0:00:00.000,0:00:00.330


0:00:00.330,0:00:03.150
In the last video, we talked[br]about how the cell uses a

0:00:03.150,0:00:08.280
sodium potassium pump and ATP[br]to maintain its potential

0:00:08.280,0:00:11.110
difference between the inside[br]of the cell or the inside of

0:00:11.110,0:00:13.680
the neuron and the outside-- and[br]in general, the outside is

0:00:13.680,0:00:15.330
more positive than the inside.

0:00:15.330,0:00:19.290
You have a -70 millivolt[br]potential difference from the

0:00:19.290,0:00:21.000
inside to the outside.

0:00:21.000,0:00:25.340
It's minus because the outside[br]is more positive.

0:00:25.340,0:00:27.480
Less positive minus more[br]positive, you're going to get

0:00:27.480,0:00:29.730
a negative number[br]and it's by -70.

0:00:29.730,0:00:32.880
Now, I said that this was the[br]foundation for understanding

0:00:32.880,0:00:35.690
how neurons actually[br]transmit signals.

0:00:35.690,0:00:38.730
And to understand that, I'll[br]kind of lay a foundation over

0:00:38.730,0:00:39.490
that foundation.

0:00:39.490,0:00:42.710
I think then just the actual[br]neuron transmission will make

0:00:42.710,0:00:43.760
a lot of sense.

0:00:43.760,0:00:45.910
Even better, it'll make a lot[br]of sense why they even have

0:00:45.910,0:00:49.070
these myelin sheaths and these[br]nodes of Ranvier and why we

0:00:49.070,0:00:50.120
have all of these dendrites.

0:00:50.120,0:00:52.390
Hopefully it'll all[br]fit together.

0:00:52.390,0:00:55.250
So there are two types of[br]ways that kind of a

0:00:55.250,0:00:57.160
potential can travel.

0:00:57.160,0:00:59.290
So there's two types[br]of signal transfer.

0:00:59.290,0:01:02.290
I'll just call it[br]signal transfer.

0:01:02.290,0:01:04.260
I don't know what the[br]best word for it is.

0:01:04.260,0:01:06.710
The first one I'll talk[br]is electrotonic.

0:01:06.710,0:01:08.190
It sounds very fancy,[br]but you'll see it's

0:01:08.190,0:01:09.440
a very simple idea.

0:01:09.440,0:01:21.020


0:01:21.020,0:01:22.670
And the other one I'm[br]going to go over

0:01:22.670,0:01:25.350
is an action potential.

0:01:25.350,0:01:27.770
And they both have their own[br]positives and negatives in

0:01:27.770,0:01:32.760
terms of being able to[br]transmit a signal.

0:01:32.760,0:01:36.360
We're talking about within the[br]context of in a cell or across

0:01:36.360,0:01:38.270
a cell membrane.

0:01:38.270,0:01:39.940
Let's understand what[br]these mean.

0:01:39.940,0:01:41.780
So let me get my membrane[br]of a cell.

0:01:41.780,0:01:46.310
Let's say it's a nerve cell or[br]a neuron, just to make it all

0:01:46.310,0:01:49.130
fit together in this context.

0:01:49.130,0:01:51.050
And we know it's more[br]positive on the

0:01:51.050,0:01:52.300
outside than the inside.

0:01:52.300,0:01:55.520
We know that there's a lot of[br]sodium on the outside or a lot

0:01:55.520,0:01:58.280
more sodium on the outside[br]than on the inside.

0:01:58.280,0:01:59.320
There might be a little bit.

0:01:59.320,0:02:05.660
And we know there's a lot more[br]potassium on the inside than

0:02:05.660,0:02:08.180
the outside, but we know[br]generally that the outside is

0:02:08.180,0:02:11.070
more positive then the inside[br]because our sodium potassium

0:02:11.070,0:02:13.630
pump will pump out three[br]sodiums for every two

0:02:13.630,0:02:15.430
potassiums it takes in.

