Need some help understanding membrane potential

[BigToque]

My professor put something up the other day that I'm having trouble understanding since it does seem counter-intuitive. Why would adding sodium to the inside of a cell make it hyperpolarize, and adding it to the outside of the cell make it depolarize?

The math seems to work based on the examples he's written, but I'm having trouble understanding what the equations are actually saying about what's happening.

Put a whole bunch of positively charged Na into a cell, and logic is telling me that the inside is going to become positive relative to the outside, and you'll get depolarization.

 

[fralexandr]

it has something to do with the sodium-potassium pump and not the actual Na+ ion, though i've never personally learned any of this in enough depth to explain it
http://en.wikipedia.org/wiki/Na+/K+-ATPase

something along the lines of: the differing sodiumotassium concentrations are used as potential energy by certain cell membrane proteins (might be the transport proteins themselves, or some other protein in the chain of events)
the relative voltage/potential difference is measured as "negative" because the cell membrane proteins developed/oriented themselves that way, or because humans created their measurement devices backwards or prefer + vs - numbers (your pick)

that's the basic introductory college biology explanation anyway

membrane potential might help also
https://en.wikipedia.org/wiki/Membrane_potential

a video about it
http://highered.mcgraw-hill.com/sit...annels_and_the_action_potential__quiz_1_.html

google & wikipedia - when textbooks aren't enough

 

[mshan]

http://www.youtube.com/watch?v=7EyhsOewnH4

 

[Turbonium]

Ugh... I'm a bio major and I hate this stuff...

 

[yhelothar]

I do neuron simulation and this stumped me for a while. I think the wording made it confusing.

You should be right that when you "put" sodium or potassium into the cell, the cell will actually depolarize.

However, I don't think that's what your professor is referring to here.

You have to first understand what Vm is actually referring to here in the GHK equation. Vm here is not referring to the transient potential, but rather the equilibrium potential. The intracellular and extracellular ionic concentrations in these equations refer to the experimentally derived ionic concentrations for when the neuron is at equilibrium. This is not the transient ionic concentrations when you when you inject sodium or potassium into the cell, in which the nernst potential does not change. In fact, the Nernst potentials, Ena, Ek, Ecl, are constants for a given neuron.

So your prof didn't actually say you "put" sodium or potassium in the cell. I think what he meant was a higher concentration of sodium or potassium*at equilibrium* in the cell, would result in a hyperpolarized *equilibrium* potential.


To understand what the equations are saying, recall that the Nernst potential refers to the potential required to counteract the concentration difference in/out to keep the cell in equilibrium. Remember at rest, sodium has high concentration outside the cell, and low concentration inside the cell. Thus, if you add sodium inside the cell, the concentration gradient is lower, and thus the Nernst potential would also be lower.

 

[TheVrolok]

http://www.youtube.com/watch?v=7EyhsOewnH4

Was going to write up something .. then I watched this video to see what it had to offer and .. yeah, just watch the video. Nice and simple.

 

[yhelothar]

Was going to write up something .. then I watched this video to see what it had to offer and .. yeah, just watch the video. Nice and simple.

The video actually doesn't answer his question and only affirms his intuition that sodium entering the cell depolarizes it rather than hyperpolarizes it.

 

[mshan]

I looked more carefully at what OP said and I think his prof is talking about changing conditions determining resting membrane potential (keeping K and Cl gradients constant, but decreasing Na concentration gradient), thus getting more net hyperpolarization because K effect is constant, but Na effect has been reduced.


(I think confusion is that prof is not talking about action potentials and depolarization (Na channels opening and Na rushing into cell) or repolarization (K channels) in that setting, just resting membrane potential, like we, and OP assumed).

 

[TheVrolok]

The video actually doesn't answer his question, and only affirms his intuition that sodium entering the cell depolarizes it rather than hyperpolarizes it.

.. sure it does. It explains "Why would adding sodium to the inside of a cell make it hyperpolarize, and adding it to the outside of the cell make it depolarize?" Which is his question verbatim.

Edit: Ah, I also just read mshan's last post and think I fell victim to the same misread.

 

[yhelothar]

.. sure it does. It explains "Why would adding sodium to the inside of a cell make it hyperpolarize, and adding it to the outside of the cell make it depolarize?" Which is his question verbatim.

Edit: Ah, I also just read mshan's last post and think I fell victim to the same misread.

