Electrotonic and Action Potentials

WEBVTT
Kind: captions
Language: en

00:00:00.330 --> 00:00:03.150
In the last video, we talked
about how the cell uses a

00:00:03.150 --> 00:00:08.280
sodium potassium pump and ATP
to maintain its potential

00:00:08.280 --> 00:00:11.110
difference between the inside
of the cell or the inside of

00:00:11.110 --> 00:00:13.680
the neuron and the outside-- and
in general, the outside is

00:00:13.680 --> 00:00:15.330
more positive than the inside.

00:00:15.330 --> 00:00:19.290
You have a -70 millivolt
potential difference from the

00:00:19.290 --> 00:00:21.000
inside to the outside.

00:00:21.000 --> 00:00:25.340
It's minus because the outside
is more positive.

00:00:25.340 --> 00:00:27.480
Less positive minus more
positive, you're going to get

00:00:27.480 --> 00:00:29.730
a negative number
and it's by -70.

00:00:29.730 --> 00:00:32.880
Now, I said that this was the
foundation for understanding

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

00:00:35.690 --> 00:00:38.730
And to understand that, I'll
kind of lay a foundation over

00:00:38.730 --> 00:00:39.490
that foundation.

00:00:39.490 --> 00:00:42.710
I think then just the actual
neuron transmission will make

00:00:42.710 --> 00:00:43.760
a lot of sense.

00:00:43.760 --> 00:00:45.910
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

00:00:49.070 --> 00:00:50.120
have all of these dendrites.

00:00:50.120 --> 00:00:52.390
Hopefully it'll all
fit together.

00:00:52.390 --> 00:00:55.250
So there are two types of
ways that kind of a

00:00:55.250 --> 00:00:57.160
potential can travel.

00:00:57.160 --> 00:00:59.290
So there's two types
of signal transfer.

00:00:59.290 --> 00:01:02.290
I'll just call it
signal transfer.

00:01:02.290 --> 00:01:04.260
I don't know what the
best word for it is.

00:01:04.260 --> 00:01:06.710
The first one I'll talk
is electrotonic.

00:01:06.710 --> 00:01:08.190
It sounds very fancy,
but you'll see it's

00:01:08.190 --> 00:01:09.440
a very simple idea.

00:01:21.020 --> 00:01:22.670
And the other one I'm
going to go over

00:01:22.670 --> 00:01:25.350
is an action potential.

00:01:25.350 --> 00:01:27.770
And they both have their own
positives and negatives in

00:01:27.770 --> 00:01:32.760
terms of being able to
transmit a signal.

00:01:32.760 --> 00:01:36.360
We're talking about within the
context of in a cell or across

00:01:36.360 --> 00:01:38.270
a cell membrane.

00:01:38.270 --> 00:01:39.940
Let's understand what
these mean.

00:01:39.940 --> 00:01:41.780
So let me get my membrane
of a cell.

00:01:41.780 --> 00:01:46.310
Let's say it's a nerve cell or
a neuron, just to make it all

00:01:46.310 --> 00:01:49.130
fit together in this context.

00:01:49.130 --> 00:01:51.050
And we know it's more
positive on the

00:01:51.050 --> 00:01:52.300
outside than the inside.

00:01:52.300 --> 00:01:55.520
We know that there's a lot of
sodium on the outside or a lot

00:01:55.520 --> 00:01:58.280
more sodium on the outside
than on the inside.

00:01:58.280 --> 00:01:59.320
There might be a little bit.

00:01:59.320 --> 00:02:05.660
And we know there's a lot more
potassium on the inside than

00:02:05.660 --> 00:02:08.180
the outside, but we know
generally that the outside is

00:02:08.180 --> 00:02:11.070
more positive then the inside
because our sodium potassium

00:02:11.070 --> 00:02:13.630
pump will pump out three
sodiums for every two

00:02:13.630 --> 00:02:15.430
potassiums it takes in.

00:02:15.430 --> 00:02:18.060
Now in the last video, I told
you that there are these

00:02:18.060 --> 00:02:21.485
things called-- well, we could
call them a sodium gate.

00:02:21.485 --> 00:02:23.810
A sodium ion gate, right?

00:02:23.810 --> 00:02:24.430
These are all ions.

00:02:24.430 --> 00:02:25.990
They're charged.

00:02:25.990 --> 00:02:28.420
Now let's say that there's some
reason, some stimulus--

00:02:28.420 --> 00:02:29.670
let me label this.

00:02:35.430 --> 00:02:39.990
That right there is my
sodium ion gate.

00:02:39.990 --> 00:02:41.660
And it's in its closed position,
but let's say

00:02:41.660 --> 00:02:43.480
something causes it to open.

00:02:43.480 --> 00:02:45.700
We'll talk maybe in this video
or maybe this video and the

00:02:45.700 --> 00:02:47.330
next about the different
things that

00:02:47.330 --> 00:02:48.200
could cause it to open.

00:02:48.200 --> 00:02:51.100
Maybe it's some type of stimulus
causes this to open.

00:02:51.100 --> 00:02:52.910
Actually, there's a whole bunch
of different stimuluses

00:02:52.910 --> 00:02:55.480
that would cause it to open.

00:02:55.480 --> 00:02:56.370
But let's say it opens.

00:02:56.370 --> 00:03:00.560
What's going to happen
if it opens?

00:03:00.560 --> 00:03:02.560
So let's say we open it.

00:03:02.560 --> 00:03:04.790
Some stimulus opens-- what's
going to happen?

00:03:04.790 --> 00:03:06.880
We have more positive on the
outside than the inside, so

00:03:06.880 --> 00:03:08.460
positive things want
to move in.

00:03:08.460 --> 00:03:12.400
And this is a sodium gate so
only sodium can go through it.

00:03:12.400 --> 00:03:14.960
So it's kind of a convoluted
protein structure that only

00:03:14.960 --> 00:03:17.000
sodium can make its
way through.

00:03:17.000 --> 00:03:19.840
And on top of that, we have a
lot more sodium on the outside

00:03:19.840 --> 00:03:20.510
than on the inside.

00:03:20.510 --> 00:03:22.580
So the diffusion gradient's
going to want to make sodium

00:03:22.580 --> 00:03:23.320
go through it.

00:03:23.320 --> 00:03:26.650
And the fact that sodium's a
positive ion, the outside is

00:03:26.650 --> 00:03:28.270
more positive, they're going to
want to run away from that

00:03:28.270 --> 00:03:30.340
positive environment.

00:03:30.340 --> 00:03:32.810
So if you open this, you're just
going to have a lot of

00:03:32.810 --> 00:03:35.040
sodium ions start to
flood through.

00:03:40.250 --> 00:03:43.190
Now as that happens, what's
going to happen if we go

00:03:43.190 --> 00:03:44.500
further down the membrane?

00:03:44.500 --> 00:03:47.110
Let's zoom out.

00:03:47.110 --> 00:03:50.650
So let's say that this is
my membrane right there.

00:03:50.650 --> 00:03:54.160
Let's say that this is my open
gate right here and that it's

00:03:54.160 --> 00:03:56.990
open for some reason and a bunch
of sodium is flowing in.

00:03:56.990 --> 00:04:01.050
So all of this is becoming
much more positive.

00:04:01.050 --> 00:04:04.980
Let's say we had a voltmeter
right here.

00:04:04.980 --> 00:04:07.230
We're measuring the potential
difference between the inside

00:04:07.230 --> 00:04:10.420
of the membrane a
and the outside.

00:04:10.420 --> 00:04:13.580
Let me do a little chart.

00:04:13.580 --> 00:04:17.190
I'm going to do the chart
here on my voltmeter.

00:04:17.190 --> 00:04:20.579
And this is going to be the
potential difference-- or

00:04:20.579 --> 00:04:26.750
we'll call it the membrane
voltage or the voltage

00:04:26.750 --> 00:04:28.000
difference across the
membrane-- and

00:04:28.000 --> 00:04:31.150
let's say this is time.

00:04:31.150 --> 00:04:33.460
Let's say I haven't opened
this gate yet.

00:04:33.460 --> 00:04:35.030
So it's in its resting state.

00:04:35.030 --> 00:04:37.100
Our sodium potassium
pumps are working.

00:04:37.100 --> 00:04:39.090
Things are leaking back and
forth, but it's staying at

00:04:39.090 --> 00:04:41.790
that minus 70 millivolts.

00:04:41.790 --> 00:04:46.770
So that right there is
minus 70 millivolts.

00:04:46.770 --> 00:04:51.110
Now as soon as this gate that's
way down some other

00:04:51.110 --> 00:04:53.770
part of the cell opens, what's
going to happen?

00:04:53.770 --> 00:04:55.910
And let's say that's the
only thing that's open.

00:04:55.910 --> 00:04:58.370
So this, all of a sudden, is
going to become more positive.

00:04:58.370 --> 00:05:01.180
So positive charges that's
already here-- so other

00:05:01.180 --> 00:05:07.480
positive charges, whether
they're sodiums or potassiums,

00:05:07.480 --> 00:05:09.260
they're going to want to run
away from that point because

00:05:09.260 --> 00:05:11.480
this area hasn't had a flood
of positive things.