0:02:15.430,0:02:18.060
Now in the last video, I told[br]you that there are these

0:02:18.060,0:02:21.485
things called-- well, we could[br]call them a sodium gate.

0:02:21.485,0:02:23.810
A sodium ion gate, right?

0:02:23.810,0:02:24.430
These are all ions.

0:02:24.430,0:02:25.990
They're charged.

0:02:25.990,0:02:28.420
Now let's say that there's some[br]reason, some stimulus--

0:02:28.420,0:02:29.670
let me label this.

0:02:29.670,0:02:35.430


0:02:35.430,0:02:39.990
That right there is my[br]sodium ion gate.

0:02:39.990,0:02:41.660
And it's in its closed position,[br]but let's say

0:02:41.660,0:02:43.480
something causes it to open.

0:02:43.480,0:02:45.700
We'll talk maybe in this video[br]or maybe this video and the

0:02:45.700,0:02:47.330
next about the different[br]things that

0:02:47.330,0:02:48.200
could cause it to open.

0:02:48.200,0:02:51.100
Maybe it's some type of stimulus[br]causes this to open.

0:02:51.100,0:02:52.910
Actually, there's a whole bunch[br]of different stimuluses

0:02:52.910,0:02:55.480
that would cause it to open.

0:02:55.480,0:02:56.370
But let's say it opens.

0:02:56.370,0:03:00.560
What's going to happen[br]if it opens?

0:03:00.560,0:03:02.560
So let's say we open it.

0:03:02.560,0:03:04.790
Some stimulus opens-- what's[br]going to happen?

0:03:04.790,0:03:06.880
We have more positive on the[br]outside than the inside, so

0:03:06.880,0:03:08.460
positive things want[br]to move in.

0:03:08.460,0:03:12.400
And this is a sodium gate so[br]only sodium can go through it.

0:03:12.400,0:03:14.960
So it's kind of a convoluted[br]protein structure that only

0:03:14.960,0:03:17.000
sodium can make its[br]way through.

0:03:17.000,0:03:19.840
And on top of that, we have a[br]lot more sodium on the outside

0:03:19.840,0:03:20.510
than on the inside.

0:03:20.510,0:03:22.580
So the diffusion gradient's[br]going to want to make sodium

0:03:22.580,0:03:23.320
go through it.

0:03:23.320,0:03:26.650
And the fact that sodium's a[br]positive ion, the outside is

0:03:26.650,0:03:28.270
more positive, they're going to[br]want to run away from that

0:03:28.270,0:03:30.340
positive environment.

0:03:30.340,0:03:32.810
So if you open this, you're just[br]going to have a lot of

0:03:32.810,0:03:35.040
sodium ions start to[br]flood through.

0:03:35.040,0:03:40.250


0:03:40.250,0:03:43.190
Now as that happens, what's[br]going to happen if we go

0:03:43.190,0:03:44.500
further down the membrane?

0:03:44.500,0:03:47.110
Let's zoom out.

0:03:47.110,0:03:50.650
So let's say that this is[br]my membrane right there.

0:03:50.650,0:03:54.160
Let's say that this is my open[br]gate right here and that it's

0:03:54.160,0:03:56.990
open for some reason and a bunch[br]of sodium is flowing in.

0:03:56.990,0:04:01.050
So all of this is becoming[br]much more positive.

0:04:01.050,0:04:04.980
Let's say we had a voltmeter[br]right here.

0:04:04.980,0:04:07.230
We're measuring the potential[br]difference between the inside

0:04:07.230,0:04:10.420
of the membrane a[br]and the outside.

0:04:10.420,0:04:13.580
Let me do a little chart.

0:04:13.580,0:04:17.190
I'm going to do the chart[br]here on my voltmeter.

0:04:17.190,0:04:20.579
And this is going to be the[br]potential difference-- or

0:04:20.579,0:04:26.750
we'll call it the membrane[br]voltage or the voltage

0:04:26.750,0:04:28.000
difference across the[br]membrane-- and

0:04:28.000,0:04:31.150
let's say this is time.

