Ascending Tracts | Spinothalamic Tract

Hi, Niche Nerds. In this video, we are going to talk about the anterolateral system or the spinothalamic tract. Okay, so this is going to be a very, very interesting tract, very in-depth. I'm not going to lie, we're going to cover a lot of stuff. So I want you guys to just stay focused with me and hang in there. Let's get started, guys. So when we talk about this system, and again, which system are we talking about? We're talking about the antero. It has a couple different names, just so that you guys know. We're talking about the anterolateral system. Another name for it is also referred to as what's called the spinothalamic tract. Okay, and it's actually broken up into two parts and we'll discuss that on the anterior or ventral and the lateral spinothalamic tract. We'll talk about this, I promise. We'll get there. All right, so now first things first, before we even start going into the tracts, we have to know What is the function of this anterior lateral system? What is its function? So now, what I want to do is, let's come over here in the middle, and let's write down the basic components of this. The spinothalamic tract. And again, you can call it the anterior lateral system or the spinothalamic tract, but you have to understand, there's two divisions here of the spinothalamic tract. Okay, there's the ventral or anterior, and then there's the lateral spinothalamic. my tract. Now, earlier studies revealed that these two carried separate types of information, and we're going to stick to that, but I want you to keep your minds open because they're finding now more research that these two aren't as clear-cut as we thought they were. What do I mean? The anterior is believed to only carry sensations with respect to crude touch or very, very light touch. In other words, you can't discriminate this type of touch. Okay? And another type of sensation called pressure, but specifically more along the superficial pressure. Okay? That's what they believe was the responsibility of the anterior spinothalamic tract, or another word is ventral spinothalamic tract. Okay? The lateral, it is believed to carry specifically pain and temperature sensations. Okay? And there's two different types of pain. There's fast or pin prick pain and then there's slow pain. We'll talk about that. And then there's temperature sensations, right? So the extreme temps, cold or hot. And we'll talk about that again. But I want you to keep your minds open. They're finding that this isn't always the case. They're finding that some of these sensations are kind of mixed and intertwined. So that they actually, that's why they treat it as the entire anterolateral system. Because the entire anterolateral system carries crude touch, pressure, pain. and temperature. But just for the sake of simplicity, we're going to keep it separate in this situation, okay? For simplicity's sake. Okay, so we know that the spionthalamic tract has anterior and lateral. We know that anterior carries crude touch and pressure, lateral's pain and temperature. Now the next thing that we have to discuss is how do we, because this is going to be the main focus here, is the pain and temperature pathway. How do we stimulate this pathway? What is the way that we can stimulate this pathway? So the next thing you need to know is in order for us to be able to sense pain or sense extreme temperatures, we have to have some type of receptor to pick up that stimulus. So now we have to have some type of receptor. Okay, now what kind of receptor is this that actually picks up pain and temperature? You know, there's different types of receptors. There's nociceptors, there's mechanoreceptors, there's thermoreceptors. There's so many different types. Enteroreceptors. This type of receptor that we're going to discuss here is going to be of what's called the nociceptors. Okay, nociceptors. These are basically going to be the ones that are picking up tissue damage or extreme temperatures. Okay, so these nociceptors are picking up two different types of stimuli, right? What is that? It's going to be picking up tissue damage. Maybe this tissue damage is because of... Chemical burns are because of some type of mechanical situation, like you freaking slam your finger in the pliers or something like that. Or it can respond to extreme temperatures. Okay, you're outside, you know, with no gloves on, throwing around snowballs, or you stick your hand in, you know, really hot water, whatever. It's responding to those types of situations. So we know the receptors that are responding to it. We know what they're responding to it. But we have to dive in a little bit more. more into that. We have to understand how does tissue damage, how does extreme temperatures activate these nociceptors, okay? So now, we have to understand that there's three different ways that we can stimulate these nociceptors. But before we do that, we have to have two different types, okay? So here, this blue one, this is going to be one that we're going to refer to as A-delta, okay? These are A-delta fibers. A-delta fibers are specifically for what's called fast pain or pinprick pain. Okay, so like I take a needle and I stab your hand, right? I just stab the epidermis. These guys right there are located within the epidermis. They're found in many different places. Epidermis, dermis, you can find them even in your cornea. You can find these suckers everywhere, all right? So now these A-delta fibers are going to respond to that pinprick pain. Well, what types of things really stimulate these guys? Let me tell you. There's only two main things that stimulate these guys. One is temperature, and the other one is thermal sensations. So, let's say that I want to stimulate this guy. And if I want to stimulate him, I need to activate him by mechanical stimuli. In other words, take that example. I smash my finger with pliers on accident, alright? So there's mechanical stimuli because this one mainly responds to that. Another one is really cold temperatures. Okay, so really, really cold temperatures. The way these things work is, let's pretend for a second. I kind of zoom in on this receptor here. Let's say I zoom in this receptor right here. Here's the receptor. Here's like the bulb of it, right? You have different types of little channels on this receptor. Let's pretend I put a channel right here. Here's a channel, and here's another channel. Let's say I smash my finger, right? Once I smash my finger, there's certain types of channels that are mechanically activated, that they can deform them, all right? So let's say that you give, you smash your finger with a pliers, it mechanically changes the shape of this protein. If it mechanically changes the shape of this protein, what can enter in? Maybe sodium ion is getting around. If sodium ions enter in, what is it going to do to this guy? It's going to stimulate him and send these action potentials down this actual peripheral process or this A delta fiber. What about temperature? How does that do it? You know, there's different types of channels here that are very sensitive to cold temperatures or even hot temperatures. You know, there's different types of thermal receptors here. You know, they're under a category like TRPV receptors. They're transient cation receptors. channels and their permeability changes due to the fluctuations in temperature. So if there's extreme cold temperature it might change the permeability of this channel and then guess what? Maybe some sodium ions start flowing in. All right because of the extreme cold and what does it do? Cause the cell to become more positive and triggers action potential. So that's really cool. So now we know that a delta fibers or fast pain are activated by cold temperature and mechanical stimuli. We have another one because if we have these a delta we're gonna have another one called C fibers. Okay, and this is our slow pain. This is for the slow pain. Now, how would you describe the slow pain? It's more of your burning pain, the aching pain, the dull pain, and you're not able to localize this pain as well as you could pinprick pain or fast pain. So what can stimulate these guys? So pretty much similar to whatever the A delta is. So for example, let's say that there's actually going to be really hot temperatures. So we're taking the opposite here, so hot temperatures. If there's