The Na/K pump works to increase the extracellular concentration of Sodium and increase the concentration of Potassium inside the axon.
Some how this makes the resting potential negative if we were to probe the membrane with a potentiometer leaving the reference probe inside the cell.
I figured this was because relative to the outside of the cell, the the inside is LESS positive (because of the 2K:3Na ratio).
I think I've been wrong. I read if you block the K-leak channels, the membrane will depolarize itself because the inside will just become more positive.
Does this mean, the negative resting potential is actually from the MOVEMENT of potassium OUT of the axon via K-leak channels? And depolarization is from the MOVEMENT of sodium INTO the cell?
Is this charge separation alone not sufficient to create a negative resting potential? Does potassium have to be constantly leaking in order for it to have a negative resting potential?
If this is the case, how does the cell actually maintain this resting potential for an extended period of time? It can't be from the Na/K pump, because Potassium is the only one leaking, not sodium.
I think you need some evidence for the bolded...there could be other processes which allow Na
+ in, some leakage, etc.
But aside from that, you need to remember what the potential represents. The extracellular volume is large enough that its relative ion concentration is fairly stable. Therefore, we can mostly consider the change in concentration of the
inside of the cell when discussing this.
A couple of things to clarify: current is defined as the movement of positively charged particles across the membrane. In neuro, a positive current is when there is net cation movement INTO the cell.
Potential IS charge separation; that is its definition. So yes, 'charge separation is sufficient' to create the membrane potential. However, there is a lot of behind-the-scenes stuff going on to stabilize and maintain the baseline membrane potential. You've touched on a lot of it; Na/K pump, K leak, etc.
A quick note here, potential is defined as 0 when the cation concentration on either side of the membrane is equal. As mentioned before, the extracellular [cations] is relatively stable, so we primarily consider the intracellular fluid when determining potential...more cations inside than out = positive potential, fewer inside = negative potential.
Now, let's look at the equilibrium potentials of the relevant ions. The equilibrium potential is the charge difference needed to prevent the movement of an ion down its concentration gradient. So, a charge difference of ~80mV is needed before the rate of potassium movement down the concentration gradient is balanced equally by the rate of potassium movement away from the area of more positive charge. In a neuron, potassium's concentration gradient is from inside to outside, so in order for it to be at equilibrium, there would have to be a large positive charge on the OUTSIDE of the cell, pushing K+ back in. This is why the equilibrium potential of potassium is actually -80mV.
Sodium's is positive (about 60mV) because there would have to be a higher positive charge inside the cell in order to balance the inward movement caused by the concentration gradient.
Cool, so...how do we get from equilibrium potential to membrane potential? Well, equilibrium potential only applies when only 1 ion can move across the membrane, and it can do so freely. At rest, however, neither potassium nor sodium can move freely across the membrane, there is active transport, and each ion is driving the membrane potential in a different direction. That last part is the key:
membrane potential is largely determined by the relative permeability of the membrane to potassium and sodium.
If you increase the sodium permeability or decrease the potassium permeability, you drive the membrane potential towards the equilibrium potential of sodium...which is UP from the normal balance...and vice versa.
I think it would be overly simplistic to assume that either potassium or sodium is ever truly 100% blocked from crossing the membrane...the important part is the relative permeability of each. So yes, if you close any given population of K+ channels, you will increase the potential from the resting potential, and yes, K+ mvmt contributes a lot to the stability of the resting membrane potential (you can tell by how close it is to the equilibrium potential of potassium).