0:04:31.150,0:04:33.460
Let's say I haven't opened[br]this gate yet.

0:04:33.460,0:04:35.030
So it's in its resting state.

0:04:35.030,0:04:37.100
Our sodium potassium[br]pumps are working.

0:04:37.100,0:04:39.090
Things are leaking back and[br]forth, but it's staying at

0:04:39.090,0:04:41.790
that minus 70 millivolts.

0:04:41.790,0:04:46.770
So that right there is[br]minus 70 millivolts.

0:04:46.770,0:04:51.110
Now as soon as this gate that's[br]way down some other

0:04:51.110,0:04:53.770
part of the cell opens, what's[br]going to happen?

0:04:53.770,0:04:55.910
And let's say that's the[br]only thing that's open.

0:04:55.910,0:04:58.370
So this, all of a sudden, is[br]going to become more positive.

0:04:58.370,0:05:01.180
So positive charges that's[br]already here-- so other

0:05:01.180,0:05:07.480
positive charges, whether[br]they're sodiums or potassiums,

0:05:07.480,0:05:09.260
they're going to want to run[br]away from that point because

0:05:09.260,0:05:11.480
this area hasn't had a flood[br]of positive things.

0:05:11.480,0:05:13.840
So it's less positive[br]than this over here.

0:05:13.840,0:05:16.670
So maybe we have some potassiums[br]and maybe we have

0:05:16.670,0:05:19.960
some sodiums. Everything is[br]going to want to move away

0:05:19.960,0:05:23.310
from the place where[br]this is opened.

0:05:23.310,0:05:25.590
The charge is going to[br]want to move away.

0:05:25.590,0:05:28.910
So as soon as this happens, as[br]soon as we open this gate,

0:05:28.910,0:05:30.460
we're going to have a[br]movement of positive

0:05:30.460,0:05:31.230
charge in this direction.

0:05:31.230,0:05:34.830
So all of a sudden-- this was[br]at minus 70 millivolts.

0:05:34.830,0:05:37.240
So more positive charge[br]is coming its way.

0:05:37.240,0:05:41.680


0:05:41.680,0:05:45.120
Almost immediately, it's going[br]to become less negative or

0:05:45.120,0:05:46.470
more positive.

0:05:46.470,0:05:48.590
The potential difference between[br]this and this is going

0:05:48.590,0:05:49.800
to become less.

0:05:49.800,0:05:51.980
So this is this point[br]over here.

0:05:51.980,0:05:57.900
Now if we took this point, if we[br]did the same thing-- if we

0:05:57.900,0:06:01.120
measured the voltage at this[br]point right here, maybe it was

0:06:01.120,0:06:06.150
at minus 70 millvolts, maybe a[br]fraction of a minute amount of

0:06:06.150,0:06:09.560
time later, the positive charge[br]starts affecting it so

0:06:09.560,0:06:13.020
it becomes more positive, but[br]the effect is diluted, right?

0:06:13.020,0:06:14.660
Because these positive charges,[br]they're going to

0:06:14.660,0:06:16.240
radiate in every direction.

0:06:16.240,0:06:17.410
So the effect is diluted.

0:06:17.410,0:06:20.130
So the effect on this thing[br]is going to be less.

0:06:20.130,0:06:21.860
It's going to become[br]less positive.

0:06:21.860,0:06:26.160
So an electrotonic potential--[br]what happens is at one point

0:06:26.160,0:06:30.390
in the cell, a gate opens,[br]charge starts flooding in, and

0:06:30.390,0:06:32.460
it starts affecting the[br]potential at other

0:06:32.460,0:06:33.630
parts of the cell.

0:06:33.630,0:06:40.970
But the positive of it is, it's[br]very fast. As soon as

0:06:40.970,0:06:41.820
this happens.

