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Sal Khan 创建




in the last video I showed you what a neuron looked like and we talked about the different parts of a neuron and I gave you the general idea of what a neuron does and get stimulated at the dendrites and you know the stimulation we'll talk about in future videos on what exactly that means and that that impulse that information that signal gets added up if there's multiple stimulation points on various dendrites it gets added up and if it meets some threshold level it's going to create this action potential or signal that travels across the axon that travels across the axon and maybe stimulates other neurons or muscles because these these terminal points of the axons might be connected to dendrites of other neurons or to muscle cells or who knows what but what I want to do in this video is to kind of lay the building blocks for exactly what the signal is or how does a how does a neuron actually transmit this information across the axon or really how does it go from the dendrite all the way to the axon before actually even talk about that we need to kind of lay the ground rules of or a ground understanding of the actual voltage potential across the membrane of a neuron and actually all cells have some voltage potential difference but it's especially relevant when we talk about a neuron and its ability to send signals so let's zoom in on a neurons cell so let me just zoom in let me just zoom in right there I could zoom in on any point on this cell that's not covered by a myelin sheath I'm gonna zoom in on its membrane so let's say that this let me go down here let's say that this is the membrane of the neuron just like that that's the new membrane this is outside outside the neuron or the cell and then this is inside inside the neuron or the cell now you have sodium and potassium ions floating around I'm going to draw a sodium like this sodium is going to be a circle so that's sodium and they're positively charged ions have a plus one charge and then potassium I'll draw them as little triangles so let's say that's potassium symbol for potassium miss Kay it's also positively charged and you have them just lying around let's say we start off both inside and outside of the cell so listen mean draw some potassium I'll just do it as a triangle I want it shouldn't be a time sign should be plus so there's some potassium inside the cell and then there's some potassium outside there's a plus charge and then there's some sodium inside the cell they're all positively charged sodium inside some sodium outside now it turns out that cells have more positive charge outside of their membranes then inside of their membranes that there's actually a potential difference that if the membrane wasn't there negative charges would want to escape or positive charges or positive ions would want to get in so let me write this down the outside ends up being more positive and we're gonna talk about why more positive and we could say less positive less positive so this is an electrical potential gradient right if you if this is less positive than that if I have a positive if I have a positive charge here it's gonna want to go to the less positive side it's gonna go on to go away from the other positive charges it's repelled by the other positive charges likewise if I had a negative charge here I want to go to the other side or positive charge I guess would be happier being here then over here but the question is how does that happen because I left of their own devices the charges would disperse so you wouldn't have this potential gradient somehow we have to put energy into the system in order to produce the cyst this this state where we have more positive on the charge on the outside than we do on the inside and that's done by sodium potassium pumps and I'm gonna draw them a certain way so I'm gonna draw them like this and this is obviously not how the protein actually looks but it'll give you a sense of how it actually pumps things out so let me draw it like I'll draw that side of the protein maybe it looks like this and you'll have a sense of why I drew it like this so that side of the protein or the or the enzyme and then the other side I'll draw it like this it looks something like this and of course the real protein doesn't look like this you've seen me show you what proteins really look like they look like big clusters of things hugely complex different parts of the proteins can bond to do four things and when things bond to proteins they change shape but I'm doing a very simple diagram here and what I want to show you is this is our sodium potassium pump in kind of it's inactivated state and what happens in this situation is that we have these nice places where our sodium can bind to so in this situation sodium can bind to these locations on our enzyme or on our protein and if we just had the sodium vine and we didn't have any energy go to the system nothing would happen they would just stay in this in this situation on the actual protein might look like something crazy you know it might the actual protein might be this big cloud of protein and then your sodium's you know maybe your sodium's bond there there and there maybe it's inside the protein somehow but still nothing's gonna happen just when the sodium bonds on this side of the protein in order for to do anything all right for it to pump anything out it has to be take it has to it uses the energy from ATP so we you know had all those videos on respiration and I told you that ATP was the currency of energy in the cell wall this is something useful for ATP to do so ATP that's you know the adenosine triphosphate it might go to some other part of our of our enzyme but in this diagram maybe it goes to this part of the enzyme I'll do it in different color so that's maybe our ATP and then this end up time it's a type of ATP ace and when I say ATPase it breaks off a phosphate from the ATP and that's just by virtue of its shape it's able to plunk it off when it plunks off the phosphate it changes shape so let's write this down so let me write down the steps just so we remember and I think it'll help so step one we have we have sodium ions and actually let's let's keep count of them we have three sodium these are the actual ratios three sodium ions from inside from inside the cell or the neuron they bond to pump which is really a protein that crosses that crosses our membrane now step two we have also ATP ATP gets broken into ADP plus phosphate on the actual pro Dean and that that changes the shape so that also provides energy energy to change shape change pumps shape pumps shape now this is when the pump was before now hour after hour pump might look something like