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神经元突触 (化学)

神经元如何在化学突触刺激 (或抑制) 另一个神经元. Sal Khan 创建




I think we have a decent idea of how signal is transmitted a longer neuron we saw that you know 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 open that's probably the trigger that channel allows ions to be released into the cell or actually there's situations where ions can be released out of the cell would be inhibitory but let's take the case where ions are released into the cells in an electro tonic fashion it changes the charge or the voltage gradient across the membrane and if the combined effects of the change in the voltage gradient it's just enough at the axon hillock to meet that threshold then the sodium channels over here will open up the sodium floods in and then you know we have the situation where because the chart the voltage becomes very positive potassium channels open up to change things again but by the time we went very positive then that electro tonically affects the next the next sodium pump and then we have this 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 we said that this dendrite gets triggered or gets excited and 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 another either another cell it could be a muscle cell or in probably most cases of the human body it's exciting it's exciting another neuron and so how does it do that so this this is the terminal end of the axon there could be the dendrite of another neuron right here so this is another neuron right there I'm not drawing it that well but I think you get the other idea this is another neuron with its own acts on its own cell and so this would trigger the dendrite right there so the question is how does that happen how does the signal go from one neurons axon to the next neurons 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 accident agenda right 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 in at the base the terminal end of this axon and let's and then let's zoom in on this whole area and 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've zoomed in big-time it looks something like that this is the terminal end of the neuron this is inside this inside the neuron and then the next dendrite let me draw it right here the next dendrite the dendrite of the next neuron we put it right there so we've really zoomed in so this is dendrite dendrite of next neuron this is inside the first neuron so we have this action potential that keeps traveling along eventually you know for maybe right over here and if you can zoom in which would be over here the action potential makes the electrical potential or the the voltage potential across this membrane just positive enough to trigger this sodium this sodium channel so then this sort of so I'm I'm really you know maybe I'm getting really 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 it allows a flood of sodium and then 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 truck it can trigger another channel and it could try to trigger another sodium channel if there's other sodium channels further down but near the end of the axon there are actually calcium channels calcium channels I'll do that in pink so this is a calcium channel this is a calcium channel that is traditionally closed so this is a calcium ion channel Jim has a +2 charge calcium ion channel it tends to be closed but it's also voltage-gated when when the voltage gets high enough it's very similar to a sodium gated or a sodium voltage-gated channel is that if if it comes 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 they're positive to 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 was becoming positive because of all of 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 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 you know this is a phospholipid layer membrane so maybe I should just draw it maybe I'll draw two layers here just so you realize that it's a bilayer membrane let me draw it like that it makes it look a little bit more realistic although the whole thing is not very realistic and this is also going to be a bi lipid membrane I can get the idea but let me just do it just to make the point clear so there are also these calcium ion pumps that are there are also subsets of ATP ASE's which they really did just like the sodium potassium pumps you give them one ATP one ATP and a calcium will bond to someplace else a calcium will bond someplace else and it'll 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 or essentially it what was the the calcium will bond and then it'll open up so the calcium can only exit the cell so that's what the it's just like the sodium potassium pumps but just it's good to know that in the resting state you have a high concentration of calcium ions it's all driven by ATP a much higher concentration on the outside then you have on the inside 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 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 these two where this axon is meaning with this dendrite this is the synapse this is the synapse this is the synapse or you can kind of view it as that 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 pre synaptic presynaptic neuron this is the postsynaptic neuron post synaptic neuron and the space between the two neurons between this axon and this dendrite this is called a synaptic cleft synaptic cleft I'll just write is ass in synaptic cleft it's a really small space in the terms of so what we're gonna deal with in this video is a chemical synapse in general when people talk about synapses they're talking about chemical synapses there are also are electrical synapses but I won't go into detail on those this is kind of the one the most traditional one that people talk about so your synaptic cleft and can in chemical synapses about 20 nanometers which is really small if you think about the average the average width of a cell is about well depending on your view at 10 to 100 microns that's 10 a micron is 10 to the minus 6 this is 20 times 10 to the minus meter 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 this on this the the the pre-synaptic neuron near the terminal end you have these vesicles remember what vesicles were these are just membrane background these are just membrane brown things inside of the cell so you have these vesicles they also have their phospho by lipids by lipid layers the little membrane so you have these vesicles so these are just you can kind of view them as containers I'll just draw I'll draw one more now I'll draw one more just like that and they can train near these molecules called neurotransmitters and I'll draw the neurotransmitters in orem already use that I'll do it in green so they have these molecules called neurotransmitters in them you've probably heard the word before in fact a lot of a lot of drugs that you know people use for depression or other other things related to our mental state they they they affect neurotransmitters and I won't go into detail there but they contain these neurotransmitters and when the calcium gets the calcium channels they are 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 vesicles so these little vesicles they're docked to the synaptic they're docked to the presynaptic membrane or to this axon terminal membrane right there and 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 conformation of the proteins just enough that these proteins bring these vesicles closer to the membrane they bring these vesicles closer to the membrane and also kind of pull apart the two membranes so that the membranes so let me do a zoom in of that just to make it clear what's going on so after they've bonded what 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 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 pulled 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 of this is happening is this allows those neurotransmitters to be dumped into the synaptic cleft so those things that those neurotransmitters that were inside of our vesicle then get dumped into the synaptic cleft so they get dumped into the synaptic cleft this process right here is called exocytosis it's exiting the the the cytoplasm you could say of the the presynaptic neuron but these neurotransmitters and you've probably heard the specific names of many of these sirata ninh dopamine epinephrine which is also adrenaline but that's also a hormone but it also acts as a neurotransmitter norepinephrine also a both a hormone and a neurotransmitter so these are things that words that you've probably heard before but anyway these enter into the synaptic cleft and then they bond they bond to parts they bond on the surface of the membrane of the postsynaptic neuron or this dendrite so they'll bond let's say they bond I was doing yellow so 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 it that will trigger ion channels so let's say that this neuron is is exciting this dendrite so when 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 its 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 that's the quit you know if we go to the original one that's like this getting excited we come a little bit positive and then if it's enough positive it'll electro tonically increase the potential at this point on the axon hillock and then we'll have another we'll have another neuron in this case this neuron being stimulated so that's essentially how it's happens it actually could be inhibitory you could imagine if if if this instead of instead of triggering a sodium ion channel if it triggered a patek it a potassium ion channel if it triggered a potassium ion channel potassium ions gradient 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 the if you go further down the neuron it'll become less positive and so it'll be even harder for things to for the action potential to start up because it'll hat it'll need even more positive someplace else to make the threshold gradient I hope I'm not confusing 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 their contents into the synaptic cleft and then that will make other sodium gates open up maybe sodium gates 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 and there's there's you know house about to say there's millions of synapses no but that being correct there's trillions of synapses the best estimate of the number of synapses in our cerebral cortex is 100 to 500 trillion trillion synapses synapses just in the in the cerebral cortex and the reason why we can have so many is that one neuron can actually form many many many many synapses I mean you could imagine if there's this original drawing of cell you might have a synapse here synapse here synapse there you have hundreds or thousands of synapse even into one neuron or going out of one neuron this might be a synapse with one neuron another one and another one another one so you have many many many many many connections and so synapses are really what give us the complexity of you know what would probably make us tick in terms of our human mind and all of that but anyway hopefully found this useful