[0:07]All right, Ninja Nurd, in this video we're going to talk about descending tracks. So what are descending tracks? Descending tracks, they basically start for the most part, we're going to talk about in this video within the cortex. We're going to talk about cortical spinal tracks. So when we talk about descending tracks, right? So we have descending, descending tracks, we're going to talk about two different types. Right? We're going to talk about corticospinal tracks, and we're going to talk about another one in in separate videos, which are going to be different subcortical, subcortical tracks. We'll talk about these like rubrospinal, vestibulospinal. We'll talk about the ponto reticulospinal and the medullary reticulospinal. But we're going to focus right now on the corticospinal tracks. And there's going to be two types here. One is you're going to have what's called the lateral corticospinal tract. And the other one is we're going to talk about the anterior or ventral corticospinal tract. We're going to talk about these two. The more important one, which is going to be controlling like your distal limb musculature, is going to be the lateral corticospinal tract and the anterior is more for the axial musculature and we'll talk about that. Now, something else we have to understand with corticospinal tracks, where do they start? Let's say here, I draw a crude diagram, really quick one. I'm going to have here central nervous system and then your spinal cord, right? From up at the top, the cerebral cortex, we're going to talk about this in more detail, but for simplicity sake, you have a bunch of neurons here. And these neurons are called your upper motor neurons. We're going to abbreviate them for simplicity sake as UMN, upper motor neurons. From here, they can send down fibers that move down through the cerebral cortex through the brainstem and synapse on cell bodies that are located within the actual spinal cord, specifically in the anterior or the ventral gray horn. From there, these axons will come out and go innervate a special muscle. Generally, we're going to be talking about skeletal muscle, okay? And it's going to go and innervate a muscle. Now, what is this neuron right here called, located within the spinal cord? We call this guy a lower motor neuron, okay? We'll abbreviate this as LMN. So, we have to understand that we're basically going to be talking about in this video is the corticospinal tracks, the upper motor neurons starts from the cortex, moves its way down to a lower motor neuron located within the spinal cord. Now, there is other cortical tracks, we're not going to discuss them in this video, but you do want I do want you to understand that as these fibers can go down, they can give off little connections to nuclei that are located within the brain stem, different cranial nerves, like maybe the the 12th cranial nerve or the 11th, or the third, or the fourth. So you might have here maybe third cranial nerve, maybe fourth, maybe the um 11th cranial nerve, maybe 12th cranial nerve, whatever. From here, these fibers can go out and innervate different muscles, like maybe extraocular eye muscles, maybe the trapezius, maybe the tongue muscles, it all depends. But since this is going from the cortex, cerebral cortex, sending descending fibers that go to the different cranial nerve nuclei that are going to go innervate skeletal muscles. All right, but our main focus is going to be on the corticospinal tracks.
[4:24]So, let's go ahead and get started here. Before we do that though, we have to understand a little bit about the cerebral cortex. How does it know to send a command down? How does it do that? So, if we take over here, we're going to have a section of the brain, right? So I'm taking a section of the brain where I'm doing like a sagittal section and we're looking at one half. So over here, you're going to have temporal lobe, right? Here's your lateral sulcus or lateral fissure. And then right here, you're going to have another sulcus or fissure called the central sulcus. And then over here, you can have another one called the parietal occipital sulcus, right? So then you have occipital lobe, temporal lobe, parietal lobe, frontal lobe, right? Now, we already discussed in the ascending tracks that the parietal lobe, that postcentral gyrus was associated with sensations. In the anterior or frontal lobe, anterior to that central sulcus, there's a special lobe here. So look at this. You have a special gyrus just anterior to that central sulcus. What is that gyrus called? They call this gyrus the precentral gyrus. This precentral gyrus has a bunch of different cell bodies, which are going to be important in being able to send down those descending tracks. So it's the primary area of where the motor fibers begin, those upper motor neurons. So we actually call the precentral gyrus functionally, we call it the primary motor cortex. Okay? We call it the primary motor cortex. So precentral gyrus is the structural gyrus. The functional aspect of that gyrus is that it's called the primary motor cortex. Most of the actual axons coming from this cortex controls voluntary or conscious awareness of the movement of our skeletal muscles, okay? But that's not the only area. There's two other really important areas. And there's another one that's really shocking that usually confuses people. There's one right here. There's a little bit anterior to this precentral gyrus, there's another part here, just anterior to it. And this one is actually called the premotor cortex. So this one right here is called the premotor cortex. This one is more associated with learned and repetitive or planned motor movements, okay? So it's associated with learned or repetitive or planned motor movements. And this one also has descending axons that can come down. There's another one just a little bit superior to that. So just a little bit superior to that one, and this one right here is called the supplementary. It's called the supplemental motor area.
