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HAZEL SIVE: All right, let's
look at some of your questions.
A bunch of them
are-- you know what,
I'm probably going to use
that screen most of the time.
Because this one's not fitting.
But let's look on this screen.
Your questions really focused
around IPS cells and the kind
of magic of IPS cells.
And there were a couple of
major questions that came up.
What's the problem with using
IPS cells therapeutically?
Well, there are a number.
One, they're so
new that really we
don't know what kinds of cell
types these IPS cells can make,
so it's not clear
how to use them.
Another problem actually,
which my colleague,
Professor Jaenisch
works on is the question
of how to actually
grow these cells.
I have a voice
issue this morning.
So you know, the quieter
you are, the more you'll
be able to hear my words.
IPS cells, human cells, grow
very slowly in the laboratory.
And it's very
difficult to grow them.
So there are some basic
questions in biology,
as to how to grow these
cells to enough numbers that
would actually be useful.
But here's another one
that's very important.
The transcription
factors that we
use to convert adult
cells into IPS cells
included an oncogene
called c-Myc.
And Myc is the kind of gene
you don't want floating
around your body nice
and active in your cells,
because it will give you cancer.
So the challenge is
how to actually use
Myc and other
potent transcription
factors to turn adult
cells into stem cells
but not have the stem cells
give the recipient cancer.
And there's some
very clever ways
that people are trying
to get around this.
It is really impossible
to teach you,
if there are groups
of you talking.
I just cannot do it.
So those of you
talking, please don't.
Thank you.
All right, so what's the
big deal about IPS cells?
Well, the big deal is
they can be your own.
In theory-- and maybe
in practice, a decade,
five years maybe from now--
your own cells, your skin cells
could be removed from you,
could be dealt with
in the laboratory,
turned into your own
stem cells, and put back
into your own body,
and they'd be your own.
So of course, they wouldn't
be rejected by you.
And that is an enormous deal.
They're called autologous cells.
There's an ethical
issue, in that
you don't have to harvest any
embryos to get stem cell lines.
And that's a big deal.
And here's a conceptual deal.
When we talked
about development,
we talked about this
directional pathway,
where uncommitted
cells became committed
became differentiated.
It was really thought that
that was a one-way pathway.
But what IPS cell
technology has shown--
let's look at this--
all right, we can get the idea
from one of these two screens.
What IPS cell
technology has shown
is that you can reverse
differentiation.
You can take
differentiated adult cells
and with the right
regulatory factors,
you can turn them
into stem cells.
And so conceptually,
that's a big deal.
Very good, great questions.
My next office hours
are on Monday, 12-1.
Come along or email me.
But now we're going to
turn to a new module,
and that is the nervous system.
And this is where we
are in the course.
You've done all the
foundational material.
We've talked about formation.
And now we're going
to talk about systems
and the nervous
system in particular.
Let's start with the
question of what a system is.
Building up, the
complexity of what
we're talking about in life.
A system refers to many
organs that work together
with one common function.
Many organs working together
with one overall function.
And you will talk about
two in the course.
You'll talk about the nervous
system and the immune system
with Professor Jacks.
The nervous system, which is
the topic of the next three
lectures, has to do
with communication--
has to do with communication
from the outside to the inside
of an animal's body, has to
do with communication within
the body, and has to do
with communication to get
the organism-- the animal,
because plants don't have
nervous system--
the animal to do something.
So the nervous system
is about communication
to or from or within
the body of an animal.
We've had a number of electrical
analogies in the course.
You will remember the
famous signaling analogy
of turning on the light
switch over there on the wall.
But now we move on to
another electrical analogy,
which is actually a truer one.
And I'm going to use the
analogy for the nervous system
of talking about
wires that transmit
the signal, by which
cells communicates.
I'm going to talk about
connectors between the wires.
And then I'm going to
talk about circuits.
And those are going to be
the topics of the next three
lectures.
So wires, connectors,
and circuits.
And those will be
nervous system 1 to 3,
the lectures in this module.
Today, we're going to
talk about the wires.
And there will be three topics.
The first is the
cell type that's
got something to do with the
wiring for this communication.
The second is something
called the action potential,
which is the signal
by which cells
communicate within themselves.
And the third has to do with
the ion channels and pumps.
But let's start by
phrasing the problem
in a kind of a cool way.
If you look in the human--
let's look on this screen.
If you look in the human,
and you outline the nerves.
