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We've already seen that when a neuron is in its resting state there's a voltage difference across the membrane. And so in these diagrams right over here, this right over here 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 voltage minus this voltage right over here, the voltage between this and that, you would get negative-- let's say for the sake of argument, let's say it would measure, it would average about negative 70 millivolts. So this is in millivolts, negative 70. 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 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'll want to flood in is because there's a higher concentration of sodium on the outside than on the inside. So it'll just go down its concentration gradient. And 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 this spike in positive charge flowing. And then 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. And this is not just in the rightward direction. It's really going to be in all directions. In all directions the positive charge, 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 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 as the concentration of these positive charges spread out, you're going to see the voltage start to increase. And then as they fully get spread out then it might return to something of an equilibrium. And then if we go a little bit further down the neuron a little more time will pass before you see a voltage increase, but because this thing is just getting spread out across more and more distance, the effect is going to be more limited. You're not going to see as much of a bump in the voltage over here than 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. Or this is the spread of an 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. 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. Now let's play out this 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 sodium channel. Let's say it opens at negative 55 millivolts. So that would be 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 at positive 40 millivolts, right over there. I'm just trying to show the threshold. And let's say we also have a potassium channel too, right over here. So this is a potassium channel, the infamous leaky potassium channels, which are the true reason why we have this voltage difference 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 to give you the idea, at positive 40 millivolts. And let's say it closes at negative 80 millivolts. So that one opens up here, and then it closes down here. Now what is going to happen? Well just like we saw before-- Let's let our positive charge flood in here at the left side of this neuron, I guess we could say, and then because of electrotonic spread, a little bit later you're going to have 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 just 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 the opening of this sodium channel. So the sodium channel is going to open because the voltage got high enough, and so you're going to have sodium flood in again. So what's that going to do? Well that's going to spike up the voltage. So it's going to look something like that. It's going to keep flowing in, 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 more charge. It's more positive outside than the inside so it's going to go across a voltage gradient, or go down the voltage gradient, or the electro potential gradient, but also there's a higher 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 voltage gradient, but because of the concentration gradient it's 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 positively-charged potassium ions want to get out, want to get out from this positive environment. And so the voltage is going to get more and more negative, and it's going to go beyond neutral because potassium is going to want to go down, 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 it'll also want to go down its concentration gradient. There's a higher concentration of potassium on the inside than on the outside because of the sodium potassium pump. So the potassium will just keep going out, and out, and out, and out, and then at negative 80 millivolts the potassium channel closes, and then we can 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, and if you get far enough it would be very hard to notice that signal. And so what this essentially just did is it just boosted the signal again. It just boosted the signal, and now, a few moments later, 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 were to measure the potential difference across the membrane where this yellow voltmeter is, then you're going to have-- So where that yellow one is, before it had just a little dissipated bump here, but now 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 kind of very active boosting of the voltage, this is called an action potential. You could view this as the boosting of the signal. The signal is spreading, 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, in order to obviously to spread passively, but then to boost it so that the signal can cover over long distances.