In the last video, we talked about how the cell uses a
sodium potassium pump and ATP to maintain its potential
difference between the inside of the cell or the inside of
the neuron and the outside-- and in general, the outside is
more positive than the inside.
You have a -70 millivolt potential difference from the
inside to the outside.
It's minus because the outside is more positive.
Less positive minus more positive, you're going to get
a negative number and it's by -70.
Now, I said that this was the foundation for understanding
how neurons actually transmit signals.
And to understand that, I'll kind of lay a foundation over
that foundation.
I think then just the actual neuron transmission will make
a lot of sense.
Even better, it'll make a lot of sense why they even have
these myelin sheaths and these nodes of Ranvier and why we
have all of these dendrites.
Hopefully it'll all fit together.
So there are two types of ways that kind of a
potential can travel.
So there's two types of signal transfer.
I'll just call it signal transfer.
I don't know what the best word for it is.
The first one I'll talk is electrotonic.
It sounds very fancy, but you'll see it's
a very simple idea.
And the other one I'm going to go over
is an action potential.
And they both have their own positives and negatives in
terms of being able to transmit a signal.
We're talking about within the context of in a cell or across
a cell membrane.
Let's understand what these mean.
So let me get my membrane of a cell.
Let's say it's a nerve cell or a neuron, just to make it all
fit together in this context.
And we know it's more positive on the
outside than the inside.
We know that there's a lot of sodium on the outside or a lot
more sodium on the outside than on the inside.
There might be a little bit.
And we know there's a lot more potassium on the inside than
the outside, but we know generally that the outside is
more positive then the inside because our sodium potassium
pump will pump out three sodiums for every two
potassiums it takes in.
Now in the last video, I told you that there are these
things called-- well, we could call them a sodium gate.
A sodium ion gate, right?
These are all ions.
They're charged.
Now let's say that there's some reason, some stimulus--
let me label this.
That right there is my sodium ion gate.
And it's in its closed position, but let's say
something causes it to open.
We'll talk maybe in this video or maybe this video and the
next about the different things that
could cause it to open.
Maybe it's some type of stimulus causes this to open.
Actually, there's a whole bunch of different stimuluses
that would cause it to open.
But let's say it opens.
What's going to happen if it opens?
So let's say we open it.
Some stimulus opens-- what's going to happen?
We have more positive on the outside than the inside, so
positive things want to move in.
And this is a sodium gate so only sodium can go through it.
So it's kind of a convoluted protein structure that only
sodium can make its way through.
And on top of that, we have a lot more sodium on the outside
than on the inside.
So the diffusion gradient's going to want to make sodium
go through it.
And the fact that sodium's a positive ion, the outside is
more positive, they're going to want to run away from that
positive environment.
So if you open this, you're just going to have a lot of
sodium ions start to flood through.
Now as that happens, what's going to happen if we go
further down the membrane?
Let's zoom out.
So let's say that this is my membrane right there.
Let's say that this is my open gate right here and that it's
open for some reason and a bunch of sodium is flowing in.
So all of this is becoming much more positive.
Let's say we had a voltmeter right here.
We're measuring the potential difference between the inside
of the membrane a and the outside.
Let me do a little chart.
I'm going to do the chart here on my voltmeter.
And this is going to be the potential difference-- or
we'll call it the membrane voltage or the voltage
difference across the membrane-- and
let's say this is time.
Let's say I haven't opened this gate yet.
So it's in its resting state.
Our sodium potassium pumps are working.
Things are leaking back and forth, but it's staying at
that minus 70 millivolts.
So that right there is minus 70 millivolts.
Now as soon as this gate that's way down some other
part of the cell opens, what's going to happen?
And let's say that's the only thing that's open.
So this, all of a sudden, is going to become more positive.
So positive charges that's already here-- so other
positive charges, whether they're sodiums or potassiums,
they're going to want to run away from that point because
this area hasn't had a flood of positive things.
So it's less positive than this over here.
So maybe we have some potassiums and maybe we have
some sodiums. Everything is going to want to move away
from the place where this is opened.
The charge is going to want to move away.
So as soon as this happens, as soon as we open this gate,
we're going to have a movement of positive
charge in this direction.
So all of a sudden-- this was at minus 70 millivolts.
So more positive charge is coming its way.
Almost immediately, it's going to become less negative or
more positive.
The potential difference between this and this is going
to become less.
So this is this point over here.