0:06:41.820,0:06:50.000
further down the cell, it starts[br]becoming more and more

0:06:50.000,0:07:00.820
positive, but the further you[br]go, the effect gets dissipated

0:07:00.820,0:07:02.570
with distance.

0:07:02.570,0:07:05.050
So if you care about speed,[br]you'd want this

0:07:05.050,0:07:06.240
electrotonic potential.

0:07:06.240,0:07:08.620
As soon as it happens, it'll[br]start affecting the rest of

0:07:08.620,0:07:11.750
the cell, but if you wanted[br]this potential change to

0:07:11.750,0:07:14.660
travel over large distances--[br]for example, let's say if we

0:07:14.660,0:07:18.030
got all the way to this point of[br]the neuron and we wanted to

0:07:18.030,0:07:20.210
measure it, it might not[br]have any impact.

0:07:20.210,0:07:22.410
Maybe a little bit later, but[br]it's not having any impact

0:07:22.410,0:07:24.920
because all of this gets diluted[br]by the time it gets--

0:07:24.920,0:07:26.960
it's increasing the charge[br]throughout the cell.

0:07:26.960,0:07:30.200
So it's a impact far away from[br]the initial place where the

0:07:30.200,0:07:30.750
gate opened.

0:07:30.750,0:07:32.240
It's going to be a lot less.

0:07:32.240,0:07:36.110
So it's really not good for[br]operating over distance.

0:07:36.110,0:07:38.040
Now let's try to figure[br]out what's going on

0:07:38.040,0:07:40.080
with an action potential.

0:07:40.080,0:07:43.310
And you might understand, this[br]might involve more action.

0:07:43.310,0:07:45.510
So let's start off with[br]the same situation.

0:07:45.510,0:07:51.500
We have a sodium gate that gets[br]opened by some stimulus.

0:07:51.500,0:07:54.730
What I'm going to do-- let me[br]draw two membranes here.

0:07:54.730,0:07:55.980
So this is the outside.

0:07:55.980,0:07:59.500


0:07:59.500,0:08:02.440
This is the inside.

0:08:02.440,0:08:05.680
And let me draw-- maybe we're[br]dealing with a-- and we'll go

0:08:05.680,0:08:06.270
in more detail.

0:08:06.270,0:08:11.870
Maybe this is an axon or[br]something, but let me-- let's

0:08:11.870,0:08:14.810
say we have another sodium[br]gate right here.

0:08:14.810,0:08:18.810


0:08:18.810,0:08:21.700
And then they're alternating,[br]essentially.

0:08:21.700,0:08:26.090
So they're alternating so then[br]I have another sodium gate.

0:08:26.090,0:08:38.520


0:08:38.520,0:08:39.980
I don't want to do[br]a bunch of these.

0:08:39.980,0:08:42.980
I think I just have to draw one[br]round of it for you to get

0:08:42.980,0:08:44.730
what's going on.

0:08:44.730,0:08:46.350
Let me draw another[br]potassium gate.

0:08:46.350,0:08:52.140


0:08:52.140,0:08:54.290
And let's say that they[br]all start closed.

0:08:54.290,0:08:56.240
So they're all in the[br]closed position.

0:08:56.240,0:08:58.750
Now let's say that this sodium[br]gate gets stimulated.

0:08:58.750,0:09:00.000
It gets opened.

0:09:00.000,0:09:03.220


0:09:03.220,0:09:05.740
Let's say that guy right[br]there gets opened.

0:09:05.740,0:09:07.670
It gets stimulated by something[br]to get opened.

0:09:07.670,0:09:10.860
We'll talk about the things[br]that-- let's say in particular

0:09:10.860,0:09:20.160
this thing gets opened-- let's[br]say the stimulus-- it has to

0:09:20.160,0:09:21.210
be a certain voltage.

0:09:21.210,0:09:26.170
And let's say they become open[br]when we are at minus 55

0:09:26.170,0:09:27.420
millivolts.

