Initiation of Action Potential

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[@tucc40095]

What kind of stimulus causes a depolarization. Following the sequence of events after depolarization is fairly straightforward, but in trying to conceptualize action potential its helpful to understand (to whatever small extent) where the stimulus comes from and how it decreases the membrane potential. Is it cause by ions flowing out of the membrane, decreasing the potential? If so, through what channel do these ions flow out, what causes them to flow and thus eventuate in a threshold value?

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[@tomchan5643]

May I ask some questions? 1. during the hyper-polarization period, the potential difference is lower than the usual resting potential, this means the cytoplasm is much more negative. If we want to restore it (increase voltage back to -70mV), aren't we going to bring more positive charges into the cytoplasm rather than bringing 3Na+ out and 2K+ into the axon? 2. If the stimulus is stronger, the frequency of this action potential cycle increases. Can I say if the stimulus is stronger (very painful after hitting something), the neurones use more ATP? (increased frequency of hyper-polarization using Na+/K+ ATP-ase. Thank you very much

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[@user-xr6pq2vw5g]

the relatively refractory period is it in which the canal voltage sodium will be closed or open ?or the active atpase canal will be closed?

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[@michellechristian7406]

First off, thank you for all your efforts and willingness to share your time and talents. I'm still not quite clear on how the return to resting membrane potential occurs after hyperpolarization. Many sources cite the Na+/K+ pump. This is confusing to me because if 3 positive ions (Na+) are leaving the -90mV cell and 2 positive ions (K+) enter the cell than in my mind the ICF would still have a negative defect in relation to the ECF. Now, we know that K+ is 25-30 times more permeable to the membrane than Na+ is...so is the return to resting potential (-70mV) achieved by lots of K+ crossing the membrane into the cell via leak channels?

[@michellechristian7406]

+Michelle Christian Ah, yes I think you gave the critical info in the previous lecture: Resting membrane potential, but would just like confirmation. After hyperpolarization, the cells ability to return to resting membrane potential of -70mV is r/t the protein leak channels that are specific to K+ being present in vast numbers in relation to the other protein channels. So K+ is entering the cell in large quantities, allowing for RMP to be achieved. Am I understanding this concept correctly?

[@brownie4985]

+Michelle Christian indeed the Na+/K+ pump causes a net negative charge in the cell as 2 K+ is pumped in while 3Na+ is pumped out. however due to the fact that there are more K+ leak channels than Na+ leak channels, more K+ influx occurs than the Na+ efflux, resulting in repolarisation.

[@rasalkilani8528]

Thank you so much........but voltage gated k+ channel has only one gate not two........

[@anumsiddiqui3986]

Beautiful description..I have a question what happens at the threshold that it immediately drops from -70 to -45?

[@violetlavi2207]

Super late, but -70 mV is actually the resting membrane potential, while -45 mV is the threshold potential (and -70 mV is less than -45 mV, to be clear). So what happens is a stimulus is applied, and if it reaches -45 mV, an action potential is initiated. If it doesn't reach -45 mV, no action potential occurs

[@lukecampbell1127]

When the voltage drops below -70mV due to the K ions leaving the inside of the cell, how does the Na/K pump make the inside of the cell less negative if it is pushing out 3 Na ions with a +1 charge and only receiving 2 K ions with a +1 charge? Seems like the voltage on the inside of the cell would decrease to me. Can you clarify? Thanks

[@GeorgeSandle]

Aside from the Na/K ATPase pump, there are also non-gated leakage channels which allow the diffusion of Sodium and Potassium ions in and out of the cell. At an axon, there are typically more Potassium non-gated channels than sodium ones. The effect of this is a diffusion of Potassium back into the cell as well as active transportation form the ATPase pump. :)

[@debasmitanayak6485]

Shouldn't the the depolarization stop when the system reaches equilibrium i.e. potential difference be 0?

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[@phamduong2862]

threshold is -55mV or -45mV?

[@B1SpkTKD]

You said that after reaching -45 mV (before hiperpolarization), the NaK-Pump starts to work. But when does the Voltage-gated Potassium channel get closed? Does it get closed by detecting any specific voltage?

[@AKLECTURES]

Hey Brian. No, I said after the membrane reaches - 45 mV, some of the inactivated Na voltage-gated channels begin to recover! Remember, the Na channels need to recover so that we can initiate another action potential after the first one is over. The Na-K pump begins to work at around the time the potassium voltage-gated channels begin to close. Potassium channels close at the end up the repolarization period, when the membrane potential is lowered to slightly below the normal membrane potential. Hope that helped! :)

[@B1SpkTKD]

Thanks! So, there isn't an specific voltage to get the channel closed? is it approximately below rest potential? I ask you this, cause I haven't found nothing yet in textbooks (I haven't searched that much neither). Thanks for the quick answer!

[@AKLECTURES]

Brian Díaz Flores The potassium voltage gated channels begin to close at around the membrane potential (approximately the - 70 mV range). As they close, the Na-K pump helps reestablish the normal membrane potential.

[@vinnyfig1]

Many references have the threshold point as -55mV not -45mV??

[@ashutoshpadhy2363]

sir ..i have a doubt ...is resting potential is -90mv or -70mv ...some book state that its around -90mv ...please clarify my doubt sir

[@maggiec7418]

When do VOLTAGE GATED K+ channels close?

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[@Mleczyn1992]

most of the books gives me threshold value equal -55mv .is ur value also correct ? @AK LECTURES

[@Volvo990]

Bichemistry, Stryer, gives -60mV. I guess just stick to what your book says as your exams are judged based on your syllabus books.

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more like 2 lmao

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