Action Potentials - Online Tutor, Practice Problems & Exam Prep

Action potentials are rapid changes in membrane potential crucial for neuron signaling. Starting at resting potential of -70 mV, graded potentials lead to depolarization when the threshold of -55 mV is reached, opening voltage-gated sodium channels. Sodium influx raises the potential to +30 mV, followed by repolarization as potassium exits the cell. This process can result in hyperpolarization, dropping below -70 mV. Eventually, the sodium-potassium pump restores resting potential, maintaining homeostasis in neuronal activity.

Action Potential

Okay. So in this video, we are finally going to be talking about action potentials. We have officially moved into the axon of our neuron up here. I'm going to be going through the sequence or the steps of an action potential and, as I do that, I'm going to 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 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-gated 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. Okay? Now, you may have also noticed there's this dotted line over here between steps 2 and 3, and that is just there because you already know steps 1 and 2, and then steps 3 through 6 will be more specific to action potentials. So let's get started.

Alright, so at step 1 our neuron is at rest. Right? We're always going to be starting at rest and we know that that is negative 70 millivolts. We are just sitting pretty at resting potential to start. Eventually, we're going to be receiving some graded potentials, right? Some EPSPs are going to come in, they're going to start depolarizing our membrane. Nothing too crazy but they're going to 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. 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 going to get really positive up to about positive 30 millivolts.

Now at positive 30 millivolts, we're going to see a shift where our voltage-gated sodium channels are going to close and our voltage-gated potassium channels are going to open. At this point, there's a lot 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. Once those 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 repolarize to return to resting potential, right? We're going to get negative again down down down but in that process, we're going to overshoot that repolarization and we're going to hyperpolarize, get more negative than resting potential.

We're going to go down here to about negative 80 or 90 millivolts. Now, what's happening here with our channels, our voltage-gated potassium channels actually start closing at around negative 50 millivolts, but they respond a little bit slowly. 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 repolarize, 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 going to 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 going to cause sodium, positive sodium, to come rushing into our cell, and that's going to 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 going to 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 going to 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 hyperpolarization, our potassium channels are in the process of closing. So they start closing at around negative 50, and when we're hyperpolarizing, they're still closing. There might be a couple stragglers that are still open. So I'm going to 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.

Here’s what students ask on this topic:

What is an action potential and how does it occur?

An action potential is a rapid change in the membrane potential of a neuron, essential for transmitting signals. It starts at the resting potential of -70 mV. When graded potentials depolarize the membrane to the threshold of -55 mV, voltage-gated sodium channels open, allowing Na⁺ ions to rush in, raising the potential to +30 mV. This is followed by repolarization as K⁺ ions exit the cell through voltage-gated potassium channels. The membrane potential may briefly hyperpolarize below -70 mV before the sodium-potassium pump restores the resting potential.

What are the steps involved in generating an action potential?

The steps in generating an action potential are: 1) Resting potential at -70 mV. 2) Graded potentials depolarize the membrane to -55 mV (threshold). 3) Voltage-gated sodium channels open, Na⁺ influx raises potential to +30 mV. 4) Sodium channels close, potassium channels open, K⁺ efflux causes repolarization. 5) Membrane potential may hyperpolarize below -70 mV. 6) Sodium-potassium pump restores resting potential.

What role do voltage-gated sodium and potassium channels play in action potentials?

Voltage-gated sodium channels open when the membrane potential reaches -55 mV, allowing Na⁺ ions to enter the neuron, causing depolarization to +30 mV. These channels then close, and voltage-gated potassium channels open, allowing K⁺ ions to exit the neuron, leading to repolarization. The potassium channels may cause hyperpolarization before closing, after which the sodium-potassium pump restores the resting potential.

What is the significance of the threshold potential in action potentials?

The threshold potential, typically around -55 mV, is crucial for initiating an action potential. When the membrane potential reaches this threshold, voltage-gated sodium channels open, leading to a rapid influx of Na⁺ ions and a subsequent spike in membrane potential to +30 mV. If the threshold is not reached, an action potential will not occur, making it a critical point for neuronal signaling.

How does the sodium-potassium pump restore resting potential after an action potential?

After an action potential, the sodium-potassium pump helps restore the resting potential by actively transporting 3 Na⁺ ions out of the neuron and 2 K⁺ ions into the neuron against their concentration gradients. This active transport process, which requires ATP, helps re-establish the negative resting potential of -70 mV, maintaining the neuron's readiness for subsequent action potentials.