this maybe I'll draw it let me clear out some space right here I'll draw the after pump right there clear that out and so this is before after the 80 the the phosphate gets split off of the ATP it might look something like this instead of being in that in that configuration it kind of opens in the other direction so now it might look something like this and of course it's carrying these phosphate groups they have positive charge it's carrying those phosphate groups and it's open like this this side now looks like this so now the phosphates are released to the outside so they've been pumped to the outside remember this required energy because it's going against the natural gradient you're taking positive charge and you're pushing them to an environment that is even more positive and you're also taking it to an environment where there's already a lot of sodium and you're putting a more sodium there so you're getting going against the charge gradient and you're going against the sodium gradient but now I guess we call it step 3 the sodium gets released outside the cell so here and when the this changes shape it's not so good at bonding with the sodium anymore so maybe these can become a little bit different too so that the sodium can't even bond in this configuration now that the protein has changed shape due to the due to the ATP so step 3 the 3 na plusses sodium ions are released are released outside released outside now once it's in this configuration we have all these positive ions out here these positive ions want to get really as far away from each other as possible they actually probably be attracted to the cell itself because the cell is less positive on the inside so these positive ions and in particular the potassium can bond at this side of the protein when it's in this I guess we could call its you know this activated configuration so now I guess we could call it step 4 step four we have two's sodium ions bond two I guess we could call it the activated pump activated or changed pump or maybe we could say it's in its open form so they come here and that when they bond their reach angel it retains the shape of this protein back to this shape back to that open shape now when it goes back to the open shape these guys aren't here anymore but we have these two guys sitting here and in this shape right here all of a sudden these two divots or you know maybe they're not divots they're actually things in this big you know cluster of protein they're not as good at staying bonded or holding on to the sodium so these sodium's get released into into the cell so step 5 the pump this changes shape of pump so pump changes shape to original changes shape to original changes shape to original and then once we're in the original those two sodium ions released released inside inside the cell we're gonna see in the next few videos why it's useful to have those sodium ions on the inside you might say well why don't we just keep pumping things on the outside in order to have a potential difference but we'll see these sodium ions are actually also very useful so what's the net effect that's going on we end up with a lot more we end up with more more sodium ions on the outside and we end up with more potassium ions on the inside but I told you that the inside is less positive than the outside but hey these are both positive I don't care what if I have more potassium or sodium but if you if you paid attention to the ratios that I talked about every time we use an ATP we're pumping out three sodium's and we're only pumping in three or were only pumping in to potassium right we pumped out three sodium's and two potassium each of them have a plus one charge but every time we do this we're adding a net one charge to the outside right three on the outside to the inside we have a net1 charge we have a plus one to the outside so we're making the outside more positive especially relative to the inside and this is what creates that potential difference and if you actually took a voltmeter a voltmeter measures electrical potential difference and you took the voltage if you took the voltage difference between that point and this point or more specifically between this point and that point if you were to subtract the voltage here from the voltage there you will get minus 70 millivolts minus 70 millivolts which is generally considered the resting voltage difference the potential difference across the membrane of a neuron when it's in its resting state so in this video I kind of laid out laid out the foundation that of why and how a cell using ATP using energy is able to maintain a potential difference across this membrane where the outside is slightly more positive than the inside so we actually have a negative potential difference if we're comparing the inside to the outside positive charge would want to move in if they were allowed to and negative charge would want to move out if it was allowed to now there might be one last question you might say well gee you know if this kept happening if we just kept adding charge out here our voltage difference would get really negative this would be a much more negative than the outside you know why does it stabilize at minus 72 they answer that question we're gonna these are going to come into play in a lot more detail in future videos you also have channels which are really protein structures that are that when they're in their open position will allow sodium to go through them and there are also channels that are in their open position would allow potassium to go through them I'm drawing over in their closed position and we're gonna talk in the next movie about what happens when they open but in their closed position they're still a little bit leaky they're still leaky they're still leaky and they say the concentration of potassium becomes too high down here you know and too high meaning well you know when they start to reach this threshold of minus 70 millivolts or even better when the sodium gets too high out there a few of them will start to leak down when the concentration gets really high and this is really positive just because of the electrical potential some of them will just be shoved through so it'll keep us right at around my knee - 70 millivolts and if we go below maybe some of the potassium gets leaked through the other way so even though when these are shut if if it becomes too ridiculous if we go to minus eighty millivolts or minus 90 millivolts all of a sudden to be a huge incentive for some of this stuff to leak to leak through their respective channels so that's what allows us to stay at that stable voltage potential in the next video we're gonna see what happens to this voltage potential when the neuron is actually stimulated