Membrane Potentials

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Introductory Human Physiology

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Duke University

Introductory Human Physiology

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In this course, students learn to recognize and to apply the basic concepts that govern integrated body function (as an intact organism) in the body's nine organ systems.

From the lesson

The Nervous System

We hope you are enjoying the course! Last week's lectures can be challenging because we introduce many concepts that may be new to you. This module will allow you to apply some of the concepts that you learned last week and provide you with more concrete examples. In this module we will begin our tour of the various organ systems with the nervous system. We start by considering the function of the individual cells (neurons) and then how they interact as an integrative system. The nervous system provides rapid communication throughout the body coordinating the actions of trillions of cells. It responds to internal changes to the body as well as to changes in our external environment. This is a busy week. The things to do this week are to watch the 5 videos, to answer the in-video questions, to read the notes, and to complete the Nervous System problem set. We suggest that you read the notes, watch the videos, and answer the in-video questions before you start on the problem sets. The problem sets require you to apply your knowledge from the lectures so it is best to be fairly familiar with the material before tackling them. The problem sets are not graded, and there is no due-date for them.

Membrane Potentials21:16

Meet the Instructors

Jennifer Carbrey
Assistant Research Professor
Department of Cell Biology
Emma Jakoi
Associate Research Professor
Department of Cell Biology

Hello.

Welcome to our second session about the nervous system.

We're gonna talk about membrane potentials.

So, membrane potentials are gonna be changes or

differences in the amount of charge on either side of the membrane of cells,

and in particular, we're gonna talk about neurons.

And this is gonna be our first introduction into

the method of communication in the nervous system.

So, Dr. Dacoy has already talked to you about the electrochemical gradient.

So, these are going to being chemical gradients of electrically charged

molecules, and she's told you how the sodium potassium ATPase is going to be so

important in establishing these gradients, and we'll talk more about that later.

And the whole point of this,

our bodies spend a significant portion of their ATP setting up this gradient,

and they do it so that they can harness it to do different jobs.

So, sometimes they use the potential energy that's in that gradient

to help transport certain substances across the membrane.

We'll be hearing about that in other organ systems.

But the other thing it can do is use this gradient to allow ions

to flow across the membrane and form a current that can be used in signaling.

And so that's what we're gonna be talking about for the next several minutes.

So, one thing that you need to remember, and you don't

need to remember these concentrations down here at the bottom of the slide.

But what you do need to remember is that the sodium potassium ATPase is going

to make it so that there's a larger concentration of

sodium on the outside of the cell compared to the inside of the cell.

And there's much more potassium on the inside of the cell

compared to the outside of the cell.

And we'll see in a few minutes why this is really helpful.

We've taken two different positive ions and

segregated them to two different sides of the membrane, basically.

And we'll see how that's advantageous.

The other thing to keep in mind is that in neurons and

in many other cells of our body, when those cells are at rest, there is what

we call a negative membrane potential, and that's what's shown right here.

And what a negative membrane potential refers to is the fact that

there are more negative charges inside the cell than outside of the cell.

And so, we're gonna talk about that in much greater detail as well.

So, we've established that in a normal cell, we're gonna

have a lot more sodium on the outside and a lot more potassium on the inside.

That's what's shown here in a simplified system on this slide, where,

here, we have a 150 millimolar potassium chloride on the inside of the cell and

150 millimolar sodium chloride on the outside of the cell.

Since these are potassium chloride and sodium chloride, and we've got

equal numbers of positive and negative charges on each side of the membrane,

right now, the membrane potential equals zero millivolts.

There's no charge difference between the sides of the membrane.

Also, you'll note in this figure that we have some ion channels, which are these

blue boxes, and there are several things to keep in mind about ion channels.

One is that they're specific.

And so, these happen to be potassium channels, which means that they're only

going to transport potassium for all intents and purposes, that really, sodium

is not going to be able to go through these channels in any perceivable manner.

So they're specific for potassium and they're going to be gated.

This makes sense.

If you spend ATP to segregate sodium and potassium and to build this

electrochemical gradient, you don't want ion channels just sitting there, open and

unregulated, and letting ions flow down their concentration gradient.

You want ion flow to be highly regulated if you're gonna put so

much energy into building the gradient.

And so, these will be gated, and

we'll be talking about the different modalities that can gate these channels.

Then the other thing to keep in mind is that when the channels are open,

they're bi-directional.

Potassium's gonna be able to move in either direction across these channels,

depending on both the chemical gradient that's present and

the electrical gradient.

So, in step 2, that's what's happening.

We are opening a potassium channel.

And as we do that, we've got no electrical gradient, and

we have a very large chemical gradient for potassium.

And so, potassium is going to leave the cell.

As soon as one potassium leaves, then all of a sudden,

we've got a excess of negative charge on the inside

of the cell versus the outside, and we've got a negative membrane potential.

