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Resting Cell Potentials and the Neuron


Cell Membrane Potential and Neurons. We're in 3A, and we are right around here. Resting Membrane Potential, this is a diagram of the cell membrane that you might find in a neuron. And, in particular, we're looking at ion concentration differences inside and outside of the cell.

Inside the cell, we have cytosol. And the chemical environment of the inside of the cell is quite different from what we find outside. With ions, we see a lot of sodium ions outside of the cell, we see chloride ions, where as inside, we see potassium. And we also see these A minuses, and that just stands for any variety of nonionic substances in the cell, such as maybe they could be negatively charged nucleic acids or proteins, that contribute to a negative charge of the cell.

What this all adds up to as a matter of fact, one of the more important things to know about this is that the inside of this cell has a relatively more negative charge than the outside. -70 millivolts is a pretty typical charge for the inside of a cell to have. And that's what called the resting membrane potential. One of the ways that this resting membrane potential is sustained, is through the sodium potassium pump which you might remember.

What this pump does is, it allows two potassium ions to enter the cell. This is inside here, while shooting out three sodium ions. It's expelling three, and it's taking in two, and both of these have a value of plus one charge. And so, it is maintaining a slightly more negative environment. It's also in that process, using up one ATP.

And, that's part of what allows this negative resting cell membrane potential. Now, this relative charge difference and ion concentration difference between the inside and the outside of the cell, contribute to the electrochemical gradient. And there are three things that affect how ions move across a membrane. The first is just the permeability of the membrane, and membranes tend to be selectively permeable to particular ions.

For instance, potassium tends to have a really high permeability or rather, membranes are very permeable to potassium ions. The relative concentration of ions on one side of the cell versus the other. And the relative charge difference between this two sides. For example, let's say a chloride ion channel opened up. If this happened, the chloride ions would want to move into the cell, because there are fewer chloride ions on the inside, and that's one of the things an ion wants to do.

Moving to a lower concentration, but it also wants to move to a place where the charge value is opposite from itself. As chloride ions move into the cell, the cell is going to have an increasingly negative charge. And at some point, the chloride is going to be less drawn into the cell, because that negative charge value of the inside of the cell makes it less attractive.

We can figure out at what point any particular ion reaches that state of equilibrium, at which the charge value and the diffusion value or the osmosis, cancel one another out, and we can do that using the Nernst equation. The E stands for the actual cell potential, the voltage. And, here, the RT over zF, this is really a way of calculating the charge value. We see the ideal gas constant, temperature.

And, that's divided by Faraday's constant times the ion's charge. And then, this last portion here the natural log of that ion concentration, the difference between the outside inside, this is the diffusion force. Will you need to know this on the MCAT? Possibly. I don't think it's extremely likely to appear, but I definitely think you should be familiar with the Nernst Equation.

You should know what it is. You should be able to recognize it if you see it. And, for the MCAT in general, you do want to get more practice using natural log and log base 10, being really familiar with those and translating between the two. Because you won't have a calculator on the MCAT. One thing the MCAT might want you to understand is, just how these different ions effect the resting cell membrane potential.

They're more interested in that than they are in having you calculate various equations. So, I'm gonna go ahead and just show you a couple simplified equations that might come in handy. They're easier to manipulate, anyway. If we're looking at sodium and potassium, which are two of the particularly relevant ions when it comes to neuronal cells.

We can find the charge value by taking the log of the ion concentration on the outside, dividing it by the inside, just like in the last equation. And, that's the log of that times 61.5. So, taking a lot of the numbers out of the equation just using this one. And for chloride ions, it's almost the same, it's just negative 61.5, and that's because these all have a charge of one, but up here it's positive and here it's negative.

You might need to know calcium or just be kind of familiar with it. Calcium has a plus two charge. And what that does to this constant here is it reduces it in half, so you wind up in 30.75, otherwise the equation's the same and this is what happens when you plug it in. For potassium, this is the only ion were we have, remember its outside divided by inside, where we have more ions inside of the cell relative to outside.

Therefore, we wind up with a negative voltage, a negative resting potential. And that's because we're taking the log of a number that's between zero and one. Sodium, we get around 67 millimoles, and with chloride 86, and this negative of course, is just coming from this multiplication constant. More important is just understanding how this fits with actual movement of ions, and how these ions affect the cell resting potential.

The membrane potential is always described according to what happening inside of the cell. And that's because as these particles move inside and outside the cell, the external environment doesn't really change charge. It's too vast. It's too negligible.

So, everything is always described according to the inside. Sodium, remember, we had a +67 voltage. And that makes sense because we have sodium moving into the cell. That's the direction it moves. It's bringing positive charge inside. Potassium is moving outside, it's moving positive away, so the resting potential is negative.

And with chloride ions, it's bringing negative charges into the cell. Goldman-Hodgkin-Katz equation. This one is, I have not heard it yet coming up on the MCAT, as something you need to solve. But I think it's definitely worth knowing about, definitely worth recognizing. And probably, most importantly just sort of understanding how this fits with intense to calculate resting cell potential.

In the examples before, we were just looking at the permeability, the resting potential of one particular ion at a time. Of course, that's not how the real world works. We have multiple ions that are all contributing to the resting cells potential. This calculation takes that into effect, and it also takes permeability into effect.

And therefore it's a more accurate way of describing the overall cellular resting cell potential. Talking about these equations also sets us up to talk about the other subject of the lesson, which is how neurons react to these different potentials. Suppose the permeability of one of the ions changed? Then, the electrical potential of the cell would change and this is partly what happens with neurons.

Here we have a graph of what can happen in a typical neuron, the dendrites contain what are called Ligand-gated ion channels. And these are ion channels that open when they bind to a certain molecule. Most of the times that's some kind of a neurotransmitter. This changes the permeability of the membrane to that ion, and with that, it changes the potential.

The typical resting potential of a neuron is around negative 70, like we've said. Suppose, we open up just a few sodium ion channels. All right? So, we're bringing more positive value into the cell. Suppose, we open up a few sodium ion channels in the dendrites, in response to some kind of stimulus.

Then, we'd expect to see a slow spread of sodium ions into the cell body. And the membrane potential would become slightly less negative, or more positive. This is called the movement of depolarization, as we see here. And this potential change will spread out from the spot where the sodium ions are coming in from. However, if the number of channels open is too small, then nothing much is gonna happen, the charge difference won't be great enough to push it to this, say, -55 measure of a threshold.

And then, we have what's called a failed initiation. However, suppose that several of the neuron's neighbors are all tossing neurotransmitters at it. A bunch are open at once, lots of positive ions coming in, then these graded potentials will sum up together, and we don't have a failed initiation, we have an action potential.

And that's when the charge reaches the axon hillock, goes down the axon and sets off some cascade of activity in the neuronal system. Question. Increased permeability to ions of which of these elements would not depolarize the cell? Sodium, chlorine, calcium, or we cannot answer this question unless we know the relative ion concentrations.

Pause the video if you'd like. The answer is B. Chlorine or chloride ions. And, the reason is because chlorine forms anions. They're negatively charged, and that means they've fuller polarize the cell. They don't contribute to its depolarization.

And why can't D, be the answer? Well, because we're not trying to solve for a particular equation for a particular cell potential. The relative ion concentrations don't really matter, as long as we know that chlorine is always in greater concentration on the outside of the cell. And so, it's trying to always move into the inside.

Which means that it's going to create a more negative internal cellular environment, compared to if the permeability was last or acquired ion. Therefore, we can eliminate D, as well.

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