00:05:11.480 --> 00:05:13.840
So it's less positive
than this over here.

00:05:13.840 --> 00:05:16.670
So maybe we have some potassiums
and maybe we have

00:05:16.670 --> 00:05:19.960
some sodiums. Everything is
going to want to move away

00:05:19.960 --> 00:05:23.310
from the place where
this is opened.

00:05:23.310 --> 00:05:25.590
The charge is going to
want to move away.

00:05:25.590 --> 00:05:28.910
So as soon as this happens, as
soon as we open this gate,

00:05:28.910 --> 00:05:30.460
we're going to have a
movement of positive

00:05:30.460 --> 00:05:31.230
charge in this direction.

00:05:31.230 --> 00:05:34.830
So all of a sudden-- this was
at minus 70 millivolts.

00:05:34.830 --> 00:05:37.240
So more positive charge
is coming its way.

00:05:41.680 --> 00:05:45.120
Almost immediately, it's going
to become less negative or

00:05:45.120 --> 00:05:46.470
more positive.

00:05:46.470 --> 00:05:48.590
The potential difference between
this and this is going

00:05:48.590 --> 00:05:49.800
to become less.

00:05:49.800 --> 00:05:51.980
So this is this point
over here.

00:05:51.980 --> 00:05:57.900
Now if we took this point, if we
did the same thing-- if we

00:05:57.900 --> 00:06:01.120
measured the voltage at this
point right here, maybe it was

00:06:01.120 --> 00:06:06.150
at minus 70 millvolts, maybe a
fraction of a minute amount of

00:06:06.150 --> 00:06:09.560
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?

00:06:13.020 --> 00:06:14.660
Because these positive charges,
they're going to

00:06:14.660 --> 00:06:16.240
radiate in every direction.

00:06:16.240 --> 00:06:17.410
So the effect is diluted.

00:06:17.410 --> 00:06:20.130
So the effect on this thing
is going to be less.

00:06:20.130 --> 00:06:21.860
It's going to become
less positive.

00:06:21.860 --> 00:06:26.160
So an electrotonic potential--
what happens is at one point

00:06:26.160 --> 00:06:30.390
in the cell, a gate opens,
charge starts flooding in, and

00:06:30.390 --> 00:06:32.460
it starts affecting the
potential at other

00:06:32.460 --> 00:06:33.630
parts of the cell.

00:06:33.630 --> 00:06:40.970
But the positive of it is, it's
very fast. As soon as

00:06:40.970 --> 00:06:41.820
this happens.

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

00:06:50.000 --> 00:07:00.820
positive, but the further you
go, the effect gets dissipated

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

00:07:02.570 --> 00:07:05.050
So if you care about speed,
you'd want this

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

00:07:06.240 --> 00:07:08.620
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

00:07:14.660 --> 00:07:18.030
got all the way to this point of
the neuron and we wanted to

00:07:18.030 --> 00:07:20.210
measure it, it might not
have any impact.

00:07:20.210 --> 00:07:22.410
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.

00:07:26.960 --> 00:07:30.200
So it's a impact far away from
the initial place where the

00:07:30.200 --> 00:07:30.750
gate opened.

00:07:30.750 --> 00:07:32.240
It's going to be a lot less.

00:07:32.240 --> 00:07:36.110
So it's really not good for
operating over distance.

00:07:36.110 --> 00:07:38.040
Now let's try to figure
out what's going on

00:07:38.040 --> 00:07:40.080
with an action potential.

00:07:40.080 --> 00:07:43.310
And you might understand, this
might involve more action.

00:07:43.310 --> 00:07:45.510
So let's start off with
the same situation.

00:07:45.510 --> 00:07:51.500
We have a sodium gate that gets
opened by some stimulus.

00:07:51.500 --> 00:07:54.730
What I'm going to do-- let me
draw two membranes here.

00:07:54.730 --> 00:07:55.980
So this is the outside.

00:07:59.500 --> 00:08:02.440
This is the inside.

00:08:02.440 --> 00:08:05.680
And let me draw-- maybe we're
dealing with a-- and we'll go

00:08:05.680 --> 00:08:06.270
in more detail.

00:08:06.270 --> 00:08:11.870
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.

00:08:18.810 --> 00:08:21.700
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.

00:08:38.520 --> 00:08:39.980
I don't want to do
a bunch of these.

00:08:39.980 --> 00:08:42.980
I think I just have to draw one
round of it for you to get

00:08:42.980 --> 00:08:44.730
what's going on.

00:08:44.730 --> 00:08:46.350
Let me draw another
potassium gate.

00:08:52.140 --> 00:08:54.290
And let's say that they
all start closed.

00:08:54.290 --> 00:08:56.240
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.

00:09:03.220 --> 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

00:09:26.170 --> 00:09:27.420
millivolts.

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.

00:10:36.210 --> 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.

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

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.

00:12:54.810 --> 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.

00:15:11.920 --> 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.

00:17:47.880 --> 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.

00:18:02.330 --> 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.


Sodium-potassium pump

WEBVTT
Kind: captions
Language: en

00:00:00.580 --> 00:00:03.360
In the last video, I showed you
what a neuron looked like

00:00:03.360 --> 00:00:05.790
and we talked about the
different parts of a neuron,

00:00:05.790 --> 00:00:08.480
and I gave you the general
idea what a neuron does.

00:00:08.480 --> 00:00:11.270
It gets stimulated at the
dendrites-- and the

00:00:11.270 --> 00:00:13.730
stimulation we'll talk about
in future videos on what

00:00:13.730 --> 00:00:16.480
exactly that means-- and
that that impulse, that

00:00:16.480 --> 00:00:19.450
information, that signal
gets added up.

00:00:19.450 --> 00:00:22.480
If there's multiple stimulation
points on various

00:00:22.480 --> 00:00:25.660
dendrites, it gets added up and
if it meets some threshold

00:00:25.660 --> 00:00:28.310
level, it's going to create
this action potential or

00:00:28.310 --> 00:00:34.380
signal that travels across the
axon and maybe stimulates

00:00:34.380 --> 00:00:38.550
other neurons or muscles because
these terminal points

00:00:38.550 --> 00:00:41.280
of the axons might be connected
to dendrites of

00:00:41.280 --> 00:00:44.550
other neurons or to muscle
cells or who knows what.

00:00:44.550 --> 00:00:46.690
But what I want to do in this
video is kind of lay the

00:00:46.690 --> 00:00:50.100
building blocks for exactly
what this signal is or how

00:00:50.100 --> 00:00:54.350
does a neuron actually transmit
this information

00:00:54.350 --> 00:00:56.980
across the axon-- or really,
how does it go from the

00:00:56.980 --> 00:00:59.630
dendrite all the way
to the axon?

00:00:59.630 --> 00:01:01.960
Before I actually even talk
about that, we need to kind of

00:01:01.960 --> 00:01:07.400
lay the ground rules-- or a
ground understanding of the

00:01:07.400 --> 00:01:10.120
actual voltage potential
across the

00:01:10.120 --> 00:01:11.060
membrane of a neuron.

00:01:11.060 --> 00:01:14.070
And, actually, all cells have
some voltage potential

00:01:14.070 --> 00:01:15.660
difference, but it's especially
relevant when we

00:01:15.660 --> 00:01:18.110
talk about a neuron and its
ability to send signals.

00:01:18.110 --> 00:01:20.745
Let's zoom in on a
neuron's cell.

00:01:25.580 --> 00:01:27.870
I could zoom in on any point
on this cell that's not

00:01:27.870 --> 00:01:30.410
covered by a myelin sheath.

00:01:30.410 --> 00:01:32.520
I'm going to zoom in
on its membrane.

00:01:32.520 --> 00:01:39.470
So let's say that this is
the membrane of the

00:01:39.470 --> 00:01:43.260
neuron, just like that.

00:01:43.260 --> 00:01:45.060
That's the membrane.

00:01:45.060 --> 00:01:50.970
This is outside the neuron
or the cell.

00:01:50.970 --> 00:01:56.730
And then this is inside the
neuron or the cell.

00:01:56.730 --> 00:01:59.220
Now, you have sodium
and potassium

00:01:59.220 --> 00:02:00.370
ions floating around.

00:02:00.370 --> 00:02:01.920
I'm going to draw sodium
like this.

00:02:01.920 --> 00:02:03.900
Sodium's going to be a circle.

00:02:03.900 --> 00:02:06.770
So that's sodium and their
positively charged ions have a

00:02:06.770 --> 00:02:09.840
plus one charge and then
potassium, I'll draw them as

00:02:09.840 --> 00:02:11.340
little triangles.

00:02:11.340 --> 00:02:13.150
So let's say that's potassium--
symbol for

00:02:13.150 --> 00:02:14.320
potassium is K.

00:02:14.320 --> 00:02:16.190
It's also positively charged.

00:02:16.190 --> 00:02:17.710
And you have them just
lying around.

00:02:17.710 --> 00:02:19.340
Let's say we start off
both inside and

00:02:19.340 --> 00:02:21.230
outside of the cell.