I guess my post was too windy to be understood. :\

 

[TheVrolok]

I guess my post was too windy to be understood. :\

I'm sleepy and very quickly TLR'd it

 

[BigToque]

I'm currently working on simulating neurons in a lab and this stumped me for a while. I think the wording made it confusing.

You should be right that when you "put" sodium or potassium into the cell, the cell will actually depolarize.

However, I don't think that's what your professor is referring to here.

You have to first understand what Vm is actually referring to here in the GHK equation. Vm here is not referring to the transient potential, but rather the equilibrium potential. The intracellular and extracellular ionic concentrations in these equations refer to the experimentally derived ionic concentrations for when the neuron is at equilibrium. This is not the transient ionic concentrations when you when you inject sodium or potassium into the cell, in which the nernst potential does not change. In fact, the Nernst potentials, Ena, Ek, Ecl, are constants for a given neuron.

So your prof didn't actually say you "put" sodium or potassium in the cell. I think what he meant was a higher concentration of sodium or potassium*at equilibrium* in the cell, would result in a hyperpolarized *equilibrium* potential.


To understand what the equations are saying, recall that the Nernst potential refers to the potential required to counteract the concentration difference in/out to keep the cell in equilibrium. Remember at rest, sodium has high concentration outside the cell, and low concentration inside the cell. Thus, if you add sodium inside the cell, the concentration gradient is lower, and thus the Nernst potential would also be lower.

This professor usually seems a little light on the details, so I appreciate the input.

The context in which this professor presented these slides was "If I stab you with a needle and inject a bolus of sodium into you, why are you going to die?", and then proceeded to try and explain why adding sodium inside the cell would cause it to hyperpolarize. It was an odd lecture

For the purposes of what I'm going to be tested on, I'm not sure I need to know the exact details. I hate it when I can't understand the actual mechanism, so at this point, I'm just trying to satisfy my curiosity. I'm gonna think about it a bit more.

 

[yhelothar]

This professor usually seems a little light on the details, so I appreciate the input.

The context in which this professor presented these slides was "If I stab you with a needle and inject a bolus of sodium into you, why are you going to die?", and then proceeded to try and explain why adding sodium inside the cell would cause it to hyperpolarize. It was an odd lecture

For the purposes of what I'm going to be tested on, I'm not sure I need to know the exact details. I hate it when I can't understand the actual mechanism, so at this point, I'm just trying to satisfy my curiosity. I'm gonna think about it a bit more.

Hmmm I don't know about injecting a bolus of sodium to kill you, but injecting a bolus of potassium is actually how lethal injection is done.

Potassium is an electrolyte, 98% of which is intracellular. The 2% remaining outside the cell has great implications for cells that generate action potentials. Doctors prescribe potassium for patients when there is insufficient potassium, called hypokalemia, in the blood. The potassium can be given orally, which is the safest route; or it can be given intravenously, in which case there are strict rules and hospital protocols on the rate at which it is given.
The usual intravenous dose is 10–20 mEq per hour and it is given slowly since it takes time for the electrolyte to equilibrate into the cells. When used in state-sanctioned lethal injection, bolus potassium injection affects the electrical conduction of heart muscle. Elevated potassium, or hyperkalemia, causes the resting electrical potential of the heart muscle cells to be lower than normal (less negative). Without this negative resting potential, cardiac cells cannot repolarize (prepare for their next contraction).
Depolarizing the muscle cell inhibits its ability to fire by reducing the available number of sodium channels (they are placed in an inactivated state). ECG changes include faster repolarization (peaked T-waves), PR interval prolongation, widening of the QRS, and eventual sine-wave formation and asystole. Cases of patients dying from hyperkalemia (usually secondary to renal failure) are well known in the medical community, where patients have been known to die very rapidly, having previously seemed to be normal.

So the bolbus injection isn't an injection into the cell, it's an injection into the bloodstream, which would then change the resting ionic concentrations.

This is much different than injecting ions into one cell, in which it would not change the resting membrane potential.

 

[BigToque]

Hmmm I don't know about injecting a bolus of sodium to kill you, but injecting a bolus of potassium is actually how lethal injection is done.

So the bolbus injection isn't an injection into the cell, it's an injection into the bloodstream, which would then change the resting ionic concentrations.

This is much different than injecting ions into one cell, in which it would not change the resting membrane potential.

You're right about potassium. I meant to say that, but I was thrown off by the slides I've been reading all night.

 

[yhelothar]

Don't blame ya. I was thrown off for a while as well, and I work on this stuff everyday, although we pretty much never touch the resting membrane potential stuff.

Hope it makes sense to you now.