WEBVTT

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

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sodium potassium pump and ATP
to maintain its potential

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difference between the inside
of the cell or the inside of

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

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more positive than the inside.

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You have a -70 millivolt
potential difference from the

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inside to the outside.

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It's minus because the outside
is more positive.

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

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a negative number
and it's by -70.

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Now, I said that this was the
foundation for understanding

00:00:32.880 --> 00:00:35.690
how neurons actually
transmit signals.

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

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that foundation.

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I think then just the actual
neuron transmission will make

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a lot of sense.

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

00:00:45.910 --> 00:00:49.070
these myelin sheaths and these
nodes of Ranvier and why we

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have all of these dendrites.

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Hopefully it'll all
fit together.

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So there are two types of
ways that kind of a

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potential can travel.

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So there's two types
of signal transfer.

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I'll just call it
signal transfer.

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

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The first one I'll talk
is electrotonic.

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

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a very simple idea.

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And the other one I'm
going to go over

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is an action potential.

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And they both have their own
positives and negatives in

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terms of being able to
transmit a signal.

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

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a cell membrane.

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Let's understand what
these mean.

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So let me get my membrane
of a cell.

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

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fit together in this context.

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And we know it's more
positive on the

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outside than the inside.

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

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more sodium on the outside
than on the inside.

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There might be a little bit.

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

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the outside, but we know
generally that the outside is

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more positive then the inside
because our sodium potassium

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pump will pump out three
sodiums for every two

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potassiums it takes in.

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

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

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A sodium ion gate, right?

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These are all ions.

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They're charged.

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

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let me label this.

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That right there is my
sodium ion gate.

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

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something causes it to open.

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

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next about the different
things that

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could cause it to open.

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

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Actually, there's a whole bunch
of different stimuluses

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that would cause it to open.

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But let's say it opens.

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What's going to happen
if it opens?

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So let's say we open it.

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

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

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positive things want
to move in.

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

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

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sodium can make its
way through.

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

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than on the inside.

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

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go through it.

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

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

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positive environment.

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

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sodium ions start to
flood through.

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

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further down the membrane?

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Let's zoom out.

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

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

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

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So all of this is becoming
much more positive.

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Let's say we had a voltmeter
right here.

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We're measuring the potential
difference between the inside

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of the membrane a
and the outside.

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Let me do a little chart.

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

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

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we'll call it the membrane
voltage or the voltage

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difference across the
membrane-- and

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let's say this is time.

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

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So it's in its resting state.

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Our sodium potassium
pumps are working.

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

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that minus 70 millivolts.

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So that right there is
minus 70 millivolts.

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

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

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

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

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

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positive charges, whether
they're sodiums or potassiums,

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

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

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So it's less positive
than this over here.

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So maybe we have some potassiums
and maybe we have

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some sodiums. Everything is
going to want to move away

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from the place where
this is opened.

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The charge is going to
want to move away.

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

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we're going to have a
movement of positive

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charge in this direction.

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

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So more positive charge
is coming its way.

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

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more positive.

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The potential difference between
this and this is going

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to become less.

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So this is this point
over here.

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

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

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

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time later, the positive charge
starts affecting it so

00:06:09.560 --> 00:06:13.020
it becomes more positive, but
the effect is diluted, right?

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Because these positive charges,
they're going to

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radiate in every direction.

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So the effect is diluted.

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

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It's going to become
less positive.

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

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

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it starts affecting the
potential at other

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parts of the cell.

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

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this happens.

00:06:41.820 --> 00:06:50.000
further down the cell, it starts
becoming more and more

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

00:07:00.820 --> 00:07:02.570
with distance.

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

00:07:05.050 --> 00:07:06.240
electrotonic potential.

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

00:07:08.620 --> 00:07:11.750
the cell, but if you wanted
this potential change to

00:07:11.750 --> 00:07:14.660
travel over large distances--
for example, let's say if we

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

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measure it, it might not
have any impact.

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

00:07:22.410 --> 00:07:24.920
because all of this gets diluted
by the time it gets--

00:07:24.920 --> 00:07:26.960
it's increasing the charge
throughout the cell.

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

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gate opened.

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It's going to be a lot less.

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

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

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with an action potential.

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And you might understand, this
might involve more action.

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So let's start off with
the same situation.

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

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

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So this is the outside.

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This is the inside.

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

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in more detail.

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

00:08:11.870 --> 00:08:14.810
say we have another sodium
gate right here.

