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Hello, and welcome back to
Fundamentals of
Human Physiology.
My name is Charles Sevigny.
Today, we're going to be talking
about action potentials.
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An action potential is
a very specific signal
generated by an excitable cell.
In our context,
we'll talk about
that in neurons.
But it also applies to
skeletal muscle cells,
other types of muscle cells,
other specialised cells
in the body that we'll
learn more about as we
move through the course.
But what an action
potential does is
carry a signal along
the cell membrane
representing a change
in the charge or
the membrane potential
across that membrane.
Now, we need to think back
to our last activity,
when we talked about
how we maintain
resting membrane
potential across
a membrane by the balance of
permeability between sodium,
which wants to move into the
cell and make it more positive
and potassium, which wants to
move out of the cell
making it more negative.
Simply by opening
up permeability
to one or the other
of those ions,
we can change the charge
across the membrane.
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Just to review some of
that, think about this.
We have a mystery ion,
it doesn't matter what it is,
with a positive
charge and it has
an equilibrium
potential of -60 mV.
Now, if membrane potential is
currently sitting at -70,
what will the ion do?
Pause and think about that.
In that case, the ion will have
net movement into the cell.
If you got that right, you're
doing pretty well so far.
What we know about this ion is
that it has a positive charge.
If the charge inside
the cell was at -60 mV,
the ion wouldn't move
in or out of the cell.
Whatever its concentration
gradient may be,
that's being
perfectly opposed by
this charge of -60 mV.
But if the charge is
more negative than that,
then that's making a more
attractive charge environment
for that positive ion to
want to move into the cell.
As a result, the ion
will move into the cell
towards that more
negative environment
and continue to move in along
with its positive charge
making the inside of the
cell more and more positive
until ultimately
it reaches -60 mV.
Now, if you chose 'not enough
information is provided'
because I didn't
tell how permeable
the membrane was to that ion,
you're also doing very well
and you're one step ahead of me.
The membrane, of course,
has to be permeable
to that ion in order
for it to move across.
We know that a normal
cell membrane,
without any channels in it,
an ion can't get through.
We need specific protein
channels called leak channels
for that ion to be able to
move through the membrane.
Now let's try another one.
Same mystery ion with
a positive charge,
same equilibrium
potential of -60 mV,
but now our membrane
potential is -40 mV.
What's that ion going to do now?
If you said that
it's going to have
net movement out of the
cell, then you're correct.
Because now, if that ion needs
the inside of the cell to
be at -60mV to hold it in,
but the membrane potential
is actually at -40 mV,
it's not negative enough to
hold that positive
ion in the cell.
As a result, the ion will
start to leak out of the cell.
Leaking out, making
the cell more negative
because it's taking its
positive charge out of the cell
until that cell reaches -60 mV.
That's why we also call
equilibrium potentials
reversal potentials,
because if the membrane
potential is actually more
negative than -60 in this case,
then the ion will
move into the cell.
If it's more positive than -60,
it will move out of the cell.
By knowing the equilibrium
or reversal potential
of that ion and knowing
the membrane potential,
we can predict whether that ion
will move in or out of a cell.