Membrane Potentials

[CookieZine]

TBR mentions that the concentrations of cations and anions balance each other out. This occurs both inside and outside the cell and represents electroneutrality. If both the inside and outside of a cell are electroneutral then what is it that creates the negative resting membrane potential? Is it the relative concentrations of Na+ and K+? I don’t understand how we can have electroneutrality and still have a membrane potential.

They use the Nernst equation and calculate the voltage for two cases: one in which the membrane is just permeable to K+ and one in which the membrane is just permeable to Na+. For K+ there calculated value is V=-87 mV. In the illustration above it they show the inside of the cell being -87 mV and the outside of the cell being +87 mV. In the case of Na+ the calculated value is V=+60 mV. In the illustration above it they show +60 mV inside the cell and -60 mV outside the cell. The Nernst equation, although not tested on the MCAT, is the only way I can explain why the voltages are equal in magnitude but opposite in sign. Without the Nernst equation I want to use the potential difference equation. In the case of K+ this would yield V=+/-174. What exactly am I missing here? If we say that the resting membrane potential is V= -80mV does this mean that the outside of the cell has a voltage of V=+80 mV?

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[studyq]

I'm not sure about the Nernst equation or voltage potential, but I know that there is an electric gradient because of negatively charged proteins inside the cell. This gives the inside of the cell a net negative charge.

 

[lumpyduster]

This is simplified, but the way I understand it is that the membrane is semipermeable to some ions (I usually just remember potassium). If potassium leaves the cell (going down its concentration gradient), then there will be some anions missing their counter ion. This is what makes the inside of the cell more negative than the outside. BUT, unpaired positive ions on the outside of the cell will line up with unpaired negative ions on the inside of the cell (with the membrane in between them) and this is why they talk about the system being electrically neutral.

And with the voltages - the -80 mV is relative to the outside. So it's not +80 mV outside and -80 mV inside - potentials are always relative to something else. It's like an adjective ending in "-er" like "newer" or "higher." A single pair of shoes cannot be "newer." There has to be another pair of shoes to compare them to. I haven't looked at membrane potentials from a physics standpoint, so this is how I understand it.

I think those Nernst equations tell you the equilibrium potential for K and Na, or the potential the cell will be for K and Na to stop flowing into or out of the cell. So for K, the eq. potential is -80 mV; you know that resting potential is around -60 mV or so. For K to reach its equilibrium potential, the inside of the cell has to get even more negative than the outside. How does that happen? Well, K has to leave the cell.

Similarly, for Na+ the equilibrium potential is +60 mV. If the resting potential of the cell is -60 mV, then how does Na need to move to make the cell more positive? Into the cell! These equilibrium potentials are the driving forces of action potentials. This is why sodium will move into the cell once its voltage-gated ion channels are open and why potassium will move out of the cell once its voltage gated ion channels are open.

 

[Czarcasm]

TBR mentions that the concentrations of cations and anions balance each other out. This occurs both inside and outside the cell and represents electroneutrality. If both the inside and outside of a cell are electroneutral then what is it that creates the negative resting membrane potential? Is it the relative concentrations of Na+ and K+? I don’t understand how we can have electroneutrality and still have a membrane potential.

Bare in mind there are a number of highly negatively charged molecules confided within a cell: DNA, RNA, amino acids, proteins, etc. all carry a negative charge. For instance, you have DNA confined in the nucleus, and some protein channels/carriers attached to various organelles. Moreover, these molecules are not readily diffuse through the plasma membrane and are predominately the basis for the resting membrane potential being negative in cells.

What exactly am I missing here? If we say that the resting membrane potential is V= -80mV does this mean that the outside of the cell has a voltage of V=+80 mV?

Yes.

 

[Concrete Jungle]

Think of it this way. At the resting membrane potential, the membrane is much more permeable to K+ than Na+, ~100x more. The concentration gradient drives the K+ out of the cell but the negative charged ions/proteins within the cell attract the K+ and work against the diffusion. The Nernst equation allows us to figure out the electrical potential that counters a given concentration difference, so its equilibrium point. The Nernst equation though is only if the membrane is permeable to 1 ion. This is not the case in humans(and all living things but who knows). The potassium and sodium channels permeability are a function of the local potential. So we normally use the Goldman equation that takes into account all ions that are permeable as well as the their relatively permeability to each other (remember this ratio changes as potential changes). That is why resting membrane potential is ~ -80mV, and the Nernst equation gives us -87mV and +60mV for potassium and sodium respectively. The Goldman will give us -80mV and this is intuitive because we know that at resting potential 100 potassium channels are open for every sodium, so the resting membrane potential is close to the theortical potassium only but slightly more positive due to some sodium permeability. Concentration difference + permeability = potential.

Anytime we talk about electrical potential the convention is to set some reference to 0 volts. So inside is -80mV the outside is 0 volts ...

You should think about how leak channels and pumps work into potentials.. and if you want to dig a little deeper look up driving force for an ion. The details really don't matter its how you can think through this stuff in novel ways that matters.