Two gates of Na+ channel

[chiddler]

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What is the significance of the two gates or doors of the voltage gated sodium channels in neurons? One closes quickly and the other slowly. How does that fit in the action potential graph?

Tell me how it works, please.

Thank you!

 

[Charles_Carmichael]

There's a fast activating gate that opens quickly in response to membrane depolarization and there's a slow inactivating gate that closes slowly in response to membrane depolarization. NOTE: The stimulus for both gates is the same thing, membrane depolarization. Both gates need to be open for Na+ influx to occur.

So, when membrane potential is depolarized to threshold, the fast activating gate opens. At this time, since the slow inactivating gate still hasn't closed (since it's slow...duh!), both the gates are open, and Na+ flows into the cell to cause the upstroke of the action potential. Once the slow inactivating gate closes (after a time delay), the influx of Na+ ends and the upstroke of the action potential ends. On the action potential chart, this is the peak of the action potential. There's no more Na+ influx. The repolarization phase now begins.

Hope this helps.

 

[chiddler]

Thanks it helped a lot.

Why is the slow gate necessary for proper action potential function?

 

[Charles_Carmichael]

Thanks it helped a lot.

Why is the slow gate necessary for proper action potential function?

Think about it. What would happen to membrane potential if there was no slow inactivating gate?

Keep in mind that both of these gates return to their normal position (ie. closed for the fast activating and open for the slow inactivating) when the membrane repolarizes to its resting level. Also, remember that depolarization causes opening of K+ channels to increase K+ conductance and repolarize the membrane back to resting potential. What would the net effect of continued Na+ influx (due to the lack of a slow inactivating gate) and K+ efflux be?

 

[chiddler]

Think about it. What would happen to membrane potential if there was no slow inactivating gate?

Keep in mind that both of these gates return to their normal position (ie. closed for the fast activating and open for the slow inactivating) when the membrane repolarizes to its resting level. Also, remember that depolarization causes opening of K+ channels to increase K+ conductance and repolarize the membrane back to resting potential. What would the net effect of continued Na+ influx (due to the lack of a slow inactivating gate) and K+ efflux be?

The net effect would be the cell will not be able to repolarize because positive influx = efflux and little/no change in charge.

OHH the slow gate's normal position is to be open! And when depolarization kicks in, fast gate opens and slow closes slowly. I thought they both start in closed position...

Ok then my question is if the gate had a single gate that opened when threshold potential is reached and closed when depolarized, quickly in both cases, that would work just as well, right?

 

[Charles_Carmichael]

The net effect would be the cell will not be able to repolarize because positive influx = efflux and little/no change in charge.

Yup. You basically maintain the membrane potential in a depolarized state, which would mean that no further action potential firing would occur.

OHH the slow gate's normal position is to be open! And when depolarization kicks in, fast gate opens and slow closes slowly. I thought they both start in closed position...

Yea, that's the important thing to keep in mind. The slow gate is normally open and the fast gate is normally closed. If you think about it, it wouldn't make sense otherwise since, if the fast gate opened and the slow gate was still closed, there wouldn't be any influx of Na+.

Ok then my question is if the gate had a single gate that opened when threshold potential is reached and closed when depolarized, quickly in both cases, that would work just as well, right?

In theory, that would work similarly. Practically, though, that'd be hard to pull off since you need the gate to change conformation and open at a specific threshold potential and then change conformation again and close at another specific depolarized potential. Having this two-gate mechanism, one with a fast activity and the other with a slow activity, is simpler.

Good work! 👍

 

[chiddler]

thanks for the help :-3