To understand how excited
a membrane can be
and I know you’re excited too to
learn about membrane excitability.
What we really need to do to look
at the current changes across
any particular membrane is you
need to use a recording electrode.
Simply, you put a recording
electrode one side of the membrane
and then on the other, or you put it
inside the cell and then ground it.
The potential across the membrane allows
for some signals to potentially be sent.
And interestingly, the signal between
or the difference between the membrane
and the outside the cell oftentimes
has a voltage potential.
This voltage potential
is determined by the ion
and out of the cell.
So by knowing the ion concentrations, you
can get inside into this voltage change.
How does this work?
Well, you need to
know a few things.
So I’m going to use a couple
of theoretical examples
first and then we’re going
to go into some practicals.
Theoretically, if we had this bath and we
had a divider in the middle of the bath
that didn’t allow any
ions to travel through.
Let’s just label one of them ECF, which
is representative of the extracellular
fluid, and the ICF as representative
of the intracellular fluid.
You put an electrode on one side,
electrode on the other side of the bath.
In this case, we’re using
potassium and chloride.
In our example here, the number of
positive and negative charges is equal.
Now, I know if you’re looking at these too
bad, you’re telling me, how is that equal?
What I’m saying here is the
number positives equal
the number of negatives
on both sides of the bath
not that the one side of the bath has a
lot more molecules on than the other.
It’s just the sum of the charges that is
important, not that particular ion number.
So when there’s no charge imbalance
across this particular membrane,
there’s no voltage.
So even though there’s a different number
of ions in one side than the other,
there’s no voltage.
Let’s now contrast that in
another condition in which
the membrane between the baths now
have little channels in them.
And these channels are going to allow
some of the ions to travel through
and let’s say those channels
were selective enough
to only let the positive
charged ions through.
Yeah, it’s exactly what you would do, you’re
only going to let positive people through
the door into your bath, you’re going to keep
the negative ones over on the other side.
If you have the positive
charges, go to one side.
If you’re recording in the opposite side,
you’ll have more negative charges there
and so membrane potential will decrease if
you’re looking at it from the ICF side.
If you are looking at it from the ECF side,
membrane potential would have increased.
So it matters where your
and where your reference
electrode is located.
If you’re recording
electrodes in one side,
you’ll only be able to determine the changes
in that side in comparison to the other.