[7:33]Okay? The supplemental motor area is also associated with certain types of motor movements. So so far we have three different areas. We have the the actual primary motor cortex on the the precentral gyrus, the supplemental motor area and the premotor cortex. All of these guys are going to help us to be able to initiate the movements. There's one shocking one though. This one usually confuses people. There's another one, and you guys have already learned this, if you guys have watched the ascending tracks videos. You know there's another area here, this is behind the central sulcus, this pink area. You know the behind the central sulcus, that was called the postcentral gyrus. Well, the postcentral gyrus is where we had what's called the primary somatosensory cortex. You know the primary somatosensory cortex? A good chunk, sometimes 30 to 40% of the motor fibers that are coming down are going to be coming from the primary somatosensory cortex. That's unbelievable. So we have four areas that I want you guys to remember that are going to be helping to control the motor movement, these descending fibers. What are they? Primary somatosensory cortex is one. The second one is going to be the primary motor cortex in the precentral gyrus. So this is going to be the second one. The third one is going to be the supplemental motor area, which is going to be just a little bit anterior to the precentral gyrus. And the fourth and final one is going to be the premotor cortex, which is just a little bit anterior and a little inferior to the precentral gyrus. Okay. So we know the areas that are going to help to be able to allow for the fibers to come down. But how do we know when to do that? Well, let's say that a thought comes into my mind. Where does the thought usually come? Let's say that I'm thinking about performing a movement. I'm thinking about flexing the biceps, right? I I had conscious control over that. The thought arose in the prefrontal cortex.
[10:05]From that, what can happen? It can send signals to these different motor areas. When it sends signals to these different motor areas, what can happen? It can stimulate these guys to send down the descending fibers. But you know there's something really important. There's a special checking mechanism, because once we have the thought that comes into our mind to move our biceps, we have to make sure that that movement, that flexion isn't super uh like low in intensity or too high in intensity. Because I want to make sure that that motor movement is just perfect, it's smooth, and it's perfectly executed. So in order for me to do that, there's special structures located deep within the cerebrum. You know, they call them uh basal nuclei. So you have a bunch of different structures here, we're just going to draw them right here, assume that they're right here. These are called your basal nuclei. And your basal nuclei, like the putamen and the globus pallidus and the caudate nucleus, all those structures, the thalamus, subthalamus, substantial nigra, which are associated with these basal nuclei. They're going to get information. So the primary motor area, the supplementor motor area, the premotor area and even the primary somatosensory cortex, are going to come and tell the basal nuclei what the motor movement that they want to perform is. The basal nuclei sifts through that information of the plan motor movement, and guess what it does? It sends up its little modified blueprint of the movement. So it says, hey, there's a couple things that I think you should fix cerebral cortex before you send this information down. So what do we have so forth? Thought, getting ready to execute the movement. Before we do that, we send it to the basal nuclei. The basal nuclei kind of modify that, prevent overshooting or undershooting of the motor movements, send it back up to the cerebral cortex. But then there's one more checking mechanism. You know, just below the cerebrum, you have another structure here, and this is called your cerebellum, right? We'll talk about these things a little bit more, but just to get the basic points here, your cerebellum also has to communicate with the cerebral cortex. So from the cerebral cortex, from these motor fibers, they have to send information to the cerebral cortex. Let the cerebral cortex know, hey, this is the plan motor movement that we want to do. You know what's so cool about the cerebral cortex though? It's just unbelievable. You know, there's a part here, right? We have the brainstem, midbrain, pons, medulla, spinal cord, right? I was going to draw double cerebellum here, but we have this whole part here, the spinal cord, right? What's really interesting about this is that the cerebellum not only gets a planned uh program of what movement that the cerebral cortex wants to do, it also is getting proprioceptive information. If you guys haven't already, go watch the actual spinocerebellar tracks, because it's also getting information from our proprioceptors. So let's pretend here's our muscle. It it could be from the muscles. You could have these those different muscle spindles. Remember we had the nuclear bag fibers and nuclear chain fibers that respond to stretching of the muscle. And you have the Golgi tendon organs, and they were responding to the stretch of the tendon. And all these guys were sending information in, right? Into the spinal cord and then up into the cerebellum, letting the cerebellum know of the position of our muscles, our tendons, our joint capsules, all the ligaments, all that stuff. So, the cerebellum says, okay, this is the plan motor movement. This is where the position of the muscles, the tendons, the joint capsules, all the ligaments are. Now, you know what I'm going to do? I'm going to send this final blueprint back up to the cerebral cortex. Let them know we now have a perfectly planned motor movement, set and ready to go. So the thought originated, got sent to this motor area. They they coordinated this