Potassium is gonna continue to diffuse through the channel,

down its concentration gradient.

So we're gonna get more and more positive charges on the outside of the cell, and

more negative membrane potential for the cell, until finally,

in step three, we've got enough positive charges outside of the cell that now

the electrical gradient is driving some potassium back into the cell.

So we've got more potassium leaving, but

now some potassium's starting to head in towards the cell.

In step four, we've got the same number of potassiums

leaving the cell as we have entering the cell.

And that's when we have now reached the equilibrium potential for potassium.

So, we still have a very large chemical gradient,

even in step four, so that the number of potassiums that

have left to form this electrical gradient is still very small.

We still have a very large chemical gradient, but

we have a large enough electrical gradient that it is opposing the chemical gradient.

And so, the membrane potential, at this state,

in step four, is going to be positive, and

it's going to be the equilibrium potential for potassium.

So, what that number is in millivolts, it's going to be a negative number.

I'm sorry, that's going to be a negative membrane potential.

And it's going to be based solely on what the potassium concentration

is on the inside of the cell versus the outside of the cell.

That's what's gonna determine the millivolt number, that's gonna be

negative, that is going to be the equilibrium potential for potassium.

So this is going to be, we're gonna look at what this means for

the cell in just a moment, but you can think of it this way.

If we have only potassium channels open, and we have a certain

concentration of potassium inside the cell versus outside the cell,

if you just open those potassium channels, and those channels only,

then the membrane potential will become The equilibrium potential for potassium.

So here's another way of looking at it where we said at rest, our membrane

potential is going to be negative, which is what's shown right here.

And we'll talk about why that is in a minute.

Now if we look at the case for sodium in a normal cell, we said it's gonna

have a much higher concentration of sodium on the outside of the cell.

And that means that if we open sodium channels,

sodium is going to want to enter the cell based on its chemical gradient

but also based on its electrical gradient.

Because we said at rest we have membrane potential.

And so that means that if we open sodium channels and

let sodium keep entering the cell, the chemical gradient is going to have

sodium come in and the electrical, until finally the inside of the cell is so

positive that now sodium does not want to enter the cell.

Because of the electrical gradient, it wants to leave.

And so, we will get to a spot which in a normal cell or

in a neuron, is often about a positive 60 millivolts where there's so

many positive charges inside the cell.

The sodium still wants to come in because it still has a chemical gradient, but

it now also wants to leave because the inside of the cell is so positive.

And so at that point you've reached the equilibrium potential for

sodium if only sodium channels are open.

We can look at the same case for potassium which we have a little bit previously

where we said it's gonna be more concentrated on the inside of the cell

which means our chemical gradient is trying to get potassium to leave.

But because our resting membrane potential is negative when we

first opened the potassium channels, potassium wants to enter the cell.

And so that means that if

we open potassium channels and let the cell come to the equilibrium potential for

potassium, that's gonna be at a negative membrane

potential when we have opposing and equal

forces from the chemical and electrical gradient.

So, basically, what the cell has done is it's taken two different positive ions,

put them on opposite sides of the membrane so

that they will have opposite charges for their equilibrium potential.

We'll see how that is gonna be harnessed in order for the cell to signal.

So we've now established what an equilibrium potential is for

a certain ion and that it's going to be based on the concentration of the ion.

But now we wanna talk about how resting membrane potential is established.

So, one thing we know is that we need to establish the electrochemical gradient,

and that the sodium potassium ATPase is going to do that.

And when it does that, it's going to move three sodiums outside of the cell and

bring in two potassiums inside the cell.

So that's going to establish our gradient, but you'll notice that is sending out

three positive charges and only bringing back in two positive charges.

So the sodium potassium ATPase is going to establish a little bit

of a charge difference, which is going to cause a little bit of a negative membrane

potential because of that difference in numbers of positive ions that it's moving.

The other issue is what channels are open at rest.

So we said if only sodium channels are open, then the membrane potential is gonna

become the equilibrium potential for sodium.

If we open only potassium channels, then the membrane potential is

going to become the equilibrium potential for potassium.

If we open them up, sodium and potassium channels, so

that their membrane is permeable equally to those two ions,

then the membrane potential will reside in between the two equilibrium potentials.

So it will be proportional to whichever ion is most permeable.

And if the membrane is equally permeable to sodium and potassium,

then the membrane potential will reside between their equilibrium potentials.

In the case of at rest,

we have the ATPase pump forming the sodium and potassium gradients.

But we also have leak channels,

which are going to be ion channels that are letting some ions through, and

they're going to be determining what the resting membrane potential is.

So that we have a fair number of potassium leak channels and

fewer sodium leak channels.

