Nernsting Around: How We Quantify Ion Movement
We can use the Nernst equation to find the equilibrium potential (Eion) of a given ion. At 37°C, for a monovalent cation, the Nernst equation is as follows:
The more general formula for all kinds of ions is as follows:
The Eion value tells us at which charge difference between the inside and outside of the cell that has chemical equilibrium for the ion in question. For example, with cellular K+ and Cl- concentrations, the Eion for K+ would be around -90mV. This tells us that, when K+ is at electrochemical equilibrium, the charge of the cell relative to the ECF is -90mV.
In essence, the force that drives diffusion of the ion from one compartment to another opposes the repulsion between like charges. If the Eion is not 0, it means that the force of diffusion takes priority over repulsion, since the concentration gradient opposes the electrical gradient. Eion can thus be thought of as a compromise between the electrical and chemical gradients of an ion.
One important thing to note is the math behind the deductions that one can make from the Eion value given. K+’s negative Eion tells us that there is more K+ inside the cell than out, since K+ wants to exit the cell really badly, as evidenced by the negative Eion (since K+ is a cation, a negative Eion suggests that at equilibrium, more K+ is outside the cell). However, it is NOT ALWAYS the case that a negative Eion suggests a higher concentration within the cell at equilibrium. For example, with an anion like Cl-, the Eion is -70mV*. However, because it is an anion, a negative Eion means there is more of it outside the cell, hoping to get into the cell.
One clarification: I’ve been saying that the Eion tells us about where an ion wants to go (concentration gradient-wise), but this is not really true. The relationship is not this direct; the previous paragraph is just an easy way to think about this concept. Here’s how it really works: we need to compare the Eion to 0. The Eion for K+ is -90mV. This is less than 0. This means that K+ wants to move out of the cell from the inside (assuming we start at a 0mV condition). Thus, there is more K+ in a cell. Below is a bullet-point summary of this logic for the three most important ions for the resting potential (Na+, K+, Cl-):
K+: Eion < 0 (-90mV). Cation wants to move out to achieve negative cell charge, so [K+] must be higher in the cell.
Na+: Eion > 0 (+60mV). Cation wants to move in to achieve positive cell charge, so [Na+] must be lower in the cell and higher in the ECF.
Cl-: Eion < 0 (-70mV). Anion wants to move in to achieve negative cell charge, so [Cl-] must be lower in the cell and higher in the ECF.
Friends, you don’t know how euphoric I am feeling as I write the conclusion to this article. The Nernst equation and resting potential is a concept that has bothered me for years, but I finally get it.
Here are some sources that helped me understand this topic:
Campbell’s Biology, Chapter 48: Neurons, Synapses, and Signaling.
https://www.youtube.com/watch?v=MplWXZTOk6o - This amazing video makes the ion balances very clear at a conceptual level.
*Coincidence? I think not! This number is the same as the resting membrane potential (-70mV), and we would interpret this similarity as follows: at resting membrane potential, Cl- is at its electrochemical equilibrium.
