Neuroscience: 1:00 - 2:00Scribe: Melissa Precise

Wednesday, January 20, 2010Proof: Matthew Davis

Dr. LesterResting Membrane PotentialPage 1 of 4

Resting Membrane Potential [S1]
A few questions on Test 1 – the closed book exam. Going to have the anatomy we had, the basic cytology, and what we talk about today and through Monday at 1:00. Finish with epilepsy which is related to ion channels, etc. So that is what is going to be on the exam. Hopefully have a new syllabus now, sent it out.

Electronic Signaling in Neurons [S2]
Key thing about neurons is that they are excitable. Can think of them as lots of brain wiring with electricity running through them connecting different parts of the brain connecting different cells within nuclei, etc. This is fundamental to how neuroscience works. Over the next few days, we will talk about synaptic inputs to the dendrite, the transfer of the signal to the soma where we will generate an action potential if we reach a certain threshold. We will get action potential propagation down the axon, which is faithful and uniformly fast to get the signal out to the terminal. These two lectures will talk about the ionic basis of the resting membrane potential, action potential initiation and its propagation.

Learning Objectives [S3]
These are the objections. If we pay close attention to these and you can understand these then you will be ok.
Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential.
Basically, we are homing in on the fact that potassium ions (IN CLASS QUESTION) are very important for the resting membrane potential of neurons. Neurons have this membrane potential across their plasma membranes.
We have to know a version of the Nernst equation, so that we can actually compute the equilibrium potential of any given ion. By any given ion, we are only talking about Sodium, Potassium, and Chloride. Not too many.
What we want to know is what will happen when we change the concentration of an ion or its relative permeability to the membrane and its effects on the membrane potential.
Potassium is the key ion for resting potential and sodium is going to be key for the action potential (lecture number 2).

Learning Objective #1 [S4]
Objective #1 – How does the membrane potential come about?

The RMP [S5]
Show a little animation that is going to talk you through it. Then we will go back through it and talk about it again.
Video:
You should play the video a few times if not familiar and think about it. That is really the basis of the entire lecture – apart from the Nernst equation. The video forms the basis of the understanding of the membrane potential and how it comes about.

The Membrane Acts to... [S6]
Would like to stress a few things. All a cell needs or a neuron needs to do to generate a membrane potential is to have a difference in the concentration of an ion species – the inside versus the outside of the cell. As the animation said, the potassium is high on the inside and lower on the outside. On the other hand, sodium is high on the outside and low on in the inside. Somehow, the cell has managed to affect the concentrations on either side of its membrane. Does this by using the Na/K exchanger. This is an energy dependant process that drives Na out of the cell and K into the cell. Concentrates K on the inside of the cell and high Na on the outside of the cell. That is the first thing that the cell needs to do – affect concentration differences on each side of the membrane.
The only other requirement for generating a resting membrane potential as the animation said is to have the membrane selectively permeable to one ion species in this case. Because if a membrane was selectively permeable to everything. Everything would just run down their concentration gradient and would not have a membrane potential. So need to be selectively permeable. Just open a few K channels.

Initial Conditions and At Equilibrium [S7]
This is the animation written down. Initially here is the membrane potential and have some sort of ion which drives Na out and K in to the cell. Basically have got high K on the inside and low K on the outside of the cell. If open ion channels that will only allow K through, as the animation said,K is going to want to leave the cell. K is going to leave behind negativity, so as ions move there is going to be a net gain of negative charge on the inside and a net gain of positive charge on the outside. The negative charge is what starts to attract the K back in. When the force due to the concentration gradient equals the force due to the electrical gradient pulling it back in – that is the resting membrane potential. As the animation said, -60mV might be a good resting membrane potential. All along the inside of the membrane, there is more negative charges compared to outside the membrane. Not to say that all one or the other, but just more of them.
Key thing is that K is important. Need those two things: the difference in concentration and selectively permeable membranes.

Electrical Difference…IN vs. OUT [S8]
Demonstrate by get getting an electrode and sticking it inside the cell. Will be measuring a membrane potential difference or a trans-membrane potential (measuring difference between inside and outside of cell). If start with a electrode outside of the cell and a reference electrode outside the cell. If set the reference to zero (ground –zero). The other electrode is zero because it is also outside of the cell. If the electrode is then placed inside the cell then the resting membrane potential of -70 will be measured.