00:08:14.810 --> 00:08:18.810


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And then they're alternating,
essentially.

00:08:21.700 --> 00:08:26.090
So they're alternating so then
I have another sodium gate.

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

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

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what's going on.

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Let me draw another
potassium gate.

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And let's say that they
all start closed.

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So they're all in the
closed position.

00:08:56.240 --> 00:08:58.750
Now let's say that this sodium
gate gets stimulated.

00:08:58.750 --> 00:09:00.000
It gets opened.

00:09:00.000 --> 00:09:03.220


00:09:03.220 --> 00:09:05.740
Let's say that guy right
there gets opened.

00:09:05.740 --> 00:09:07.670
It gets stimulated by something
to get opened.

00:09:07.670 --> 00:09:10.860
We'll talk about the things
that-- let's say in particular

00:09:10.860 --> 00:09:20.160
this thing gets opened-- let's
say the stimulus-- it has to

00:09:20.160 --> 00:09:21.210
be a certain voltage.

00:09:21.210 --> 00:09:26.170
And let's say they become open
when we are at minus 55

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millivolts.

00:09:27.420 --> 00:09:33.400


00:09:33.400 --> 00:09:35.910
So when we're just in our
resting state, the potential

00:09:35.910 --> 00:09:38.270
difference between the inside of
the cell and the outside is

00:09:38.270 --> 00:09:40.380
minus 70, so it's not
going to be open.

00:09:40.380 --> 00:09:43.420
It's going to be closed, but if
for whatever reason, this

00:09:43.420 --> 00:09:47.520
becomes positive enough to get
to minus 55 millivolts, all of

00:09:47.520 --> 00:09:49.440
a sudden this thing
will be open.

00:09:49.440 --> 00:09:52.730
Let's write a couple of other
rules that dictate what

00:09:52.730 --> 00:09:53.630
happens to this gate.

00:09:53.630 --> 00:09:57.880
Let's say it closes-- and these
are all rough numbers,

00:09:57.880 --> 00:10:01.260
but the main idea is for you
to get the general idea.

00:10:01.260 --> 00:10:10.910
Let's say it closes at--
I don't know-- plus 35

00:10:10.910 --> 00:10:11.870
millivolts.

00:10:11.870 --> 00:10:20.303
And let's say that our potassium
gate opens at plus

00:10:20.303 --> 00:10:24.280
40 millvolts, just to give
an idea of things.

00:10:24.280 --> 00:10:33.110
Let's say it closes at--
I don't know-- minus 80

00:10:33.110 --> 00:10:34.360
millivolts.

00:10:34.360 --> 00:10:36.210


00:10:36.210 --> 00:10:37.010
So what's going to happen?

00:10:37.010 --> 00:10:40.040
Lets say that, for whatever
reason, the voltage here has

00:10:40.040 --> 00:10:41.620
now become minus 55.

00:10:41.620 --> 00:10:44.830
Let me do a chart just
like I did down here.

00:10:44.830 --> 00:10:46.855
So I want to have space
to draw my chart.

00:10:46.855 --> 00:10:54.110


00:10:54.110 --> 00:10:55.360
This is membrane voltage.

00:10:55.360 --> 00:11:00.370


00:11:00.370 --> 00:11:03.080
And this is time down here.

00:11:03.080 --> 00:11:05.180
And let's say we're measuring
it-- let's say this is the

00:11:05.180 --> 00:11:08.790
membrane voltage at-- let's say
right by the sodium gate

00:11:08.790 --> 00:11:09.150
right here.

00:11:09.150 --> 00:11:10.350
So we're measuring this voltage

00:11:10.350 --> 00:11:11.480
across this right here.

00:11:11.480 --> 00:11:14.200
So if it's not stimulated any
way, we're just here,

00:11:14.200 --> 00:11:18.060
flatlining at minus 70
millivolts-- and let's say

00:11:18.060 --> 00:11:19.830
some stimulus, for
whatever reason,

00:11:19.830 --> 00:11:21.620
makes this more positive.

00:11:21.620 --> 00:11:24.950
Maybe it's some type of
electrotonic effect that's

00:11:24.950 --> 00:11:26.430
making it more positive here.

00:11:26.430 --> 00:11:28.440
Maybe some positive charges
are floating by.

00:11:28.440 --> 00:11:30.860
So this becomes more positive.