Now if we took this point, if we did the same thing-- if we
measured the voltage at this point right here, maybe it was
at minus 70 millvolts, maybe a fraction of a minute amount of
time later, the positive charge starts affecting it so
it becomes more positive, but the effect is diluted, right?
Because these positive charges, they're going to
radiate in every direction.
So the effect is diluted.
So the effect on this thing is going to be less.
It's going to become less positive.
So an electrotonic potential-- what happens is at one point
in the cell, a gate opens, charge starts flooding in, and
it starts affecting the potential at other
parts of the cell.
But the positive of it is, it's very fast. As soon as
this happens.
further down the cell, it starts becoming more and more
positive, but the further you go, the effect gets dissipated
with distance.
So if you care about speed, you'd want this
electrotonic potential.
As soon as it happens, it'll start affecting the rest of
the cell, but if you wanted this potential change to
travel over large distances-- for example, let's say if we
got all the way to this point of the neuron and we wanted to
measure it, it might not have any impact.
Maybe a little bit later, but it's not having any impact
because all of this gets diluted by the time it gets--
it's increasing the charge throughout the cell.
So it's a impact far away from the initial place where the
gate opened.
It's going to be a lot less.
So it's really not good for operating over distance.
Now let's try to figure out what's going on
with an action potential.
And you might understand, this might involve more action.
So let's start off with the same situation.
We have a sodium gate that gets opened by some stimulus.
What I'm going to do-- let me draw two membranes here.
So this is the outside.
This is the inside.
And let me draw-- maybe we're dealing with a-- and we'll go
in more detail.
Maybe this is an axon or something, but let me-- let's
say we have another sodium gate right here.
And then they're alternating, essentially.
So they're alternating so then I have another sodium gate.
I don't want to do a bunch of these.
I think I just have to draw one round of it for you to get
what's going on.
Let me draw another potassium gate.
And let's say that they all start closed.
So they're all in the closed position.
Now let's say that this sodium gate gets stimulated.
It gets opened.
Let's say that guy right there gets opened.
It gets stimulated by something to get opened.
We'll talk about the things that-- let's say in particular
this thing gets opened-- let's say the stimulus-- it has to
be a certain voltage.
And let's say they become open when we are at minus 55
millivolts.
So when we're just in our resting state, the potential
difference between the inside of the cell and the outside is
minus 70, so it's not going to be open.
It's going to be closed, but if for whatever reason, this
becomes positive enough to get to minus 55 millivolts, all of
a sudden this thing will be open.
Let's write a couple of other rules that dictate what
happens to this gate.
Let's say it closes-- and these are all rough numbers,
but the main idea is for you to get the general idea.
Let's say it closes at-- I don't know-- plus 35
millivolts.
And let's say that our potassium gate opens at plus
40 millvolts, just to give an idea of things.
Let's say it closes at-- I don't know-- minus 80
millivolts.
So what's going to happen?
Lets say that, for whatever reason, the voltage here has
now become minus 55.
Let me do a chart just like I did down here.
So I want to have space to draw my chart.
This is membrane voltage.
And this is time down here.
And let's say we're measuring it-- let's say this is the
membrane voltage at-- let's say right by the sodium gate
right here.
So we're measuring this voltage
across this right here.
So if it's not stimulated any way, we're just here,
flatlining at minus 70 millivolts-- and let's say
some stimulus, for whatever reason,
makes this more positive.
Maybe it's some type of electrotonic effect that's
making it more positive here.
Maybe some positive charges are floating by.
So this becomes more positive.
So let's say this becomes more positive and then the ATP
pumps-- the sodium potassium pumps pump it out so it
doesn't get to the threshold of minus 55, so then nothing
will happen, right?
It didn't get to the threshold.
But then let's say there's another electrotonic or maybe
a bunch of them and just there's a lot of positive
charge here so we get to the minus 55 millvolts.
Remember, when positive charge comes by,
we become less negative.
The potential difference becomes less negative.
We get to that minus 55 volts-- this
thing opens then, right?
This was closed before.
It was closed when we were just at minus 70.
So let me write here.
So at this point, our sodium gate opens.
Now, what's going to happen when our sodium gate opens?
When that opens-- we've seen this show before-- all the
positively charged sodium is going to go down there, both
electric gradient and diffusion gradient, and
there's going to flood into the cell.
There's so much sodium out there, it's so positive out
there, they just want to come in.
So as soon as they hit that threshold, even though this
might've only gotten us to minus 55 or maybe minus 50,
all of a sudden that gate opens and we have all of this
positive charge flooding into the cell.