And if you went to see
the living, plasticized
human exhibits, you would have
seen the plasticized nerves.
They're really extraordinary.
The network of nerves, which
are the unit of communication
throughout the body.
Is enormous.
And it embraces every single
part of the body, almost.
The cell that's involved
in communication--
we'll draw this out in a
moment-- is the neuron.
And one of the
things about neurons
is that they've got very
long processes, called axons,
that we will deal
with in depth today.
But let's phrase things
more intuitively.
This is what our
brains look like.
They in fact do not have
nerves innovating them.
And so if you actually
have brain surgery,
you can really be
awake during surgery.
Because there are no nerves
within the brain, or at least
no pain receptors
within the brain.
Of course, the brain has
nerves, but there are
no pain receptors in the brain.
Human brain.
Billions and billions
and trillions of neurons.
We'll talk about numbers
in future lectures.
But let's look a little
more deeply in the brain,
so you can see how
packed it is with neurons
and how the connections
between the neurons
are so unbelievably
complex that thinking
about how you use
circuitry to construct
the human nervous system--
or indeed that of most animals--
is an enormous problem.
Professor Sebastian Seung here--
whose name is off the screen,
and I apologize to him,
it's the screen snafu, it
will be on your power points
when you download
these from the web--
is working on the task
of putting together
all the connections
in the human brain.
And starting with a
little cube of brain--
this is about 100 microns,
it's not quite a cube,
but it's approximately
a 100-micron cube--
he's worked to try to
figure out what all the cell
connections are and
what all the cells are,
within this little
tiny cube of brain,
which is just a tiny, tiny
fraction of your brain.
So look at this.
This is an electron
micrograph that he's
put a bunch of
electron micrographs
together, to build
this 3D structure.
And now his students and
those of Professor Lichtman
at Harvard go and
outline a particular cell
in serial sections,
through this cube.
These are very tiny sections.
These are about five
nanometer sections.
And then they put these
sections together.
And you can get the
three dimensional
structure of the neuron, as
it's going through the cube.
And you can start to map
how this neuron lies next
to other neurons.
OK, there it's going backwards.
And there are two cells
lying next to one another
that you can get
in 3D rendering,
with this very painstaking
process of first,
getting sections of
the brain, putting them
all together into a chunk,
and then deconstructing
the individual cells
shapes within that chunk.
If you do that for the
whole chunk of tissue,
this is what you get.
There's that red and
that green neuron.
Here is a blue one,
there's a yellow one, lime,
purple, red, dark
blue, yellow, orange.
It's daunting.
There you go.
In that tiny little
chunk of brain,
that is how packed
the cells are.
And the connections
between them are enormous.
And that is just less than
a millionth of your brain.
So to figure out
all the connections,
all the circuitry in the nervous
system, is an enormous task.
And we don't know it.
We'll talk about what
we do know later on.
But I wanted to frame
the problem for you,
so that you have a sense of
where we're trying to go.
Let's go back to the neuron.
And let's talk about
cell type and how
cell type is important for
thinking about signaling
in the nervous system.
So like everything
else in the body,
communication uses
cells as its currency.
And the cell type
is the special kind
of cell type, which
is the neuron.
So neurons are the
connecting/wiring cells.
There's a second type of cell
in the nervous system that
is really pivotal for
nervous system function that
are called glia.
And these are cells which are
referred to as support cells.
But that's not really fair.
They guide neurons, and
as we'll mention later,
they also insulate
neurons, so that the wires
don't short circuit.
So they guide and
insulate neurons.
The structure of the neuron
is important to understand
its function.
Like all cells, it's got
a nucleus and cytoplasm.
And here it is, the
nucleus, the cytoplasm.
And this region of the neuron
is called the cell body.
But unlike other cells,
coming out from this cell body
there are processes.
And they are very
substantial processes.
On one side of the cell body are
usually fairly short processes.
They can be branched, and
there can be bunches of them.
And these are called dendrites.
Dendrites are processes
that receive a signal.
So they are the place where
there is a signaling--
let me get rid of
that dendrite--
there is a signaling input.
The signal that
dendrites receive moves
through the cell body and
into another process, which
is called the axon
and is very long.
And the fact that
it is very long
is actually what this
lecture is all about.
So the axon is the wire.
Axons can be up to a meter long.
Here's the axon.