00:11:30.860 --> 00:11:33.540
So let's say this becomes more
positive and then the ATP

00:11:33.540 --> 00:11:38.520
pumps-- the sodium potassium
pumps pump it out so it

00:11:38.520 --> 00:11:41.780
doesn't get to the threshold of
minus 55, so then nothing

00:11:41.780 --> 00:11:42.620
will happen, right?

00:11:42.620 --> 00:11:43.640
It didn't get to
the threshold.

00:11:43.640 --> 00:11:45.710
But then let's say there's
another electrotonic or maybe

00:11:45.710 --> 00:11:47.950
a bunch of them and just there's
a lot of positive

00:11:47.950 --> 00:11:54.480
charge here so we get to
the minus 55 millvolts.

00:11:54.480 --> 00:11:55.960
Remember, when positive
charge comes by,

00:11:55.960 --> 00:11:57.130
we become less negative.

00:11:57.130 --> 00:11:59.110
The potential difference
becomes less negative.

00:11:59.110 --> 00:12:01.710
We get to that minus
55 volts-- this

00:12:01.710 --> 00:12:03.950
thing opens then, right?

00:12:03.950 --> 00:12:05.200
This was closed before.

00:12:05.200 --> 00:12:07.380
It was closed when we were
just at minus 70.

00:12:07.380 --> 00:12:09.230
So let me write here.

00:12:09.230 --> 00:12:20.320
So at this point, our
sodium gate opens.

00:12:20.320 --> 00:12:23.040
Now, what's going to happen when
our sodium gate opens?

00:12:23.040 --> 00:12:25.780
When that opens-- we've seen
this show before-- all the

00:12:25.780 --> 00:12:28.500
positively charged sodium is
going to go down there, both

00:12:28.500 --> 00:12:31.490
electric gradient and diffusion
gradient, and

00:12:31.490 --> 00:12:34.340
there's going to flood
into the cell.

00:12:34.340 --> 00:12:36.200
There's so much sodium out
there, it's so positive out

00:12:36.200 --> 00:12:37.910
there, they just want
to come in.

00:12:37.910 --> 00:12:41.350
So as soon as they hit that
threshold, even though this

00:12:41.350 --> 00:12:44.910
might've only gotten us to minus
55 or maybe minus 50,

00:12:44.910 --> 00:12:47.440
all of a sudden that gate opens
and we have all of this

00:12:47.440 --> 00:12:49.170
positive charge flooding
into the cell.

00:12:49.170 --> 00:12:50.480
So the potential difference
becomes

00:12:50.480 --> 00:12:51.800
much, much more positive.

00:12:51.800 --> 00:12:54.810


00:12:54.810 --> 00:12:57.380
So they keep flooding in,
becomes much, much more

00:12:57.380 --> 00:13:00.220
positive, but as it gets
more positive, it

00:13:00.220 --> 00:13:15.340
closes at plus 35 millvolts.

00:13:15.340 --> 00:13:17.880
So let's say that we're dealing
here-- let's say that

00:13:17.880 --> 00:13:21.310
this up here is plus
35 millvolts.

00:13:21.310 --> 00:13:24.720
So here it closes and at the
same time, that stuff I just

00:13:24.720 --> 00:13:30.550
deleted-- I set at plus 40
millvolts-- or let's say at

00:13:30.550 --> 00:13:32.320
plus 35, just for the
sake of argument.

00:13:32.320 --> 00:13:34.840
Let's say at plus 45
millvolts, our

00:13:34.840 --> 00:13:39.520
sodium gates open.

00:13:39.520 --> 00:13:40.650
So what's happened here?

00:13:40.650 --> 00:13:43.990
All of a sudden, we're at plus
35 or maybe plus 40 millivolts

00:13:43.990 --> 00:13:47.430
so this is-- let's just say plus
40, I think you get the

00:13:47.430 --> 00:13:51.840
idea either way so we'll say
plus 40-- either way.

00:13:51.840 --> 00:13:56.100
So at plus 40, this guy's
going to close.

00:13:56.100 --> 00:13:58.640
No more positive ions are coming
in, but now we are at

00:13:58.640 --> 00:14:01.340
more positive inside, at least
locally at this point on the

00:14:01.340 --> 00:14:03.070
membrane, than we are outside.

00:14:03.070 --> 00:14:05.720
And so this gate will open.

00:14:05.720 --> 00:14:08.220
So then our sodium
gate will open.

