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8.3 Synaptic trasmission III
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Из курса от партнера Peking University
Advanced Neurobiology I
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Synaptic transmission & Synapitic plasticity
Let's continue with synaptic transmission and synapitic plasticity.
Познакомьтесь с преподавателями
Yan ZhangProfessor
School of Life Sciences, Peking University
Yulong LiResearch Fellow
School of Life Sciences, Peking University
In parallel, our naval systems also exist,
the so-called inhibitory synapse.
Okay.
And, in principle,
analogous to the excitatory synapse, why is it a transmitter release?
That you will actuate the receptor.
But in this case, the transmitter's different.
Usually it's a GABA or glycine.
And if you directly activate a leak ungated channels they
are chloride sensitive.
So once [INAUDIBLE] to a receptor,
you will allow chloride to come in to the cell.
Okay?
And chloride carries a negative charge, okay?
And what it does, it's going to increase the minus,
the negative charge In the membrane to hyperpolarize the cell, okay.
So this require inhibitory synapse, okay.
The inhibitory synapse that will generally this
is so-called IPSP, inhibitory post synaptory potential.
In a mechanism similar again because the chloride, we lost potential.
Is very close to the resting membrane potential.
Okay?
So, activating it very close or even more hypopolarized.
Activating it is going to clan
the membrane potential near the chloride membrane.
With rest potential which is far away from the threshold, so
the cell will be inhibitory, will be less likely to fire action potential.
So the fundamental mechanism of post-synaptic sensing by
Derived, [INAUDIBLE] channel is just as I describe okay?
Once the [INAUDIBLE] receptor,
the receptor depends on which type, you will conduct ions.
For example if its [INAUDIBLE] Those are so called excitatory neurotransmitters.
And they are disengaging on channel.
Most likely are the non-selective positively charged ions.
Okay?
So those ions most potential will be 0 meter volt.
So when they get activated you will drive the membrane.
Potential to want their reverse potential, 0 millivolts so they are excitatory.
Especially once they reach the threshold, you will generate action potential.
And for the direct, lichen guided inhibitor synapse,
using the receptors, they are permeable to chloride.
And therefore, while those receptors get activated you will
tend to drive the membrane potential close to the chloride reverse potential.
And most likely you will be inhibitory.
They are some, there are a few exceptions.
There is, when the cells,
if they have a different color concentration due in development, okay,
due in development, then if that concentration's different,
and if the worst potential is reach the threshold.
Sometimes in a few exception condition, chloride can be excited because simply
because of doing development on people identify that a [INAUDIBLE] concentration,
actually outside is lower, so once the [INAUDIBLE] is sort of open,
the channel is open, the [INAUDIBLE] gets out, okay.
So intuitive development inside unique conditions.
But generally chloride gating inhibitory carbon receptor usually is called
carbon a receptor that will be inhibitory and impermeable to chloride.
Okay so in that condition we already have excite rate synapse.
And inhibitory synapse.
And in a very simple neurocircuitry that we already discussed earlier.
For example, in a motor neuron, the stretch reflex circuit.
And then we can just put this Simply [INAUDIBLE] and
inhibitory synapse into usage.
For example, this is a muscle and if you touch or
hit here, and then this matter [INAUDIBLE] is going to get activated.
And activating these sensor neurons which are excitatory
it will release the excitatory transmitter glutamate
to the downstream neuron okay.
So for example you will directly activate this neuron.
This is a motor neuron.
That in the way there's a muscle, you will cause this muscle to, Contract, okay?
So activate this sensory neuron, so
this excitatory neuron will cause this muscle to contract, but at the same time
this essential neuron actually bifurcate, it has two different targets.
It will activate this nerve, control the same nerve.
But on the other hand, another synapse.
From this we're in the way inhibitory neuron.
Okay and this inhibitory neuron once it activate, it's going to control
this different hamstring muscle.
Okay and this will lead to this muscle to relax.
Okay so if we are doing this recording.
So your simulator nerve and
then you can record in these presynaptic sensory neurons.
And then you can record this post synaptic neuron.
In this case, it is the sensory neuron circuit
excitatory circuit, so you can record.
The prison has action potential, propagate in this essential neuron.
And then post net modern neuron, okay actuate it,
that will be an [INAUDIBLE] post peptic potential.
But if we are recording in this, sort of black muscle cells,
sorry, black or dark inhibited neurons,
then you observe a so the IPST inhibitory.
And indeed, is this the integration
of a different excitatory and inhibitory synapse at the same neuron.
So your envision a neuron that can so it's dendrite or
cell body or axonal initial segment simultaneously receive
different excitatory input or inhibitory input, and
the total response for a neuron will depend on the relative
weight of those excitatory and inhibitory inputs, okay?
