Propagation of Action Potentials - Online Tutor, Practice Problems & Exam Prep

Action potentials propagate along axons through two main mechanisms: continuous conduction and saltatory conduction. Continuous conduction occurs in unmyelinated axons, where each segment depolarizes sequentially, resembling a knight traversing mountains. In contrast, saltatory conduction, found in myelinated axons, allows the action potential to leap between nodes of Ranvier, significantly speeding up transmission. This process is efficient due to the absence of leakage in myelinated segments, enhancing neural communication. Understanding these mechanisms is crucial for grasping how signals travel in the nervous system.

Types of Propagation: Continuous Conduction

So in this video, we're gonna be talking about the propagation of action potentials. Now propagation is a word that just describes the unidirectional spread of the action potential down the plasma membrane of the axon. And up until this point, you may have kind of gotten the impression that an action potential is just sort of one event that happens on the axon, and that's not totally accurate. An action potential is actually a series of events that happen all the way down the membrane. So one way to kind of picture this in your head is to imagine 4 light bulbs all in a line and they're all off. Now if those light bulbs were to turn on one after the other very rapidly, boom boom boom boom, it would look a little bit as though one light was traveling kind of right down that line, but you would know that it was actually 4 distinct events. Even if it looked as though it was one light traveling all the way down, that was actually 4 light bulbs turning on, right, just one after the other. And that's kind of how propagation works.

Now there are two types of propagation. We have continuous conduction, which is sometimes called continuous propagation, and saltatory conduction or saltatory propagation. And if you're wondering like, why is it called continuous conduction? That's a really good question. So propagation is the more accurate term here, but the older term is conduction, and so this is kind of a leftover, term that stuck around. But if you hear someone talking about continuous conduction, they are describing the process of propagation, so just so you know. Now continuous conduction is the relatively slow propagation that we see along unmyelinated axons. So if we look down at our figure here, the yellow piece is our axon, and let's just say that this action potential just started. So this green area here is our depolarizing membrane. You can see our voltage-gated sodium channel is open and little purple sodium ions are rushing into the cell and depolarizing it, and that little piece of membrane, that piece of axon, is depolarizing. That's an action potential.

Now, those positive sodium ions are going to be attracted over here because this membrane is negative, this membrane is still resting. Okay? So the rest of the axon is still at rest, it's still negative, and those sodium ions are going to be attracted to that negative charge. And so they're going to start moving down the axon in a current and they're going to get all the way down here to the next set of voltage-gated channels and they're going to depolarize that membrane and they're going to open up those channels and then sodium is going to come rushing in through those channels. We're going to have more sodium. It's going to move down, down, down in that current attracted to that negative resting membrane. Then the same thing is gonna happen all the way down the axon, just depolarizing segments of the axon one after the other. Now, keep in mind the current can't go backward because this area behind the action potential is in its refractory period. So that current has to keep moving forward in the same direction that unidirectional spread. So one way to kind of picture this in your head is to imagine we have a knight. He's on a quest. He has to get his message over to this castle over here, and he has to get over 3 mountains. And with continuous conduction, there are no shortcuts. Right? We don't have any myelin helping us out. Nothing to make it easy. He's gonna have to go up the mountain, down the mountain, up the mountain, down the mountain, up the mountain, down the mountain to get to that castle. It's gonna work. It's still a good method of travel. Right? But there's nothing fast about it. This is pretty slow compared to our other type of conduction, saltatory conduction. And I'll see you guys in our next video to talk more about that.

Types of Propagation: Saltatory Conduction

Okay. So, let's dive into saltatory conduction now. Saltatory conduction is the rapid propagation that we see along myelinated axons. You guys remember myelin makes neural communication faster. Right? That is what is making saltatory conduction faster than continuous conduction. In saltatory conduction, only the nodes of Ranvier get depolarized. Unlike in continuous conduction, where each segment of the axon has to get depolarized all the way down, here only those nodes are going to get depolarized. If we look at our figure here, we can see these bluish kind of blobs are our myelin, and in between our myelin, we have our nodes, right? And that is where our voltage-gated sodium channels are going to be. So, if we move over here to this first image, we can see it's the exact same thing that we had with continuous conduction where we have our depolarizing membrane over here highlighted in green. We can see our voltage-gated sodium channel is open, sodium ions are rushing into our cell, and over here adjacent to that is our resting membrane. This part of the axon is still at rest. It still has that negative membrane potential. And so it's the same process as continuous conduction, but because of the presence of myelin, a few things are different. Just like before, those positive sodium ions are attracted to that negative membrane right next to it, so they're going to go traveling down the axon. This time, however, because that myelin is enclosing our axon, there's no leak channels. There's nowhere for that current to kind of leak out of, nothing to slow it down. So they're going to kind of zoom down that myelinated segment pretty fast and then they're going to get to this node. Once they get to this node, it's going to depolarize the membrane, our voltage-gated channels are going to open up, more sodium is going to come rushing into our cell, and then the same process will keep happening all the way down the axon at each of those nodes. That's what it looks like. Now it's called saltatory because the Latin word, saltare, means to leap. And this almost looks as though our action potential is leaping from node to node, so kind of like it's just kind of jumping over those myelinated segments and just depolarizing those nodes. A great way to kind of picture this in your head is to imagine our same scenario with our mountains. We are, you know, we're way over here and we have to get a message to this kingdom way on the other side of these three mountains, but this time each mountain has a watchtower along the top. So we're going to get the message up to here and then this guy is going to light a fire and then this guy over here is going to see that and light his fire. This guy over here sees that and lights his fire. And within a matter of seconds, the kingdom has received our message. So it's much faster and much more efficient than that continuous conduction where we are just continually propagating down the entire axon. Here we're just leaping from mountaintop to mountaintop, right? Much faster, much more efficient. So, that is saltatory conduction and I'll see you guys in our next video.

