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8.2 Synaptic trasmission II
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En provenance du cours de Peking University
Advanced Neurobiology I
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À partir de la leçon
Synaptic transmission & Synapitic plasticity
Let's continue with synaptic transmission and synapitic plasticity.
Rencontrer les enseignants
Yan ZhangProfessor
School of Life Sciences, Peking University
Yulong LiResearch Fellow
School of Life Sciences, Peking University
Well, this indeed is not just an.
If you systematically measured the peak,
which is probably here, the peak, 1 2 3 4 5 and
this is showing the distance along to the endplate.
This is the place closest to the endplate and
then to one you are moving far away from left end,
to eight, you are moving far away to the right end.
As you can see that near the endplate, number five and six,
the peak is the smallest.
It doesn't matter whether you're moving to the left, which is number one.
You have a pretty big peak.
And if it's number eight, it's almost the same as number one, okay?
So here's the problem.
Why the so-called action potential peak have different height?
We have already learned that the action potential is all or none, right?
And the height of the action potential mainly
are determined by the sodium channels to reverse potential, okay?
So, doing a depolarization is a sodium channel are open and the action
potential's peak depends on the sodium reverse potential and they are all numb.
So it makes sense number one and number eight, their height is similar.
Or number two is similar but number five and
six that's the region very close to the endplate.
The membrane potential actually is lower, it just doesn't make sense, isn't it?
Right, because there you have a lot of endplates.
You have the ion channel that can receive the transmitter from [INAUDIBLE].
So why the endplate potential there is lower?
This is a problem.
What do you guys think?
Near the endplate there is this
ligand-gated ion channel that can sense acetylcholine.
Or let's just put it this way.
In the endplate there is an acetylcholine-gated ion channel that can
be opened and closed, okay?
Once the transmitter gets released, you get activated.
And then in a far away place, there's this voltage-gated sodium channel for example.
And there, they can sort of generate action
potential according to the classic work.
It's print by and okay?
But, She,
using an elaborate way to explain why this peak is lower, that I didn't get it.
But let's just discuss these two possibility.
According to one person, one person said there are two voltage-gated sodium channel
a different form near the endplate or far away from the endplate.
Now here's the problem.
If they are both very specific sodium ion channel voltage-gated sodium ion channel,
then is there going to be a big difference in terms of the peak
of a action potential in number five and number eight?
Will it be?
Because we already discussed the peak of an action
potential as determined by, largely by voltage.
By the sodium conductance, by the sodium reversal potential,
if they are all the same they are different voltage-gated sodium but
they are all the same in terms of sodium selectivity.
The sodium reverse potential is going to be the same.
Isn't it?
And therefore, most likely the peak will be the same.
Because if there's been determined by the sodium reverse potential, but
correctly pointed out, if there's a different
ion channel which is acetylcholine-activated, okay?
And if the acetylcholine-activated
ion channel that has different permeability than sodium or
it has a different reverse potential than sodium.
Then this makes sense because near the endplate you might have
a different type of ion channel that has different permeability.
Okay?
For example if we probability is no specific for sodium.
If we both selected for sodium and potassium.
Then the reverse potential for this mixture will be close to zero millivolt.
And if they are totally open,
then the peak will reach to zero millivolt.
Okay, zero millivolt close here to zero millivolt,
but if the endplate just a few small amount of
voltage-gated sodium channel,
then the endplate potential here will be less than here.
But you will be larger than zero.
Okay.
So indeed, this is a case
Through the work by [INAUDIBLE]
and [INAUDIBLE]
and other scientists.
So, they can precisely measure the permeability
of this acetylcholine-gated ion channel, okay.
How do you do it?
Well again this is similar as what we discussed for
sodium and potassium channel.
You can just simply change the ion concentration,
and measure how this reversal potential
change according to different ion concentration.
And indeed so this work.
People understand the ion channel,
the acetylcholine receptor in the neurovascular
junction, that actually are nonselective
meaning that it's permeable both to sodium,
but also to potassium, calcium.
So its reverse potential is zero millivolt.
So we are recording closer to the endplate.
You have a lot of acetylcholine-gated ion channel open and even you will
depolarize the membrane because of that reverse potential is close to zero, okay?
They will only reach close to zero millivolt if they are fully open okay?
And If that is enough to cause the action potential,
then the sodium channel will kick in.
But at the same time, at this peak, let us know,
only just the what you get the sodium channel open but they also this.
Now so that you can with a reverse potential close to
zero millivolt to drag this peak down,
because while they will be reaching the equilibrium will be zero millivolt.
So the peak will be smaller than this
much pure voltage-gated sodium channel determined peak.
Because if you are two millimeter away, outside of the endplate
there is no such legand-gated acetylcholine-gated ion channel.
Only the voltage-gated sodium channel and potassium channel are there.
And this also illustrates that the cell must have a neat
mechanism to deliver different ion channel to different part of the cell right?
In the plasma membrane it needs to have this voltage-gated sodium
potassium channel.
But near the synapse, near the neuromuscular junction,
it's only in reach for the acetylcholine-gated receptors.
So this is an efficient way because even the cell only
non-specifically is present as acetylcholine-gated everywhere.
Then, there's no transmitter rays from there anyway.
You are probably wasting all of your energy.
So there has some neat mechanism to sense that.
So this illustrates the electrical signaling,
the postsynaptic electrical signalling.
That is the the opening of this acetylcholine-gated
announce that [INAUDIBLE] will depolarize the membrane.
See they will depolarize it because of that be worse potential since zero
millivolt, much larger than the threshold.
So, once the cell reaches threshold you will find action potential, okay?
And this is the synapse called excitatory synapse.
[FOREIGN] Because releasing the transmitter will
activate a post synaptic cell.
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