information with the basal nuclei, and they coordinated with the cerebellum, who's also getting information from our proprioceptors. So that that the final pathway that we're going to send down is completely checked, it's a perfectly executed movement. Let's go ahead and follow it. Okay. So if we start up here in the cerebral cortex, we said that right here, we're taking a section. We're we're doing a coronal section here, right? And you can see here that we have the parietal lobes, then we got your temporal lobe, the midbrain, the pons, medulla, and cross sections of the spinal cord here. In the actual parietal lobe, we're we're going to say that there's a bunch of different types of nuclei here. Okay? Bunch of different types of motor neurons that we're going to talk about here. And these motor neurons, their cell bodies are located in a special uh lamina or section. You know the cerebral cortex is like many different layers, six different lamina sections, partitions, right? In the fifth lamina of the cerebral cortex, you find these special cell bodies, okay? And these guys, these these cell bodies, they're actually called the they're called pyramidal cells, pyramidal cells. Now, some of them are a little bit bigger than others, pretty big. And those are called your cells of Betts. So you might have and these pyramidal cells, you might have special ones called cells of Betts. And these are the really big ones, and they're heavily myelinated too. But for simplicity's sake, just remember that these cells that are located within the fifth lamina of the cerebral cortex are called your pyramidal cells. Some of them that are a little bit bigger than others are called the cells of Betts, and they're heavily myelinated. Now, what do these guys do? Okay, we have the thought from the prefrontal cortex. It's being organized. We had all the coordination and the uh modifications from the basal nuclei and the cerebellum. We're ready to send the motor movement down. Okay. From here, these guys give off their axons, these pyramidal cells. And some of them, which are a little bit bigger than others, again, are called the cells of Betts. Very, very highly myelinated. These guys come down in like a fan, basically like a radiating crown. What do you call this? They call that the corona radiata. Now, the corona radiata then actually condenses and compacts and moves through a special white matter structure, which is called the internal capsule. So you know this right here is our internal capsule. In the actual posterior limb of the internal capsule, these fibers funnel down. They kind of move down through the internal capsule. And as they funnel down through the internal capsule, they get really, really condensed. And there's a reason why that's clinically important because if you remember, we talked about this in the ascending tract videos. There's an artery called the lenticulostriate artery, and sometimes in people who have chronic hypertension, that artery can actually get blocked, and it can affect the blood flow to this area, and it can cause a lot of different types of problems, possibly paralysis of the muscles, because this is going to go down to the muscles, okay? So now, these fibers are coming down. Same way, what do we having over here? You're having the pyramidal cells. I'm not going to draw as many as I did on the left side, but we're going to have same thing here. And then what are these guys doing? They're giving off their axons, and their axons are coming together from this corona radiata. They're condensing, and then what are they doing? They're running through the posterior limb of the internal capsule.
[18:03]And now, as they run through the internal capsule, they come down, and they're going to start descending. And as they start descending, they go through a part of the brainstem. What part of the brainstem were we getting ready to enter into? We're getting ready to enter into what's called the midbrain. So now we're at the midbrain, right here, okay? To understand this a little bit better of how they're coursing through, let's come over here to the right, because I want to show you guys a special structure that it actually enters through, because all this stuff is is clinically relevant, because remember, any problem in which there's a change in the normal physiology can lead to pathology. So we got to we have to know some of these things here. So here's our midbrain, right? And here, these little stocks of white matter, they're called your cerebral peduncles.
[18:54]Right? Now the cerebral peduncles has this part here, where these fibers, these motor fibers, move through. They really condense, and they start moving through this area. That area of the cerebral peduncles is called the crus cerebri, okay? So those fibers that are coming from the actual cerebral cortex, they're coming down through the internal capsule. They're going to start moving through the crus cerebri. And they're going to start funneling down through these cerebral peduncles, and then they're going to enter into the pons. So that's how this is working. So I just want you to understand that, that where are they entering? They're entering into the actual crus cerebri of the cerebral peduncles, and then moving continuously inferiorly. Now, we get to an important point here. Remember I told you that the cerebellum is constantly coordinating with the actual cerebral cortex to make sure that there's a proper planned movement. We see these nuclei here that are located within the pons. These are called your pontine nuclei. Simple, right? So these are called your pontine nuclei. They're constantly getting informed from the cerebral cortex. When they're constantly informed from the cerebral cortex of what the plan motor movement is, guess what they do? They take that information. So let's say that the information is coming down from here, right, from these fibers. And these guys can take this information of what all this motor movement is.
