>> Hi everyone and welcome to the Penguin Prof Channel today.
We're going to talk about,
as you can see, the action potential.
Ok, so we're going to try and get a handle
on some very difficult topics by comparing these new concepts
to things you already know.
And as you know that's a theme that I have.
We're going to talk about some good stuff
and some bad stuff dealing with action potential.
So here we go.
This is of course where it all happens.
This is a giant multi [Inaudible] neuron
and as a quick review, neurons are unidirectional
signaling devices.
This is the input end.
These are the dendrites.
Dendrite means tree-like.
All these signals coming into this neuron will come
in through the dendrites and then be focused on one spot
which we're going to look at here, the axon hillock.
You can also call that the trigger zone.
If this trigger zone reaches threshold then this neuron
will fire.
What that means is we're going to send an action potential
down the axon to the axon terminals shown down here,
and in this case this neuron is talking to another neuron.
But of course, neurons talk to all cells
of the body not just other neurons.
In this video, we're going to try and get a handle
on what kind of signal the action potential is actually is
and how the neuron actually codes for information.
Of course the nervous system drives people crazy
because you're talking about changes that happen
over milliseconds, thousandths of a second and movements
of ions which are not really easy to visualize.
So this kind of looks like a mess, but as we try and look
at what the ions are doing, primarily sodium and potassium,
there are two main forces
that are responsible for their movements.
So let's try and break this down a little bit and see
if we can get a handle on it.
The first gradient that's going to cause the ions to move
in any particular direction is one that is very easy
to recognize; it's a concentration gradient.
And a concentration gradient goes from where particles
or in this case children, are to where they aren't.
That's the direction of the gradient.
So, you know, basic diffusion you can look at movements of,
you know, food coloring in a beaker
of water you'll see the same thing.
So everything moves from high concentration to areas
of low concentration until equilibrium is reached.
So when we talk about an ion or any particle,
but in this case an ion's concentration gradient,
that's what we're talking about, is moving from where they are
to where they aren't until there's equilibrium point.
The other gradient that we have to worry
about is the voltage gradient and all you got
to remember there is this is a basic property
of opposite charges attracting each other.
So similar charges repel each other but opposites attract.
So now if we go back to this figure of a cell
and cross section, we start to look at these particular ions,
hopefully makes a little bit more sense
when you break it down.
So let's look at sodium.
For sodium the story is actually pretty clear
because sodium is being concentrated all the time
by this three sodium two potassium pump
and it's being concentrated outside the cell.
What this pumps does, it's an active transporter,
it's powered by ATP.
Every time an ATP molecule gets hydrolyzed three ions
of sodium get pumped out of the cell and two ions
of potassium get pumped in.
So this is what creates the sodium concentration gradient.
That is why there is so much sodium; you see it all out here,
why there's so much sodium outside the cell
and basically none inside the cell.
There are leak channels for sodium but you notice
that is blocked when the cell is at rest.
It's critical.
The other thing, the other gradient of course,
is the voltage gradient and you notice that the cell
at rest is inside negative.
That's what all of these negative signs represent
and the cell is positive outside of course.
So for sodium the story is pretty clear.
The driving force for sodium is a combination
of the voltage gradient and the concentration gradient drives
sodium to want to go into the cell very very badly.
That driving force is actually quite high.
So that's the reason why sodium has
such a strong desire to get into the cell.
Another way of looking at this, a little table format,
what we've just said the concentration gradient
for sodium drives it in; that's based on where sodium wants
to go because of its numbers; just being concentrated
in numbers outside the cell means it wants to go in based
on the voltage gradient.
Sodium also wants to go
in because the cell is inside negative.
And sodium of course, is positive.
The story for potassium is a little bit different.
So potassium is always being concentrated inside the cell,
because again; its three sodium, two potassium pump;
the same pump of course.
Every time an ATP pump gets hydrolyzed two ions
of potassium get pumped in.
So the concentration gradient for potassium is outward,
but looks at the voltage gradient.
It's the same voltage gradient that we just saw, right?
The cell is inside negative with respect to the outside.
So for potassium the story's a little bit unclear.
If you add the concentration gradient
and the voltage gradient together,
what you actually get is a small driving force outward
for potassium.
In other words
that concentration gradient effect is a little bit stronger
than the voltage gradient.
But again just to compare, the purpose of clarity I hope,
the concentration gradient for potassium is outward,
that's based on numbers, where potassium wants to go.
But the voltage gradient wants to keep potassium in.
In other words potassium, just like sodium,
is a monovalent cation.
So its, its happy point is being surrounded by negativity right?
So it would actually like to stay in.
So you can see why that the degree of the driving force
for each of the ions is different.
Ok, so let's put all this together.
Basically everything, all the signals,
coming into this neuron are going to be focused all
of the changes in memoring [Assumed spelling] voltage,
get focused right here at the axon hillock.
The axon hillock really is the magic spot and if
that particular spot on the cell body reaches threshold,
this neuron will fire.
And if it doesn't, it won't.
So this is what we're going to look at in terms of the graph
of millivolts on the Y, the membrane potential.
How inside negative or how inside positive is the cell
and then we've got time in milliseconds
on the X. What I want everyone to notice is
that normally text books will draw a membrane at rest
or a dendrite at rest as a flat line,
usually they'll average it around minus seventy.
So that's inside negative minus seventy millivolts.
The truth is those dendrites are constantly getting information.
Like I said, and what kind of information is that?
Well from any given point on the Y you have two choices right?
You can go up or you can go down.
That's pretty much it.
So from minus seventy when the neuron gets any kind
of information that pushes this down,
that is to say making the cell even more inside negative
than it already is, we call that a hyper polarization.