And there are axons that
start in your spinal cord
and move all the way down
your leg, from a single cell.
We'll talk about why
that is in a moment.
These axons branch
at their ends.
And they connect to another
neuron or something else,
but we'll draw another neuron.
Here's another neuron with its
dendrites and another axon.
The connection-- here is
neuron 1 and neuron 2.
And the connection between
axons and dendrites--
or as you'll discover,
axons in the cell body--
is called a synapse.
Some people say synapse--
that's the connection--
either is OK.
And the thing is
that this input that
is way over on the left
hand side of the board
is transmitted, along the
axon, into the next neuron,
and then along the next neuron.
This is the signal.
And we have to think
about why cells
might want to have these very
long processes to do this,
and then how the
signal is transmitted.
The reason that cells have got
these long processes rather
than--
well, let's actually
step back a moment.
Let's think about
how cells might
communicate with one another.
You could imagine a whole
bunch of little round cells,
all lined up, so that
there a meter of them
that go from your spinal
cord down to your leg.
And that would give you
a chain of communication
from your spinal
cord to your leg.
And you could have one back
to your brain and so on.
And that in theory
would work OK.
But it turns out that cell-cell
communication is very slow,
and that cells have
figured out a way
to transmit a signal along their
own length that is very rapid.
You know that there is a
finite time between getting
a stimulus and a response.
You know, you touch
something hot,
you can tell it actually takes
a moment before you figure out
it's hot.
That's the speed of
transmission of the signal, up
into your brain.
And you say, wow, that's
hot, move my finger.
If you had cells that were
connecting rather than long
processes of one cell, it
would take you that much longer
to actually make that--
maybe 10 times longer--
to make that connection.
And you'd get a
bad burned finger.
So the axon is the thing that
allows rapid transmission
of the signal.
So the axon is long.
It leads to an
intracellular signal.
And this is very rapid, relative
to an intercellular signal.
But how do you transmit
a signal along a cell?
Well, axons do this by
using movement of ions.
So the signal along the axon
is due to movement of ions.
And this is called an action
potential, as we will discuss.
And this process draws on
a property of all cells
that neurons have
capitalized on.
So almost all cells have
got a potential difference
across their membrane, because
there is a charge difference
across the plasma membrane.
So almost all cells
have what is called
a membrane potential, which is
a membrane potential difference.
And in general, cells are more
positive outside than they
are inside.
So outside the cell--
and this is worth
your remembering--
it is more positive.
And it's more positive because
there's a lot of sodium ion.
You'll see the why
this is important.
There's low potassium ion, and
there's some high chloride ion.
But really, the thing
that's important
is that there's very high sodium
concentration outside the cell.
Conversely, inside the cell
is obviously more negative.
Sodium is low.
Potassium is higher but
still not very high.
And there's a bunch of ions
that are kind of trapped
in the cell.
Why is this important?
Let me see what
I have next here.
OK, most cells show a
potential difference.
Here it is.
Here is written the
potential difference.
Neurons are somewhere
between minus 70
and minus 60
millivolts, where you
are talking about the
relative potential difference
inside to outside,
that's why it's negative.
And you can see
tumor cells actually
have got a very low membrane
potential, which may or may not
be significant.
What is the nature of
the signal that neurons
use to transmit from the
input, along the axon,
to the next cell?
Particularly, what
is the signal that's
transmitted along the axon?
And the answer is something
called an action potential.
It's the signal transmitted
along the axon, the wire
that I referred to.
And an accident potential, which
I'm going to abbreviate as AP,
will define--
and you'll understand
this in a moment--
as a local, transient
depolarization.
Local, transient--
just lasts a moment--
depolarization, change
in membrane potential.
And I'm going to do most
of this on the board.
You have a hand-out,
but I'm going
to do most of this on the
board because it works better
as a conversation
than a demonstration.
Let's draw a a bit of an axon.
Here is the axon.
Two plasma membranes--
outside, inside, outside.
So this is the axon.
Plasma membrane, PM.
And what we are going to do--
and here is a cytoplasm--
what we are going to do is
take a chunk of the axon
and blow it up and focus on
just one plasma membrane,
on one side of the axon,
and look and see in detail
what is going on there.
So let's take this
chunk and blow it up,
so that we now have the plasma
membrane--and you remember
it's a lipid bilayer.
But I'm drawing it
as a single line,
because it's really
a pain control
to draw it as a lipid bilayer.