0:09:27.420,0:09:33.400


0:09:33.400,0:09:35.910
So when we're just in our[br]resting state, the potential

0:09:35.910,0:09:38.270
difference between the inside of[br]the cell and the outside is

0:09:38.270,0:09:40.380
minus 70, so it's not[br]going to be open.

0:09:40.380,0:09:43.420
It's going to be closed, but if[br]for whatever reason, this

0:09:43.420,0:09:47.520
becomes positive enough to get[br]to minus 55 millivolts, all of

0:09:47.520,0:09:49.440
a sudden this thing[br]will be open.

0:09:49.440,0:09:52.730
Let's write a couple of other[br]rules that dictate what

0:09:52.730,0:09:53.630
happens to this gate.

0:09:53.630,0:09:57.880
Let's say it closes-- and these[br]are all rough numbers,

0:09:57.880,0:10:01.260
but the main idea is for you[br]to get the general idea.

0:10:01.260,0:10:10.910
Let's say it closes at--[br]I don't know-- plus 35

0:10:10.910,0:10:11.870
millivolts.

0:10:11.870,0:10:20.303
And let's say that our potassium[br]gate opens at plus

0:10:20.303,0:10:24.280
40 millvolts, just to give[br]an idea of things.

0:10:24.280,0:10:33.110
Let's say it closes at--[br]I don't know-- minus 80

0:10:33.110,0:10:34.360
millivolts.

0:10:34.360,0:10:36.210


0:10:36.210,0:10:37.010
So what's going to happen?

0:10:37.010,0:10:40.040
Lets say that, for whatever[br]reason, the voltage here has

0:10:40.040,0:10:41.620
now become minus 55.

0:10:41.620,0:10:44.830
Let me do a chart just[br]like I did down here.

0:10:44.830,0:10:46.855
So I want to have space[br]to draw my chart.

0:10:46.855,0:10:54.110


0:10:54.110,0:10:55.360
This is membrane voltage.

0:10:55.360,0:11:00.370


0:11:00.370,0:11:03.080
And this is time down here.

0:11:03.080,0:11:05.180
And let's say we're measuring[br]it-- let's say this is the

0:11:05.180,0:11:08.790
membrane voltage at-- let's say[br]right by the sodium gate

0:11:08.790,0:11:09.150
right here.

0:11:09.150,0:11:10.350
So we're measuring this voltage

0:11:10.350,0:11:11.480
across this right here.

0:11:11.480,0:11:14.200
So if it's not stimulated any[br]way, we're just here,

0:11:14.200,0:11:18.060
flatlining at minus 70[br]millivolts-- and let's say

0:11:18.060,0:11:19.830
some stimulus, for[br]whatever reason,

0:11:19.830,0:11:21.620
makes this more positive.

0:11:21.620,0:11:24.950
Maybe it's some type of[br]electrotonic effect that's

0:11:24.950,0:11:26.430
making it more positive here.

0:11:26.430,0:11:28.440
Maybe some positive charges[br]are floating by.

0:11:28.440,0:11:30.860
So this becomes more positive.

0:11:30.860,0:11:33.540
So let's say this becomes more[br]positive and then the ATP

0:11:33.540,0:11:38.520
pumps-- the sodium potassium[br]pumps pump it out so it

0:11:38.520,0:11:41.780
doesn't get to the threshold of[br]minus 55, so then nothing

0:11:41.780,0:11:42.620
will happen, right?

0:11:42.620,0:11:43.640
It didn't get to[br]the threshold.

0:11:43.640,0:11:45.710
But then let's say there's[br]another electrotonic or maybe

0:11:45.710,0:11:47.950
a bunch of them and just there's[br]a lot of positive

0:11:47.950,0:11:54.480
charge here so we get to[br]the minus 55 millvolts.

0:11:54.480,0:11:55.960
Remember, when positive[br]charge comes by,

0:11:55.960,0:11:57.130
we become less negative.

