Propagation of Action Potentials - Video Tutorials & Practice Problems

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 that 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 four 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, boom, it would look a little bit as though one light was traveling you kind of right down that line. But you would know that it was actually four distinct events, even if it looked as though it was one light traveling all the way down, that was actually four 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 um 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 um 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. OK. So the rest of the axon is still at rest, it's still negative. And those sodium ions are gonna be attracted to that negative charge. And so they're gonna 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 gonna open up those channels and then sodium is gonna come rushing in through those channels. We're gonna have more sodium. It's gonna move down, down, down in that current, attracted to that negative resting membrane. And 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 backwards 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 uni directional spread. So one way to kind of picture this in your head is to imagine we have a knight. He is on a quest. He has to get his message over to this castle over here and he has to get over three mountains and with continuous conduction, there's no shortcuts, right? We don't have any my in 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, of travel, right? But nothing fast about it. This is pretty slow compared to our other type of, of conduction, solito conduction. And I'll see you guys in our next video to talk more about that.

Types of Propagation: Saltatory Conduction

OK. So let's dive into saltatory conduction. Now. So saltatory conduction is the rapid propagation that we see along myelinated axons. So you guys remember myelin makes neural communication faster, right? So that is what is making saltatory conduction faster than continuous conduction and in saltatory conduction, what we'll see is that only the nodes of R va get depolarized. So unlike in continuous conduction where each segment of the axon has to get depolarized all the way down here, only those nodes are gonna get depolarized. So 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. So 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. So just like before those positive sodium ions are attracted to that negative membrane right next to it. So they're gonna 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, so they're gonna kind of zoom down that myelinated segment pretty fast. And then they're, they're gonna 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 gonna come rushing into our cell and then the same process will keep happening all the way down the axon at each of those nodes. So that's what it looks like like now it's called saltatory because the Latin word saltaire 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, a great way to kind of picture this in your head is to imagine our same scenario with our mountain. So 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 watch tower 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 mountain top to mountain top, right. So much faster, much more efficient. So that is saltatory conduction. And I'll 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.