00:02:38.550 --> 00:02:40.910
They're all positively
charged.

00:02:40.910 --> 00:02:43.800
Sodium inside, some
sodium outside.

00:02:43.800 --> 00:02:48.010
Now it turns out that cells
have more positive charge

00:02:48.010 --> 00:02:50.130
outside of their membranes than

00:02:50.130 --> 00:02:51.410
inside of their membranes.

00:02:51.410 --> 00:02:53.830
So there's actually a potential
difference that if

00:02:53.830 --> 00:02:56.450
the membrane wasn't there,
negative charges would want to

00:02:56.450 --> 00:02:59.380
escape or positive charges
or positive ions

00:02:59.380 --> 00:03:01.130
would want to get in.

00:03:01.130 --> 00:03:03.620
The outside ends up being more
positive, and we're going to

00:03:03.620 --> 00:03:04.870
talk about why.

00:03:10.140 --> 00:03:14.480
So this is an electrical
potential gradient, right?

00:03:14.480 --> 00:03:18.300
If this is less positive than
that-- if I have a positive

00:03:18.300 --> 00:03:19.790
charge here, it's going to
want to go to the less

00:03:19.790 --> 00:03:20.540
positive side.

00:03:20.540 --> 00:03:21.820
It's going to want to
go away from the

00:03:21.820 --> 00:03:22.870
other positive charges.

00:03:22.870 --> 00:03:25.310
It's repelled by the other
positive charges.

00:03:25.310 --> 00:03:27.610
Likewise, if I had a negative
charge here, it'd want to go

00:03:27.610 --> 00:03:30.290
the other side-- or a positive
charge, I guess, would be

00:03:30.290 --> 00:03:32.530
happier being here
than over here.

00:03:32.530 --> 00:03:34.160
But the question is, how
does that happen?

00:03:34.160 --> 00:03:36.570
Because left to their own
devices, the charges would

00:03:36.570 --> 00:03:38.840
disperse so you wouldn't have
this potential gradient.

00:03:38.840 --> 00:03:41.910
Somehow we have to put energy
into the system in order to

00:03:41.910 --> 00:03:46.860
produce this state where we
have more positive on the

00:03:46.860 --> 00:03:49.070
charge of the outside than
we do on the inside.

00:03:49.070 --> 00:03:52.530
And that's done by sodium
potassium pumps.

00:03:52.530 --> 00:03:55.560
I'm going to draw then
a certain way.

00:03:55.560 --> 00:03:57.810
This is obviously not how the
protein actually looks, but

00:03:57.810 --> 00:04:00.870
it'll give you a sense of how it
actually pumps things out.

00:04:03.700 --> 00:04:05.230
I'll draw that side
of the protein.

00:04:05.230 --> 00:04:08.760
Maybe it looks like this and
you'll have a sense of why I

00:04:08.760 --> 00:04:10.050
drew it like this.

00:04:10.050 --> 00:04:14.010
So that side of the protein or
the enzyme-- and then the

00:04:14.010 --> 00:04:15.640
other side, I'll draw
it like this.

00:04:15.640 --> 00:04:18.380
It looks something like this,
and of course the real protein

00:04:18.380 --> 00:04:19.250
doesn't look like this.

00:04:19.250 --> 00:04:22.510
You've seen me show you what
proteins really look like.

00:04:22.510 --> 00:04:25.620
They look like big clusters
of things, hugely complex.

00:04:25.620 --> 00:04:27.570
Different parts of the proteins
can bond to different

00:04:27.570 --> 00:04:31.150
things and when things bond to
proteins, they change shape.

00:04:31.150 --> 00:04:33.565
But I'm doing a very simple
diagram here and what I want

00:04:33.565 --> 00:04:37.170
to show you is, this is our
sodium potassium pump in its

00:04:37.170 --> 00:04:38.580
inactivated state.

00:04:38.580 --> 00:04:40.680
And what happens in this
situation is that we have

00:04:40.680 --> 00:04:44.290
these nice places where our
sodium can bind to.

00:04:44.290 --> 00:04:49.940
So in this situation, sodium can
bind to these locations on

00:04:49.940 --> 00:04:52.320
our enzyme or on our protein.

00:04:52.320 --> 00:04:54.470
And if we just had the sodiums
bind and we didn't have any

00:04:54.470 --> 00:04:56.070
energy going into the system,
nothing would happen.

00:04:56.070 --> 00:04:58.270
It would just stay in
this situation.

00:04:58.270 --> 00:05:01.650
The actual protein might look
like something crazy.

00:05:01.650 --> 00:05:05.290
The actual protein might be this
big cloud of protein and

00:05:05.290 --> 00:05:09.200
then your sodiums bond there,
there, and there.

00:05:09.200 --> 00:05:11.940
Maybe it's inside the protein
somehow, but still, nothing's

00:05:11.940 --> 00:05:15.500
going to happen just when the
sodium bonds on this side of

00:05:15.500 --> 00:05:16.670
the protein.

00:05:16.670 --> 00:05:18.730
In order for it to do anything,
in order for it to

00:05:18.730 --> 00:05:24.490
pump anything out, it uses
the energy from ATP.

00:05:24.490 --> 00:05:27.580
So we had all those videos on
respiration and I told you

00:05:27.580 --> 00:05:30.530
that ATP was the currency of
energy in the cell-- well,

00:05:30.530 --> 00:05:33.030
this is something useful
for ATP to do.

00:05:33.030 --> 00:05:38.280
ATP-- that's adenosine
triphosphate-- it might go to

00:05:38.280 --> 00:05:42.160
some other part of our enzyme,
but in this diagram maybe it

00:05:42.160 --> 00:05:44.220
goes to this part
of the enzyme.

00:05:47.170 --> 00:05:50.390
And this enzyme, it's
a type of ATPase.

00:05:50.390 --> 00:05:54.760
When I say ATPase, it breaks off
a phosphate from the ATP--

00:05:54.760 --> 00:05:56.410
and that's just by virtue
of its shape.

00:05:56.410 --> 00:05:57.910
It's able to plunk it off.

00:05:57.910 --> 00:06:01.270
When it plunks off the
phosphate, it changes shape.

00:06:06.320 --> 00:06:11.510
So step one, we have sodium
ions-- and actually, let's

00:06:11.510 --> 00:06:13.070
keep count of them.

00:06:13.070 --> 00:06:16.310
We have three sodium-- these are
the actual ratios-- three

00:06:16.310 --> 00:06:23.570
sodium ions from inside the
cell or the neuron.

00:06:23.570 --> 00:06:29.330
They bond to pump, which is
really a protein that crosses

00:06:29.330 --> 00:06:30.920
our membrane.

00:06:30.920 --> 00:06:34.590
Now, step two, we
have also ATP.

00:06:34.590 --> 00:06:41.040
ATP gets broken into ADP plus
phosphate on the actual

00:06:41.040 --> 00:06:43.350
protein and that changes
the shape.

00:06:43.350 --> 00:06:51.635
So that also provides energy
to change pump's shape.

00:06:55.780 --> 00:06:57.430
Now this is when the
pump was before.

00:06:57.430 --> 00:07:01.920
Now after, our pump might look
something like this.

00:07:01.920 --> 00:07:03.570
Let me clear out some
space right here.

00:07:03.570 --> 00:07:05.200
I'll draw the after
pump right there.

00:07:07.860 --> 00:07:09.400
And so this is before.

00:07:09.400 --> 00:07:13.460
After the phosphate gets split
off of the ATP, it might look

00:07:13.460 --> 00:07:14.830
something like this.

00:07:14.830 --> 00:07:18.360
Instead of being in that
configuration, it opens in the

00:07:18.360 --> 00:07:19.130
other direction.

00:07:19.130 --> 00:07:21.420
So now it might look something
like this.

00:07:24.170 --> 00:07:27.500
And of course it's carrying
these phosphate groups.

00:07:27.500 --> 00:07:28.775
They have a positive charge.

00:07:31.470 --> 00:07:32.310
It's open like this.

00:07:32.310 --> 00:07:36.390
This side now looks like this.

00:07:36.390 --> 00:07:38.940
So now the phosphates are
released to the outside.

00:07:38.940 --> 00:07:40.300
So they've been pumped
to the outside.

00:07:40.300 --> 00:07:42.190
Remember, this is required
energy because it's going

00:07:42.190 --> 00:07:43.870
against the natural gradient.

00:07:43.870 --> 00:07:46.290
You're taking positive charge
and you're pushing them to an

00:07:46.290 --> 00:07:50.670
environment that is even more
positive and you're also

00:07:50.670 --> 00:07:53.740
taking it to an environment
where there's already a lot of

00:07:53.740 --> 00:07:55.625
sodium, and you're putting
more sodium there.

00:07:55.625 --> 00:07:58.370
So you're going against the
charge gradient and you're

00:07:58.370 --> 00:07:59.980
going against the
sodium gradient.

00:07:59.980 --> 00:08:03.550
But now-- I guess we call it
step three-- the sodium gets

00:08:03.550 --> 00:08:07.090
released outside the cell.