0:11:57.130,0:11:59.110
The potential difference[br]becomes less negative.

0:11:59.110,0:12:01.710
We get to that minus[br]55 volts-- this

0:12:01.710,0:12:03.950
thing opens then, right?

0:12:03.950,0:12:05.200
This was closed before.

0:12:05.200,0:12:07.380
It was closed when we were[br]just at minus 70.

0:12:07.380,0:12:09.230
So let me write here.

0:12:09.230,0:12:20.320
So at this point, our[br]sodium gate opens.

0:12:20.320,0:12:23.040
Now, what's going to happen when[br]our sodium gate opens?

0:12:23.040,0:12:25.780
When that opens-- we've seen[br]this show before-- all the

0:12:25.780,0:12:28.500
positively charged sodium is[br]going to go down there, both

0:12:28.500,0:12:31.490
electric gradient and diffusion[br]gradient, and

0:12:31.490,0:12:34.340
there's going to flood[br]into the cell.

0:12:34.340,0:12:36.200
There's so much sodium out[br]there, it's so positive out

0:12:36.200,0:12:37.910
there, they just want[br]to come in.

0:12:37.910,0:12:41.350
So as soon as they hit that[br]threshold, even though this

0:12:41.350,0:12:44.910
might've only gotten us to minus[br]55 or maybe minus 50,

0:12:44.910,0:12:47.440
all of a sudden that gate opens[br]and we have all of this

0:12:47.440,0:12:49.170
positive charge flooding[br]into the cell.

0:12:49.170,0:12:50.480
So the potential difference[br]becomes

0:12:50.480,0:12:51.800
much, much more positive.

0:12:51.800,0:12:54.810


0:12:54.810,0:12:57.380
So they keep flooding in,[br]becomes much, much more

0:12:57.380,0:13:00.220
positive, but as it gets[br]more positive, it

0:13:00.220,0:13:15.340
closes at plus 35 millvolts.

0:13:15.340,0:13:17.880
So let's say that we're dealing[br]here-- let's say that

0:13:17.880,0:13:21.310
this up here is plus[br]35 millvolts.

0:13:21.310,0:13:24.720
So here it closes and at the[br]same time, that stuff I just

0:13:24.720,0:13:30.550
deleted-- I set at plus 40[br]millvolts-- or let's say at

0:13:30.550,0:13:32.320
plus 35, just for the[br]sake of argument.

0:13:32.320,0:13:34.840
Let's say at plus 45[br]millvolts, our

0:13:34.840,0:13:39.520
sodium gates open.

0:13:39.520,0:13:40.650
So what's happened here?

0:13:40.650,0:13:43.990
All of a sudden, we're at plus[br]35 or maybe plus 40 millivolts

0:13:43.990,0:13:47.430
so this is-- let's just say plus[br]40, I think you get the

0:13:47.430,0:13:51.840
idea either way so we'll say[br]plus 40-- either way.

0:13:51.840,0:13:56.100
So at plus 40, this guy's[br]going to close.

0:13:56.100,0:13:58.640
No more positive ions are coming[br]in, but now we are at

0:13:58.640,0:14:01.340
more positive inside, at least[br]locally at this point on the

0:14:01.340,0:14:03.070
membrane, than we are outside.

0:14:03.070,0:14:05.720
And so this gate will open.

0:14:05.720,0:14:08.220
So then our sodium[br]gate will open.

0:14:08.220,0:14:11.320
K-plus ion gate opens.

0:14:11.320,0:14:12.720
Now when that opens,[br]what happens?

0:14:12.720,0:14:15.800
We have all of these[br]sodium ions here.

0:14:15.800,0:14:19.370
We already saw from the sodium[br]potassium pump that the

0:14:19.370,0:14:22.510
potassium-- we have all of these[br]potassium ions here.

0:14:22.510,0:14:25.150
We saw from the sodium potassium[br]pump that it makes

0:14:25.150,0:14:27.280
the sodium concentration on[br]the outside higher and the

0:14:27.280,0:14:30.300
potassium concentration[br]on the inside higher.

