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Neuronal Synapses (Chemical) : How one neuron can stimulate (or inhibit) another neuron at a chemical synapse
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- I think we have a decent idea of how a signal is transmitted
- along the neuron.
- We saw that a couple of dendrites, maybe that one and
- that one and that one, might get excited or triggered.
- And when we say it gets triggered, we're saying that
- some type of channel gets opened.
- That's probably the trigger.
- That channel allows ions to be released into the cell-- or
- actually, there are situations where ions can be released out
- of the cell.
- It would be inhibitory, but let's take the case where ions
- are released into the cells in an electrotonic fashion.
- It changes the charge or the voltage gradient across the
- membrane and if the combined effects of the change in the
- voltage gradient is just enough at the axon hillock to
- meet that threshold, then the sodium channels over here will
- open up, sodium floods in, and then we have the situation
- where the voltage becomes very positive.
- Potassium channels open up to change things again, but by
- the time we went very positive, then that
- eletrotonically affects the next sodium pump.
- But then we have the situation where that will allow sodium
- ions to flood in and then the signal keeps getting
- transmitted.
- Now the next natural question is, what happens at the neuron
- to neuron junctions?
- We said that this dendrite gets
- triggered or gets excited.
- In most cases, it's getting triggered or excited by
- another neuron.
- It could be something else.
- And over here, when this axon fires, it should be exciting
- either another cell.
- It could be a muscle cell or-- in probably most cases of the
- human body-- it's exciting another neuron.
- And so how does it do that?
- So this is the terminal end of the axon.
- There could be the dendrite of another neuron right here.
- This is another neuron with its own axon, its own cell.
- This would trigger the dendrite right there.
- So the question is, how does that happen?
- How does the signal go from one neuron's axon to the next
- neuron's dendrite?
- It actually always doesn't have to go from axon to
- dendrite, but that's the most typical.
- You can actually go from axon to axon, dendrite to dendrite,
- axon to soma-- but let's just focus on axon to dendrite
- because that's the most traditional way that neurons
- transmit information from one to the other.
- So let's zoom in.
- Let's zoom in right here.
- This little box right there, let's zoom at the base, the
- terminal end of this axon and let's zoom in
- on this whole area.
- Then we'll also zoom in-- we're also going to get the
- dendrite of this next neuron-- and I'm going to rotate it.
- Actually, I don't even have to rotate it.
- So to do that, let me draw the terminal end.
- So let's say the terminal end looks something like this.
- I'm zoomed in big time.
- This is the terminal end of the neuron.
- This is inside the neuron and then the next dendrite-- let
- me draw it right here.
- So we've really zoomed in.
- So this is the dendrite of the next neuron.
- This is inside the first neuron.
- So we have this action potential that
- keeps traveling along.
- Eventually for maybe right over here-- I don't know if
- you can zoom in-- which would be over here, the action
- potential makes the electrical potential or the voltage
- potential across this membrane just positive enough to
- trigger this sodium channel.
- So actually, maybe I'm really close.
- This channel is this one right here.
- So then it allows a flood of sodium to enter the cell.
- And then the the whole thing happens.
- You have potassium that can then take it out, but by the
- time this comes in, this positive charge, it can
- trigger another channel and it could trigger another sodium
- channel if there's other sodium channels further down,
- but near the end of the axon there are
- actually calcium channels.
- I'll do that in pink.
- So this is a calcium channel that is traditionally closed.
- So this is a calcium ion channel.
- Calcium has a plus 2 charge.
- It tends to be closed, but it's also voltage gated.
- When the voltage gets high enough, it's very similar to a
- sodium voltage gated channel is that if it becomes positive
- enough near the gate, it will open up and when it opens up,
- it allows calcium ions to flood into the cell.
- So the calcium ions, their plus 2 charge, to
- flood into the cells.
- Now you're saying, hey Sal, why are calcium ions flooding
- into the cells?
- These have positive charge.
- I just thought you said that the cell is becoming positive
- because of all the sodium flowing in.
- Why would this calcium want to flow in?