Electrical Difference…IN vs. OUT [S9]
If the electrode is then placed inside the cell then the resting membrane potential of -70 will be measured.

Can We Calculate the Potential? [S10]
Can we calculate that potential precisely based on the conditions?
Nernst – will give a simplified version of this to remember. Just wanted to show this version because it is derived from the basic idea that we have to have about the concentration gradient and the electrical gradient being equal. Nernst who developed this equation in the late 19th century had the equation for predicting the work or the force generated by the difference in concentrations. Then the similar equation for the electrical difference, so have a difference in the electrical balance of the ions.
When put those two equations together, get the electrical expression and the concentration expression which when set to being equal – get the Nernst equation. The Nernst equation determines the voltage at which the electrical and chemical forces for an ion are balanced – so it is at equilibrium. There is no net movement of ions. Potassium ions might move in and out, but for every ion that moves in another moves out so it kind of stays the same.

Learning Objective #2 [S11]
So we want to be able to use that.

The Nernst Potential for K+ [S12]
Take a reasonable example where K has been concentrated 10x higher on the inside than the outside. On the take home exam sent out today, it will have examples of these questions. Going to try to keep to relatively simple concentration differences so it is easy to compute. For example, might have 100mmol inside and 10mmol outside. 10-fold difference in concentration.
Going to simplified expression – this is the equation that you should remember.
Have 10 K outside and 100 K in. This is allows us to go from natural log to log base 10. The electrical terms kind of simplifies to 60 roughly over Z. Z is the valence. For K and Na – that is just going to be 1 and positive. For chloride, it changes things a little because it becomes negative. So remember is using chloride need to put a -1 for Z. That is it. Use that equation.
For reasonable physiological condition, this equation predicts a Nernst potential for K at -60. Since the resting membrane potential is primarily determined by the K ion. The Nernst potential for K is roughly the resting membrane potential.
If can do this problem, then will be fine.
IN CLASS QUESTION: What would happen if lowered the external 10 fold to 1mmol. Would the membrane potential change, would it stay the same? If it changes will it become more hyperpolarized(that is more negative) or will it become depolarized (more positive)?
Actually has clinical significance because there are conditions where people have too much K or too little K in their CSF. That will therefore change the excitability of neurons – either making them too excitable or not excitable enough.
Most people think it will change, and most people think it will change in the correct way (Hyperpolarized).
If want a point to memorize – memorize this – if K goes up on the outside then will depolarize the membrane potential and get more excitable.
If K goes up too much, then will actually get to a point where are unexcitable again. Will talk about that in the second lecture.
A little bit of depolarization, it is much easier to trigger an Action Potential so it is more excitable.

The Nernst Potential for K+ [S13]
All have to do is substitute those numbers in. Have 1 over 100 and get -120. So -120mV. Again as mentioned, hyperkalemia, which would be an increase in K, would have an opposite effect and depolarize the cell.

Learning Objective #3 [S14]
Have done part of this learning objective already. We have predicted the concentration of an ion. We haven’t changed the relative permeability on the membrane potential.
This is where we start to run into the basis of the action potential.

Other Ions Affect RMP [S15]
Going to complicate things a little bit and talk about terms that we need to know before we start talking about the Action Potential itself. As we mentioned, the cell uses the ATP in the Na/K exchange to concentrate ions against their natural will. Ions will just want to be kind of all diffused so there is equal Na and K on both sides. Can change that by putting pumps of various types in.
Various pumps, not just the Na/K exchange that account for the imbalance of ions across the membrane. As mentioned, they are not uniformly permeable. So the membrane is not permeable or leaky. Can say, that the more ion channels that we have open – the more leaky it is for ions to leak out of the cell.
The relative permeability of an ion will determine its contribution to the resting membrane potential. So the complicating factor is the small permeability of Na and Cl that offsets some of the potential set up by K.
Once given the real concentrations in physiological conditions for K in and out, if that was true the resting membrane potential would be much more negative than other cells. It would be much down around -90 or so. Might be closer to that in muscle cells, but in most neurons it is around -60 to -70mV. That is because there is a little bit of leak of other ions to compensate some of the K.