hot temperatures, that could stimulate the C-fibers. Or mechanical stimuli. Again, mechanical stimuli. This also can cause the activation of these receptors, these noise receptors. But there's one more. This one's a little bit more prominent, and that's chemical factors. The chemical stimuli. So whenever there's tissue damage, you know how they found that out? Let's say that I... slap my hand, right? I slap my hand. There's going to be a little bit of tissue damage there. That tissue damage is going to cause a lot of inflammatory chemicals to be released. They found that out because if you go and you take a sample of that, they'll find a lot of different chemicals. What are some of those chemicals, for example? Well, some of them, the most important one, one of the big ones, is protons. So you're going to have a lot of protons in this area. Protons are basically coming from the metabolic acids, which are inside of your cells. What else? You know 97% of our cells are rich in potassium. So potassium is also going to be released out in that area. What else? Another chemical called bradykinin. This is another one. What else? Histamines. Histamines are also another important one. And these guys are going to be released and they're going to stimulate this actual pain fibers, these C fibers, the slow pain fibers. So if you have to remember, remember the chemical factors, C chemical are pretty much the entire stimulation for the C fibers. They do respond to some mechanical and hot temperature, but their main stimulation is through chemical factors. A delta respond to cold temp mechanical stimuli. Not very much of it responds to chemical. All right? Now, since we're kind of grouping this together, we also have other types of receptors, which are going to be picking up crude touch and pressure. What are those? You know you have different types of receptors over here, which are called, you have what's called Merkel's. You know you have what's called Merkel's disks. Merkel's disks. These are for kind of your superficial pressure and your fine touch. And then there's another one, which is your peritracheal nerve endings. These are basically surrounding hair follicles. And then another one, which are pretty much the most important ones, is your free nerve endings. Okay? Your free nerve endings can also respond to touch and pressure. So you have free nerve endings, which is the big ones. The peritracheal nerve endings, which are around your hair follicles, they respond to bending of the hair. and then Merkel's disc which kind of responds to more of the superficial pressure and some of the touch. So now these fibers, they can also pick up that crude touch. So what can they respond to? They can respond to crude touch and they can respond to pressure. These can be stimulatory factors for these guys. So now we know what is actually going to be picking up. We know how we're actually picking up this tissue damage. or these extreme temperatures and then for the last situation we know how we're picking up crude touch and some superficial pressure. We know these big points here the C fibers the A delta fibers. Very very quickly why do they call them A delta why do they call them C and if you really want to know because I know you guys are always interested in learning more these are generally again they can be A delta and C fibers also you can have a mixture of these but now why do they really quickly why do they call them A delta and C? It's dependent upon the myelination. So you know when you talk about myelination, let me get this out of the way here guys. So myelination, axons can be myelinated to varying degrees. So when we talk about nerve fibers, there is of three types. We're mainly only going to focus on A and C, but there's A, B, and C. We're mainly going to be focusing on A. A is highly myelinated. B is moderately myelinated. This is more for the sympathetic. motor neurons here and parasympathetic. C are going to be not myelinated. So myelination determines the speed of the actual action potential. So A would be the fastest, B would be next, and C would be the slowest. That's the fact of slow pain. They're not very heavily myelinated, so they don't move very fast for the action potentials. A, there's many different types. We'll talk about this more in the dorsal column, but there's A alpha, A beta, and A delta. A delta is the one that's picking up the actual fast pain or the cold temperatures. C is the one that's actually going to be the slow pain. So that's why I just wanted to mention that really quickly, where this A delta and where this C is coming from, because I don't want to just throw that out there and just accept that you guys know it. All right, so again, A, heavily myelinated. C, lightly myelinated. It's really, really important because it determines the speed of the action potential. Okay, cool. So now we know... What this tract is responding to, we know the receptors that are picking it up, now we have to talk about how is it going into the spinal cord. So I wanted to, before, because it's going to get kind of messy, so I want to blow up a big cross-section of the spinal cord so we can see how these fibers are coming in and so it's not too messy. So if we look at the cross-section of the spinal cord here, we're not going to talk about it in great detail, but they kind of section it out into different, like, what's called lamina. They call them rexed lamina. If you really want to know what's called rexed. Lamina, we're not going to talk about that in great detail. We're going to talk about a couple of them that are really important. But it goes like this. You have Rex lamina 1, 2, 3, 4, and it just keeps going. What's important here is mainly, actually let me put one more here, 5. The only really thing that's important about this is kind of remembering where these actual fiber synapse in this point for the pain pathway. Okay? So let's say we take, for example, the C fibers. Let me get this out of the way here. Okay? Let me get that there. So now, let's say here we have the C fibers, right? And the C fibers here was the endings, right? That was the one that was responding to the chemical stimuli or the hot temperatures or the mechanical stimuli. And what happens is this is the peripheral process, this part here, picking up the stimulus. This is the pseudo-unipolar neuron, right, the cell body there. And then it has the central process, which is going into the central nervous system. What happens is it goes in, right? And we're going to talk about this in a second. And it actually ascends and descends. We'll talk about that in a second. It's called the tract of LeSaur. LeSaur, sorry. So tract of LeSaur. But when it does, when it ascends or descends, it synapses on specific points. For the C fibers, it mainly synapses in Rex lamina 2 and in Rex lamina 3. And then from there... It goes through kind of a cascade of different types of neurons here. Throughout this entire process, it kind of goes through a cascade of multiple different synapses. And then eventually, what happens is these fibers cross. They go to the contralateral side. And when they do that, they go through what's called the anterior commissure and then go up. So right here, where these fibers cross over, it's called the anterior commissure. So that's for the C-fibers. So if you really had to remember the important ones, remember 2 for the C-fibers, and then remember 3 for the C-fibers, okay? If you really want to know, they call it 2, they call it the substantia gelatinosa of Rolando, and they call 3, they call it a part of the nucleus propius. All right? So that's for the C-fibers. Now the next one is the A-delta. Where is A-delta going in? A-delta is actually going to be, here's again. Receptor part, picking up the stimulus. Peripheral process, pseudo-unipolar neuron. Central process. And again, tractal lacer, lacer, sorry. And that can go into specific parts here. Mainly one. Rex lamina one and rex lamina five. And then from here, these guys, again, they cross over through the anterior white commissure, come over here into the anterior lateral system, and move upwards. And that's what we're going to talk about. But I want you guys to get an idea of where these things are moving, what parts they're going into. So now, remember for this one, for the fast pain, it's 1 and 5. Okay? 1, if you really want to know, is the