period of time, the interior of the cell actually becomes more positive.

I did find this, however:
http://www.psych.uiuc.edu/~etaylor4/action_potential.jpg

At point 6 on their graph, it seems as though it states that the K+ does in fact diffuse away to help the cell become less negative to achieve its resting potential after hyperpolarization. However, it looks like the graph was made by a student, so I'm not sure how reliable the source is.

 

[Akarat]

To build onto the question...

I know that Na+ is responsible for depolarization, while K+ is responsible for repolarization, and even hyperpolarization. So while at the most hyperpolarized, or most negative, state... which ion is responsible for bringing the cell back to it's 'less negative' resting potential? I think I remember reading something once that some of the extracellular K+ diffuses away so that the cell regains its resting potential, but could someone confirm this?

When the cell is hyperpolarized, too negative, the electrical gradient is less than the chemical gradient for K+. The electrical gradient wants K+ to enter into the cell because the cell is relatively negative. However, the chemical gradient wants K+ to leave the cell because there's too many K+ in the cell. Therefore, the net electrochemical gradient points out of the cell.

A neuron has passive channels, or leak channels, that are always open. These channels constantly leak out K+, while Na+ constantly leaks in. When the cell is hyperpolarized, more K+ leaves the cell via these channels. These channels are also the reason why cells have Na+/K+ pumps. After all, if the ions never leaked, then the Na+/K+ pumps wouldn't need to be in constant operation.