00:14:08.220 --> 00:14:11.320
K-plus ion gate opens.

00:14:11.320 --> 00:14:12.720
Now when that opens,
what happens?

00:14:12.720 --> 00:14:15.800
We have all of these
sodium ions here.

00:14:15.800 --> 00:14:19.370
We already saw from the sodium
potassium pump that the

00:14:19.370 --> 00:14:22.510
potassium-- we have all of these
potassium ions here.

00:14:22.510 --> 00:14:25.150
We saw from the sodium potassium
pump that it makes

00:14:25.150 --> 00:14:27.280
the sodium concentration on
the outside higher and the

00:14:27.280 --> 00:14:30.300
potassium concentration
on the inside higher.

00:14:30.300 --> 00:14:34.330
And now that we've gotten to
this plus 40 millvolt range,

00:14:34.330 --> 00:14:37.140
we're also now more positive
on the inside.

00:14:37.140 --> 00:14:38.360
So this opens.

00:14:38.360 --> 00:14:40.320
These guys want to escape
because there's

00:14:40.320 --> 00:14:42.200
less potassium outside.

00:14:42.200 --> 00:14:44.290
They want to go down their
concentration gradient.

00:14:44.290 --> 00:14:45.940
It's also very positive
on the inside.

00:14:45.940 --> 00:14:47.970
We're at plus 40 millvolts.

00:14:47.970 --> 00:14:49.480
So they also want to escape.

00:14:49.480 --> 00:14:51.500
So they start escaping
the cells.

00:14:51.500 --> 00:14:54.350
So positive charges starts
exiting the cell from the

00:14:54.350 --> 00:14:56.210
inside to the outside.

00:14:56.210 --> 00:14:59.920
So we become less
positive again.

00:14:59.920 --> 00:15:01.550
So let me write what
happens here.

00:15:01.550 --> 00:15:07.530
So at this point, our sodium
gate closes and our potassium

00:15:07.530 --> 00:15:09.140
gate opens.

00:15:09.140 --> 00:15:11.920


00:15:11.920 --> 00:15:14.250
And then the positive charge
starts flooding out of the

00:15:14.250 --> 00:15:18.300
cell again and maybe it'll
overshoot because it's only

00:15:18.300 --> 00:15:21.670
going to close maybe once we
get to minus 80 millvolts.

00:15:21.670 --> 00:15:30.660
So maybe our potassium gate
closes at minus 80.

00:15:30.660 --> 00:15:35.340
And then our sodium potassium
pump might get us back to our

00:15:35.340 --> 00:15:36.550
minus 70 millvolts.

00:15:36.550 --> 00:15:40.940
So, this is what's happening
just at this point in the

00:15:40.940 --> 00:15:44.720
cell, just near that
first sodium gate.

00:15:44.720 --> 00:15:47.040
But what's going to happen
in general, right?

00:15:47.040 --> 00:15:49.680
As this became very positive--
we went to 40

00:15:49.680 --> 00:15:51.340
millivolts over here.

00:15:51.340 --> 00:15:54.490
We went to 40 millvolts in
this area of the cell.

00:15:54.490 --> 00:15:56.970
Because of-- I guess you could
almost view it as a short term

00:15:56.970 --> 00:16:00.120
or very short distance
electrotonic potential, this

00:16:00.120 --> 00:16:02.760
area is going to become
more positive, right?

00:16:02.760 --> 00:16:03.910
This is going to become
more positive.

00:16:03.910 --> 00:16:05.300
These positive charges
are going to go

00:16:05.300 --> 00:16:06.970
where it's less positive.

00:16:06.970 --> 00:16:09.040
So this is going to become
more positive.

00:16:09.040 --> 00:16:11.970
This was at minus 70, but it's
going to become more positive.

00:16:11.970 --> 00:16:18.200
It'll go to minus 65, minus 60,
minus 55-- and then bam.

00:16:18.200 --> 00:16:19.780
This guy will get
triggered again.

00:16:19.780 --> 00:16:22.060
Then this guy gets opened.

00:16:22.060 --> 00:16:23.440
Then this guy gets opened.

00:16:23.440 --> 00:16:25.430
Sodium floods in through here.