- And the reason why it wants to flow in is because the cell
- also-- just like it pumps out sodium and pumps in potassium,
- the cell also has calcium ion pumps and the mechanism is
- nearly identical to what I showed you on the sodium
- potassium pump, but it just deals with calcium.
- So you literally have these proteins that are sitting
- across the membrane.
- This is a phospobilipid layer membrane.
- Maybe I'll draw two layers here just so you realize it's
- a bi-layer membrane.
- Let me draw it like that.
- That makes it look a little bit more realistic, although
- the whole thing is not very realistic.
- And this is also going to be a bilipid membrane.
- You get the idea, but let me just do it to
- make the point clear.
- So there are also these calcium ion pumps that are
- also subsets of ATPases, which they're just like the sodium
- potassium pumps.
- You give them one ATP and a calcium will bond someplace
- else and it'll pull apart the phosphate from the ATP and
- that'll be enough energy to change the confirmation of
- this protein and it'll push the calcium out.
- Essentially, what was the calcium will bond and then
- it'll open up so the calcium can only exit the cell.
- It's just like the sodium potassium pumps, but it's good
- to know in the resting state, you have a high concentration
- of calcium ions out here and it's all driven by ATP.
- A much higher concentration on the outside than you have on
- the inside and it's driven by those ion pumps.
- So once you have this action potential, instead of
- triggering another sodium gate, it starts triggering
- calcium gates and these calcium ions flood into the
- terminal end of this axon.
- Now, these calcium ions, they bond to other proteins.
- And before I go to those other proteins, we have to keep in
- mind what's going on near this junction right here.
- And I've used the word synapse already--
- actually, maybe I haven't.
- The place where this axon is meeting with this dendrite,
- this is the synapse.
- Or you can kind of view it as the touching point or the
- communication point or the connection point.
- And this neuron right here, this is called
- the presynaptic neuron.
- Let me write that down.
- It's good to have a little terminology under our belt.
- This is the post-synaptic neuron.
- And the space between the two neurons, between this axon and
- this dendrite, this is called the synaptic cleft.
- It's a really small space in the terms of-- so what we're
- going to deal with in this video is a chemical synapse.
- In general, when people talk about synapses, they're
- talking about chemical synapses.
- There also are electrical synapses, but I won't go into
- detail on those.
- This is kind of the most traditional one that people
- talk about.
- So your synaptic cleft in chemical synapses is about 20
- nanometers, which is really small.
- If you think about the average width of a cell as about 10 to
- 100 microns-- this micron is 10 to the minus 6.
- This is 20 times 10 to the minus 9 meters.
- So this is a very small distance and it makes sense
- because look how big the cells look next
- to this small distance.
- So it's a very small distance and you have-- on the
- presynaptic neuron near the terminal end,
- you have these vesicles.
- Remember what vesicles were.
- These are just membrane bound things inside of the cell.
- So you have these vesicles.
- They also have their phospobilipid layers, their
- little membranes.
- So you have these vesicles so these are just-- you can kind
- of view them as containers.
- I'll just draw one more just like that.
- And they can train these molecules called
- neurotransmitters and I'll draw the
- neurotransmitters in green.
- So they have these molecules called
- neurotransmitters in them.
- You've probably heard the word before.
- In fact, a lot of drugs that people use for depression or
- other things related to our mental state, they affect
- neurotransmitters.
- I won't go into detail there, but they contain these
- neurotransmitters.
- And when the calcium channels-- they're voltage
- gated-- when it becomes a little more positive, they
- open calcium floods in and what the calcium does is, it
- bonds to these proteins that have docked these vesciles.
- So these little vesicles, they're docked to the
- presynpatic membrane or to this axon terminal membrane
- right there.
- These proteins are actually called SNARE proteins.
- It's an acronym, but it's also a good word because they've
- literally snared the vesicles to this membrane.
- So that's what these proteins are.
- And when these calcium ions flood in, they bond to these
- proteins, they attach to these proteins, and they change the
- confirmation of the proteins just enough that these
- proteins bring these vesicles closer to the membrane and
- also kind of pull apart the two membranes so that the
- membranes merge.