But you know it's
a lipid bilayer.
On one side-- and here
is outside the cell
and inside the cell.
On one side, there are
lots of positive charges.
And on the other side, there
are fewer and relatively more
negative charges.
When the axon looks like
this, with this balance
of positive and
negative charges,
it's said to be at
resting potential.
And resting potential is
about minus 60 millivolts.
OK, what's our goal?
Our goal is to start
here, at this asterisk,
and to transmit a
signal along the length
of this piece of axon and
to transmit the signal
in a directional way.
So our goal is to transmit
a directional signal.
Let's draw three time
points, each of which
have a plasma membrane of this
particular segment of axon.
And let us--
I'm going to move over a bit,
to this side of the board--
let's have them
here, here, and here.
And so we're going to have a
time vector going diagonally
across the board.
And we started
off with something
that looked like
it did on the board
above at resting potential.
And now, we're going to-- over a
very short segment of membrane,
we're going to reverse
the membrane charge,
or the cell's going to do it.
So that on the
outside now, there's
a little part of
the membrane that's
negative outside
and positive inside.
And the rest is positive
outside and negative inside.
Over time, this is called a
depolarization, a reversal
of the membrane potential.
Over time, that depolarization,
that initial depolarization
is going to rectify.
It's going to go
back to how it was.
So you'll get positive
charges outside again.
But the segment of membrane next
door is going to depolarize,
so it now becomes negative
outside and positive inside.
And the rest is positive
outside and negative inside.
That little piece of
membrane, the second--
so that's depolarization
1, here's depolarization 2.
Again, over time, you're
going to get rectification
of that second depolarization.
And you've got
the idea now, it's
going to move further
down the axon.
So here is depolarization 3.
And you recall that this is
outside and inside the axon.
So if you look at my diagram--
I haven't given
you any mechanism--
but you can see here we
have got a signal that
is moving in this
direction, along the axon.
Each of these depolarizations
that I've drawn
is called an action potential.
And I'll give you, in a
moment, some more properties,
so that you'll know an action
potential when you see one.
But there are a couple
of questions that arise
from this easy-to-draw diagram.
Firstly, how does
this really happen?
How do charges reverse
across the membrane?
Secondly, why is the
signal unidirectional?
Why doesn't it go backwards?
And thirdly, how do you
reset the depolarization
once it's happened?
And all of these
things you will see
are connected, but let's
raise these questions.
So how does this happen?
Why is it unidirectional?
And what does it mean--
or what is the
mechanism of resetting
the membrane potential, after
a depolarization has happened?
As for example here, you have
reset the membrane potential.
So the answer to all
of this is complex.
And we'll answer it in chunks,
as we tend to do in this class.
And the first thing we'll
answer with respect to
is changing membrane potential.
Let's start off again with
our axon chunk, with outside
and inside, and the
charge distribution
that is at resting potential.
And along comes
some kind of input.
It might be touch, it
might be another neuron
touching a second neuron.
It might be vision,
light, that comes along,
some kind of input.
Here it is.
And this input acts on a very
local part of the membrane.
And it changes the
membrane potential
just a tiny bit, such that
the membrane potential might
reach something called
threshold potential.
So let's look,
let's draw it out.
Here it is.
Just over a one little
tiny bit of the membrane,
there's some kind
of charge reversal.
The positive charges come from
outside, and they move inside.
And we'll call this potential
difference threshold potential.
And if you want a number, it's
about minus 55 millivolts.
What happens after threshold?
Well threshold, you
understand what threshold is.
It means something
happens because you have
reached a point of no return.
And what happens is that there
is now an action potential,
and there is a massive
movement of the positive sodium
ions into the cell.
So from threshold
potential there
is a massive movement,
again out and in.
So now you've got instead
of this little tiny region
of depolarization, you've got a
large region of depolarization.
So this is a small, depol--
for depolarization-- leading
to a very large depolarization
of the kind that I drew
on the board before.
This large depolarization is
called the action potential.
And it has several properties.
The action potential reverses
the membrane potential
almost completely.
So now in this region
of the membrane,
it's at about plus
60 millivolts.
So it's a massive
depolarization.
You completely reverse
the ion distribution
over a small region
of the membrane.
It's very local, however.
An action potential
occurs over--
or this massive
depolarization occurs over
about a micron of membrane.
It takes one to two
milliseconds to set up.