[20:34]And guess what they do? They're really weird. They say, I'm going to take this to the opposite cerebellum. So they take this information, and they go through the middle cerebellar peduncles and alert the contralateral cerebellum. So again, over here, it might get some stimulation from the cerebral cortex, and then guess what they do? They come over here, they come to the opposite side, contralateral side, and they go into the actual cerebellum through the middle cerebellar peduncles and alert the cerebral cortex of this movement, right?
[21:32]So you have to understand here that these pontine nuclei, they're constantly interacting with the cerebellum through the actual middle. What are these structures right here? These white matter fibers here that is connecting the pons to the cerebellum. They call this the middle cerebellar peduncles. And what's important is that the cerebral cortex is constantly coming down, giving information to these pontine nuclei, which send the information to the cerebellum, let the cerebellum know of what the actual plan motor movement is, so that the cerebellum can modify it. But we want this this actual motor movement to come down. So what happens is it has to actually kind of like scatter. Some of these fibers kind of scatter around the pontine nuclei, but eventually they come back into a condensed form. So some of them are going to scatter across the actual pontine nuclei, but again, afterwards, they're going to start condensing. Now, these fibers, once they start condensing, we've already gone past the actual pontine nuclei, where this ponto cerebellar connection is actually, right? Now, they're going to move through this next part. What's the next part? The medulla. Now, let's come over here to the other diagram over here for a second. So I want to show you something with the medulla. In the medulla, right here, let's make a section right here. And then let's do it like this, like this. If we kind of do like this, you'll notice here that we have kind of a special structure. These are actually called pyramids. These are your pyramids. And over here, you're going to have the olives. So here's your two pyramids, and here's the two olives. What happens is, if we follow those fibers, those corticospinal fibers right over here, right? What they do is they start moving through these actual pyramids. So some of they actually start moving through the pyramids, they really get condensed there. As they start moving through the pyramids, kind of towards the distal part of the pyramids, some of the fibers will cross to the opposite side and move down. So let's here, let's follow these ones. If we follow these guys, they're going to come down. They're going to enter into the pyramids. As they enter into the pyramids, they're going to start really, really condensing. And some of the fibers about 80% of the fibers will cross over at the distal half of the pyramids and go down. A small percentage, 15 to 20% of them will stay uncrossed, and they'll go into the ventral part of the spinal cord. So if you look here, what's happening at the pyramids? The fibers are decating. They're crossing at the pyramids. So what we need to know here that it's happening. What's happening here at the medulla, where the pyramids are? These fibers are very, very good chunk of them. About how much again is important to remember? About 80%. 80% of these corticospinal fibers are actually going to cross and go to the contralateral side. So a good, good chunk of these guys are going to cross. The remaining part, the remaining, you know, 15 to 20% are not going to cross, and they're going to stay ipsilateral. Okay? So about good maybe 15, 20% here is going to stay ipsilateral and they're going to go into the ventral part of the spinal cord. And this one is more associated with the axial musculature. So it's for more of gross movements or large motor movements, okay?
[24:59]And the second one is going to be the anterior or the ventral corticospinal tract. This one is more associated with the axial musculature. So it's more associated with the axial musculature. So it's for more of gross movements or large motor movements, okay? So lateral corticospinal, distal limb, limb musculature, and the anterior is more for the axial musculature, okay?
[30:40]So lateral corticospinal, distal limb musculature. So more of the actual arm movements, the finger movements. So it's for very fine movements.
[31:01]Very fine, um, and precise movements, okay? Very fine and precise movements. The other one though, that's actually what we're going to talk about next, is going to be called the the actual anterior or the ventral corticospinal tract. This one is more associated with the axial musculature. So it's more associated with the axial musculature. So it's for more of gross movements or large motor movements, okay? All right, Ninja Nurse, so we pretty much went over for the most part, all of the actual corticospinal tracks. I hope it made sense. I really do hope that you guys enjoyed it. If you guys learned a lot, please hit that like button, comment down in the comment section. Please subscribe. Also, guys, if you have the time, please go check out our Facebook, our Instagram, even our Patreon account. All right, Ninja Nerds, as always, until next time.