0:14:30.300,0:14:34.330
And now that we've gotten to[br]this plus 40 millvolt range,

0:14:34.330,0:14:37.140
we're also now more positive[br]on the inside.

0:14:37.140,0:14:38.360
So this opens.

0:14:38.360,0:14:40.320
These guys want to escape[br]because there's

0:14:40.320,0:14:42.200
less potassium outside.

0:14:42.200,0:14:44.290
They want to go down their[br]concentration gradient.

0:14:44.290,0:14:45.940
It's also very positive[br]on the inside.

0:14:45.940,0:14:47.970
We're at plus 40 millvolts.

0:14:47.970,0:14:49.480
So they also want to escape.

0:14:49.480,0:14:51.500
So they start escaping[br]the cells.

0:14:51.500,0:14:54.350
So positive charges starts[br]exiting the cell from the

0:14:54.350,0:14:56.210
inside to the outside.

0:14:56.210,0:14:59.920
So we become less[br]positive again.

0:14:59.920,0:15:01.550
So let me write what[br]happens here.

0:15:01.550,0:15:07.530
So at this point, our sodium[br]gate closes and our potassium

0:15:07.530,0:15:09.140
gate opens.

0:15:09.140,0:15:11.920


0:15:11.920,0:15:14.250
And then the positive charge[br]starts flooding out of the

0:15:14.250,0:15:18.300
cell again and maybe it'll[br]overshoot because it's only

0:15:18.300,0:15:21.670
going to close maybe once we[br]get to minus 80 millvolts.

0:15:21.670,0:15:30.660
So maybe our potassium gate[br]closes at minus 80.

0:15:30.660,0:15:35.340
And then our sodium potassium[br]pump might get us back to our

0:15:35.340,0:15:36.550
minus 70 millvolts.

0:15:36.550,0:15:40.940
So, this is what's happening[br]just at this point in the

0:15:40.940,0:15:44.720
cell, just near that[br]first sodium gate.

0:15:44.720,0:15:47.040
But what's going to happen[br]in general, right?

0:15:47.040,0:15:49.680
As this became very positive--[br]we went to 40

0:15:49.680,0:15:51.340
millivolts over here.

0:15:51.340,0:15:54.490
We went to 40 millvolts in[br]this area of the cell.

0:15:54.490,0:15:56.970
Because of-- I guess you could[br]almost view it as a short term

0:15:56.970,0:16:00.120
or very short distance[br]electrotonic potential, this

0:16:00.120,0:16:02.760
area is going to become[br]more positive, right?

0:16:02.760,0:16:03.910
This is going to become[br]more positive.

0:16:03.910,0:16:05.300
These positive charges[br]are going to go

0:16:05.300,0:16:06.970
where it's less positive.

0:16:06.970,0:16:09.040
So this is going to become[br]more positive.

0:16:09.040,0:16:11.970
This was at minus 70, but it's[br]going to become more positive.

0:16:11.970,0:16:18.200
It'll go to minus 65, minus 60,[br]minus 55-- and then bam.

0:16:18.200,0:16:19.780
This guy will get[br]triggered again.

0:16:19.780,0:16:22.060
Then this guy gets opened.

0:16:22.060,0:16:23.440
Then this guy gets opened.

0:16:23.440,0:16:25.430
Sodium floods in through here.

0:16:25.430,0:16:27.840
So if you wanted to plot this[br]guy's, the potential

0:16:27.840,0:16:33.240
difference of what's going on[br]across this, this all happened

0:16:33.240,0:16:36.630
as soon as-- maybe as soon as a[br]sodium started going in this

0:16:36.630,0:16:41.440
first dude, the second guy-- he[br]gets triggered here because

0:16:41.440,0:16:45.640
the second guy a little bit[br]later in time-- because of all

0:16:45.640,0:16:47.700
this flow a little bit to[br]the left of him, his

0:16:47.700,0:16:48.540
potential goes up.