Propagation of Action Potentials Example 1

Okay. So our example asks us, during saltatory conduction, action potentials are generated blank. So let's see what we have here. Alright. So A is "regardless of if threshold is reached," and we know that that is wrong, because action potentials only happen if we hit threshold. Super important. So A is definitely out. We also have B "when the entire axolemma depolarizes," but we know that that's not how propagation works. Right? It doesn't depolarize the whole axon all at once. It's a series of depolarizations that kind of depolarize chunks of membrane at a time. So B is out. C is "along the length of the entire unmyelinated axon." The keyword here is unmyelinated. Saltatory conduction takes place on myelinated axons, so C is out and we're left with D "only at the nodes of Ranvier of myelinated axons," and both of those terms are important. So saltatory conduction takes place on myelinated axons and the action potential itself, the depolarization, takes place at the nodes of Ranvier. So if you guys remember, if we're looking at our axon and we have a chunk of myelin, a chunk of myelin, a chunk of myelin, and we have our voltage-gated channels right here at our nodes. And so our sodium ions zoom right down the myelin, they're going to depolarize right here at the node, this little chunk of membrane. More sodium ions are going to come rushing in. They're going to zoom right down that myelin. They're going to depolarize that node, that little piece of membrane, and so on and so forth. And because of these chunks of myelin, it's going to look like our action potential is leaping boing boing over the myelin. And remember, saltere means to leap and that's where saltatory comes from. It looks like our action potential is just jumping from node to node. So our answer is D and I will see you guys in our next video.

In unmyelinated axons, action potentials are propagated by_________ conduction.

• A. Continuous
• B. Saltatory

Dr. Roberts is testing a new drug that blocks sodium channels in the axons of a neuron. Which of the following statements is correct?

• A. The drug would prevent both the generation of graded potentials and the propagation of action potentials.
• B. The drug would prevent the generation of graded potentials.
• C. The drug would prevent the propagation of action potentials.
• D. The drug would only have a mild effect on the neuron because potassium channels are still operational.

Here’s what students ask on this topic:

What is the difference between continuous conduction and saltatory conduction?

Continuous conduction occurs in unmyelinated axons, where the action potential propagates by sequentially depolarizing each segment of the axon. This process is relatively slow because each segment must be depolarized one after the other. In contrast, saltatory conduction occurs in myelinated axons and is much faster. Here, the action potential 'leaps' from one node of Ranvier to the next, bypassing the myelinated segments. This is more efficient because the myelin sheath prevents ion leakage, allowing the action potential to travel quickly between nodes.

How does myelin affect the speed of action potential propagation?

Myelin significantly increases the speed of action potential propagation by insulating the axon and preventing ion leakage. In myelinated axons, the action potential jumps from one node of Ranvier to the next in a process called saltatory conduction. This 'leaping' mechanism is much faster than the continuous conduction seen in unmyelinated axons, where each segment must be depolarized sequentially. The presence of myelin allows for rapid and efficient neural communication.

What role do the nodes of Ranvier play in saltatory conduction?

The nodes of Ranvier are crucial for saltatory conduction. These are small gaps in the myelin sheath where voltage-gated sodium channels are concentrated. During saltatory conduction, the action potential 'leaps' from one node to the next. When the action potential reaches a node, it depolarizes the membrane, opening the sodium channels and allowing sodium ions to rush in. This process repeats at each subsequent node, enabling rapid and efficient propagation of the action potential along the myelinated axon.

Why is continuous conduction slower than saltatory conduction?

Continuous conduction is slower than saltatory conduction because it occurs in unmyelinated axons, where each segment of the axon must be depolarized sequentially. This step-by-step process takes more time as the action potential moves down the axon. In contrast, saltatory conduction occurs in myelinated axons, where the action potential 'leaps' from one node of Ranvier to the next. The myelin sheath prevents ion leakage, allowing the action potential to travel quickly between nodes, making the process much faster and more efficient.

How does the refractory period ensure unidirectional propagation of action potentials?

The refractory period ensures unidirectional propagation of action potentials by temporarily inactivating the voltage-gated sodium channels in the segment of the axon that has just been depolarized. This prevents the action potential from traveling backward. During the refractory period, the axon segment cannot be depolarized again immediately, ensuring that the action potential only moves forward along the axon. This mechanism is crucial for the proper transmission of neural signals.