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Current time:0:00Total duration:10:18

Video transcript

we've already seen that when a neuron is in its resting state there is a voltage difference across the membrane and so and these diagrams right over here this right over here is the membrane that right over is the membrane this right over here is the inside of the neuron and this right over here is the outside that's the outside and of course this is the outside this is the outside as well so if you had a voltmeter measuring the potential difference across the membrane so if you took this this voltage minus this voltage right over here the voltage referring this and that you would get negative let's say for the sake of argument let's say you would measure it would average about negative 70 millivolts so this is in millivolts negative 70 so that's negative 70 negative 70 millivolts and I'll do it actually for both of these graphs we're going to use both of these to describe slightly different or actually quite different scenarios and you could have another voltmeter out here in yellow and that's a little further out but that's also as that's also going to register negative 70 millivolts now let's make something interesting happen let's say that for some reason let's say that the membrane becomes permeable to sodium so sodium just starts flooding through it's going to flood through for two reasons one it is a positive ion it's more positive on the outside than the inside so positive charge will want to flood in and the other reason why it will want to flood in is because there's a higher concentration of sodium on the outside than on the inside so just across its constant just it'll just go down its concentration gradient the reason why we have a higher concentration gradient on the sodium on the outside than the inside we've already seen is because of the sodium potassium pump but anyway so you're going to have this increase you're going to really have the spike and positive charge flowing in and then what's cool what are going what's going to be the dynamic then inside the neuron well if you have all this positive charge right over here the other positive charge in the neuron is going to want to get away from it it's going to want to get away from it other positive charge is going to want to get away from it and so the positive charge is going and this is not just in the rightward Direction is really going to be in all directions in all directions the positive they're going to want to get away from each other so this one's going to move that way and then that's going to make that one want to move that way which is going to make that one we want to move that way so if we let some time pass what's the voltage going to look like on this blue voltmeter well after some time because more and more positive charges are trying to get away from these other ones right over here is the concentration of these parts positive charges spread out you're going to see the voltage start to increase you're going to see the voltage start to increase and then as they fully get spread out then it might return it might return to something of an equilibrium and then as we if we go a little bit further down the neuron as you go a little bit further down then you're on a little more time will pass before you see a voltage increase but because this thing is getting spread out across more and more distance that the effect is going to be more limited you're not going to see as much of a bump in the voltage over here then you saw over here and this type of spread of I guess you could say a signal is called electrotonic spread let me write that down electro electro Tonique spread or this is a spread of an electro tonic potential electrotonic potential so there's a couple of characteristics here one it's passive this part that we drew right here this isn't the electrotonic spread what happened the electrotonic spread is what happens after that once you have this high concentration here the fact that a few moments later you're going to have a higher concentration of positive charge here and a few moments later a higher positive concentration here this is a passive phenomenon so this thing right over here it is passive and it also dissipates the signal gets weaker and weaker the further and further you get out because this stuff just gets further and further spread out so it's passive and it dissipates and it dissipates now let's play out the scenario again but let's also throw in some voltage-gated ion channels right over here so let's say this right over here that I'm drawing let's say this is a voltage-gated let's say this is a voltage-gated sodium channel and this one right over here so this is a sodium channel sodium channel let's say it opens let's say it opens at negative 55 millivolts so let's see that would be right around right around there so that is when it opens at negative 55 millivolts let me draw that threshold there and let's say it closes let's say it closes at positive 40 millivolts so it closes at positive 40 millivolts right over there I'm just trying to show the threshold and let's say we also have some potassium let's say we have a potassium channel - right over here right over here and let's say so this is a potassium channel the infamous leaky potassium channels which are the the true reason why we have this voltage difference across a across the membrane but this potassium channel let's say it opens when this one closes so it opens just for the sake of argument these aren't going to be the exact numbers but give you the idea at positive 40 millivolts and let's say it closes at negative at negative 80 millivolts so that one opens up here and then it closes and it closes it closes down here now what is going to happen well just like we saw before so let's let's let's just let's let our positive charge flood in here at the left side of our at the left side of this the left side of this of this neuron I guess we could say and then because of electrotonic spread you're going to have the put a little bit later you're going to have the potential between the potential across the membrane at this point is going to start to become less negative the potential difference is going to become less negative just like we saw right over here so it's going to become less negative but it's not just going to be a little bump and then go back down because what happens right when the potential hits negative 55 millivolts well then it's going to trigger this it's going to trigger the opening of this sodium channel so this sodium channel is going to open because the voltage not high enough and so you're going to have sodium flood in again sodium flood in again and so what's that going to do well that's going to spike up spike up the voltage so it's going to look something like that's going to keep flowing in keep flowing keep flowing in the voltage is going to get more and more positive because remember this is going to be flowing in for two reasons one there's just a there's just more char it's more positive outside than the inside so it's going to go across a voltage gradient or go down the voltage gradient but also are the electro electro potential gradient but also there's a higher put concentration of sodium out here than there is in here because of the sodium potassium pump and so it'll also want to go down its concentration gradient so it's just going to keep flowing in even past the point at which you have no electro no voltage gradient but because of the concentration gradient going to keep going but then as you get to positive 40 millivolts this channel is going to close so that's going to stop flooding in and you also have the potassium channel opening and the potassium channel now you're more positive on the inside than the outside at least locally right over here and so now you're going to have this positive positively charged potassium ions want to get out want to get out from this pot from this positive environment and so the voltage is going to get more and more negative it's going to go beyond a neutral because potassium is going to want to go down not just not just its voltage gradient it's going to do that while it's positive on the inside and negative on the outside or more positive on the inside than it is on the outside but there's also it'll also want to go down its concentration gradient there's more Pratap there's a higher concentration of potassium on the inside than on the outside because of the sodium potassium pump so it'll just keep the potassium will just keep going out and out and out and out and then a negative 80 millivolts the potassium channel closes and then we can get back to we can get back to our equilibrium state we could get back to our equilibrium state now why is this interesting well we had the electrotonic spread up to this point but the signal would just keep dissipating and keep dissipating if you get further if you get far enough it would be very hard to notice that signal and so what this essentially just did is boosted the signal again it just boosted the signal and now a few moments later if you were to measure if you were to measure the potential difference because these things are trying to get away from each other again once again you have electrotonic spread if you try if you were to measure the potential difference across this yellow across the membrane where this yellow voltmeter is then you're going to have so where that yellow one is before it had a just a little dissipated bump here but now it's going to have quite a nice it's going to have quite a nice bump and if you actually had another voltage-gated channel right over here then that would boost it again and so this this this this kind of very active boosting of the of of the of the of the voltage this is called an action potential action action potential you could view this as the boosting of the signal the signal spreading electrotonic electrotonic spread then you trigger a channel a voltage-gated channel then that boosts the signal again and as we'll see the neuron uses a combination just the way we described it here in order to spread a signal in order for it to have the signal spread but in order to be also in order to see the spread passively but then to boost it so that the signal can cover over long distances