marginal nucleus. And 5 is referred to as the reticular nucleus. Okay? Or that lamina area. Okay. So now we know that. The last one here, and this is, again, this blue one here. Let me mark this down. This is for A delta. The last one here that I want to finish off with is for these actual crude touch. Again, it's not a super big one here, but again, same concept here. Peripheral process, pseudo-unipolar neuron, central process comes in here. It's not specifically that important where they go in here. They believe it could go to 3, 4, maybe 5. But just know that it comes in here. Again, it could go to 3, 4, 5. And then what happens is it crosses over. Now, it does cross. goes to the contralateral side of the spinal cord and it goes through the anterior white commissure but it comes a little bit more anterior so you know over here we have the lateral white column and then over here we have the ventral or the anterior white column this one mainly kind of goes situated within the ventral or the anterior white column and then goes up okay so just remember that so now this is for this pathway is for the crude touch and pressure. Sweet deal. So now we know the pathway, we know what it's carrying, we know the receptors, we know how it's entering into the spinal cord, but we have to do one more little touch. Really, really important point, super important point is that ascending and descending system I was telling you about. And the only reason it's super, super important is because of its relationship with like certain types of lesions. Let's use this marker here. So let's pretend I take that pain fiber right there. Okay, here was its peripheral process which is picking up the stimulus. When it does that, it goes in to the posterior gray horn. But right when it goes into the posterior gray horn, it gives off ascending branches and it gives off descending branches. And what these do is they can go down. They can actually go down maybe one, two, tops three spinal cord segments or they can go up. One, two, tops three spinal cord segments. And they can synapse on the nuclei in this area that cross over and go up. Again. Let's pretend that this one's coming over here. This one can come up. This one comes over here, goes up, goes down, stimulates here, crosses over, moves up. So it develops this tract, if you get an idea of this. If I were to draw it at every single section, it starts making this tract that's kind of going up and down. You see that? That tract right there is called the tract of La Sour. Okay, it's called the tract of the sour. Why is this important? Really briefly, let's pretend that you have a lesion in the spinal cord around T6. T6. The area that's actually going to be affected is maybe two or three spinal cord segments below that on the contralateral side. That's why this is important. It's important for clinical aspects because if it's ascending and descending fibers, if there's a lesion at T6, the actual, if there's Elysian at T6, right? The problem is going to be two segments, maybe three, down and on the contralateral side because of this tract. Okay, I think we got it down for this part. So now let's come over here and see how this ascending pathway moves up and the different nuclei that it synapses on, right? So let's start here first with that ventral. Let's do the crude touch. Let's get that one out of the way. So here, let's put crude touch right here. Here's that. Pseudo-unipolar neuron, it's responding to what type of stimulus here? Crude touch, which could be coming from the Merkel's disc, the peritracheal nerve endings, the free nerve endings. And it's also responding to a little bit of pressure. And again, what happens here? It comes in and it goes to this actual cell body. This is your first order neuron, the first point. It goes into the dorsal gray horn, synapses on the cell bodies of a... nucleus somewhere within the dorsal gray horn, and then does what? Crosses over through the anterior white commissure and goes up, all right? And this right here, they refer to this as the, I'm going to abbreviate it, the ventral spinothalamic tract, okay? We're going to abbreviate it as the ventral spinothalamic tract. And just remember, this thing right here, this pseudo-unipolar neuron, it's actually located within the dorsal root. ganglion, which is kind of located outside of the spinal cord. Okay, so we have the first order neuron, which is this pseudo-unipolar neuron, which is located within the dorsal root ganglion. We have the second order neuron, which is located within the dorsal grey horn, and then the axons, crossover anterior white commissure, ventral white matter here, and then goes up. and then it ascends. Okay? Now, let's do the next one. Let's do this one. Let's do it in that baby blue. Okay? And we're going to keep this pretty much the same here. Let's say here we have the pain pathway here, right? Let's say this is our A Delta. Okay? Our A Delta does what? Picks up pain and cold temperatures. So we'll put cold temp. Okay? And that pain could be through cold temperatures, mechanical stimuli, right? Sends the information inwards. And again, it can have that tractalisor where it can go down or it can ascend. So it can ascend and descend one to two to three segments. Synapses here on the cell bodies, right, of the dorsal gray horn. Then from here, it crosses over through the anterior white commissure and then goes upwards. And specifically, where is it going? It's going into the lateral white column. So this is going to be a component of what's called the lateral spinothalamic tract. So you have the ventral spinothalamic tract, the lateral spinothalamic tract. All right. One more thing, though. We need the red. My red marker. Okay. This last one here is the C fibers. And the C fibers, these are the ones, remember, they're responding to slow pain, that dull pain, achy pain. They're going to be coming in. synapsing on cell bodies in the dorsal root ganglion, sorry, dorsal gray horn, and then doing what? Crossing over via the anterior white commissure and then moving upwards. Now, the lateral spinothalamic tract, let me actually do this, is actually both of these things, right? So the lateral spinothalamic tract is really both of these things. But I got to give us one more thing because... The A, the A delta, right, this pin prick pain. It's more of a newer kind of more modern concept. So because of that, they believe that this A-delta is a part of, they believe it's more specifically what's called the neospinothalamic pathway. That's specifically for the A-delta. And they refer to the C-fibers because they're more of a primitive kind of concept here. They're called the... ... paleo-spinothalamic pathway. If you don't really want to remember that, that's fine. It's just giving you an idea of the origins, the concepts of this, okay? So C is mainly carrying fibers into the anterior lateral system, but specifically in the lateral spinothalamic tract or the lateral white column. And if it's C, it's more of the older concept, the older aspect of, they believe it, the paleo-spinothalamic pathway. Whereas the A-delta, It's more of a newer concept of pain, right? That's the neospinothalamic pathway. Now, what happens is, is all of these guys come together. They all come as one big system. That's why we call it the anterolateral system. So now let's bring this guy together like this for the crude touch and the pressure. We'll bring this red guy right here and we'll bring the blue one right there smack dab in the middle. Okay, so now we have this whole thing here. This whole anterolateral system. Some books, they even refer to this group of structure here as what's called the spinal leminiscus. There's other fibers that are moving up in this, and we'll talk about those, like spinotectal, spinomesencephalic, spinal hypothalamic, spinal reticular. There's so many of these darn things. But for right now, let's keep it with just this part, the ventral spinothalamic tract. And the lateral spinothalamic tract, which is broken up into two different types of pathways, paleospinothalamic, which is the C, neospinothalamic, which is the A-delta. As they come upwards, they're going to give off certain types of structures. So this ventral spinothalamic tract, it's mainly going to go up to a specific nucleus within the thalamus. And that is called the ventral posterior lateral