So now we come, in segment 5 of week 2, to the most complex topic that we'll discuss in the present week.
How does the membrane make use of these ionic differences
that have been established by the sodium-potassium pumps
in order to create a membrane voltage?
It does so in the following series of steps;
First, it adjusts itself, so that it is
permeable to potassium only; in other words, not sodium.
Now of course, it can't do that perfectly,
But what we mean by that is it adjusts itself
so that the permeability of potassium is much greater than the permeability of sodium
or, in other words, ions of potassium can pass through the membrane relatively
easily as compared to ions of sodium.
It's what it does first. It's not an easy trick to do,
because what we're asking the membrane to do--here is the membrane
, and here's a
potassium channel...right here...
that's the channel, here comes a potassium!
HEre comes a potassium, and we're asking the membrance to let that potassium go right on through.
The potassium is coming fast. Fast, fast, fast!
It's not just drifting over there; think in terms of a car traveling very fast.
It's coming at high speed; the membrane recognizes it, and lets it
though the channel.
On the other hand, what we're asking the membrane to do
is if we have a sodium coming, a sodium ion coming, up to that same channel
well, we wanted to X out the sodium and bounce it back
so that teh sodium ion does not get through.
It's not an easy trick to do,
because these are both small ions
Both a positive charge--just has to recognize it and act quickly.
But it is able to do it, so it's a
great accomplishment.
So the membrane begins by being permeable to potassium
by a much higher degree than it is to sodium.
Then, the memrbane lets potassium ions <i>leak</i> back
out.
through these channels where it is permeable to ]
potassium only.
So, in a, uh, conceptual or pictorial sense, you'd say,
"What's happening here
is that the membrane
The membrane is letting potassium leak, from the inside to the outside.
And when it lets it leak from the inside to the outside,
it carries a positive charge with it.
So the result of the potassium ion moving out
is that there is excess positive outside, excess
negative inside.
The corollary is that an electric field is established,
pointing inward, from higher potential to lower potential.
So I've drawn an arrow here representing the direction of the
electric field.
Every time a potassium ion goes out,
The electric field gets stronger
If one describes this with a mathematical equation,
one has the equation that is, uh, given here.
The movement of potassium outward because of the diffusion gradient
is the term here on the left.
The electric field, on teh other hand,
is pushing potassium back in
That's the term over here on the right.
This is inward.
Here.
This is outward. There.
Both processes are occurring at the same time,
to the degree that they are not in balance--
there is a flux of potassium ions, indicated by the J<u>p symbol</u>
that is moving across the membrane.
Dr. Roger BarrAnderson-Rupp Professor of Biomedical Engineering and Associate Professor of Pediatrics