It involves the movement of
about 10 to the 5th ions,
from the outside in.
And I've told you the potential.
The other thing that
is really critical
that you have to understand is
something called all or none.
The depolarization you get
with an action potential
is either complete,
or it doesn't happen.
If you reach threshold,
you reverse polarity,
and you get this complete
depolarization to plus 60
millivolts from minus 60.
You do not get a
partial depolarization
to plus 10 or 20 or
30 sometimes, or 35.
For a given neuron, you get
a specific action potential.
And it either happens,
or it doesn't happen.
So all on none, very important.
No little or big
action potentials.
And now we've got
a depolarization,
but we haven't
answered two questions.
We haven't answered the
question of unidirection,
and we haven't answered
the question of resetting.
So let's do that
on the next board.
And let's start off, actually,
with an action potential.
And I'm actually going to
draw an action potential, kind
of in the middle of this axon.
You'll see why.
When you get an action
potential, what's happening
is that sodium ions are moving
from the outside inside.
And those sodium ions--
because they're just ions--
will start to diffuse
in the cytoplasm.
And as they diffuse
next door, they'll
change the membrane
potential, which
will reach threshold,
which will trigger
an action potential in the
membrane chunk next door.
And then those
ions will diffuse,
to make the chunk of
membrane next door
reach threshold potential.
And you'll get an action
potential triggered, and so on.
But the ions, of course,
because they're just ions,
can move in either direction.
So the ions can move back.
If this is where your
action potential took place,
the ions could move
in that direction
and trigger an action potential
going back, up the axon,
towards the cell body.
Why doesn't that happen?
It doesn't happen
because once you've
triggered an action potential,
that membrane becomes
refractory, unable to
trigger another action
potential for a while.
And during that time
where the membrane
is unable to respond and make
another action potential,
the ions have diffused away
and gone on down the axon.
And so you get a
unidirectional propagation.
Let's try to draw that out.
So here's an action potential.
And the ions that are
moving in will diffuse.
And they will take the membrane
next door to threshold.
And so they'll trigger an
action potential next door.
Those ions fusing backwards
can't do anything.
Because once the membrane
has had an action potential,
it can't have another
one for a while.
So this membrane here, next
door to the action potential,
is refractory to
depolarization--
that is a really
horrendous spelling
job there, depolarization--
for some period of time.
Let's say for about a second
or a little less than that,
but somewhere around there.
And so that means that
the action potential
is unidirectional.
The ions can diffuse
in both directions,
but the action potential
can only go in one.
So that gives you a
direction of your signal.
And also, I have
cavalierly drawn on there
that the membrane potential
reverses and resets itself,
where the action potential
previously occurred.
And we'll talk about
that more in a moment.
So here, the membrane
potential has reset.
So this is a theoretical walk
through action potentials.
And I gave you a
bunch of handouts.
But I'm not going to go through
them, because you can use them
as a test or as an
exercise after class,
to see how much you understood.
One of the things about
conductance along an axon
is that it's very quick.
It takes a very short time
from touching that hot thing
to realizing you've touched it.
But one of the
reasons it's so short
is because you're not sending
an action potential all the way
along an axon like
we're drawing.
You don't really
get successive parts
of the membrane depolarizing.
Because that actually,
although it's faster
than intercellular connections,
is still quite slow.
So there's a way that a
cell insulates itself,
an axon insulates itself, to
give you action potentials
just to particular places.
And that really speeds up
the rate of transmission
of an action potential.
And I've got that
on the slide here,
and we're right on the board.
So that during
depolarization and after
and all the time, ions
leak from the axon.
And this decreases the frequency
of action potential formation.
And so what cells do, it's
kind of like a short--
no, it's not quite
short circuiting,
but it's a bad electrical wire.
And so what the cell has
done is to insulate itself
with layers of fatty cells.
And these cells are
really kind of amazing.
Most of the cells that
insulates the neurons
in the nervous system
wrap around the neurons,
as in this diagram.
You can see here is a cell.
And these lines are because a
single cell has wrapped itself
around the neuron.
You know that the plasma
membrane is lipid.
It doesn't conduct
ions, and so you've
got really a fatty
layer of insulation.
And the thing that insulates
the cells is something
called a myelin sheath, which
is lipid plus some protein.
But it's a really
hydrophobic layer
that wraps around the
neurons and insulates them.
Along the neurons
that are insulated,
there are specific places
where there is no insulation.