You're taking the polarization, the inside negativeness,
if you will, and you're making it more,
right, hyper means more.
That's one type of information that the cell could be getting.
On the other hand, this can also deflect upwards.
We're making the inside of the cell less negative
than it already is or more on the inside positive,
depending on how you want to look at it.
Any kind of signal that does that,
we call that depolarizing stimulus.
So those are really the only options; that's the kind
of information the neurons will get.
What this all comes down to is will the cell be depolarized;
will it come all the way to threshold voltage
at the axon hillock, yes or no.
That's the part of the neuron that acts like a switch;
it's either on or off.
So when we look at all this we say,
as this is when the cell is at rest.
All that means is you notice it's not,
it doesn't really look restful to me;
it's going up, it's going down.
So again, here right here it would be depolarizing.
Here it's hyperpolarizing.
The cell is getting information, saying no, no, no,
don't fire now, but look what happens.
It's getting depolarized and depolarized.
It's getting less and less inside negative
and its getter more and more, closer and closer to threshold
and finally at this point right here something amazing happens.
Sodium gates open.
Those are voltage regulated sodium gates and sodium
if you recall, is the ion that wants to get in so badly,
but the membrane at rest is not permeable to sodium.
But all of a sudden that threshold, these little proteins
in the membrane change shape and allow sodium to come in.
Boy does it ever.
So here are these sodium channels all of a sudden open,
all of the sodium rushing in due to the concentration gradient
and the voltage gradient and wow that causes the upsweep
of the action potential.
So if you think about it this part
of the action potential is due to the fact
that sodium is rushing in making the cell more and more
and more inside positive.
Now where sodium actually wants the membrane to be,
that is the equilibrium value for sodium is way up here;
way above where the neuron actually ends up.
Why it wants to go so high you want to watch this video
on driving forces and equilibrium potentials.
This video really explains why the ions want
to go where they do.
So check that out.
Anyway, back to this.
At the peak of the action potential two events occur.
The gates for sodium which open the threshold now close.
And now we've got tons
of voltage sensitive potassium gates, they open.
And now the situation
for potassium has changed dramatically.
Now the situation is this.
The concentration gradient for potassium is outwards,
which it was before so in that sense it's just like rest.
But what has changed is the membrane voltage.
Remember at rest, the cell is inside negative, but at the peak
of the action potential it's now inside positive right?
Sodium took it to inside positive; so now potassium,
kind of like sodium at the very beginning of the story,
now potassium has a huge driving force,
both the concentration gradient
and the voltage gradient re pushing potassium out.
So potassium now floods out and that's what you see as the
down sweep of the action potential;
that is potassium leaving.
Wow, where the potassium wants the membrane to be,
its equilibrium value is way down here at minus ninety.
And again to understand why that is and what that means you want
to check out that driving forces video.
Ok, so we've got an action potential, now what?
Now we've got to spread the love, all the way down the axon.
So what happens is as you look at the entire length of the axon
and remember axons can be extremely long,
every single region of depolarization,
it's just like a line of dominos that's going
to cause the next region just downstream to depolarize
and you're going to get action potentials all the way down.
But there's a little problem.
Actually what I learned in Jamaica is there are no
such things as problems, just situations.
We don't call it a problem we call it a situation.
The situation is that current is not unit directional.
Current change in a membrane will flow both upstream,
that is to say back towards the cell body as well as downstream
and that's a big problem because those neurons have
to be unidirectional signaling devices.
So the solution to this problem is called rest.
It's called the refractory period.
But it's basically a resting phase.
So what actually happens as you look down an axon;
here's a region and it's going
through an action potential right now;
as the action potential spreads,
it does influence both the region that just fired as well
as the region that's going to fire next.
The reason why this region does not fire again is
because it can't.
It is having its resting phase.
So what you look at down here in the pink is
that this region is recovering
from the action potential it just had.
That's why it can't fire again.
There is actually what we call an absolute refractory period
during which time the excitability of the region
of the membrane is actually zero.
That means no matter how strongly you stimulate this
region it cannot have an action potential and that's
because it's basically having one.
Right, this is the action potential.
These are the two ions' that are involved.
It has to in a sense, reset.
This is a relative refractory period; this shows the time,
you notice it's only a couple of milliseconds long right?
When the excitability is slowly increasing
and now it's perfectly excitable again.
In other words it's back to its original excitability state.
So the refractory period is what prevents the action potential
from going backwards, and that's pretty cool.
Another problem, I mean situation,
is that signal strength decreases over distance.
And like I said, axons can be extremely long.
If you've worked at all with electrical wiring you know
that we insulant electrical wires
with a single piece of insulation.
We cannot do that with an axon.
And that's because the axon is so long and the signal is
so weak, that if you actually expect it
to send this big depolarizing signal all the way down an axon,
by the time it gets to the end, I mean this could be,
the cell body could be in your spinal cord
and the axon terminal could be in your little toe, you know
and by the time it gets that distance,
the signal would die out.
So what we actually have is a situation
where through the myelin region
of axon you only have the depolarizing stimulus.
The problem with that even though it goes very fast,
is that it does decrease in strength over distance.
But that's ok, because at the next node of Ranvier that's
where the action potential will occur.
So the signal gets re-energized, if you will,
then the next region that's myelinated,
you get a very fast spread of depolarization.
It's fast but it dies out over distance.
But that's ok because now we have another node.
So at every node of Ranvier the signal gets re-amplified.
And a lot of people do think of the nodes
as little mini amplifiers and that is what prevents the loss
of the signal intensity over the entire length of the axon.
It's because the nodes of Ranvier act
to amplify the signal all the way down which is pretty cool.
My voice is tired.
I know that's a lot of information.
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