Membrane Potentials

Here are your questions that you have sent me regarding membrane potentials.  As always, feel free to send me any others via email and/or comment on this blog post.

1.  I have a question regarding the following question:

49) Which of the following scenarios would most likely depolarize a neuron the most?

A) Open chloride channels 

B) Open voltage gated potassium channels 

C) GABA binding to post-synaptic receptors opening a chloride channel 

D) Efflux of K+ 

E) Open voltage gated sodium channels

Off the back I was able to eliminate the first three choices, but that's when I hit a road block. 

I understand Sodium's role in depolarization if the Voltage Gated Sodium Channels are open then more positive comes in to a negative cell. 

I don't understand why an efflux of K won't have the same effect… When I read Efflux I assumed K+ leaving the cell and depolarization as any voltage less than the resting membrane potential. If K is leaving the cell than the cell is becoming more positive.

Remember, K+ is a positively charged ion (a cation) and so if it leaves the cell it is removing a + from the cell, leaving behind a - charge and therefore causing the inside of the cell to become more negative, or hyperpolarize.

2.  Will you give us the full equations or the simplified equations on an exam?

Remember that you are responsible for memorizing the Nernst Equation, so I will not give you this equation at all.  If, however, I were to give you either the Goldman-Hodgkin-Katz Equation or the Chord Conductance Equation they would be in a form you could utilize.  In other words, do not memorize the universal gas constant or faraday's constant or even the equation, but it would be given to you.

 

3.  I was confused a little bit in on the slide where you say Chemical drive = Electrical drive. 

-I have in my notes that at equilibrium a chemical drive is equal and opposite to and electrical drive.

- Can you clarify that for me just a bit. what I'm thinking is that once the chemical drive is reached lets say at 20 mM then the electrical drive will be (-)20 mM?

- I wasn't quite clear on that.

The value of the chemical drive in terms of a vector is equal to the value of the electrical drive.  Now, you can calculate the value of the electrical drive by calculating the equilibrium potential and that is the difference in charges where those two driving forces would be equal and opposite.

Second question

- on slide 16 you have both Na and K equilibrium. 

- is this on the same cell? because the charges are different. 

- so what Im thinking is that K (due to its chemical gradient) will have a (-) charge on the inside and Na will have a (+) charge on the inside. 

- so my question is, are we looking at the ions independently but yet still in the same cell? 

- because the whole cell should be (-) right? [if we were looking a a neural cell] 

I suspect you actually mean slide 21 because slide 16 discusses only the movement of K+, but in slide 21 we are looking at where both Na+ and K+ would be at equilibrium given the concentrations on slide 19.  As you can see in the figure the top indicates where K+ would be at equilibrium and that is if the inside of the cell is negative with respect to the outside (as you indicate it should be) and the bottom represents where Na+ would be at equilibrium and that is where the inside is + with respect to the outside.  Therefore, this is not one cell and two points, but what it would look like for each individually.  From a typical resting cell, however, you are NOT at the equilibrium potential for any ion so their electrochemical drive would be net in the direction of which way the ion would need to move to bring the membrane potential to its equilibrium potential.

4.  For some reason I'm missing something to tie everything together. So I need your help. I'm going to try to explain equilibrium potential for K+ and please tell me what I'm getting wrong or what I'm missing.

In today's lecture you explain that if we had two compartments that were initially equal in ions charges, osmolarity but not permeable to any ion, there would be no membrane potential. But if we made K+ permeable it would diffuse down its chemical gradient causing an electrical gradient which in turns causes a membrane potential.

Now I understand that the electrical gradient is going to try to reach equilibrium by moving charges across the plasma membrane. Does these movement of charges means that K+ is not the only ion being move to reach an electrical equilibrium? Because I'm trying to understand how for K ions that there would be an equilibrium potential, which means that both the chemical gradient & electrical gradient are balance. I always assume that chemical equilibrium means concentration of that ions are equal. 

THIS IS KEY, CONCENTRATIONS DO NOT BECOME EQUAL FOR IONS TO REACH EQUILIBRIUM!!!!! In order for ions to reach an equilibrium there will STILL be a concentration difference and therefore a concentration gradient and also an electrical difference and electrical gradient.  It is where THESE TWO GRADIENTS ARE EQUAL that ions can be at equilibrium.  THIS IS KEY!!!  As I mentioned in class, you want to believe they will get to a place where the concentrations are equal but THAT IS NOT TRUE for charged molecules (i.e. ions).  For ions, it is the equal balance of the two driving forces that occurs at equilibrium.  So, at that place there is a difference in charges which is equal to the equilibrium potential that can be calculated using the Nernst Equation.

5.  Potassuim leaves the cell so its electric potential (-90) will equal the cell's resting membrane potential (-70)? So would the electrical gradient of potassuim go out of the cell? Or is the electrochemical gradient out of the cell?

If you make the cell (with a resting membrane potential of -70mV) permeable to potassium with an equilibrium potential of -90mV, you are correct it will leave the cell to try to bring the resting membrane potential towards its equilibrium potential of -90mV.  This is potassium moving with its electrochemical gradient.  Ions ALWAYS move with their electrochemical gradient.  In this case that is in the same direction as the chemical gradient, but not in the same direction as the electrical gradient. 

6.  If the membrane potential is becoming less negative or more positive why would that cause inactivation of the sodium channels, because sodium's membrane potential is positive 60?

A woman with sever muscle weakness is hospitalized.  The only abnormality in her laboratory values is an elevated serum K+ concentration.  Why does the elevated serum K+ causes muscle weakness?

A.  The resting membrane potential is more negative than normal

B.  The K+ equilibrium potential is more negative than normal

C.  The Na+ equilibrium potential is more negative than normal

D.  Na+ channels are inactivated by a movement in the membrane towards less negative values

E.  K+ channels are inactivated by a movement in the membrane towards less negative values

 

Hopefully today's lecture cleared this up.  Voltage-gated Na+ channels need to have a change in voltage to open, but become inactivated if they are open long enough.  In order for them to then go back to closed, however, there needs to be another change in voltage that is repolarizing and if that does not occur then the Na+ channels can get stuck inactivated causing a refractory period for the action potential leading to lack of action potentials in the muscle and therefore lack of contraction.

 

7.  For today's membrane potential lecture, on slide 23's notes section you give two examples that I copied and pasted here to save you time:

Example 1:  Were the membrane potential -70 mV and an ion channel for calcium were to open, the ion must diffuse in the direction that will produce a +127 mV membrane potential.  Since calcium has a +2 charge, it would influx, that is, move towards the more negative cytoplasm by facilitated diffusion. The total difference in potential {127 – (-70)} of 197 mV between the membrane potential and the equilibrium potential is extremely great and would provide a very strong motive force for diffusion.

Example 2:  Same membrane potential, but the ion channel that opens is for chloride.  The ion must diffuse in the way that will produce a -88 mV membrane potential  Chloride has -1 charge and the equilbrium potential is more negative than the current membrane potential, so the anion must influx.  The difference in potential is only 18 mV {-88 – (-70)}, less than 1/10 of the driving force for calcium, so it will provide a weaker motive force for diffusion.

I think I understand the concept here, but where did you get the values +127 mV and -88 mV from? Is this something that we're able to calculate from the given information or is it just given?

There was not any information given in these examples for you to know that Ca2+ equilibrium potential was +127 and Cl- equilibrium potential was -88mV.  Those ARE their approximate equilibrium potentials under typical resting concentrations, but that is not something I expect you to memorize at this point.  I do expect, however, that if I gave you concentrations for these two ions you could calculate those values if asked.  So in essence, this information in this instance was just given.