How well do you remember and understand the Resting Membrane Potential in cells? Get ready to think back to your Biology prerequisite days, because this basic idea is the foundation to understanding nursing concepts as diverse as electrolyte imbalances, muscle movement, and myasthenia gravis.
The key to understanding Resting Membrane Potentials is to remember two things: 1) that Potassium (K+) is mostly on the inside of the cell, and the inside of the cell is negatively charged; and 2) that Sodium (Na+) is mostly on the outside of the cell, and the outside of the cell is positively charged.
Since both K+ and Na+ are positively charged ions, it can be a little confusing to keep track of which side of the cell wall is positive and which side is negative unless you also understand some of the other factors involved. The human body is wonderfully complex, and so there are a lot of other ions and negatively charged particles floating around besides Na+ and K+.
Each of these other ions also has a charge, either positive or negative. The positive and negative ions have special names that you’ve probably heard before. Knowing these special names can help make it a little more clear when we’re talking about this stuff. Here’s a trick to help you remember those special names (and keep track of what they mean!):
– Cation = contains the letter “t,” which looks like a “+,” which means it is positively charged
– Anion = has no “t,” therefore no “+,” which means it is negatively charged
So if you’re keeping track, that means that both Na+ and K+ are cations. So why aren’t both sides of the cell wall positive? Because there are also plenty of anions floating around with them. For example, Cl– is more abundant in the extracellular fluid than the intracellular fluid…but there’s still some Cl– located in both locations. And there are negatively charged metabolites and macromolecules located inside cells that have negative charges, too. That’s why, if you were to add up of the positive and negative charges inside the cell, the net charge would be negative because of those metabolites and macromolecules, even though our most numerous ion (K+) is positive. And if you were to add up all of the charges located outside of the cell, the net charge there is be positive.
Trick to Remember
One way to help you remember that the inside of the cell is the negative side is to think about where the anions are “imprisoned.” You’ll notice in the picture that there are some pink blobby things floating around in the cytoplasm (which is inside the cell wall). Those represent large anions, which are too big to leave the cell. Since they are anions (and therefore negatively charged), and they are “imprisoned” inside the cell, then the inside of the cell will always be negative when it is at rest. The anions “on the outside” (aka in the extracellular fluid) always have the option to just float away if they want to 🙂
So there are cations AND anions both inside AND outside the cell. But Na+ is the most common ion on the outside, and K+ is the most common ion on the inside of the cell. When you add up the charges for the cations and anions for all of the ions on the outside, the total charge is positive. When you add up the charges for the cations and anions on the inside of the cell, the total charges ends up negative.
It’s important to understand how these pieces all fit together so that you have the foundational nursing knowledge you need in order to understand more advance concepts that you’ll need to know for Med-Surg. For example, just remembering that Potassium is the most common ion inside the cell can help you understand why a burn patient is at risk for hyperkalemia…when all those burned cells die and pop open, they release a lot of extra potassium that was inside of them!
What do you think would happen if a client had hyponatremia (low sodium)? How might that affect their Resting Membrane Potentials?