nucleus. And there's another one called the ventral posterior inferior nucleus. But these crude touch and temperature sensations, these are mainly going to be going to a structure, a lot of them are going to be going to a structure in the thalamus, which is called the ventral posterior lateral, and there's another one called the ventral posterior inferior nucleus. Okay, you'll see as we talk that there's going to be a lot of collaterals. Okay, now the C fibers, here's the important thing. The C fibers are really interesting. As they're moving upwards, some of them make it up to the thalamus, but about 85% of them terminate in this big orange structure here. What is this big orange structure here called? This big orange structure right here is called the reticular formation. So this big orange structure here is called the reticular formation. And it's a piece of gray matter that extends. throughout all parts of the brainstem, midbrain, pons, and medulla. So a good portion, how much of it again? About 85% of the fibers terminate in the reticular formation. Only about 15% of these fibers go up to the thalamus. Here's the thing though, these fibers here, these red fibers which are coming from the C fibers, right, the slowpane, they go to specific nuclei Non-specific nuclei in the thalamus. In the thalamus, just a brief thing here, so I draw a little thalamus here. There's different parts of the thalamus, alright, so you have different parts, but this structure right here, this Y-shaped structure, it's called the intermedullary lamina, right? And there's a whole bunch of nuclei that are located within this lamina. Okay, they're called intralaminar nuclei. And I have that pink representing that. So you're going to have some nuclei which are lodged in this lamina. And these nuclei here are called your intralaminar nuclei. And these are really important. There's a bunch of them, but the main ones that we're going to be focusing on here, if you really like to know that, is the centro. median nucleus, and another one called the parafasiculus. Okay, so these are the two big... The ones that are actually a part of the intralaminar nuclei is the central media nucleus and the parafascicular, parafasciculus nucleus. And again, they're nonspecific. What does that mean? From here, they do a lot of different things. They go to different parts from this structure here. So from the intralaminar nuclei, they can actually kind of go out and supply different areas. They can supply the cortex, like the primary somatosensory cortex, but they go to two really important structures. One of them, and I'm going to do it like this to represent its significance, is it can go to a part of the actual cortex, which is associated with the emotional aspect of pain. Okay? This right here, let's say that this structure here is called the cingulate gyrus. There's another one that it could go to. There's another part here, which is kind of tucked deep within the temporal lobe, and that one is called... The anterior, this one's called the anterior insular cortex. Okay, so some of these fibers can go out to different parts like the somatosensory cortex, the primary and the secondary association, the secondary association cortex. But two big ones is it goes to the cingulate gyrus and it goes to the anterior insular cortex. And the reason why this is important is because they play a role within the emotional aspects of pain. Holy crap. Okay. Well, we're going to come back to the C fibers because there's a lot more that we got to talk about yet. But let's go to this A delta. So these A delta fibers, they're also coming up, right? Some of them, very little of them give off some collaterals to the reticular formation. Most of them ascend upwards and go to this nucleus here within the thalamus. And again, what is that nucleus there called? It's called the ventral posterior lateral. This is the main one. And some of them can go to the ventral posterior inferior nucleus. From there what happens? These fibers can radiate. So we're taking with us fast pain and we're taking crude touch and pressure to this ventral posterior inferior and ventral posterior lateral. From here they're going to radiate. They're going to go up. Here you have the internal capsule. And you have the anterior limb and the posterior limb. There's a part here of the posterior limb. Which is the posterior one third. of the internal capsule, it can actually hop on that and then go to the corona radiata and supply large parts of the cortex. What is that part of the cortex that it's supplying? I'm going to abbreviate them, but it goes to what's called S1 and it goes to what's called S2, okay? And these areas of the cerebral cortex, they're actually located on specific part of the brain. You know, you have what's called the central sulcus. So let's imagine here I draw a small little diagram. Here's the central sulcus here. I'm going to draw it like this. Here's your central sulcus. Behind the central sulcus, you're going to have this gyrus here, which is called the postcentral gyrus. The postcentral gyrus, there's another area just a little bit behind it, which is called the secondary, you know, secondary association cortex, right? The secondary somatosensory cortex. So this right here is S. This right here is S1. What I'm doing is, is we're taking a slice. We're taking a coronal section here. And if I take a coronal section right here, we're going to see that we're focusing on all this part right here. This is all parietal lobe. Okay? So it's going to be supplying a large part of the parietal lobe and taking this actual information, these sensations, to different parts of the cortex. S1 and S2 mainly. It's going to help us to become consciously aware and to understand. The concept of that pain, all right? Okay, so we got the information going to the cerebral cortex, primary somatosensory cortex, secondary somatosensory cortex. We got branches off to the cingulate gyrus and the anterior insular cortex. Sweet, stinking deal. Now, we have to come back to this over here, the C-fibers. Along the way, there's other components, right? Well, we're good with the C-fibers, but we have to talk about one more thing. Along the way... There's other fibers that are associated with this entire anterolateral system. So if we talk about that, let me add these ones in. So in the anterolateral system, so again we're going to go ALS, okay? This is not amyotropic lateral sclerosis, we're talking about the anterolateral system. There's different fibers, right? We're going to be carrying with us the lateral spinothalamic tract, the anterior spinothalamic tract, and there's some other ones, right? Another one is we're carrying with us what's called the Spino-Tactile Tractile. We're also carrying with this anterior lateral system the spino-mesencephalic, which is really important, tract. These are just fibers that are part of this whole system here. And there's another one, and this one is actually going to be called the spino-hypothalamic tract. And there's actually one more here. And this one is actually going to be called the spino-reticular tract. We've already kind of covered that, though. These little branches, these collaterals that are coming off from the anterolateral system. But now we have to cover the spinal tectal and the spinal mesencephalic really quickly. So coming up through this, you have, let's do the spinal tectal first. So where is the tectum? You know, if you look here, you take like a different type of section here of the brain. We take like a sagittal section. We can look at the central nervous system like this. And on the back of the midbrain, you have these two little swellings. Okay, they call it the capora quadrigemina, which is made up of the superior colliculus and the inferior colliculus. We're going to take information to the superior colliculus specifically so that it can help to move our eyes or our head to wherever this actual stimulus is. Maybe I stepped on a Lego or something like that, and I moved my head and my eyes towards that area. So now... Coming up through here, here's the midbrain, we have to give off collaterals. And these collaterals that we're going to give off is going to go to this structure right here. Look at this structure right here. This structure right there, we're going to call this right here the superior colliculus. But it's a part of