And so because we have a greater permeability of the membrane to potassium,

because we have more potassium ions leaving through the leak channels then we

have sodiums coming in through the sodium leak channels, then that means our

membrane potential at rest is much closer to the equilibrium potential of potassium.

Now if we had only potassium leak channels,

then the resting membrane potential would be the equilibrium potential of potassium.

But since we have a few sodium leak channels operating,

then the resting membrane potential is just a little bit above

the equilibrium potential for potassium.

So it's another demonstration of- to determine what the membrane potential will

be at any certain time, you need to know the concentrations of the ions,

because those determine the equilibrium potentials.

And then you need to know which ion channels are open,

or how permeable the membrane is to one ion versus the other.

When we talk about membrane potential changing,

we have certain terms that we use.

I'll be using them a lot so this figure is to help you with that.

When we're at resting membrane potential,

we say that the cell is polarized because there's a difference in charge

across the membrane and it happens to be more negative.

So, at rest a cell is called polarized.

If for instance we open sodium channels so that sodium is going to rush in and

our membrane potential is going to increase and head towards the equilibrium

potential for sodium, then we say that the cell is depolarizing.

It's becoming less polarized.

The membrane potential is less negative.

If it keeps going in that direction and then

the membrane potential becomes positive, then you can call that an overshoot.

If then we close the sodium channels, and then open potassium channels, which means

that the membrane potential is going to head towards the equilibrium potential for

potassium, then the membrane potential will head toward zero,

and then become more negative, and that is called repolarizing.

Because now, the cell is becoming polarized again.

If we keep those potassium channels open, and then we go more negative

than the resting membrane potential and closer to the equilibrium potential for

potassium, then the cell is hyperpolarizing.

It's becoming even more polarized.

Than it is at rest, even more negative.

So I'll be using these terms, so it's good to become familiar with them because you

will hear them a lot when anyone's talking about changes in membrane potential.

We're gonna finish up this section

talking just a little bit about a type of change in membrane potential.

So it gets confusing I think because we've talked about membrane potential,

equilibrium potential.

But what we're gonna start talking about now are types

of changes in membrane potential.

And so this type that we're gonna spend just this image talking about,

are called graded potentials.

They're called graded because the amount of change

in the membrane potential depends on the strength of the stimulus.

So it can be graded, it can be a small change or large change,

depending on the strength of the stimulus.

I think the easiest thing to think about when you're thinking about a graded

potential is to imagine that you have a ligand-gated ion channel.

Meaning that it's gated by binding a ligand,

which is very often a molecule like a neurotransmitter.

So let's say we have in this image, a sodium channel that's gated

by a neurotransmitter, and it's sitting in the membrane, and we add to that little

spot in the membrane just the little small amount of that neurotransmitter.

So it's going to diffuse and right where we added it,

where it's most concentrated, it's going to be able to bind to these sodium

channels and open them, which is going to cause sodium to rush,

which is then going to cause a small depolarization of the membrane.

The membrane potential is going to be less negative in that spot, and

then as go you farther out from that spot in the membrane,

the change in the membrane potential is going to be be less and less.

The ligand, the neurotransmitter is diffusing, and

so its concentration is less.

So it's going to be less likely to open sodium channels.

And then we have sodium rushing in at this spot, they're going to diffuse,

and so their effect on the membrane potential and

increasing it is going to decrease as you go farther and farther from that site.

So that's what's important to realize about these graded potentials,

is that they are going to decay over space along the membrane,

and over time as you've just put a little pulse of that neurotransmitter, and

everything's diffusing away and then these ion channels close.

Then as we've already mentioned with graded potentials,

if you give a greater stimulus, then you're gonna have a bigger response.

So you're going to add more neurotransmitters the second time.

It's gonna be a stronger stimulus.

It's going to open more of these ligand-gated sodium channels,

let more sodium in, cause a larger increase

in the membrane potential and affect the membrane farther out.

And so that's what we see with this red curve, where we have a bigger response.

And so we'll see why these properties of graded potentials are so

important in the nervous system.

But in our next session,we'll be talking about a different type of change in

the membrane potential which you're probably familiar with, action potentials.

So to sum up this session,

we've talked about the equilibrium potential of a certain ion, which is when

the chemical gradient and the electrical gradient are gonna be opposing each other.

We talked about how the resting membrane potential is established,

how we need the sodium potassium ATPase to make the gradients of the ions.

And that we will have the potassium and sodium leak channels

that are also gonna be important in establishing at what

membrane potential our resting membrane potential sits.

However, if we inactivated the sodium ATPase

and we just let those leak channels happen or if we let those function,

then eventually we would dissipate the resting membrane potential.

It would become zero because we would have no more sodium or

potassium gradient without the sodium potassium ATPase, so

it's an extremely important molecule in signaling.

And then we finished up talking about graded potentials,

which are going to vary with the strength of the stimulus, and

they're going to decay over time and space along the membrane.

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