So the potential difference becomes
much, much more positive.
So they keep flooding in, becomes much, much more
positive, but as it gets more positive, it
closes at plus 35 millvolts.
So let's say that we're dealing here-- let's say that
this up here is plus 35 millvolts.
So here it closes and at the same time, that stuff I just
deleted-- I set at plus 40 millvolts-- or let's say at
plus 35, just for the sake of argument.
Let's say at plus 45 millvolts, our
sodium gates open.
So what's happened here?
All of a sudden, we're at plus 35 or maybe plus 40 millivolts
so this is-- let's just say plus 40, I think you get the
idea either way so we'll say plus 40-- either way.
So at plus 40, this guy's going to close.
No more positive ions are coming in, but now we are at
more positive inside, at least locally at this point on the
membrane, than we are outside.
And so this gate will open.
So then our sodium gate will open.
K-plus ion gate opens.
Now when that opens, what happens?
We have all of these sodium ions here.
We already saw from the sodium potassium pump that the
potassium-- we have all of these potassium ions here.
We saw from the sodium potassium pump that it makes
the sodium concentration on the outside higher and the
potassium concentration on the inside higher.
And now that we've gotten to this plus 40 millvolt range,
we're also now more positive on the inside.
So this opens.
These guys want to escape because there's
less potassium outside.
They want to go down their concentration gradient.
It's also very positive on the inside.
We're at plus 40 millvolts.
So they also want to escape.
So they start escaping the cells.
So positive charges starts exiting the cell from the
inside to the outside.
So we become less positive again.
So let me write what happens here.
So at this point, our sodium gate closes and our potassium
gate opens.
And then the positive charge starts flooding out of the
cell again and maybe it'll overshoot because it's only
going to close maybe once we get to minus 80 millvolts.
So maybe our potassium gate closes at minus 80.
And then our sodium potassium pump might get us back to our
minus 70 millvolts.
So, this is what's happening just at this point in the
cell, just near that first sodium gate.
But what's going to happen in general, right?
As this became very positive-- we went to 40
millivolts over here.
We went to 40 millvolts in this area of the cell.
Because of-- I guess you could almost view it as a short term
or very short distance electrotonic potential, this
area is going to become more positive, right?
This is going to become more positive.
These positive charges are going to go
where it's less positive.
So this is going to become more positive.
This was at minus 70, but it's going to become more positive.
It'll go to minus 65, minus 60, minus 55-- and then bam.
This guy will get triggered again.
Then this guy gets opened.
Sodium floods in through here.
So if you wanted to plot this guy's, the potential
difference of what's going on across this, this all happened
as soon as-- maybe as soon as a sodium started going in this
first dude, the second guy-- he gets triggered here because
the second guy a little bit later in time-- because of all
this flow a little bit to the left of him, his
potential goes up.
He gets triggered, same exact thing happens to him, right?
When the sodium flows in here, becomes really positive around
here, that makes the cell around here, the voltage
around here, the charge around here a little bit more
positive, triggers this next sodium gate to open and then
this whole same thing happens, same cycle.
Then the potassium gates open to make it negative again, but
by the time that's happened, it's become positive over here
to trigger another sodium gate.
So one after another, you have these sodium gates opening and
closing, but it's transmitting that information, it's
transmitting that potential change.
So what's going on here?
So this is slower and it actually involves energy.
So this was-- the electrotonic was very fast. This is slow.
An action potential is slower.
I don't want to say it's slow.
It's slower because it has to involve these opening and
closing of gates and it also involves energy.
It also requires more energy.
And you're also going to have to keep changing the potential
in your cell and you actively have your sodium potassium
pumps being very active.
But it's good.
The positive is, it's good at covering distance.
When you have something like this-- we saw with the
electrotonic, as we get further and further away from
where the stimulus happened, the change in potential
becomes more and more dissipated.
It actually exponentially declines.
It becomes more and more dissipated as we get further
and further away so it's not good for long distance.
This thing can just continue forever because every time it
stimulates the next gate, it's like we're starting all over
again and so this gate-- it's going to have a flood of ions
come in and those ions are going to make it a little less
negative over here.
Then the next gate's going to open.
We're going to have the cycle over and over again.
So this is really good for traveling long distances.
So now we have really the foundation to understand
exactly what's happening in a neuron and I'm going to go
over that in the next video to show you how electrotonic
potentials and action potentials can combine to have
a signal travel through a neuron.