00:08:07.090 --> 00:08:10.350
And when this changes shape,
it's not so good at bonding

00:08:10.350 --> 00:08:11.490
with the sodium anymore.

00:08:11.490 --> 00:08:13.660
So maybe these can become a
little bit different too, so

00:08:13.660 --> 00:08:16.410
that the sodium can't even bond
in this configuration now

00:08:16.410 --> 00:08:20.080
that the protein has changed
shape due to the ATP.

00:08:20.080 --> 00:08:26.110
So step three, the three Na
plusses, sodium ions-- are

00:08:26.110 --> 00:08:28.590
released outside.

00:08:31.700 --> 00:08:33.760
Now once it's in this
configuration, we have all

00:08:33.760 --> 00:08:35.820
these positive ions out here.

00:08:35.820 --> 00:08:38.480
These positive ions want to get
really as far away from

00:08:38.480 --> 00:08:39.240
each other as possible.

00:08:39.240 --> 00:08:42.320
They'd actually probably be
attracted to the cell itself

00:08:42.320 --> 00:08:44.820
because the cell is less
positive on the inside.

00:08:44.820 --> 00:08:48.590
So these positive ions-- and in
particular, the potassium--

00:08:48.590 --> 00:08:52.170
can bond this side of the
protein when it's in this-- I

00:08:52.170 --> 00:08:55.550
guess we could call it this
activated configuration.

00:08:55.550 --> 00:08:58.570
So now, I guess we could
call it step four.

00:09:01.400 --> 00:09:07.260
We have two sodium ions bond
to-- I guess we could call it

00:09:07.260 --> 00:09:12.680
the activated pump--
or changed pump.

00:09:12.680 --> 00:09:15.320
Or maybe we could say it's
in its open form.

00:09:15.320 --> 00:09:18.930
So they come here and when they
bond, it re-changes the

00:09:18.930 --> 00:09:24.430
shape of this protein back
to this shape, back

00:09:24.430 --> 00:09:25.590
to that open shape.

00:09:25.590 --> 00:09:27.660
Now when it goes back to the
open shape, these guys aren't

00:09:27.660 --> 00:09:30.510
here anymore, but we have these
two guys sitting here

00:09:30.510 --> 00:09:33.930
and in this shape right here,
all of a sudden these divots--

00:09:33.930 --> 00:09:34.660
maybe they're not divots.

00:09:34.660 --> 00:09:37.860
They're actually things in this
big cluster of protein.

00:09:37.860 --> 00:09:42.230
They're not as good at staying
bonded or holding onto these

00:09:42.230 --> 00:09:47.290
sodiums so these sodiums get
released into the cell.

00:09:47.290 --> 00:09:53.310
So step five, the pump-- this
changes shape of pump.

00:09:53.310 --> 00:09:56.636
So pump changes shape
to original.

00:10:05.200 --> 00:10:09.220
And then once we're in the
original, those two sodium

00:10:09.220 --> 00:10:15.920
ions released inside the cell.

00:10:15.920 --> 00:10:17.990
We're going to see in the next
few videos why it's useful to

00:10:17.990 --> 00:10:19.690
have those sodium ions
on the inside.

00:10:19.690 --> 00:10:21.360
You might say, well, why don't
we just keep pumping things on

00:10:21.360 --> 00:10:23.420
the outside in order to have
a potential difference?

00:10:23.420 --> 00:10:24.500
But we'll see these
sodium ions are

00:10:24.500 --> 00:10:26.620
actually also very useful.

00:10:26.620 --> 00:10:28.870
So what's the net effect
that's going on?

00:10:28.870 --> 00:10:35.930
We end up with a lot more sodium
ions on the outside and

00:10:35.930 --> 00:10:40.420
we end up with more potassium
ions on the inside, but I told

00:10:40.420 --> 00:10:42.940
you that the inside is less
positive than the outside.

00:10:42.940 --> 00:10:44.300
But these are both positive.

00:10:44.300 --> 00:10:46.850
I don't care if I have more
potassium or sodium, but if

00:10:46.850 --> 00:10:50.760
you paid attention to the ratios
I talked about, every

00:10:50.760 --> 00:10:54.290
time we use an ATP, we're
pumping out three sodiums and

00:10:54.290 --> 00:10:58.860
we're only pumping in two
potassiums, right?

00:10:58.860 --> 00:11:02.880
We pumped out three sodiums and
two potassiums. Each of

00:11:02.880 --> 00:11:06.610
them have a plus-1 charge, but
every time we do this, we're

00:11:06.610 --> 00:11:08.960
adding a net-1 charge to
the outside, right?

00:11:08.960 --> 00:11:10.910
3 on the outside,
2 to the inside.

00:11:10.910 --> 00:11:14.560
We have a net-1 charge-- we have
a plus-1 to the outside.

00:11:14.560 --> 00:11:17.770
So we're making the outside
more positive, especially

00:11:17.770 --> 00:11:19.520
relative to the inside.

00:11:19.520 --> 00:11:21.840
And this is what creates that
potential difference.

00:11:21.840 --> 00:11:24.000
If you actually took a
voltmeter-- a voltmeter

00:11:24.000 --> 00:11:27.770
measures electrical potential
difference-- and you took the

00:11:27.770 --> 00:11:31.800
voltage difference between that
point and this point-- or

00:11:31.800 --> 00:11:34.440
more specifically, between this
point and that point, if

00:11:34.440 --> 00:11:37.310
you were to subtract the voltage
here from the voltage

00:11:37.310 --> 00:11:43.710
there, you will get -70
millivolts, which is generally

00:11:43.710 --> 00:11:46.650
considered the resting voltage
difference, the potential

00:11:46.650 --> 00:11:49.910
difference across the membrane
of a neuron when it's in its

00:11:49.910 --> 00:11:51.010
resting state.

00:11:51.010 --> 00:11:55.180
So in this video, I kind of laid
out the foundation of why

00:11:55.180 --> 00:11:59.190
and how a cell using ATP,
using energy, is able to

00:11:59.190 --> 00:12:02.290
maintain a potential difference
across its membrane

00:12:02.290 --> 00:12:06.490
where the outside is slightly
more positive than the inside.

00:12:06.490 --> 00:12:08.540
So we actually have a negative
potential difference if we're

00:12:08.540 --> 00:12:10.810
comparing the inside
to the outside.

00:12:10.810 --> 00:12:13.260
Positive charge would want to
move in if they were allowed

00:12:13.260 --> 00:12:15.880
to, and negative charge would
want to move out if it was

00:12:15.880 --> 00:12:17.030
allowed to.

00:12:17.030 --> 00:12:18.680
Now there might be one
last question.

00:12:18.680 --> 00:12:23.380
You might say, well, if we just
kept adding charge out

00:12:23.380 --> 00:12:25.930
here, our voltage difference
would get really negative.

00:12:25.930 --> 00:12:29.290
This would be much more negative
than the outside.

00:12:29.290 --> 00:12:32.030
Why does it stabilize at -70?

00:12:32.030 --> 00:12:34.240
To answer that question-- these
are going to come into

00:12:34.240 --> 00:12:37.550
play in a lot more detail in
future videos-- you also have

00:12:37.550 --> 00:12:42.530
channels, which are really
protein structures that in

00:12:42.530 --> 00:12:46.740
their open position will allow
sodium to go through them.

00:12:46.740 --> 00:12:48.550
And there are also channels
that are in their open

00:12:48.550 --> 00:12:51.120
position, would allow potassium
to go through them.

00:12:51.120 --> 00:12:53.020
I'm drawing it in their
closed position.

00:12:53.020 --> 00:12:54.890
And we're going to talk in the
next video about what happens

00:12:54.890 --> 00:12:55.940
when they open.

00:12:55.940 --> 00:12:57.700
But in their closed position,
they're still

00:12:57.700 --> 00:12:58.950
a little bit leaky.

00:13:03.530 --> 00:13:06.560
And if, say, the concentration
of potassium becomes too high

00:13:06.560 --> 00:13:11.530
down here-- and too high meaning
when they start to

00:13:11.530 --> 00:13:14.560
reach this threshold of -70
millivolts-- or even better,

00:13:14.560 --> 00:13:16.820
when the sodium gets too high
out there, a few of them will

00:13:16.820 --> 00:13:17.680
start to leak down.

00:13:17.680 --> 00:13:20.060
When the concentration gets
really high and this is really

00:13:20.060 --> 00:13:22.560
positive just because of the
electrical potential, some of

00:13:22.560 --> 00:13:23.990
them will just be
shoved through.

00:13:23.990 --> 00:13:26.930
So it'll keep us right around
-70 millivolts.

00:13:26.930 --> 00:13:29.790
And if we go below, maybe some
of the potassium gets leaked

00:13:29.790 --> 00:13:30.580
through the other way.

00:13:30.580 --> 00:13:33.240
So even though when these are
shut-- if it becomes too

00:13:33.240 --> 00:13:37.150
ridiculous-- if it goes to -80
millivolts or -90 millivolts,

00:13:37.150 --> 00:13:39.410
all of a sudden, there'd be a
huge incentive for some of

00:13:39.410 --> 00:13:42.700
this stuff to leak through their
respective channels.

