Action Potential

by Jason Amores Sumpter
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There's a very special Siris of events that will happen when a cells 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 are going to result in different signal amplitude. And it's the differences in those magnitudes that air going to code information action potential is all or nothing. It is a binary signal. It's either on or off. It is zero or one, and it is a transient shift in membrane potential. So it's a, uh changing membrane potential that does not last very long, and it sends this all or none signal. So if an action potential ascent, that's a one. If there is no action potential, that's a zero and the intensity of a signal like, for example, if you want Thio, send a signal basically telling your muscle to contract just a little bit versus sending a signal to like, really, you know, super contracted the difference in those signals. They're all you know. They're all being sent his 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, all or nothing, there is no difference. An 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 Siris of events that are going to occur across the membrane of a neuron, and it's going to result in electric signal being sent down the ax on. 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 ax on membrane those they're gonna be closed now, in what's termed the rising phase, the membrane potential is going to be deep polarized, and remember, that means it's going to get mawr positive or less negative. However, you want to think about it, and this is this deep polarization is going to cause some of these voltage gated sodium channels around the membrane toe actually open. Now there is a special membrane potential, which we call the threshold, and this is essentially a point a point in membrane potential that, if crossed its 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 gonna be thrown open. And because of the way the membrane potential has been established, right where sodium ions want to move into the cell both because of their concentration radiant. That's one trying to symbolize here. Concentration, radiant and because of the negative charge inside the cell. So when threshold is reached and those voltage gated that voltage gated sodium channels, open sodium is just gonna rush into the cell full steam ahead. Now, remember that the inside of the cell is negative, but we have a huge influx of cat ions. This is going to dip, polarize the membrane potential. So here, if we look at our chart, we started off with resting phase right, We have our voltage. This is our essential er membrane potential Here it's resting at this negative value, but due to deep polarization zones, 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 at the ah, essentially, when the sodium channels reach this super deep polarized point, they're going to become inactivated. And the potassium channels which are also voltage gated but gated toe open at thes, you know, deep polar deep polarized potentials. So here in the falling phase, we're gonna have our potassium ions flowing out of the cell. They're gonna rush out of the cell because the voltage gated potassium channels open. Now, remember, at rest, those potassium ions are more or less gonna be at their equilibrium potential right there. Their concentration radiant causes them toe, want to leave the cell, and the electrical radiant 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're now deep polarized, like we're about plus 40 million volts so essentially the inside of ourselves has become positive at this point. So now potassium is gonna have the double whammy that sodium had before. Right now, potassium is gonna want to move, so sodium is gonna go in the cells. Potassium is gonna wanna go out of the cell because of the concentration, radiant. 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 F flux of cat lines, all these leaving cat lines cause re polarization 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, uh, this refractory period, which is essentially the time during which another action potential cannot be generated because our potassium channels air inactivated and no potassium is getting through some sodium channels air still open. Right? So that's actually going to cause this hyper polarization. That's what we refer to this as. It's a hyper polarization, because we're actually going to go past resting membrane potential. Right? Resting membrane potential is up here in our graph, we're gonna undershoot that and the cell will actually have toe work to get back to resting memory and potential. And this time between Thea undershoot and actually getting back to the resting state. That is our refractory period. And it's important because it ensures that there won't be another action potential until the Selcan 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 will allow those sodium channels toe open, no sodium still gonna get through because they've been inactivated because that ball and chain has plugged up the channel. So even if the voltage will allow them to open, no sodium is getting through. And that means no action potential until we hit this resting phase again. Okay, that is those of the phases of the action potential. Let's actually flip the page