There's a very special series of events that will happen when a cell's membrane potential hits a certain specific point and leads to what's called an action potential. Some cells, however, will have shifts in their membrane potential that vary in magnitude and actually aren't going to generate this action potential; we call those potential fluctuations graded potentials. And basically, graded potentials actually code their information based on the signal amplitude. So essentially, the magnitude of these membrane potential shifts is going to result in different signal amplitudes, and it's the differences in those magnitudes that are going to code the information. Now, action potential is all or nothing. It is a binary signal. It's either on or off. It is 0 or 1. And it is a transient shift in membrane potential, so it's a change in membrane potential that does not last very long and sends this all-or-none signal. So, if an action potential is sent, that's a 1. If there is no action potential, that's a 0. And the intensity of a signal, like, for example, if you want to send a signal basically telling your muscle to contract just a little bit, versus sending a signal to contract it really hard, the difference in those signals, they're all sent as action potentials, but the difference in those intensities is coded by the frequency of the action potentials. Because action potentials, again, are a binary signal. It's all or nothing. There is no difference in amplitude. There's no magnitude to be calculated or to be factored in; it's just one thing.
So what is the action potential? Well, it's basically a series of events that occur across the membrane of a neuron and results in an electric signal being sent down the axon. So essentially, you're going to start off in a resting state. This is just the cell at resting potential, nothing's happening; it's just chilling out. And at that state, these voltage-gated sodium and potassium channels that are going to be all over the axon membrane, those are going to be closed. Now, in what's termed the rising phase, the membrane potential is going to be depolarized. And remember, that means it's going to get more positive or less negative, however you want to think about it. And this depolarization is going to cause some of these voltage-gated sodium channels, around the membrane to actually open. Now, there is a special membrane potential which we call the threshold. And this is essentially a point in membrane potential that if crossed, it's action potential time. It is on. If you don't reach it, no action potential. So you have to cross this threshold in order to actually have an action potential. Now if the threshold potential is reached, all those voltage-gated sodium channels are going to be thrown open. And because of the way the membrane potential has been established, where sodium ions want to move into the cell, both because of their concentration gradient and because of the negative charge inside the cell. So when the threshold is reached and those voltage-gated sodium channels open, sodium is just going to rush into the cell full steam ahead. Now, remember that the inside of the cell is negative, but we have a huge influx of cations. This is going to depolarize the membrane potential.
So here, if we look at our chart, we started off with the resting phase. Right? We have our voltage, this is our membrane potential here. It's resting at this negative value, but due to depolarizations, it will cross the threshold. And then we have the rising phase. Right? Where the membrane potential shoots up because of all of those sodium ions entering the cell. Now, essentially, when the sodium channels reach this super depolarized point, they are going to become inactivated. And the potassium channels, which are also voltage-gated but gated to open at these depolarized potentials. So here, in the following phase, we're going to have our potassium ions flowing out of the cell. They're going to rush out of the cell because the voltage-gated potassium channels are open. Now remember, at rest, those potassium ions are more or less going to be at their equilibrium potential. Right? Their concentration gradient causes them to want to leave the cell, and the electrical gradient causes them to want to enter the cell, and they're going to hit that equilibrium potential thanks to our leak channels. So here, we are now depolarized; we're at about plus 40 millivolts. So essentially, the inside of our cell has become positive at this point. So now, potassium is going to have the double whammy that sodium had before. Right? Now potassium is going to want to move, so sodium is going to go in the cell. So potassium is going to want to go out of the cell because of the concentration gradient and because now the cell interior has become positive.
So, perhaps I should express it as wanting to go out, away from the positive charge and towards the negative charge. Okay. So now, with these potassium ions rushing out of the cell, this efflux of cations, all these leaving cations, cause repolarization of the membrane potential. So our membrane potential is going to go back down. Here's the thing; it actually is going to undershoot resting membrane potential. And that's because of this refractory period, which is essentially the time during which another action potential cannot be generated because our potassium channels are inactivated. And no potassium is getting through; some sodium channels are still open, right, so that's actually going to cause this hyperpolarization, that's what we refer to this as. It's a hyperpolarization because we're going to go past the resting membrane potential. Right? Resting membrane potential is up here in our graph; we're going to undershoot that. And the cell will actually have to work to get back to resting membrane potential. And this time between the undershoot and getting back to the resting state is our refractory period. And it's important because it ensures that there won't be another action potential until the cell can stabilize itself, get back to its baseline, and it ensures that by inactivating those sodium channels. Right? They're not just closed; even if the cell were to experience a membrane potential that would allow those sodium channels to open, no sodium is still going to get through because they've been inactivated because that ball and chain have plugged up the channel. So even if the voltage would allow them to open, no sodium is getting through, and that means no action potential until we hit this resting phase again. Okay. That's the phases of the action potential. Let's actually flip the page.