the tectum. We're going to put the superior colliculus. So this tract right there, these fibers that are giving off these little collaterals, that are going to the superior colliculus, or the tectum, is called the spinal tectal fibers. Then we have some other ones. We have the spinal mesencephalic. So you see these nuclei right here? They're in the pons. This is called the parabrachial nucleus of the pons, right? So the parabrachial nucleus. This is going to give off some little collaterals, right? So we're going to give off these fibers, these collaterals that are going to come off to the parabrachial nucleus. Why is this important? Remember I told you, pain is emotional. Sometimes it's really sad, you know? So what happens with these guys? These parabrachial fibers, they can send axons upwards. Let's use this one for example, since it's kind of crowded over here. These guys can send axons upwards and they can go to a special structure located within the actual cerebral cortex. I'm sorry, within the cerebrum. Deep within the cerebrum. It's a piece of gray matter deep within the cerebrum. There's a special structure up here. Let's do this one in this green right here. There's a special structure right here that's associated with the fear and the emotional aspects of pain, right? And this is called the amygdala. Okay, this is called the amygdala. So we have a connection the parabrachial nucleus in the pons and the amygdala. And what is this little tract that's coming off here? These fibers that are coming off to supply the parabrachial nucleus and another structure. That's going to be called the spinal mesencephalic tract. So two structures the spinal mesencephalic is going to supply. One is the parabrachial nucleus in the pons, which will go up to the amygdala, which is controlling the fear and the anger and the painful aspects within pain, right? The other one we said is the superior colliculus. It's giving off fibers to that, which is called the spinotectal. Now, there's another structure here that this actual anterior lateral system is going to give to. It's right here. You see this right here? In the midbrain, you have what's called the cerebral aqueduct. And then there's a piece of gray matter that wraps around it. That piece of gray matter that's wrapping around the cerebral aqueduct is called the periaqueductal. It's actually, do it like this. Peri. PAG, periaqueductal gray matter. Let's do that. Periaqueductal gray matter. It's a piece of gray matter surrounding the cerebral aqueduct. What happens is some of these fibers, you have fibers from the spinal mesencephalic, right? Spinal mesencephalic fibers can give off collaterals to the periaqueductal gray matter. Why is this important? We'll talk about this in another video, but the periaqueductal gray matter controls your descending pain modulation pathways. They can control the pain. It has different descending fibers that can go down and release norepinephrine and serotonin that can help to modulate the pain by releasing different types of encaphalins, endorphins, and dynorphins. We'll talk about that after. Okay, so we've covered that now. One more. You know, right up here, you're going to have another structure, and that structure is called the hypothalamus. Let's just say that as we're going up here, as we're going up, we give off some collaterals here from this anterolateral system to another structure right here. And let's say that this structure here we're talking about is the hypothalamus. So this right here is called the hypothalamus. So that's called the hypo... Thalamus. So let's say that you get kicked in the nads, right? What happens? It can cause you to maybe become nauseous. It can cause you to increase your heart rate, maybe change your respiratory rate in depth. Why? Because these collaterals that are coming off and going to the hypothalamus, the spinal hypothalamic fibers, the hypothalamus controls our autonomic nervous system. So these Spinal hypothalamic fibers that are coming off here and supplying the hypothalamus controls our autonomic responses to pain. Changes in our blood pressure, changes in our heart rate, changes in our respiration, right? So different things like that. Or think about a woman giving birth. It's going to do what? It could actually trigger oxytocin release, and he controls our hormones, right? Or even the suckling reflex. So the hypothalamus controls a lot of the aspects, the autonomic effects of pain. Okay, one last thing, guys, and we are done, is this last part here. We didn't finish up on this. We said that 85% of the fibers, from the 85% of these C fibers, go to where? The reticular formation. The reticular formation is so, so, so important. What is the function of your reticular formation? It controls the alerting, the arousal, the wakefulness of the entire central nervous system. So whenever you're awake, Alright, someone screams, right? Our reticular formation can sift through all the non-specific, not really important information and help us to become aware of the most important, the most recognized situation or whatever sensation it might be. Whenever you go to sleep though, what happens to your reticular formation? It shuts off. And think about this. Let's think about me saying something out loud. And the impulses get to travel down the vestibulocochlear nerve, right? You don't hear that sound when you're in deep, deep sleep. Why? Because your reticular formation is not alerting your central nervous system of that. Or if you touch somebody and they're in really, really deep sleep and they don't wake up, does that mean that they're not able to pick up those touch sensations and send it up to the central nervous system? No. It just means that their reticular formation is not going to be alerting the cerebral cortex of that touch. That's the beauty of the reticular formation. So in certain types of pain, like think about this. It's really simplistic. If you think about it and you have arthritis or you have a lot of achy pain, right? that dull or burning pain. What's it doing to your reticular formation? Activating it. A lot of them, 85% of the fibers. Well, what is the reticular formation? What did we just say it does? It pretty much sends these fibers upwards, okay? And these fibers go to these intralaminar nuclei. The intralaminar nuclei, what do they do? They supply the primary somatosensory cortex. the secondary somatosensory cortex and it's going to go to main areas which is the anterior insular cortex and the cingulate gyrus but the big thing with this is is the reticular formation is alerting almost your entire cerebral cortex why is that important think about it when you're trying to sleep but you're in pain because of arthritis or achy or dull pain why is that it's because it's alerting your reticular formation your reticular formation is telling your entire cerebral cortex Hey, I'm in pain. So that's one of the crazy things about this, okay? One last thing I want to talk about, guys. So remember how we said that with these C-fibers, right? They come over to the dorsal regangulum. They cross over to the contralateral side and then ascend, right? As this lateral spinothalamic tract, but particularly the paleospinothalamic pathway of it. Well, just remember that it's not always contralateral. Some of the fibers, some of them, not all of them, but some of them can also stay. ipsilateral. It can stay ipsilateral and then give off collaterals to the reticular formation, right, and some of them can actually reach the intralaminar nuclei and then go to different parts of the actual cortex. So it's really important to remember that these C fibers can not only go contralateral but they can also go ipsilateral via, this is called the spinal reticular fibers, right, it's the ones that are going to the reticular formation. 85% of them, very little, maybe 15% of them. goes up to the interlaminar nuclei. Ningeners, we covered so much information. If you guys stuck in throughout the whole video, I can't say thank you enough. I hope you guys really enjoyed. I hope you learned something. If you did, please hit that like button, comment down in the comment section, please subscribe. Also guys, check out our Facebook, Instagram, and maybe even our Patreon account if you guys have the opportunity to donate. We truly appreciate it. All right, Ningeners, as always, until next time.

