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  • In the last video, we talked about how the cell uses a

  • sodium potassium pump and ATP to maintain its potential

  • difference between the inside of the cell or the inside of

  • the neuron and the outside-- and in general, the outside is

  • more positive than the inside.

  • You have a -70 millivolt potential difference from the

  • inside to the outside.

  • It's minus because the outside is more positive.

  • Less positive minus more positive, you're going to get

  • a negative number and it's by -70.

  • Now, I said that this was the foundation for understanding

  • how neurons actually transmit signals.

  • And to understand that, I'll kind of lay a foundation over

  • that foundation.

  • I think then just the actual neuron transmission will make

  • a lot of sense.

  • Even better, it'll make a lot of sense why they even have

  • these myelin sheaths and these nodes of Ranvier and why we

  • have all of these dendrites.

  • Hopefully it'll all fit together.

  • So there are two types of ways that kind of a

  • potential can travel.

  • So there's two types of signal transfer.

  • I'll just call it signal transfer.

  • I don't know what the best word for it is.

  • The first one I'll talk is electrotonic.

  • It sounds very fancy, but you'll see it's

  • a very simple idea.

  • And the other one I'm going to go over

  • is an action potential.

  • And they both have their own positives and negatives in

  • terms of being able to transmit a signal.

  • We're talking about within the context of in a cell or across

  • a cell membrane.

  • Let's understand what these mean.

  • So let me get my membrane of a cell.

  • Let's say it's a nerve cell or a neuron, just to make it all

  • fit together in this context.

  • And we know it's more positive on the

  • outside than the inside.

  • We know that there's a lot of sodium on the outside or a lot

  • more sodium on the outside than on the inside.

  • There might be a little bit.

  • And we know there's a lot more potassium on the inside than

  • the outside, but we know generally that the outside is

  • more positive then the inside because our sodium potassium

  • pump will pump out three sodiums for every two

  • potassiums it takes in.

  • Now in the last video, I told you that there are these

  • things called-- well, we could call them a sodium gate.

  • A sodium ion gate, right?

  • These are all ions.

  • They're charged.

  • Now let's say that there's some reason, some stimulus--

  • let me label this.

  • That right there is my sodium ion gate.

  • And it's in its closed position, but let's say

  • something causes it to open.

  • We'll talk maybe in this video or maybe this video and the

  • next about the different things that

  • could cause it to open.

  • Maybe it's some type of stimulus causes this to open.

  • Actually, there's a whole bunch of different stimuluses

  • that would cause it to open.

  • But let's say it opens.

  • What's going to happen if it opens?

  • So let's say we open it.

  • Some stimulus opens-- what's going to happen?

  • We have more positive on the outside than the inside, so

  • positive things want to move in.

  • And this is a sodium gate so only sodium can go through it.

  • So it's kind of a convoluted protein structure that only

  • sodium can make its way through.

  • And on top of that, we have a lot more sodium on the outside

  • than on the inside.

  • So the diffusion gradient's going to want to make sodium

  • go through it.

  • And the fact that sodium's a positive ion, the outside is

  • more positive, they're going to want to run away from that

  • positive environment.

  • So if you open this, you're just going to have a lot of

  • sodium ions start to flood through.

  • Now as that happens, what's going to happen if we go

  • further down the membrane?

  • Let's zoom out.

  • So let's say that this is my membrane right there.

  • Let's say that this is my open gate right here and that it's

  • open for some reason and a bunch of sodium is flowing in.

  • So all of this is becoming much more positive.

  • Let's say we had a voltmeter right here.

  • We're measuring the potential difference between the inside

  • of the membrane a and the outside.

  • Let me do a little chart.

  • I'm going to do the chart here on my voltmeter.

  • And this is going to be the potential difference-- or

  • we'll call it the membrane voltage or the voltage

  • difference across the membrane-- and

  • let's say this is time.

  • Let's say I haven't opened this gate yet.

  • So it's in its resting state.

  • Our sodium potassium pumps are working.

  • Things are leaking back and forth, but it's staying at

  • that minus 70 millivolts.

  • So that right there is minus 70 millivolts.

  • Now as soon as this gate that's way down some other

  • part of the cell opens, what's going to happen?

  • And let's say that's the only thing that's open.

  • So this, all of a sudden, is going to become more positive.

  • So positive charges that's already here-- so other

  • positive charges, whether they're sodiums or potassiums,

  • they're going to want to run away from that point because

  • this area hasn't had a flood of positive things.

  • So it's less positive than this over here.

  • So maybe we have some potassiums and maybe we have

  • some sodiums. Everything is going to want to move away

  • from the place where this is opened.

  • The charge is going to want to move away.

  • So as soon as this happens, as soon as we open this gate,

  • we're going to have a movement of positive

  • charge in this direction.

  • So all of a sudden-- this was at minus 70 millivolts.

  • So more positive charge is coming its way.

  • Almost immediately, it's going to become less negative or

  • more positive.

  • The potential difference between this and this is going

  • to become less.

  • So this is this point over here.