00:16:25.430 --> 00:16:27.840
So if you wanted to plot this
guy's, the potential

00:16:27.840 --> 00:16:33.240
difference of what's going on
across this, this all happened

00:16:33.240 --> 00:16:36.630
as soon as-- maybe as soon as a
sodium started going in this

00:16:36.630 --> 00:16:41.440
first dude, the second guy-- he
gets triggered here because

00:16:41.440 --> 00:16:45.640
the second guy a little bit
later in time-- because of all

00:16:45.640 --> 00:16:47.700
this flow a little bit to
the left of him, his

00:16:47.700 --> 00:16:48.540
potential goes up.

00:16:48.540 --> 00:16:53.230
He gets triggered, same exact
thing happens to him, right?

00:16:53.230 --> 00:16:56.300
When the sodium flows in here,
becomes really positive around

00:16:56.300 --> 00:16:59.630
here, that makes the cell
around here, the voltage

00:16:59.630 --> 00:17:01.180
around here, the charge around
here a little bit more

00:17:01.180 --> 00:17:04.930
positive, triggers this next
sodium gate to open and then

00:17:04.930 --> 00:17:07.280
this whole same thing
happens, same cycle.

00:17:07.280 --> 00:17:10.660
Then the potassium gates open to
make it negative again, but

00:17:10.660 --> 00:17:12.960
by the time that's happened,
it's become positive over here

00:17:12.960 --> 00:17:14.630
to trigger another
sodium gate.

00:17:14.630 --> 00:17:18.069
So one after another, you have
these sodium gates opening and

00:17:18.069 --> 00:17:20.839
closing, but it's transmitting
that information, it's

00:17:20.839 --> 00:17:23.270
transmitting that potential
change.

00:17:23.270 --> 00:17:25.130
So what's going on here?

00:17:25.130 --> 00:17:27.990
So this is slower and it
actually involves energy.

00:17:27.990 --> 00:17:32.200
So this was-- the electrotonic
was very fast. This is slow.

00:17:32.200 --> 00:17:33.820
An action potential is slower.

00:17:33.820 --> 00:17:35.350
I don't want to say it's slow.

00:17:35.350 --> 00:17:38.400
It's slower because it has to
involve these opening and

00:17:38.400 --> 00:17:41.420
closing of gates and it
also involves energy.

00:17:41.420 --> 00:17:43.275
It also requires more energy.

00:17:43.275 --> 00:17:47.880


00:17:47.880 --> 00:17:50.440
And you're also going to have to
keep changing the potential

00:17:50.440 --> 00:17:54.130
in your cell and you actively
have your sodium potassium

00:17:54.130 --> 00:17:55.700
pumps being very active.

00:17:55.700 --> 00:17:56.770
But it's good.

00:17:56.770 --> 00:17:59.480
The positive is, it's good
at covering distance.

00:17:59.480 --> 00:18:02.330


00:18:02.330 --> 00:18:03.820
When you have something like
this-- we saw with the

00:18:03.820 --> 00:18:06.160
electrotonic, as we get further
and further away from

00:18:06.160 --> 00:18:09.120
where the stimulus happened,
the change in potential

00:18:09.120 --> 00:18:10.325
becomes more and more
dissipated.

00:18:10.325 --> 00:18:12.080
It actually exponentially
declines.

00:18:12.080 --> 00:18:14.440
It becomes more and more
dissipated as we get further

00:18:14.440 --> 00:18:17.030
and further away so it's not
good for long distance.

00:18:17.030 --> 00:18:20.600
This thing can just continue
forever because every time it

00:18:20.600 --> 00:18:23.170
stimulates the next gate, it's
like we're starting all over

00:18:23.170 --> 00:18:26.940
again and so this gate-- it's
going to have a flood of ions

00:18:26.940 --> 00:18:30.550
come in and those ions are going
to make it a little less

00:18:30.550 --> 00:18:31.780
negative over here.

00:18:31.780 --> 00:18:33.110
Then the next gate's
going to open.

00:18:33.110 --> 00:18:34.750
We're going to have the cycle
over and over again.

00:18:34.750 --> 00:18:38.210
So this is really good for
traveling long distances.

00:18:38.210 --> 00:18:40.850
So now we have really the
foundation to understand

00:18:40.850 --> 00:18:43.880
exactly what's happening in a
neuron and I'm going to go

00:18:43.880 --> 00:18:46.330
over that in the next video to
show you how electrotonic

00:18:46.330 --> 00:18:49.820
potentials and action potentials
can combine to have

00:18:49.820 --> 00:18:51.970
a signal travel through
a neuron.

00:18:51.970 --> 00:18:53.220