0:16:48.540,0:16:53.230
He gets triggered, same exact[br]thing happens to him, right?

0:16:53.230,0:16:56.300
When the sodium flows in here,[br]becomes really positive around

0:16:56.300,0:16:59.630
here, that makes the cell[br]around here, the voltage

0:16:59.630,0:17:01.180
around here, the charge around[br]here a little bit more

0:17:01.180,0:17:04.930
positive, triggers this next[br]sodium gate to open and then

0:17:04.930,0:17:07.280
this whole same thing[br]happens, same cycle.

0:17:07.280,0:17:10.660
Then the potassium gates open to[br]make it negative again, but

0:17:10.660,0:17:12.960
by the time that's happened,[br]it's become positive over here

0:17:12.960,0:17:14.630
to trigger another[br]sodium gate.

0:17:14.630,0:17:18.069
So one after another, you have[br]these sodium gates opening and

0:17:18.069,0:17:20.839
closing, but it's transmitting[br]that information, it's

0:17:20.839,0:17:23.270
transmitting that potential[br]change.

0:17:23.270,0:17:25.130
So what's going on here?

0:17:25.130,0:17:27.990
So this is slower and it[br]actually involves energy.

0:17:27.990,0:17:32.200
So this was-- the electrotonic[br]was very fast. This is slow.

0:17:32.200,0:17:33.820
An action potential is slower.

0:17:33.820,0:17:35.350
I don't want to say it's slow.

0:17:35.350,0:17:38.400
It's slower because it has to[br]involve these opening and

0:17:38.400,0:17:41.420
closing of gates and it[br]also involves energy.

0:17:41.420,0:17:43.275
It also requires more energy.

0:17:43.275,0:17:47.880


0:17:47.880,0:17:50.440
And you're also going to have to[br]keep changing the potential

0:17:50.440,0:17:54.130
in your cell and you actively[br]have your sodium potassium

0:17:54.130,0:17:55.700
pumps being very active.

0:17:55.700,0:17:56.770
But it's good.

0:17:56.770,0:17:59.480
The positive is, it's good[br]at covering distance.

0:17:59.480,0:18:02.330


0:18:02.330,0:18:03.820
When you have something like[br]this-- we saw with the

0:18:03.820,0:18:06.160
electrotonic, as we get further[br]and further away from

0:18:06.160,0:18:09.120
where the stimulus happened,[br]the change in potential

0:18:09.120,0:18:10.325
becomes more and more[br]dissipated.

0:18:10.325,0:18:12.080
It actually exponentially[br]declines.

0:18:12.080,0:18:14.440
It becomes more and more[br]dissipated as we get further

0:18:14.440,0:18:17.030
and further away so it's not[br]good for long distance.

0:18:17.030,0:18:20.600
This thing can just continue[br]forever because every time it

0:18:20.600,0:18:23.170
stimulates the next gate, it's[br]like we're starting all over

0:18:23.170,0:18:26.940
again and so this gate-- it's[br]going to have a flood of ions

0:18:26.940,0:18:30.550
come in and those ions are going[br]to make it a little less

0:18:30.550,0:18:31.780
negative over here.

0:18:31.780,0:18:33.110
Then the next gate's[br]going to open.

0:18:33.110,0:18:34.750
We're going to have the cycle[br]over and over again.

0:18:34.750,0:18:38.210
So this is really good for[br]traveling long distances.

0:18:38.210,0:18:40.850
So now we have really the[br]foundation to understand

0:18:40.850,0:18:43.880
exactly what's happening in a[br]neuron and I'm going to go

0:18:43.880,0:18:46.330
over that in the next video to[br]show you how electrotonic

0:18:46.330,0:18:49.820
potentials and action potentials[br]can combine to have

0:18:49.820,0:18:51.970
a signal travel through[br]a neuron.

0:18:51.970,0:18:53.220