Action potentials

Hello, and welcome back to Fundamentals of Human Physiology. My name is Charles Sevigny. Today, we're going to be talking about action potentials.

An action potential is a very specific signal generated by an excitable cell. In our context, we'll talk about that in neurons. But it also applies to skeletal muscle cells, other types of muscle cells, other specialised cells in the body that we'll learn more about as we move through the course. But what an action potential does is carry a signal along the cell membrane representing a change in the charge or the membrane potential across that membrane. Now, we need to think back to our last activity, when we talked about how we maintain resting membrane potential across a membrane by the balance of permeability between sodium, which wants to move into the cell and make it more positive and potassium, which wants to move out of the cell making it more negative. Simply by opening up permeability to one or the other of those ions, we can change the charge across the membrane.

Just to review some of that, think about this. We have a mystery ion, it doesn't matter what it is, with a positive charge and it has an equilibrium potential of -60 mV. Now, if membrane potential is currently sitting at -70, what will the ion do? Pause and think about that. In that case, the ion will have net movement into the cell. If you got that right, you're doing pretty well so far. What we know about this ion is that it has a positive charge. If the charge inside the cell was at -60 mV, the ion wouldn't move in or out of the cell. Whatever its concentration gradient may be, that's being perfectly opposed by this charge of -60 mV. But if the charge is more negative than that, then that's making a more attractive charge environment for that positive ion to want to move into the cell. As a result, the ion will move into the cell towards that more negative environment and continue to move in along with its positive charge making the inside of the cell more and more positive until ultimately it reaches -60 mV. Now, if you chose 'not enough information is provided' because I didn't tell how permeable the membrane was to that ion, you're also doing very well and you're one step ahead of me. The membrane, of course, has to be permeable to that ion in order for it to move across. We know that a normal cell membrane, without any channels in it, an ion can't get through. We need specific protein channels called leak channels for that ion to be able to move through the membrane. Now let's try another one. Same mystery ion with a positive charge, same equilibrium potential of -60 mV, but now our membrane potential is -40 mV. What's that ion going to do now? If you said that it's going to have net movement out of the cell, then you're correct. Because now, if that ion needs the inside of the cell to be at -60mV to hold it in, but the membrane potential is actually at -40 mV, it's not negative enough to hold that positive ion in the cell. As a result, the ion will start to leak out of the cell. Leaking out, making the cell more negative because it's taking its positive charge out of the cell until that cell reaches -60 mV. That's why we also call equilibrium potentials reversal potentials, because if the membrane potential is actually more negative than -60 in this case, then the ion will move into the cell. If it's more positive than -60, it will move out of the cell. By knowing the equilibrium or reversal potential of that ion and knowing the membrane potential, we can predict whether that ion will move in or out of a cell.