So I want you guys to just stay focused with me and hang in there. Let's get started, guys. So when we talk about this system, and again, which system are we talking about? We're talking about the antero. It has a couple different names, just so that you guys know.

We're talking about the anterolateral system. Another name for it is also referred to as what's called the spinothalamic tract. Okay, and it's actually broken up into two parts and we'll discuss that on the anterior or ventral and the lateral spinothalamic tract.

We'll talk about this, I promise. We'll get there. All right, so now first things first, before we even start going into the tracts, we have to know What is the function of this anterior lateral system? What is its function?

So now, what I want to do is, let's come over here in the middle, and let's write down the basic components of this. The spinothalamic tract. And again, you can call it the anterior lateral system or the spinothalamic tract, but you have to understand, there's two divisions here of the spinothalamic tract. Okay, there's the ventral or anterior, and then there's the lateral spinothalamic.

my tract. Now, earlier studies revealed that these two carried separate types of information, and we're going to stick to that, but I want you to keep your minds open because they're finding now more research that these two aren't as clear-cut as we thought they were. What do I mean? The anterior is believed to only carry sensations with respect to crude touch or very, very light touch.

In other words, you can't discriminate this type of touch. Okay? And another type of sensation called pressure, but specifically more along the superficial pressure.

Okay? That's what they believe was the responsibility of the anterior spinothalamic tract, or another word is ventral spinothalamic tract. Okay?

The lateral, it is believed to carry specifically pain and temperature sensations. Okay? And there's two different types of pain.

There's fast or pin prick pain and then there's slow pain. We'll talk about that. And then there's temperature sensations, right?

So the extreme temps, cold or hot. And we'll talk about that again. But I want you to keep your minds open. They're finding that this isn't always the case.

They're finding that some of these sensations are kind of mixed and intertwined. So that they actually, that's why they treat it as the entire anterolateral system. Because the entire anterolateral system carries crude touch, pressure, pain. and temperature.

But just for the sake of simplicity, we're going to keep it separate in this situation, okay? For simplicity's sake. Okay, so we know that the spionthalamic tract has anterior and lateral. We know that anterior carries crude touch and pressure, lateral's pain and temperature. Now the next thing that we have to discuss is how do we, because this is going to be the main focus here, is the pain and temperature pathway.

How do we stimulate this pathway? What is the way that we can stimulate this pathway? So the next thing you need to know is in order for us to be able to sense pain or sense extreme temperatures, we have to have some type of receptor to pick up that stimulus. So now we have to have some type of receptor.

Okay, now what kind of receptor is this that actually picks up pain and temperature? You know, there's different types of receptors. There's nociceptors, there's mechanoreceptors, there's thermoreceptors. There's so many different types.

Enteroreceptors. This type of receptor that we're going to discuss here is going to be of what's called the nociceptors. Okay, nociceptors.

These are basically going to be the ones that are picking up tissue damage or extreme temperatures. Okay, so these nociceptors are picking up two different types of stimuli, right? What is that? It's going to be picking up tissue damage.

Maybe this tissue damage is because of... Chemical burns are because of some type of mechanical situation, like you freaking slam your finger in the pliers or something like that. Or it can respond to extreme temperatures.

Okay, you're outside, you know, with no gloves on, throwing around snowballs, or you stick your hand in, you know, really hot water, whatever. It's responding to those types of situations. So we know the receptors that are responding to it. We know what they're responding to it. But we have to dive in a little bit more.

more into that. We have to understand how does tissue damage, how does extreme temperatures activate these nociceptors, okay? So now, we have to understand that there's three different ways that we can stimulate these nociceptors.

But before we do that, we have to have two different types, okay? So here, this blue one, this is going to be one that we're going to refer to as A-delta, okay? These are A-delta fibers.

A-delta fibers are specifically for what's called fast pain or pinprick pain. Okay, so like I take a needle and I stab your hand, right? I just stab the epidermis.

These guys right there are located within the epidermis. They're found in many different places. Epidermis, dermis, you can find them even in your cornea.

You can find these suckers everywhere, all right? So now these A-delta fibers are going to respond to that pinprick pain. Well, what types of things really stimulate these guys?

Let me tell you. There's only two main things that stimulate these guys. One is temperature, and the other one is thermal sensations. So, let's say that I want to stimulate this guy. And if I want to stimulate him, I need to activate him by mechanical stimuli.

In other words, take that example. I smash my finger with pliers on accident, alright? So there's mechanical stimuli because this one mainly responds to that. Another one is really cold temperatures. Okay, so really, really cold temperatures.

The way these things work is, let's pretend for a second. I kind of zoom in on this receptor here. Let's say I zoom in this receptor right here.

Here's the receptor. Here's like the bulb of it, right? You have different types of little channels on this receptor. Let's pretend I put a channel right here. Here's a channel, and here's another channel.

Let's say I smash my finger, right? Once I smash my finger, there's certain types of channels that are mechanically activated, that they can deform them, all right? So let's say that you give, you smash your finger with a pliers, it mechanically changes the shape of this protein. If it mechanically changes the shape of this protein, what can enter in? Maybe sodium ion is getting around.

If sodium ions enter in, what is it going to do to this guy? It's going to stimulate him and send these action potentials down this actual peripheral process or this A delta fiber. What about temperature? How does that do it?

You know, there's different types of channels here that are very sensitive to cold temperatures or even hot temperatures. You know, there's different types of thermal receptors here. You know, they're under a category like TRPV receptors.

They're transient cation receptors. channels and their permeability changes due to the fluctuations in temperature. So if there's extreme cold temperature it might change the permeability of this channel and then guess what?

Maybe some sodium ions start flowing in. All right because of the extreme cold and what does it do? Cause the cell to become more positive and triggers action potential.

So that's really cool. So now we know that a delta fibers or fast pain are activated by cold temperature and mechanical stimuli. We have another one because if we have these a delta we're gonna have another one called C fibers. Okay, and this is our slow pain. This is for the slow pain.

Now, how would you describe the slow pain? It's more of your burning pain, the aching pain, the dull pain, and you're not able to localize this pain as well as you could pinprick pain or fast pain. So what can stimulate these guys? So pretty much similar to whatever the A delta is.