00:13:42.700 --> 00:13:44.410
So that's what allows
us to stay at that

00:13:44.410 --> 00:13:46.550
stable voltage potential.

00:13:46.550 --> 00:13:48.500
In the next video, we're going
to see what happens to this

00:13:48.500 --> 00:13:52.400
voltage potential when the
neuron is actually stimulated.


Anatomy of a neuron

WEBVTT
Kind: captions
Language: en

00:00:00.610 --> 00:00:03.620
We could have a debate about
what the most interesting cell

00:00:03.620 --> 00:00:07.540
in the human body is, but I
think easily the neuron would

00:00:07.540 --> 00:00:10.810
make the top five, and it's
not just because the cell

00:00:10.810 --> 00:00:11.830
itself is interesting.

00:00:11.830 --> 00:00:14.640
The fact that it essentially
makes up our brain and our

00:00:14.640 --> 00:00:17.420
nervous system and is
responsible for the thoughts

00:00:17.420 --> 00:00:22.010
and our feelings and maybe for
all of our sentience, I think,

00:00:22.010 --> 00:00:24.625
would easily make it the
top one or two cells.

00:00:24.625 --> 00:00:26.920
So what I want to do is first
to show you what a neuron

00:00:26.920 --> 00:00:27.450
looks like.

00:00:27.450 --> 00:00:30.290
And, of course, this is kind
of the perfect example.

00:00:30.290 --> 00:00:31.670
This isn't what all
neurons look like.

00:00:31.670 --> 00:00:33.350
And then we're going to talk
a little bit about how it

00:00:33.350 --> 00:00:37.000
performs its function, which is
essentially communication,

00:00:37.000 --> 00:00:40.760
essentially transmitting signals
across its length,

00:00:40.760 --> 00:00:43.400
depending on the signals
it receives.

00:00:43.400 --> 00:00:45.470
So if I were to draw
a neuron-- let me

00:00:45.470 --> 00:00:46.720
pick a better color.

00:00:50.750 --> 00:00:52.060
So let's say I have a neuron.

00:00:52.060 --> 00:00:53.120
It looks something like this.

00:00:53.120 --> 00:00:57.780
So in the middle you have your
soma and then from the soma--

00:00:57.780 --> 00:01:00.850
let me draw the nucleus.

00:01:00.850 --> 00:01:03.390
This is a nucleus, just like
any cell's nucleus.

00:01:03.390 --> 00:01:07.190
And then the soma's considered
the body of the neuron and

00:01:07.190 --> 00:01:09.260
then the neuron has these little
things sticking out

00:01:09.260 --> 00:01:13.680
from it that keep
branching off.

00:01:13.680 --> 00:01:16.080
Maybe they look something
like this.

00:01:16.080 --> 00:01:18.910
I don't want to spend too much
time just drawing the neuron,

00:01:18.910 --> 00:01:23.700
but you've probably seen
drawings like this before.

00:01:23.700 --> 00:01:27.810
And these branches off of the
soma of the neuron, off of its

00:01:27.810 --> 00:01:32.010
body, these are called
dendrites.

00:01:32.010 --> 00:01:34.455
They can keep splitting
off like that.

00:01:34.455 --> 00:01:36.990
I want to do a fairly reasonable
drawing so I'll

00:01:36.990 --> 00:01:39.040
spend a little time
doing that.

00:01:43.000 --> 00:01:46.140
So these right here, these
are dendrites.

00:01:46.140 --> 00:01:48.500
And these tend to be--
and nothing is

00:01:48.500 --> 00:01:49.860
always the case in biology.

00:01:49.860 --> 00:01:53.160
Sometimes different parts of
different cells perform other

00:01:53.160 --> 00:01:56.940
functions, but these tend to be
where the neuron receives

00:01:56.940 --> 00:01:58.160
its signal.

00:01:58.160 --> 00:02:01.220
And we'll talk more about what
it means to receive and

00:02:01.220 --> 00:02:03.370
transmit a signal in
this video and

00:02:03.370 --> 00:02:04.940
probably in the next few.

00:02:04.940 --> 00:02:09.630
So this is where it receives
the signal.

00:02:09.630 --> 00:02:11.160
So this is the dendrite.

00:02:11.160 --> 00:02:13.030
This right here is the soma.

00:02:13.030 --> 00:02:14.390
Soma means body.

00:02:14.390 --> 00:02:17.130
This is the body
of the neuron.

00:02:17.130 --> 00:02:20.600
And then we have kind of a--
you can almost view it as a

00:02:20.600 --> 00:02:22.100
tail of the neuron.

00:02:22.100 --> 00:02:23.350
It's called the axon.

00:02:26.330 --> 00:02:29.880
A neuron can be a reasonably
normal sized cell, although

00:02:29.880 --> 00:02:32.820
there is a huge range, but the
axons can be quite long.

00:02:32.820 --> 00:02:33.870
They could be short.

00:02:33.870 --> 00:02:36.520
Sometimes in the brain you might
have very small axons,

00:02:36.520 --> 00:02:39.610
but you might have axons that
go down the spinal column or

00:02:39.610 --> 00:02:42.190
that go along one of your
limbs-- or if you're talking

00:02:42.190 --> 00:02:44.240
about one of a dinosaur's
limbs.

00:02:44.240 --> 00:02:47.190
So the axon can actually
stretch several feet.

00:02:47.190 --> 00:02:49.160
Not all neurons' axons
are several feet,

00:02:49.160 --> 00:02:50.150
but they could be.

00:02:50.150 --> 00:02:53.760
And this is really where a lot
of the distance of the signal

00:02:53.760 --> 00:02:55.100
gets traveled.

00:02:55.100 --> 00:02:58.190
Let me draw the axon.

00:02:58.190 --> 00:03:01.620
So the axon will look
something like this.

00:03:01.620 --> 00:03:06.080
And at the end, it ends at the
axon terminal where it can

00:03:06.080 --> 00:03:10.360
connect to other dendrites or
maybe to other types of tissue

00:03:10.360 --> 00:03:13.990
or muscle if the point of this
neuron is to tell a muscle to

00:03:13.990 --> 00:03:14.910
do something.

00:03:14.910 --> 00:03:16.810
So at the end of the axon,
you have the axon

00:03:16.810 --> 00:03:19.070
terminal right there.

00:03:19.070 --> 00:03:22.170
I'll do my best to draw
it like that.

00:03:22.170 --> 00:03:22.830
Let me label it.

00:03:22.830 --> 00:03:25.270
So this is the axon.

00:03:25.270 --> 00:03:26.530
This is the axon terminal.

00:03:29.790 --> 00:03:33.090
And you'll sometimes hear the
word-- the point at which the

00:03:33.090 --> 00:03:36.590
soma or the body of the neuron
connects to the axon is as

00:03:36.590 --> 00:03:39.400
often referred to as the axon
hillock-- maybe you can kind

00:03:39.400 --> 00:03:40.380
of view it as kind of a lump.

00:03:40.380 --> 00:03:41.690
It starts to form the axon.

00:03:46.740 --> 00:03:54.610
And then we're going to talk
about how the impulses travel.

00:03:54.610 --> 00:03:58.970
And a huge part in what allows
them to travel efficiently are

00:03:58.970 --> 00:04:01.420
these insulating cells
around the axon.

00:04:04.760 --> 00:04:06.520
We're going to talk about this
in detail and how they

00:04:06.520 --> 00:04:09.230
actually work, but it's good
just to have the anatomical

00:04:09.230 --> 00:04:17.420
structure first. So these are
called Schwann cells and

00:04:17.420 --> 00:04:20.450
they're covering-- they make
up the myelin sheath.

00:04:20.450 --> 00:04:23.040
So this covering, this
insulation, at different

00:04:23.040 --> 00:04:24.850
intervals around the
axon, this is

00:04:24.850 --> 00:04:26.710
called the myelin sheath.

00:04:26.710 --> 00:04:29.540
So Schwann cells make up
the myelin sheath.

00:04:29.540 --> 00:04:31.010
I'll do one more
just like that.

00:04:40.040 --> 00:04:42.780
And then these little spaces
between the myelin sheath--

00:04:42.780 --> 00:04:46.390
just so we have all of the
terminology from-- so we know

00:04:46.390 --> 00:04:50.650
the entire anatomy of the
neuron-- these are called the

00:04:50.650 --> 00:04:51.900
nodes of Ranvier.

00:04:56.620 --> 00:04:57.950
I guess they're named
after Ranvier.

00:04:57.950 --> 00:04:59.910
Maybe he was the guy who looked
and saw they had these

00:04:59.910 --> 00:05:02.810
little slots here where you
don't have myelin sheath.

00:05:02.810 --> 00:05:05.280
So these are the nodes
of Ranvier.

00:05:05.280 --> 00:05:07.980
So the general idea, as I
mentioned, is that you get a

00:05:07.980 --> 00:05:08.860
signal here.