Concentrations of Other Ions… [S16]
Chart is pretty typical of physiological conditions. Can see the gradient is not ten fold but more like forty fold. Likewise, the gradient for Na is close to 10 fold. Close to ten fold, can see that is 60. Positive for Na because the concentrations are reversed. With K, we introduce the negative. Make it negative because it is outside over inside for the concentration, so the log of that will be a negative.
K is negative, Na is positive, and Cl is also negative near to K. Cl is another key ion that you need to know.
The resting membrane potential is not exactly at the K equilibrium potential.

General rule(s) [S17]
This is how he remembers – likes to draw the diagram on the slide where he can put on the equilibrium potentials for each ion. In the last one, all was put the concentrations into the Nernst and out popped the equilibrium potential for that ion.
Way to remember and how to understand how the membrane potential would move or what would happen if changed the concentration of an ion.
Put 0 on the line. Na is about 67mV. K is -98mV and Cl is -90mV. Resting membrane potential might be around -60mV. If the membrane potential was only permeable to K, the membrane potential would be down at the equilibrium potential for K. Because some Na tends to leak into the cell by opening up a few Na channels, so the membrane potential will start to move towards the Na equilibrium potential a little bit. Still mainly K selective. Likewise, have a little bit of chloride. Chloride is less negative than K which will tend to make the membrane potential move away from the K equilibrium potential.
Remember that the resting membrane potential is dominated by K and high permeability of the membrane at rest to K ions, but it is offset a little bit by some of the other ions because a few of those channels are open as well. Reason he tells us this is because will read K equilibrium potential but the resting membrane potential is not quite the same – so we do not get confused!

Ion Flux Explanation [S18]
Talk about things like the driving force. The driving force on an ion is the potential for that ion to move in and out of the cell if obviously have ion channels open that will let it in and out. The ion will only move in and out if have ion channels open. The potential exists only if ion channels open. Simply the difference between where the membrane potential is and the equilibrium potential for a given ion.
Can see the driving force for Na is incredibly big. Has a big driving force. If were to open a lot of Na channels all at once, then would expect a lot of Na to move into the cell because it’s driving force is large. Kind of like a battery. Have a big battery – has lots of potential difference. A 100 volt battery versus a 10V battery – so much more potential energy.
The other thing that should know is Ohm’s law which most probably do know. Voltage equals current times resistance because that is what we are dealing with when ions are moving across the membrane. Ultimately what we care about is the voltage change. The voltage change will be equal to the amount of ions that move the current times the resistance. If trying to change the membrane potential and it is very, very leaky, then will not be able to change the membrane potential very well because the ions are continuously coming out.
Transformed the equation because conductance is inverse of resistance so can change to Current = conductance times the voltage. The voltage is the driving force for a given ion. The conductance is going to be proportional to how many channels we have open. If have a lot of channels open, then have a big conductance. The amount of current that flows across the membrane will be high or will be a lot if have a big conductance times a big driving force. If open up a lot of Na channels at rest, we would predict a huge current coming into the cell carried by Na ions. The movement of the charged ion in time is the current flow so if get a lot of charge moving in a certain time then will have a high current. To do that, need a lot of ion channels open and big driving force on that ion. There will be no current if there are not any channels open (so if have no conductance). No current if have no driving force. If the membrane potential is at the equilibrium potential for the ion as demonstrate in animation, there will be no net current flow. The way to have a big current flow is to have the ion that want to get moving toward the cell be as far away from the cell as possible. If open up Na channels exclusively like 1000 fold than any other one, the resting membrane potential would move up more positive.
Reason to tell this is because it is not intuitive for those who have not seen it before, have to think it through – how the ions move depending on their concentration. Think about drawing out the model starting with high K on the outside and see what will happen. Likes this because if use and can remember this material, then will understand the ionic basis of the action potential – that is basically what we have been explaining here without talking about the action potential.

[End 37 min]