So for example, let's say that there's actually going to be really hot temperatures. So we're taking the opposite here, so hot temperatures. If there's hot temperatures, that could stimulate the C-fibers. Or mechanical stimuli.

Again, mechanical stimuli. This also can cause the activation of these receptors, these noise receptors. But there's one more.

This one's a little bit more prominent, and that's chemical factors. The chemical stimuli. So whenever there's tissue damage, you know how they found that out? Let's say that I... slap my hand, right?

I slap my hand. There's going to be a little bit of tissue damage there. That tissue damage is going to cause a lot of inflammatory chemicals to be released.

They found that out because if you go and you take a sample of that, they'll find a lot of different chemicals. What are some of those chemicals, for example? Well, some of them, the most important one, one of the big ones, is protons. So you're going to have a lot of protons in this area.

Protons are basically coming from the metabolic acids, which are inside of your cells. What else? You know 97% of our cells are rich in potassium.

So potassium is also going to be released out in that area. What else? Another chemical called bradykinin. This is another one. What else?

Histamines. Histamines are also another important one. And these guys are going to be released and they're going to stimulate this actual pain fibers, these C fibers, the slow pain fibers. So if you have to remember, remember the chemical factors, C chemical are pretty much the entire stimulation for the C fibers. They do respond to some mechanical and hot temperature, but their main stimulation is through chemical factors.

A delta respond to cold temp mechanical stimuli. Not very much of it responds to chemical. All right? Now, since we're kind of grouping this together, we also have other types of receptors, which are going to be picking up crude touch and pressure.

What are those? You know you have different types of receptors over here, which are called, you have what's called Merkel's. You know you have what's called Merkel's disks.

Merkel's disks. These are for kind of your superficial pressure and your fine touch. And then there's another one, which is your peritracheal nerve endings.

These are basically surrounding hair follicles. And then another one, which are pretty much the most important ones, is your free nerve endings. Okay?

Your free nerve endings can also respond to touch and pressure. So you have free nerve endings, which is the big ones. The peritracheal nerve endings, which are around your hair follicles, they respond to bending of the hair.

and then Merkel's disc which kind of responds to more of the superficial pressure and some of the touch. So now these fibers, they can also pick up that crude touch. So what can they respond to? They can respond to crude touch and they can respond to pressure. These can be stimulatory factors for these guys.

So now we know what is actually going to be picking up. We know how we're actually picking up this tissue damage. or these extreme temperatures and then for the last situation we know how we're picking up crude touch and some superficial pressure. We know these big points here the C fibers the A delta fibers.

Very very quickly why do they call them A delta why do they call them C and if you really want to know because I know you guys are always interested in learning more these are generally again they can be A delta and C fibers also you can have a mixture of these but now why do they really quickly why do they call them A delta and C? It's dependent upon the myelination. So you know when you talk about myelination, let me get this out of the way here guys.

So myelination, axons can be myelinated to varying degrees. So when we talk about nerve fibers, there is of three types. We're mainly only going to focus on A and C, but there's A, B, and C. We're mainly going to be focusing on A.

A is highly myelinated. B is moderately myelinated. This is more for the sympathetic.

motor neurons here and parasympathetic. C are going to be not myelinated. So myelination determines the speed of the actual action potential. So A would be the fastest, B would be next, and C would be the slowest. That's the fact of slow pain.

They're not very heavily myelinated, so they don't move very fast for the action potentials. A, there's many different types. We'll talk about this more in the dorsal column, but there's A alpha, A beta, and A delta. A delta is the one that's picking up the actual fast pain or the cold temperatures. C is the one that's actually going to be the slow pain.

So that's why I just wanted to mention that really quickly, where this A delta and where this C is coming from, because I don't want to just throw that out there and just accept that you guys know it. All right, so again, A, heavily myelinated. C, lightly myelinated. It's really, really important because it determines the speed of the action potential. Okay, cool.

So now we know... What this tract is responding to, we know the receptors that are picking it up, now we have to talk about how is it going into the spinal cord. So I wanted to, before, because it's going to get kind of messy, so I want to blow up a big cross-section of the spinal cord so we can see how these fibers are coming in and so it's not too messy. So if we look at the cross-section of the spinal cord here, we're not going to talk about it in great detail, but they kind of section it out into different, like, what's called lamina.

They call them rexed lamina. If you really want to know what's called rexed. Lamina, we're not going to talk about that in great detail. We're going to talk about a couple of them that are really important.

But it goes like this. You have Rex lamina 1, 2, 3, 4, and it just keeps going. What's important here is mainly, actually let me put one more here, 5. The only really thing that's important about this is kind of remembering where these actual fiber synapse in this point for the pain pathway. Okay? So let's say we take, for example, the C fibers.

Let me get this out of the way here. Okay? Let me get that there.

So now, let's say here we have the C fibers, right? And the C fibers here was the endings, right? That was the one that was responding to the chemical stimuli or the hot temperatures or the mechanical stimuli.

And what happens is this is the peripheral process, this part here, picking up the stimulus. This is the pseudo-unipolar neuron, right, the cell body there. And then it has the central process, which is going into the central nervous system.

What happens is it goes in, right? And we're going to talk about this in a second. And it actually ascends and descends. We'll talk about that in a second. It's called the tract of LeSaur.

LeSaur, sorry. So tract of LeSaur. But when it does, when it ascends or descends, it synapses on specific points.

For the C fibers, it mainly synapses in Rex lamina 2 and in Rex lamina 3. And then from there... It goes through kind of a cascade of different types of neurons here. Throughout this entire process, it kind of goes through a cascade of multiple different synapses.

And then eventually, what happens is these fibers cross. They go to the contralateral side. And when they do that, they go through what's called the anterior commissure and then go up.

So right here, where these fibers cross over, it's called the anterior commissure. So that's for the C-fibers. So if you really had to remember the important ones, remember 2 for the C-fibers, and then remember 3 for the C-fibers, okay? If you really want to know, they call it 2, they call it the substantia gelatinosa of Rolando, and they call 3, they call it a part of the nucleus propius.

All right? So that's for the C-fibers. Now the next one is the A-delta.

Where is A-delta going in? A-delta is actually going to be, here's again. Receptor part, picking up the stimulus.

Peripheral process, pseudo-unipolar neuron. Central process. And again, tractal lacer, lacer, sorry. And that can go into specific parts here.

Mainly one. Rex lamina one and rex lamina five. And then from here, these guys, again, they cross over through the anterior white commissure, come over here into the anterior lateral system, and move upwards. And that's what we're going to talk about. But I want you guys to get an idea of where these things are moving, what parts they're going into.

