Our example here tells us that the membrane potential inside a cell during an action potential is graphed below. So before we go on, I just want to orient myself to the graph. Alright. On the x-axis, we have time, which is measured in milliseconds, so this is really fast. And on the y-axis, we have membrane potential, which ranges from negative 90 millivolts up to 0 and then on to positive 30. Now we don't really need to worry about those values or the units. We just know that we're starting negative. We're going to cross 0 and we're going to end up slightly positively charged, again, inside the cell.
Now what it's asking us to do is to identify which part of the graph refers to depolarization and which refers to repolarization. And we have this image here showing our potassium ions and our sodium ions and our positive charge outside the cell and negative charge inside the cell that we've talked about already just to use as a reference. Alright. So as we go along here, we're going to follow this line. We see that it starts at negative 90 millivolts, so we're negatively charged inside the cell. Then the sodium channels are going to open. Remember, our cell is swimming in a salty sodium sea, so those sodium channels open and the sodium flows inside the cell because it's in high concentration outside of the cell, so it's going to diffuse to the inside and with it, it's going to bring its positive charge.
As the sodium flows into the cell, the inside of the cell gets more and more positive. It comes up to 0 and even passes 0 until it's slightly positively charged inside the cell. Well, if we started out polarized, this shift in charge, or bringing the charges on the other side, we're going to call that depolarization. Depolarization occurs when this curve increases up to the 0 mark and even overshoots it just a little bit. Now, once we've overshot it, the sodium channels are going to close, so no more sodium can enter the cell and the potassium channels are going to open. Remember, the potassium is in high concentration inside the cell, so the potassium is now going to flow out of the cell. As it flows out, it takes its positive charge with it. It takes positive charges out of the cell, so the charge on the inside of the cell is going to start falling. It's going to fall and fall, and then, the sodium channels are going to close until it levels out right where it started again. So we depolarized, and now we're getting this charge to fall back to where it started, and we're going to call that repolarization.
Again, depolarization and repolarization are key processes. This, you're going to see in much more detail when you talk about the nervous system. Following these ions, sodium coming into the cell and potassium moving out of the cell, and how they bring their charges with them is going to be really important for understanding how an action potential spreads this message through the sarcolemma. We're going to talk all about this and how it specifically refers to the muscle fiber, coming up next.