  • Now if we took this point, if we did the same thing-- if we

  • measured the voltage at this point right here, maybe it was

  • at minus 70 millvolts, maybe a fraction of a minute amount of

  • time later, the positive charge starts affecting it so

  • it becomes more positive, but the effect is diluted, right?

  • Because these positive charges, they're going to

  • radiate in every direction.

  • So the effect is diluted.

  • So the effect on this thing is going to be less.

  • It's going to become less positive.

  • So an electrotonic potential-- what happens is at one point

  • in the cell, a gate opens, charge starts flooding in, and

  • it starts affecting the potential at other

  • parts of the cell.

  • But the positive of it is, it's very fast. As soon as

  • this happens.

  • further down the cell, it starts becoming more and more

  • positive, but the further you go, the effect gets dissipated

  • with distance.

  • So if you care about speed, you'd want this

  • electrotonic potential.

  • As soon as it happens, it'll start affecting the rest of

  • the cell, but if you wanted this potential change to

  • travel over large distances-- for example, let's say if we

  • got all the way to this point of the neuron and we wanted to

  • measure it, it might not have any impact.

  • Maybe a little bit later, but it's not having any impact

  • because all of this gets diluted by the time it gets--

  • it's increasing the charge throughout the cell.

  • So it's a impact far away from the initial place where the

  • gate opened.

  • It's going to be a lot less.

  • So it's really not good for operating over distance.

  • Now let's try to figure out what's going on

  • with an action potential.

  • And you might understand, this might involve more action.

  • So let's start off with the same situation.

  • We have a sodium gate that gets opened by some stimulus.

  • What I'm going to do-- let me draw two membranes here.

  • So this is the outside.

  • This is the inside.

  • And let me draw-- maybe we're dealing with a-- and we'll go

  • in more detail.

  • Maybe this is an axon or something, but let me-- let's

  • say we have another sodium gate right here.

  • And then they're alternating, essentially.

  • So they're alternating so then I have another sodium gate.

  • I don't want to do a bunch of these.

  • I think I just have to draw one round of it for you to get

  • what's going on.

  • Let me draw another potassium gate.

  • And let's say that they all start closed.

  • So they're all in the closed position.

  • Now let's say that this sodium gate gets stimulated.

  • It gets opened.

  • Let's say that guy right there gets opened.

  • It gets stimulated by something to get opened.

  • We'll talk about the things that-- let's say in particular

  • this thing gets opened-- let's say the stimulus-- it has to

  • be a certain voltage.

  • And let's say they become open when we are at minus 55

  • millivolts.

  • So when we're just in our resting state, the potential

  • difference between the inside of the cell and the outside is

  • minus 70, so it's not going to be open.

  • It's going to be closed, but if for whatever reason, this

  • becomes positive enough to get to minus 55 millivolts, all of

  • a sudden this thing will be open.

  • Let's write a couple of other rules that dictate what

  • happens to this gate.

  • Let's say it closes-- and these are all rough numbers,

  • but the main idea is for you to get the general idea.

  • Let's say it closes at-- I don't know-- plus 35

  • millivolts.

  • And let's say that our potassium gate opens at plus

  • 40 millvolts, just to give an idea of things.

  • Let's say it closes at-- I don't know-- minus 80

  • millivolts.

  • So what's going to happen?

  • Lets say that, for whatever reason, the voltage here has

  • now become minus 55.

  • Let me do a chart just like I did down here.

  • So I want to have space to draw my chart.

  • This is membrane voltage.

  • And this is time down here.

  • And let's say we're measuring it-- let's say this is the

  • membrane voltage at-- let's say right by the sodium gate

  • right here.

  • So we're measuring this voltage

  • across this right here.

  • So if it's not stimulated any way, we're just here,

  • flatlining at minus 70 millivolts-- and let's say

  • some stimulus, for whatever reason,

  • makes this more positive.

  • Maybe it's some type of electrotonic effect that's

  • making it more positive here.

  • Maybe some positive charges are floating by.

  • So this becomes more positive.

  • So let's say this becomes more positive and then the ATP

  • pumps-- the sodium potassium pumps pump it out so it

  • doesn't get to the threshold of minus 55, so then nothing

  • will happen, right?

  • It didn't get to the threshold.

  • But then let's say there's another electrotonic or maybe

  • a bunch of them and just there's a lot of positive

  • charge here so we get to the minus 55 millvolts.

  • Remember, when positive charge comes by,

  • we become less negative.

  • The potential difference becomes less negative.

  • We get to that minus 55 volts-- this

  • thing opens then, right?

  • This was closed before.

  • It was closed when we were just at minus 70.

  • So let me write here.

  • So at this point, our sodium gate opens.

  • Now, what's going to happen when our sodium gate opens?

  • When that opens-- we've seen this show before-- all the

  • positively charged sodium is going to go down there, both

  • electric gradient and diffusion gradient, and

  • there's going to flood into the cell.

  • There's so much sodium out there, it's so positive out

  • there, they just want to come in.

  • So as soon as they hit that threshold, even though this

  • might've only gotten us to minus 55 or maybe minus 50,

  • all of a sudden that gate opens and we have all of this