There are four ions and molecules that contribute to the RMP. These are:

• Sodium ion, Na+
• Potassium ion, K+
• Chloride ion, Cl-
• Negatively charged proteins

Let’s set the stage first. Note that there are high concentrations of Na+ ions and Cl- ions outside the neuron, but high concentrations of K+ ions and negatively charged proteins inside the neuron. Also note that none of the ions or molecules can freely diffuse across the neuronal membrane. Let's also remember that the voltage inside the neuron is more negative with respect to the outside. The reason for this will become apparent shortly!

Now for the explanation. Neurons have ion channels that are selectively permeable to K+ ions. A few sentences ago we said that K+ has a high concentration inside the cell so, as you would expect, K+ ions want to diffuse out of the neuron, down their concentration gradient, using these selective ion channels. Now quite a lot of K+ ions leave by this way. Because some K+ ions are leaving the neuron, the inside becomes more negative.

So, we’ve established that some K+ ions leave the cell down their concentration gradient. But will this process go on forever until all of the K+ ions leave the cell? No, and here’s why. We are talking about ions here. Ions have an electrical charge, and therefore they have an electrical gradient too. If lots of K+ ions leave the neuron down their concentration gradient, an electrical gradient is going to be established in the opposite direction. This electrical gradient will try to push most of the K+ ions back into the neuron via the leaky K+ ion channels. The overall result of these two opposing gradients is the formation of an equilibrium (i.e. the net number of K+ ions going in equals the net number of K+ ions going out). This equilibrium occurs at -97mV for K+ ions. So, when the voltage difference is -97mV, there is no net movement of K+ ions into or out of the neuron, and most of the K+ ions are kept inside the neuron. This is shown in part 1 of Figure 1.2.1.

But the RMP is -70mV, not -90mV, so where did I get the -70mV from? The key is Na+ ions. Remember we said that Na+ ions are in a high concentration outside the neuron? Well, what happens with the K+ ions also happens with the Na+ ions. As you would expect, Na+ ions want to follow their concentration and electrical gradient into the neuron and, because there are some leaky Na+ ion channels , some Na+ ions (but not nearly as many as K+ ions) do move into the neuron. However, when the Na+ ion concentration and electrical gradients balance, most of Na+ ions stay outside the neuron and the Na+ ions reach their equilibrium at +61mV. This is shown in part 2 of Figure 1.2.1.

As Na+ ions and K+ ions both try to reach their individual equilibria, they end up compromising at -70mV. Neither ion perfectly reaches its equilibrium and so a few Na+ ions continue to leak into the neuron and some K+ ions continue to leak out. The neuron can't just let the leaks continue unchecked though, so it uses a sodium-potassium ATPase pump to help out. This is shown as part 3 in Figure 1.2.1. These pumps are located in the neuronal membrane and require ATP to work continually. These pumps force out 3 Na+ ions and force in 2 K+ ions – thereby keeping the internal part of the neuron negatively charged (and so maintaining the RMP at -70mV).

It's also worth talking about the negatively charged proteins inside the neuron. These are too large to diffuse out through the membrane and they help to maintain the negative internal state of the neuron. Chlorine ions don't have much net movement, because their equilibrium potential is -64mV (which is fairly close to the RMP).

So, in conclusion, the Na+ and K+ ion channels contribute to the formation of the RMP, but the Na+/K+ ATPase pump maintains the RMP at -70mV, by correcting the change in electrical potential caused by the leakage of Na+ and K+ ions.