00:05:08.860 --> 00:05:11.070
We're going to talk more about
what the signal means-- and

00:05:11.070 --> 00:05:13.890
then that signal gets--
actually, the signals can be

00:05:13.890 --> 00:05:16.580
summed, so you might have one
little signal right there,

00:05:16.580 --> 00:05:19.450
another signal right there, and
then you'll have maybe a

00:05:19.450 --> 00:05:23.010
larger signal there and there--
and that the combined

00:05:23.010 --> 00:05:26.780
effects of these signals get
summed up and they travel to

00:05:26.780 --> 00:05:30.430
the hillock and if they're a
large enough, they're going to

00:05:30.430 --> 00:05:34.940
trigger an action potential on
the axon, which will cause a

00:05:34.940 --> 00:05:38.280
signal to travel down the
balance of the axon and then

00:05:38.280 --> 00:05:42.920
over here it might be connected
via synapses to

00:05:42.920 --> 00:05:44.440
other dendrites or muscles.

00:05:44.440 --> 00:05:46.420
And we'll talk more about
synapses and those might help

00:05:46.420 --> 00:05:47.790
trigger other things.

00:05:47.790 --> 00:05:49.770
So you're saying, what's
triggering these things here?

00:05:49.770 --> 00:05:54.400
Well, this could be the terminal
end of other neurons'

00:05:54.400 --> 00:05:57.100
axons, like in the brain.

00:05:57.100 --> 00:05:58.910
This could be some type
of sensory neuron.

00:05:58.910 --> 00:06:02.490
This could be on a taste bud
someplace, so a salt molecule

00:06:02.490 --> 00:06:06.280
somehow can trigger it or a
sugar molecule-- or this might

00:06:06.280 --> 00:06:07.390
be some type of sensor.

00:06:07.390 --> 00:06:09.110
It could be a whole bunch of
different things and we'll

00:06:09.110 --> 00:06:11.700
talk more about the different
types of neurons.


Hemoglobin

WEBVTT
Kind: captions
Language: en

00:00:00.750 --> 00:00:04.030
I've talked a lot about the
importance of hemoglobin in

00:00:04.030 --> 00:00:06.130
our red blood cells so I thought
I would dedicate an

00:00:06.130 --> 00:00:08.340
entire video to hemoglobin.

00:00:08.340 --> 00:00:11.210
One-- because it's important,
but also it explains a lot

00:00:11.210 --> 00:00:15.410
about how the hemoglobin-- or
the red blood cells, depending

00:00:15.410 --> 00:00:18.110
on what level you want to
operate-- know, and I have to

00:00:18.110 --> 00:00:19.050
use know in quotes.

00:00:19.050 --> 00:00:22.890
These aren't sentient beings,
but how do they know when to

00:00:22.890 --> 00:00:26.060
pick up the oxygen and when
to drop off the oxygen?

00:00:26.060 --> 00:00:28.790
So this right here, this is
actually a picture of a

00:00:28.790 --> 00:00:31.710
hemoglobin protein.

00:00:36.240 --> 00:00:38.790
It's made up of four
amino acid chains.

00:00:38.790 --> 00:00:40.430
That's one of them.

00:00:40.430 --> 00:00:41.770
Those are the other two.

00:00:41.770 --> 00:00:44.010
We're not going to go into the
detail of that, but these look

00:00:44.010 --> 00:00:45.200
like little curly ribbons.

00:00:45.200 --> 00:00:47.300
If you imagine them, they're a
bunch of molecules and amino

00:00:47.300 --> 00:00:49.220
acids and then they're curled
around like that.

00:00:49.220 --> 00:00:52.590
So this on some level
describes its shape.

00:00:52.590 --> 00:00:57.140
And in each of those groups or
in each of those chains, you

00:00:57.140 --> 00:01:03.110
have a heme group
here in green.

00:01:03.110 --> 00:01:05.420
That's where you get the
hem in hemoglobin from.

00:01:05.420 --> 00:01:07.870
You have four heme groups and
the globins are essentially

00:01:07.870 --> 00:01:11.150
describing the rest of it-- the
protein structures, the

00:01:11.150 --> 00:01:12.310
four peptide chains

00:01:12.310 --> 00:01:15.100
Now, this heme group-- this
is pretty interesting.

00:01:15.100 --> 00:01:18.860
It actually is a porphyrin
structure.

00:01:18.860 --> 00:01:21.790
And if you watch the video on
chlorophyll, you'd remember a

00:01:21.790 --> 00:01:23.890
porphyrin structure, but at
the very center of it, in

00:01:23.890 --> 00:01:26.670
chlorophyll, we had a magnesium
ion, but at the very

00:01:26.670 --> 00:01:34.440
center of hemoglobin, we have an
iron ion and this is where

00:01:34.440 --> 00:01:35.720
the oxygen binds.

00:01:35.720 --> 00:01:38.810
So on this hemoglobin, you have
four major binding sites

00:01:38.810 --> 00:01:39.370
for oxygen.

00:01:39.370 --> 00:01:41.540
You have right there, maybe
right there, a little bit

00:01:41.540 --> 00:01:45.660
behind, right there,
and right there.

00:01:45.660 --> 00:01:51.510
Now why is hemoglobin-- oxygen
will bind very well here, but

00:01:51.510 --> 00:01:54.440
hemoglobin has a several
properties that one, make it

00:01:54.440 --> 00:01:57.720
really good at binding oxygen
and then also really good at

00:01:57.720 --> 00:02:01.240
dumping oxygen when it
needs to dump oxygen.

00:02:01.240 --> 00:02:04.905
So it exhibits something called
cooperative binding.

00:02:09.990 --> 00:02:14.270
And this is just the principle
that once it binds to one

00:02:14.270 --> 00:02:18.450
oxygen molecule-- let's say
one oxygen molecule binds

00:02:18.450 --> 00:02:22.710
right there-- it changes the
shape in such a way that the

00:02:22.710 --> 00:02:26.750
other sites are more likely
to bind oxygen.

00:02:26.750 --> 00:02:32.450
So it just makes it-- one
binding makes the other

00:02:32.450 --> 00:02:33.700
bindings more likely.

00:02:42.820 --> 00:02:44.300
Now you say, OK, that's fine.

00:02:44.300 --> 00:02:48.130
That makes it a very good oxygen
acceptor, when it's

00:02:48.130 --> 00:02:52.160
traveling through the pulmonary
capillaries and

00:02:52.160 --> 00:02:54.310
oxygen is diffusing
from the alveoli.

00:02:54.310 --> 00:02:57.590
That makes it really good at
picking up the oxygen, but how

00:02:57.590 --> 00:03:00.420
does it know when to
dump the oxygen?

00:03:00.420 --> 00:03:01.680
This is an interesting
question.

00:03:05.230 --> 00:03:10.050
It doesn't have eyes or some
type of GPS system that says,

00:03:10.050 --> 00:03:14.050
this guy's running right now and
so he's generating a lot

00:03:14.050 --> 00:03:17.710
of carbon dioxide right now in
these capillaries and he needs

00:03:17.710 --> 00:03:20.290
a lot of oxygen in these
capillaries surrounding his

00:03:20.290 --> 00:03:21.280
quadriceps.

00:03:21.280 --> 00:03:22.095
I need to deliver oxygen.

00:03:22.095 --> 00:03:23.840
It doesn't know it's
in the quadraceps.

00:03:23.840 --> 00:03:28.470
How does the hemoglobin know to
let go of the oxygen there?

00:03:28.470 --> 00:03:32.510
And that's a byproduct of what
we call allosteric inhibition,

00:03:32.510 --> 00:03:35.100
which is a very fancy word,
but the concept's actually

00:03:35.100 --> 00:03:36.350
pretty straightforward.

00:03:40.850 --> 00:03:44.880
When you talk about allosteric
anything-- it's often using

00:03:44.880 --> 00:03:48.170
the context of enzymes-- you're
talking about the idea

00:03:48.170 --> 00:03:50.460
that things bind
to other parts.

00:03:50.460 --> 00:03:52.550
Allo means other.

00:03:52.550 --> 00:03:55.260
So you're binding to other parts
of the protein or the

00:03:55.260 --> 00:03:59.660
enzyme-- and enzymes are just
proteins-- and it affects the

00:03:59.660 --> 00:04:02.310
ability of the protein
or the enzyme to do

00:04:02.310 --> 00:04:03.750
what it normally does.

00:04:03.750 --> 00:04:07.430
So hemoglobin is allosterically
inhibited by

00:04:07.430 --> 00:04:13.500
carbon dioxide and by protons.

00:04:13.500 --> 00:04:16.230
So carbon dioxide can bond
to other parts of the

00:04:16.230 --> 00:04:17.540
hemoglobin-- I don't
know the exact

00:04:17.540 --> 00:04:19.490
spots-- and so can protons.

00:04:19.490 --> 00:04:21.640
So remember, acidity
just means a high

00:04:21.640 --> 00:04:22.930
concentration of protons.

00:04:22.930 --> 00:04:25.760
So if you're in an acidic
environment, protons can bond.

00:04:25.760 --> 00:04:28.470
Maybe I'll do the protons
in this pink color.

00:04:28.470 --> 00:04:31.970
Protons-- which are just
hydrogen without electrons,

00:04:31.970 --> 00:04:36.200
right-- protons can bond to
certain parts of our protein

00:04:36.200 --> 00:04:39.370
and it makes it harder for them
to hold onto the oxygen.