And that's where action
potentials take place.
So action potentials take
place at nods without myelin.
And this is one way that
neurons really speed up
their conductance rate.
And so I put here action
potential frequency,
but actually that's not correct.
I'm going to talk about
rate of transmission.
So how does this
all go together?
Let's look at a movie, where
here's the neuron, and here
is the axon, transmitting
an action potential
along its length.
And here's a different way of
depicting the action potential,
as a graph of
voltage against time.
And that's something that
you'll practice in section.
But what I want you
to see is that there's
an action potential
moving along the axon.
And the axon can transmit
many, many action potentials,
one after the other,
with a short recovery
period in between.
We still have not answered
quite the question
of how action potentials work.
And the answer to that is
to consider ion channels
and pumps, because all of
this charge distribution
doesn't just happen.
It's set up by
the cell, and it's
set up by ion channels and
pumps, which we can write
Regulate Membrane Potential.
Let's review very briefly what
ion channels and pumps are.
We've talked about them a bunch.
But you need to
know some essences
for this particular module.
Ion channels allow ions across
the membrane by diffusion.
So here is an ion channel.
And I'm drawing a
channel which is open.
And the ions move by
diffusion, across the channel.
But there are other
classes of ion channels,
which are not always open.
They are called gated.
And we talked
about them, when we
talked about protein secretion,
protein localization.
So gated channels under
a particular stimulus
can be closed and then change
to the open confirmation,
after they've been given
the appropriate stimulus.
So here there is some
kind of stimulus.
And a channel that
is gated will open.
A third kind of way of getting
ions across the membrane
is to use a pump, where a pump
is localized in the plasma
membrane as well.
But instead of a
diffusion-governed process
to get ions across
the membrane, the pump
is actively moving ions
across the membrane.
So ion pumps actively
transport ions.
And they generally require
energy, ATP, in order to do so.
And all of these things
are essential to set
the membrane potential and
to change it during action
potential formation.
If we consider the resting
potential-- actually,
let me see what I have
on the slides here.
This is a really cool thing.
All right, let me get
through our board work,
and then we'll do what is
really cool after or on Monday.
In order to set up
the resting potential,
there are several kinds
of channels and pumps
that you need to be aware of.
One of them-- which is a biggie
and for which a Nobel Prize was
given some decades ago--
is called the sodium
potassium pump.
And this is really a big thing.
It's also called the
sodium potassium ATPase.
And what it does is to
pump three sodium ions out
of the cell and put two
potassium ions into the cell.
And this is an enormously
important pump for life.
And you can see what it does
is to increase the sodium
concentration outside the cell
and increase the potassium
concentration inside.
There is also something called
an open potassium channel that
will allow all this
potassium that's
being pumped in by the
sodium potassium pump
to start diffusing
out of the cell.
But in actual fact, it
doesn't all diffuse out.
Because it hits the positive
charges of the sodium
ions on the outside, and there's
an electrostatic repulsion.
And so that limits
how much potassium--
by Monday, I will either
be able to speak by Monday
or I will have
completely lost my voice.
You'll have the option.
So the open potassium
channel allows potassium out
by diffusion, until it
is repelled or stopped
by electrostatic forces
coming from the sodium ions.
And then a third
ion that's open,
an ion channel that's open is
the chloride channel, which
we won't discuss right now.
During the action
potential, there
is an enormously
important ion channel
that is the last thing
I'll mention today.
And that's called the
voltage gated sodium channel.
This is an ion channel that,
like many ion channels,
consists of a
complex of proteins.
We'll make a note of that, I'll
make a note of that next time.
But the voltage
gated sodium channel
is sensitive to
membrane potential.
And when threshold
potential is reached,
there is a change
in the confirmation
of this channel, which is closed
normally at resting content
but becomes open at
threshold potential,
to lead to the action potential.
And it becomes open through
actually the sliding
of one of the alpha
helices that make up
the proteins of the channel.
And the sliding
alpha helices slide
because their charges change,
the charges of the amino acids
change.
And that opens up the channel.
So I'm going to
show you one picture
of the last of the
sodium channel,
the voltage gated
sodium channel.
Here it is.
And then we'll finish.
Take a look at this quickly.
Here is the voltage gated
sodium channel closed.
Amino acids blocking
up the pore.
And there it opens up,
to let the ions in.
And we'll finish this on Monday.
