| Author | 6 Posts |
nadiabarati
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I'm wondering if Na activation gates are open to reach to threshold? It says they become open when the membrane has reached to threshold, if so, then how it comes to threshold
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| kuku
Forum Newbie
Topics: 4
Posts: 11
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hi
my explaination is like this
at resting membrane potential all Na channela are closed
depolarisation is the signal to open Na channels & once it reaches to threshold level AP starts
below the threshold level it the subthreshold potential which can not start AP
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| mildus
Forum Guru
Topics: 19
Posts: 614
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Voltage-gated Na channels open when the threshold is reached.
E.g. neuromuscular junction:
1. ACh is released from the nerve cell.
2. It binds to the receptors on the membrane of the muscle cell.
3. These receptors are ACh-gated ion channels, so they open and Na goes into the muscle cell.
4. This Na influx (as Na is a positive ion) will cause membrane potential to decrease.
5. In one moment, membrane potential reaches threshold.
6. Then, voltage-gated channels open and action potential happens.
So, electrotonic potential (synonym: local, graded, nonpropagating) caused by ACh binding to ACh-gated ion channels (nicotinic receptors) decreases transmembrane potential to the threshold which will be the stimulus for voltage-gated (potential-gated) Na channels to open and action potential happens.
So, membrane is depolarized to the threshold by Na influx through ligand-gated channels, and voltage-gated Na channels open and action potential happens.
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| nadiabarati
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thanks a lot dear mildus for the nice explanation. But why you say membrane potential decrease to reach to threshold? Moving from negative resting membrane potential toward positive doesn't mean increase??
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| doc179
Forum Guru
Topics: 67
Posts: 1,217
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very nice explanation. Thank you very much mildus.
Nadiabarati I think mildus means only the number and not the sign when he says the membrane potential decreases.
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| mildus
Forum Guru
Topics: 19
Posts: 614
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Why have I said decrease?
Membrane potential is, in fact, transmembrane potential i.e. potential THROUGH the membrane (=potential difference between inner and outer side of the cell membrane).
When you put two electrodes outside of the membrane, potential difference is zero.
When you put those electrodes both inside, potential difference is zero.
But when you put one electrode outside and one electrode inside, potential difference is e.g. -70mV. So, it is potential through the membrane.
So, Na influx makes inner side of the membrane more positive thus making membrane potential less negative and closer to the threshold, and since membrane potential is transmembrane potential, I said it decreased because the potential difference between inner and outer side of the memrane is decreased.
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