00:04:39.370 --> 00:04:42.730
So when you're in the presence
of a lot of carbon dioxide or

00:04:42.730 --> 00:04:46.740
an acidic environment, this
thing is going to let go of

00:04:46.740 --> 00:04:47.730
its oxygen.

00:04:47.730 --> 00:04:50.650
And it just happens to be that
that's a really good time to

00:04:50.650 --> 00:04:51.680
let go of your oxygen.

00:04:51.680 --> 00:04:55.340
Let's go back to this
guy running.

00:04:55.340 --> 00:04:57.900
There's a lot of activity in
these cells right here in his

00:04:57.900 --> 00:04:59.460
quadriceps.

00:04:59.460 --> 00:05:03.420
They're releasing a lot of
carbon dioxide into the

00:05:03.420 --> 00:05:05.310
capillaries.

00:05:05.310 --> 00:05:09.390
At that point, they're going
from arteries into veins and

00:05:09.390 --> 00:05:11.930
they need a lot of oxygen, which
is a great time for the

00:05:11.930 --> 00:05:13.900
hemoglobin to dump
their oxygen.

00:05:13.900 --> 00:05:17.560
So it's really good that
hemoglobin is allosterically

00:05:17.560 --> 00:05:19.780
inhibited by carbon dioxide.

00:05:19.780 --> 00:05:22.490
Carbon dioxide joins on
certain parts of it.

00:05:22.490 --> 00:05:24.720
It starts letting go of its
oxygen, that's exactly where

00:05:24.720 --> 00:05:26.700
in the body the oxygen
is needed.

00:05:26.700 --> 00:05:27.360
Now you're saying, wait.

00:05:27.360 --> 00:05:29.030
What about this acidic
environment?

00:05:29.030 --> 00:05:30.350
How does this come into play?

00:05:30.350 --> 00:05:32.990
Well, it turns out that most
of the carbon dioxide is

00:05:32.990 --> 00:05:34.260
actually disassociated.

00:05:34.260 --> 00:05:36.440
It actually disassociates.

00:05:36.440 --> 00:05:38.970
It does go into the plasma, but
it actually gets turned

00:05:38.970 --> 00:05:40.410
into carbonic acid.

00:05:40.410 --> 00:05:43.590
So I'll just write a little
formula right here.

00:05:43.590 --> 00:05:49.870
So if you have some CO2 and you
mix it with the water-- I

00:05:49.870 --> 00:05:53.070
mean, most of our blood, the
plasma-- it's water.

00:05:53.070 --> 00:05:54.880
So you take some carbon dioxide,
you mix it with

00:05:54.880 --> 00:05:59.960
water, and you have it in the
presence of an enzyme-- and

00:05:59.960 --> 00:06:02.155
this enzyme exists in
red blood cells.

00:06:02.155 --> 00:06:03.940
It's called carbonic
anhydrase.

00:06:10.890 --> 00:06:14.310
A reaction will occur--
essentially you'll end up with

00:06:14.310 --> 00:06:16.590
carbonic acid.

00:06:16.590 --> 00:06:17.840
We have H2CO3.

00:06:20.630 --> 00:06:21.260
It's all balanced.

00:06:21.260 --> 00:06:24.670
We have three oxygens, two
hydrogens, one carbon.

00:06:24.670 --> 00:06:27.700
It's called carbonic acid
because it gives away hydrogen

00:06:27.700 --> 00:06:28.750
protons very easily.

00:06:28.750 --> 00:06:34.020
Acids disassociate into their
conjugate base and hydrogen

00:06:34.020 --> 00:06:35.200
protons very easily.

00:06:35.200 --> 00:06:41.200
So carbonic acid disassociates
very easily.

00:06:41.200 --> 00:06:43.340
It's an acid, although I'll
write in some type of an

00:06:43.340 --> 00:06:44.910
equilibrium right there.

00:06:44.910 --> 00:06:47.500
If any of this notation really
confuses you or you want more

00:06:47.500 --> 00:06:50.680
detail on it, watch some of the
chemistry videos on acid

00:06:50.680 --> 00:06:54.180
disassociation and equilibrium
reactions and all of that, but

00:06:54.180 --> 00:06:57.540
it essentially can give away
one of these hydrogens, but

00:06:57.540 --> 00:06:59.940
just the proton and it keeps the
electron of that hydrogen

00:06:59.940 --> 00:07:03.210
so you're left with a hydrogen
proton plus-- well, you gave

00:07:03.210 --> 00:07:05.700
away one of the hydrogens so
you just have one hydrogen.

00:07:05.700 --> 00:07:08.680
This is actually a
bicarbonate ion.

00:07:11.470 --> 00:07:13.760
But it only gave away the
proton, kept the electron so

00:07:13.760 --> 00:07:15.130
you have a minus sign.

00:07:15.130 --> 00:07:18.900
So all of the charge adds up to
neutral and that's neutral

00:07:18.900 --> 00:07:19.700
over there.

00:07:19.700 --> 00:07:23.930
So if I'm in a capillary
of the leg-- let me see

00:07:23.930 --> 00:07:25.640
if I can draw this.

00:07:25.640 --> 00:07:28.890
So let's say I'm in the
capillary of my leg.

00:07:28.890 --> 00:07:30.650
Let me do a neutral color.

00:07:30.650 --> 00:07:32.410
So this is a capillary
of my leg.

00:07:32.410 --> 00:07:34.420
I've zoomed in just one
part of the capillary.

00:07:34.420 --> 00:07:36.080
It's always branching off.

00:07:36.080 --> 00:07:41.490
And over here, I have a bunch
of muscle cells right here

00:07:41.490 --> 00:07:47.000
that are generating a lot
of carbon dioxide

00:07:47.000 --> 00:07:48.580
and they need oxygen.

00:07:48.580 --> 00:07:50.440
Well, what's going to happen?

00:07:50.440 --> 00:07:53.340
Well, I have my red blood
cells flowing along.

00:07:53.340 --> 00:07:57.750
It's actually interesting--
red blood cells-- their

00:07:57.750 --> 00:08:01.280
diameter's 25% larger than
the smallest capillaries.

00:08:01.280 --> 00:08:03.370
So essentially they get squeezed
as they go through

00:08:03.370 --> 00:08:06.550
the small capillaries, which a
lot of people believe helps

00:08:06.550 --> 00:08:10.810
them release their contents and
maybe some of the oxygen

00:08:10.810 --> 00:08:11.870
that they have in them.

00:08:11.870 --> 00:08:14.010
So you have a red blood cell
that's coming in here.

00:08:14.010 --> 00:08:18.180
It's being squeezed through
this capillary right here.

00:08:18.180 --> 00:08:22.130
It has a bunch of hemoglobin--
and when I say a bunch, you

00:08:22.130 --> 00:08:25.800
might as well know right now,
each red blood cell has 270

00:08:25.800 --> 00:08:30.240
million hemoglobin proteins.

00:08:30.240 --> 00:08:32.169
And if you total up the
hemoglobin in the entire body,

00:08:32.169 --> 00:08:38.080
it's huge because
we have 20 to 30

00:08:38.080 --> 00:08:40.670
trillion red blood cells.

00:08:45.180 --> 00:08:49.820
And each of those 20 to 30
trillion red blood cells have

00:08:49.820 --> 00:08:55.330
270 million hemoglobin
proteins in them.

00:08:55.330 --> 00:08:56.880
So we have a lot
of hemoglobin.

00:08:56.880 --> 00:08:59.410
So anyway, that was a little
bit of a-- so actually, red

00:08:59.410 --> 00:09:02.240
blood cells make up roughly
25% of all of the

00:09:02.240 --> 00:09:03.080
cells in our body.

00:09:03.080 --> 00:09:04.730
We have about 100 trillion
or a little bit

00:09:04.730 --> 00:09:05.920
more, give or take.

00:09:05.920 --> 00:09:07.910
I've never sat down
and counted them.

00:09:07.910 --> 00:09:11.980
But anyway, we have 270 million
hemoglobin particles

00:09:11.980 --> 00:09:14.950
or proteins in each red blood
cell-- explains why the red

00:09:14.950 --> 00:09:18.100
blood cells had to shed their
nucleuses to make space for

00:09:18.100 --> 00:09:19.370
all those hemoglobins.

00:09:19.370 --> 00:09:21.510
They're carrying oxygen.

00:09:21.510 --> 00:09:24.730
So right here we're dealing
with-- this

00:09:24.730 --> 00:09:26.410
is an artery, right?

00:09:26.410 --> 00:09:27.610
It's coming from the heart.

00:09:27.610 --> 00:09:29.870
The red blood cell is going in
that direction and then it's

00:09:29.870 --> 00:09:32.100
going to shed its oxygen
and then it's

00:09:32.100 --> 00:09:34.080
going to become a vein.

00:09:34.080 --> 00:09:38.640
Now what's going to happen is
you have this carbon dioxide.