- Let me do a zoom in of that just to make it clear
- what's going on.
- So after they've bonded-- this is kind of before the calcium
- comes in, bonds to those SNARE proteins, then the SNARE
- protein will bring the vesicle ultra-close to
- the presynaptic membrane.
- So that's the vesicle and then the presynaptic membrane will
- look like this and then you have your SNARE proteins.
- And I'm not obviously drawing it exactly how it looks in the
- cell, but it'll give you the idea of what's going on.
- Your SNARE proteins have essentially pulled the things
- together and have pulled them apart so that these two
- membranes merge.
- And then the main side effect-- the reason why all
- this is happening-- is it allows those neurotransmitters
- to be dumped into the synaptic cleft.
- So those neurotransmitters that were inside of our
- vesicle then get dumped into the synaptic cleft.
- This process right here is called exocytosis.
- It's exiting the cytoplasm, you could say, of the
- presynaptic neuron.
- These neurotransmitters-- and you've probably heard the
- specific names of many of these-- serotonin, dopamine,
- epinephrine-- which is also adrenaline, but that's also a
- hormone, but it also acts as a neurotransmitter.
- Norepinephrine, also both a hormone and a
- neurotransmitter.
- So these are words that you've probably heard before.
- But anyway, these enter into the synaptic cleft and then
- they bond on the surface of the membrane of the
- post-synaptic neuron or this dendrite.
- Let's say they bond here, they bond here, and they bond here.
- So they bond on special proteins on this membrane
- surface, but the main effect of that is, that will trigger
- ion channels.
- So let's say that this neuron is exciting this dendrite.
- So when these neurotransmitters bond on this
- membrane, maybe sodium channels open up.
- So maybe that will cause a sodium channel to open up.
- So instead of being voltage gated, it's
- neurotransmitter gated.
- So this will cause a sodium channel to open up and then
- sodium will flow in and then, just like we said before, if
- we go to the original one, that's like this getting
- excited, it'll become a little bit positive and then if it's
- enough positive, it'll electrotonically increase the
- potential at this point on the axon hillock and then we'll
- have another neuron-- in this case, this neuron being
- stimulated.
- So that's essentially how it happens.
- It actually could be inhibitory.
- You could imagine if this, instead of triggering a sodium
- ion channel, if it triggered a potassium ion channel.
- If it triggered a potassium ion channel, potassium ion's
- concentration gradient will make it want to go
- outside of the cell.
- So positive things are going to leave the
- cell if it's potassium.
- Remember, I used triangles for potassium.
- And so if positive things leave the cell, then if you go
- further down the neuron, it'll become less positive and so
- it'll be even harder for the action potential to start up
- because it'll need even more positive someplace else to
- make the threshold gradient.
- I hope I'm not confusing you when I say that.
- So this connection, the way I first
- described it, it's exciting.
- When this guy gets excited from an action potential,
- calcium floods in.
- It makes these vesicles dump their contents in the synaptic
- cleft and then that will make other sodium gates open up and
- then that will stimulate this neuron, but if it makes
- potassium gates open up, then it will inhibit it-- and
- that's how, frankly, these synapses work.
- I was about to say there's millions of synapses, but
- that'd be incorrect.
- There's trillions of synapses.
- The best estimate of the number of synapses in our
- cerebral cortex is 100 to 500 trillion synapses just in the
- cerebral cortex.
- The reason why we can have so many is that one neuron can
- actually form many, many, many, many synapses.
- I mean, you can imagine if this original drawing of a
- cell, you might have a synapse here, a synapse here, a
- synapse there.
- You could have hundreds or thousands of synapses even,
- into one neuron or going out of one neuron.
- This might be a synapse with one neuron, another one,
- another one, another one.
- So you'd have many, many, many, many, many connections.
- And so synapses are really what give us the complexity of
- what probably make us tick in terms of our human mind and
- all of that.
- But anyway, hopefully you found this useful.