Action Potentials - Video Tutorials & Practice Problems

Action Potential

OK. So in this video, we are finally going to be talking about action potentials. So you can see we have officially moved into the axon of our neuron up here. Now I'm going to be going through the sequence or the steps of an action potential. And as I do that, I'm gonna be actually graphing the change in membrane potential. So you can see what that change looks like. There's some color coding going on here as I'm sure you saw the pinkish purple color indicates resting potential. The blue color just shows us our neuron getting more positive and this yellow color indicates our neuron getting more negative. You can see along the top of our graph here, we have our voltage, sodium and potassium channels as a reminder, our sodium channels are pink and our potassium channels are purple and you'll be able to see if those channels are open or if they are closed. OK. Now, you may have also noticed there's this dotted line over here between steps two and three and that is just there because you actually already know steps one and two and then steps three through six will be more specific to action potentials. So let's get started. All right. So at step one, our neuron is at rest, right? We're always gonna be starting at rest and we know that that is negative 70 millivolts. So we are just sitting pretty at resting potential to start. Now, eventually we're gonna be receiving some graded potentials, right? Some ep SPS are gonna come in, they're gonna start depolarizing our membrane. Nothing too, too crazy, but they're gonna start that depolarization and eventually our neuron is going to hit threshold and we know that that looks like our membrane potential getting to negative 55 millivolts. Now, when that happens, our voltage gated sodium channels are going to open and sodium is going to come rushing into our cell following its electrochemical gradient, right? And as all of those positive sodium ions come rushing into our cell, that's going to cause a massive depolarization, we're gonna get really, really positive up to about positive 30 millivolts. Now, at positive 30 millivolts, we're gonna see kind of a shift where our voltage gated sodium channels are gonna close and their voltage gated potassium channels are going to open. Now, at this point, there's lots of potassium inside the cell and our cell is positively charged just like that potassium and it wants to get away from that like charge. And so once those um potassium channels open, potassium is going to rush out of the cell. So we're losing a lot of positive ions which is going to cause repolarization, we re polarize to return to resting potential, right. So we're going to get negative again, back down, down down. But in that process, we're going to actually overshoot that repolarization and we're going to hyperpolarize get more negative than resting potential. So we're going to go down here to about negative 80 or 90 millivolts. Now, what's happening here with our chain channels are bold. Potassium channels actually start closing at around negative 50 millivolts but they respond a little bit slowly. So at this point, there's usually some potassium channels that are still open, but they are in the process of closing and eventually they will all close. And at that point, all of our channels are closed, sodium and potassium and that will allow our sodium potassium pump to do some work and we will get our cell back to resting potential. So that is the sequence of an action potential that shows you what the voltage gated channels are doing, how the ions are moving and how that membrane potential changes throughout the event. So I will see you guys in our next video. Bye bye.

Action Potentials Example 1

All right. So here we have a graph depicting the change in membrane potential during an action potential. So within this graph, we're going to be writing the state of the voltage gated sodium and potassium channels if they're open or closed during each of the main phases of our action potential. And as a reminder, the main phases are when we start at rest, then we depolarize and then we re polarize and then we hyperpolarize. So let's start at rest. Now, keep in mind we are working with voltage gated channels and when our cell is at rest, all of our voltage gated channels are closed. So I'm gonna put that here for sodium and for potassium, both of them are closed. Now, when our cell starts depolarizing and gets to negative 55 our voltage gated sodium channels are going to open. So I'm going to write open over here because during that depolarization phase, our voltage gated sodium channels are open and that's gonna cause sodium to positive sodium to come rushing into our cell and that's gonna bring up our membrane potential. Now, during this phase, the potassium channels are closed, nothing's going on there Now, once our cell has depolarized up to about positive 30 remember we have that flip where now our sodium channels are going to be closed. And at positive 30 our voltage gated potassium channels open. So we're gonna write open here and that is going to cause repolarization because all that positive potassium is going to now leave our neuron and the loss of all of those positive ions will make our cell become more negative again. Now, as we're doing that, we're gonna overshoot that repolarization and we're going to hyperpolarize. Now, at this stage, our sodium channels are in the same state, nothing has changed there. So they are closed. And remember during hyper polarization, our potassium channels are in the process of closing. So they start closing at around negative 50. And when we're hyper polarizing, they're still closing, there might be a couple stragglers that are still open. So I'm gonna write closing here just to help us remember that there might be a couple that are open, but they're in the process of closing. And right, as soon as all of them have finally closed. So all of our voltage gated sodium and potassium channels are all closed. We'll get back to rest where everything is closed again. All right. So there you have it and I will see you guys in the next one. Bye bye.

Casey is taking a new medication that blocks potassium channels. What stage of an action potential would be MOST affected by this drug? 

• A. The depolarization phase.
• B. Reaching threshold.
• C. The repolarization phase.

When an action potential is at its peak, the electrical gradient forces potassium ____________.

• A. Out of the cell.
• B. Into the cell.

What happens when the neuron reaches threshold (-55 mV)?

• A. Voltage-gated potassium channels open and potassium rushes into cell.
• B. Voltage-gated sodium channels open and sodium rushes into the cell.
• C. Voltage-gated potassium channels open and sodium channels close.
• D. The sodium potassium pump immediately establishes resting potential.