00:09:38.640 --> 00:09:41.390
You have a high concentration
of carbon dioxide in the

00:09:41.390 --> 00:09:41.880
muscle cell.

00:09:41.880 --> 00:09:45.530
It eventually, just by diffusion
gradient, ends up--

00:09:45.530 --> 00:09:52.770
let me do that same color-- ends
up in the blood plasma

00:09:52.770 --> 00:09:55.760
just like that and some of it
can make its way across the

00:09:55.760 --> 00:10:00.220
membrane into the actual
red blood cell.

00:10:00.220 --> 00:10:03.680
In the red blood cell, you have
this carbonic anhydrase

00:10:03.680 --> 00:10:07.770
which makes the carbon dioxide
disassociate into-- or

00:10:07.770 --> 00:10:09.900
essentially become carbonic
acid, which

00:10:09.900 --> 00:10:11.370
then can release protons.

00:10:14.700 --> 00:10:18.700
Well, those protons, we just
learned, can allosterically

00:10:18.700 --> 00:10:22.250
inhibit the uptake of oxygen
by hemoglobin.

00:10:22.250 --> 00:10:25.040
So those protons start bonding
to different parts and even

00:10:25.040 --> 00:10:27.650
the carbon dioxide that hasn't
been reacted with-- that can

00:10:27.650 --> 00:10:30.840
also allosterically inhibit
the hemoglobin.

00:10:30.840 --> 00:10:32.630
So it also bonds
to other parts.

00:10:32.630 --> 00:10:36.730
And that changes the shape of
the hemoglobin protein just

00:10:36.730 --> 00:10:39.370
enough that it can't hold onto
its oxygens that well and it

00:10:39.370 --> 00:10:40.640
starts letting go.

00:10:40.640 --> 00:10:43.230
And just as we said we had
cooperative binding, the more

00:10:43.230 --> 00:10:46.180
oxygens you have on, the better
it is at accepting

00:10:46.180 --> 00:10:47.480
more-- the opposite happens.

00:10:47.480 --> 00:10:49.720
When you start letting go of
oxygen, it becomes harder to

00:10:49.720 --> 00:10:50.540
retain the other ones.

00:10:50.540 --> 00:10:52.260
So then all of the
oxygens let go.

00:10:52.260 --> 00:10:55.410
So this, at least in my mind,
it's a brilliant, brilliant

00:10:55.410 --> 00:10:58.910
mechanism because the oxygen
gets let go just where it

00:10:58.910 --> 00:10:59.735
needs to let go.

00:10:59.735 --> 00:11:05.760
It doesn't just say, I've
left an artery and

00:11:05.760 --> 00:11:06.980
I'm now in a vein.

00:11:06.980 --> 00:11:09.050
Maybe I've gone through some
capillaries right here and I'm

00:11:09.050 --> 00:11:10.080
going to go back to a vein.

00:11:10.080 --> 00:11:11.920
Let me release my oxygen--
because then it would just

00:11:11.920 --> 00:11:14.180
release the oxygen willy-nilly
throughout the body.

00:11:14.180 --> 00:11:17.110
This system, by being
allosterically inhibited by

00:11:17.110 --> 00:11:20.570
carbon dioxide and an acidic
environment, it allows it to

00:11:20.570 --> 00:11:23.730
release it where it is most
needed, where there's the most

00:11:23.730 --> 00:11:28.570
carbon dioxide, where
respiration is occurring most

00:11:28.570 --> 00:11:29.870
vigorously.

00:11:29.870 --> 00:11:32.550
So it's a fascinating,
fascinating scheme.

00:11:32.550 --> 00:11:34.540
And just to get a better
understanding of it, right

00:11:34.540 --> 00:11:37.520
here I have this little chart
right here that shows the

00:11:37.520 --> 00:11:40.740
oxygen uptake by hemoglobin or
how saturated it can be.

00:11:40.740 --> 00:11:43.560
And you might see this in maybe
your biology class so

00:11:43.560 --> 00:11:45.590
it's a good thing
to understand.

00:11:45.590 --> 00:11:48.890
So right here, we have on the
x-axis or the horizontal axis,

00:11:48.890 --> 00:11:51.450
we have the partial pressure
of oxygen.

00:11:51.450 --> 00:11:54.180
And if you watched the chemistry
lectures on partial

00:11:54.180 --> 00:11:56.350
pressure, you know that partial
pressure just means,

00:11:56.350 --> 00:11:58.990
how frequently are you being
bumped into by oxygen?

00:11:58.990 --> 00:12:03.480
Pressure is generated by gases
or molecules bumping into you.

00:12:03.480 --> 00:12:05.300
It doesn't have to be gas,
but just molecules

00:12:05.300 --> 00:12:06.310
bumping into you.

00:12:06.310 --> 00:12:09.390
And then the partial pressure
of oxygen is the amount of

00:12:09.390 --> 00:12:11.310
that that's generated
by oxygen molecules

00:12:11.310 --> 00:12:11.990
bumping into you.

00:12:11.990 --> 00:12:14.370
So you can imagine as you go
to the right, there's just

00:12:14.370 --> 00:12:17.160
more and more oxygen around so
you're going to get more and

00:12:17.160 --> 00:12:18.930
more bumped into by oxygen.

00:12:18.930 --> 00:12:21.510
So this is just essentially
saying, how much oxygen is

00:12:21.510 --> 00:12:23.590
around as you go to
the right axis?

00:12:23.590 --> 00:12:27.550
And then the vertical axis tells
you, how saturated are

00:12:27.550 --> 00:12:29.150
your hemoglobin molecules?

00:12:29.150 --> 00:12:33.610
This 100% would mean all of the
heme groups on all of the

00:12:33.610 --> 00:12:37.980
hemoglobin molecules or proteins
have bound to oxygen.

00:12:37.980 --> 00:12:41.280
Zero means that none have. So
when you have an environment

00:12:41.280 --> 00:12:43.630
with very little oxygen-- and
this actually shows the

00:12:43.630 --> 00:12:48.060
cooperative binding-- so let's
say we're just dealing with an

00:12:48.060 --> 00:12:49.960
environment with very
little oxygen.

00:12:49.960 --> 00:12:53.410
So once a little bit of oxygen
binds, then it makes it even

00:12:53.410 --> 00:12:56.200
more likely that more and
more oxygen will bind.

00:12:56.200 --> 00:12:58.840
As soon as a little-- that's why
the slope is increasing.

00:12:58.840 --> 00:13:01.220
I don't want to go into algebra
and calculus here, but

00:13:01.220 --> 00:13:02.900
as you see, we're kind
of flattish, and

00:13:02.900 --> 00:13:04.550
then the slope increases.

00:13:04.550 --> 00:13:06.990
So as we bind to some oxygen,
it makes it more likely that

00:13:06.990 --> 00:13:07.605
we'll bind to more.

00:13:07.605 --> 00:13:12.420
And at some point, it's hard for
oxygens to bump just right

00:13:12.420 --> 00:13:14.730
into the right hemoglobin
molecules, but you can see

00:13:14.730 --> 00:13:17.040
that it kind of accelerates
right around here.

00:13:17.040 --> 00:13:21.620
Now, if we have an acidic
environment that has a lot of

00:13:21.620 --> 00:13:24.350
carbon dioxide so that the
hemoglobin is allosterically

00:13:24.350 --> 00:13:27.190
inhibited, it's not going
to be as good at this.

00:13:27.190 --> 00:13:31.830
So in an acidic environment,
this curve for any level of

00:13:31.830 --> 00:13:34.620
oxygen partial pressure or any
amount of oxygen, we're going

00:13:34.620 --> 00:13:36.990
to have less bound hemoglobin.

00:13:36.990 --> 00:13:39.140
Let me do that in a
different color.

00:13:39.140 --> 00:13:41.110
So then the curve would
look like this.

00:13:41.110 --> 00:13:46.930
The saturation curve will
look like this.

00:13:51.200 --> 00:13:55.360
So this is an acidic
environment.

00:13:55.360 --> 00:13:57.410
Maybe there's some carbon
dioxide right here.

00:13:57.410 --> 00:14:06.300
So the hemoglobin is being
allosterically inhibited so

00:14:06.300 --> 00:14:09.820
it's more likely to dump the
oxygen at this point.

00:14:09.820 --> 00:14:10.300
So I don't know.

00:14:10.300 --> 00:14:12.250
I don't know how exciting you
found that, but I find it

00:14:12.250 --> 00:14:15.450
brilliant because it really is
the simplest way for these

00:14:15.450 --> 00:14:17.480
things to dump their oxygen
where needed.

00:14:17.480 --> 00:14:22.080
No GPS needed, no robots needed
to say, I'm now in the

00:14:22.080 --> 00:14:23.610
quadriceps and the
guy is running.

00:14:23.610 --> 00:14:24.415
Let me dump my oxygen.

00:14:24.415 --> 00:14:26.410
It just does it naturally
because it's a more acidic

00:14:26.410 --> 00:14:28.180
environment with more
carbon dioxide.

00:14:28.180 --> 00:14:31.120
It gets inhibited and then the
oxygen gets dumped and ready

00:14:31.120 --> 00:14:33.070
to use for respiration.