At different locations, and at different times, okay?
So, this is usually called the spacial summation, or
temporal summation, okay, and eventually you will
get integrated in the so called [INAUDIBLE] where it has
a highest density of this [INAUDIBLE] sold in channel.
Okay, so most of the time the action potential will get initiated.
Just in the axon initial segment.
And then, propagate down to the axle
to activate the downstream targets, so.
A neuron, even though it is a cell, use different compartment.
Different dendrites.
Especially in the cortex.
Our brain has six different layers.
So, different part of a dendrites that it will receive different impulse
could be both excitatory and inhibitory.
The cell body can also receive special import.
Special [INAUDIBLE] cost on cell, called bask cell, most likely inhibitory
[INAUDIBLE] they only [INAUDIBLE] input In the weight the cell body okay.
Less reason surrounding a cell body or as if as a basket,
it's called basket case cell.
Okay you were sending apple.
And recently people also identify theirself, special synapse,
just target the axonal initial segment.
Okay, for example is a special cell called Chandalaya's cell.
One single cell with 1000 different input.
Just target 1000 different cells axon initial settlement.
Okay?
So people postulate this is Chandalaya's cell, it's a special cell that it can
have a Veto power, meaning that because
all of the action potential are eventually integrated and generated at a segment.
And then you have this cell just to this
segment and one cell controls 1,000 different cell,
or at an [INAUDIBLE] so this cell crew works as a gate,
to gate all the [INAUDIBLE] output for their target neurons, okay?
So, a cell use this complicated spatial and
temporal summation of excitng inhibitory input to process information.
And one of the challenges in the neural system is to
understand which cell How do they integrate different inputs.
Because they all have very different anatomy with seven different inputs and
including different transmitters, and they are working on different functions.
And there are so many of them, so
the challenge here is to understand which cell receive which input, which input?
In the behavioral or computational context, what kind of input is doing what?
This is very challenging because of the complexity
even though the most fundamental principle people can already understand
from some Motor cells, like muscle, or some simple cells.
And likewise because of this complicated integration,
the nervous system, during its development, does a lot of efforts for
the whole neurosystem So the molecular interactions to guide
those synaptic impulse into different part of the neurons.
This is how the neurodevelopments
are trying to understand that is which molecule.
At which time will control the recognition between those input and
the receiving parts.
Okay?
And how those inputs and receiving parts are shaped by our experience.
Okay? Because once they are forming
our experience, for example, linear memory.
That will the multi fiber inputs.
So this links the neuron derivement to the mechanism of
how those synapses already established and
then get modified by the experience.
And how different diseases, for example, neurodegenerative disease,
and other diseases that affect brain function by
selectively procure part of the synapse or part of the neurons.
Again, this is also a very challenging problems for
the field because of the complexity of neural system.
So therefor,e the essential part of the brain function is
how neuron communicate and then excitatory,
propagate this action potential to fuller communicate again.
And during development, one is to have this sophisticated programming and
machinery to establish those neuronal connection and maps.
And then the disease will selectively affect
part of the synapse or part of a neuron.
And again, there are reasons some of the disease have very stereotypical
behavioral phenotype because they are only affecting part of the neurons or
part of the synapse.
Okay.
And this is how I.
Simply illustrate the temporal and spacial summation.
Again, very simple okay.
If you have two inputs, depends on how closely
in tact they can integrate it in a post synaptic cells.
For example you active this a cell and then you would record in a post nav cell.
If these two input are close enough, and
once they submit together can reach the threshold, then this post [INAUDIBLE] cell
will file an action potential, okay?
And if the cell is a special cell that has a sure town constant.
Either because of the release is not enough or because of the receptor
in the post-nerve cell do not respond as fast or as potent.
Then these two impulse will not integrate in time to generate actin potential.
So there will be no output for this.
Likewise, this kind of input can be two different cells, or two different synapse.
And if again, they
are generating the output in the post-synaptic cell,
if they are sufficiently, spatially and temporally close, okay?
Space means they are in a different part of the dendrite or
different part of compartment, but still, they need to be temporally close enough.
If they are spatially separate but temporally also very separate,
they will not have the interaction to integrate the general [INAUDIBLE] and
in this case, they were general [INAUDIBLE]
So this is synapsis transmission can be
a combination of this pulse and also I'm just here illustrating the excite inputs,
there will be inhibitor inputs and they are in [INAUDIBLE],
cell body can be in axiom [INAUDIBLE] So there are many different compartments.
So a single unit of neuron can be a very complicated
processing unit with different compartments to process the information.
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