So now, remember for this one, for the fast pain, it's 1 and 5. Okay? 1, if you really want to know, is the marginal nucleus. And 5 is referred to as the reticular nucleus. Okay?

Or that lamina area. Okay. So now we know that.

The last one here, and this is, again, this blue one here. Let me mark this down. This is for A delta. The last one here that I want to finish off with is for these actual crude touch. Again, it's not a super big one here, but again, same concept here.

Peripheral process, pseudo-unipolar neuron, central process comes in here. It's not specifically that important where they go in here. They believe it could go to 3, 4, maybe 5. But just know that it comes in here. Again, it could go to 3, 4, 5. And then what happens is it crosses over.

Now, it does cross. goes to the contralateral side of the spinal cord and it goes through the anterior white commissure but it comes a little bit more anterior so you know over here we have the lateral white column and then over here we have the ventral or the anterior white column this one mainly kind of goes situated within the ventral or the anterior white column and then goes up okay so just remember that so now this is for this pathway is for the crude touch and pressure. Sweet deal.

So now we know the pathway, we know what it's carrying, we know the receptors, we know how it's entering into the spinal cord, but we have to do one more little touch. Really, really important point, super important point is that ascending and descending system I was telling you about. And the only reason it's super, super important is because of its relationship with like certain types of lesions.

Let's use this marker here. So let's pretend I take that pain fiber right there. Okay, here was its peripheral process which is picking up the stimulus.

When it does that, it goes in to the posterior gray horn. But right when it goes into the posterior gray horn, it gives off ascending branches and it gives off descending branches. And what these do is they can go down.

They can actually go down maybe one, two, tops three spinal cord segments or they can go up. One, two, tops three spinal cord segments. And they can synapse on the nuclei in this area that cross over and go up. Again. Let's pretend that this one's coming over here.

This one can come up. This one comes over here, goes up, goes down, stimulates here, crosses over, moves up. So it develops this tract, if you get an idea of this. If I were to draw it at every single section, it starts making this tract that's kind of going up and down.

You see that? That tract right there is called the tract of La Sour. Okay, it's called the tract of the sour.

Why is this important? Really briefly, let's pretend that you have a lesion in the spinal cord around T6. T6.

The area that's actually going to be affected is maybe two or three spinal cord segments below that on the contralateral side. That's why this is important. It's important for clinical aspects because if it's ascending and descending fibers, if there's a lesion at T6, the actual, if there's Elysian at T6, right?

The problem is going to be two segments, maybe three, down and on the contralateral side because of this tract. Okay, I think we got it down for this part. So now let's come over here and see how this ascending pathway moves up and the different nuclei that it synapses on, right? So let's start here first with that ventral.

Let's do the crude touch. Let's get that one out of the way. So here, let's put crude touch right here.

Here's that. Pseudo-unipolar neuron, it's responding to what type of stimulus here? Crude touch, which could be coming from the Merkel's disc, the peritracheal nerve endings, the free nerve endings. And it's also responding to a little bit of pressure.

And again, what happens here? It comes in and it goes to this actual cell body. This is your first order neuron, the first point.

It goes into the dorsal gray horn, synapses on the cell bodies of a... nucleus somewhere within the dorsal gray horn, and then does what? Crosses over through the anterior white commissure and goes up, all right?

And this right here, they refer to this as the, I'm going to abbreviate it, the ventral spinothalamic tract, okay? We're going to abbreviate it as the ventral spinothalamic tract. And just remember, this thing right here, this pseudo-unipolar neuron, it's actually located within the dorsal root. ganglion, which is kind of located outside of the spinal cord.

Okay, so we have the first order neuron, which is this pseudo-unipolar neuron, which is located within the dorsal root ganglion. We have the second order neuron, which is located within the dorsal grey horn, and then the axons, crossover anterior white commissure, ventral white matter here, and then goes up. and then it ascends. Okay?

Now, let's do the next one. Let's do this one. Let's do it in that baby blue. Okay?

And we're going to keep this pretty much the same here. Let's say here we have the pain pathway here, right? Let's say this is our A Delta.

Okay? Our A Delta does what? Picks up pain and cold temperatures.

So we'll put cold temp. Okay? And that pain could be through cold temperatures, mechanical stimuli, right?

Sends the information inwards. And again, it can have that tractalisor where it can go down or it can ascend. So it can ascend and descend one to two to three segments. Synapses here on the cell bodies, right, of the dorsal gray horn.

Then from here, it crosses over through the anterior white commissure and then goes upwards. And specifically, where is it going? It's going into the lateral white column.

So this is going to be a component of what's called the lateral spinothalamic tract. So you have the ventral spinothalamic tract, the lateral spinothalamic tract. All right.

One more thing, though. We need the red. My red marker. Okay.

This last one here is the C fibers. And the C fibers, these are the ones, remember, they're responding to slow pain, that dull pain, achy pain. They're going to be coming in. synapsing on cell bodies in the dorsal root ganglion, sorry, dorsal gray horn, and then doing what?

Crossing over via the anterior white commissure and then moving upwards. Now, the lateral spinothalamic tract, let me actually do this, is actually both of these things, right? So the lateral spinothalamic tract is really both of these things. But I got to give us one more thing because...

The A, the A delta, right, this pin prick pain. It's more of a newer kind of more modern concept. So because of that, they believe that this A-delta is a part of, they believe it's more specifically what's called the neospinothalamic pathway. That's specifically for the A-delta. And they refer to the C-fibers because they're more of a primitive kind of concept here.

They're called the... ... paleo-spinothalamic pathway.

If you don't really want to remember that, that's fine. It's just giving you an idea of the origins, the concepts of this, okay? So C is mainly carrying fibers into the anterior lateral system, but specifically in the lateral spinothalamic tract or the lateral white column. And if it's C, it's more of the older concept, the older aspect of, they believe it, the paleo-spinothalamic pathway. Whereas the A-delta, It's more of a newer concept of pain, right?

That's the neospinothalamic pathway. Now, what happens is, is all of these guys come together. They all come as one big system.

That's why we call it the anterolateral system. So now let's bring this guy together like this for the crude touch and the pressure. We'll bring this red guy right here and we'll bring the blue one right there smack dab in the middle. Okay, so now we have this whole thing here. This whole anterolateral system.

Some books, they even refer to this group of structure here as what's called the spinal leminiscus. There's other fibers that are moving up in this, and we'll talk about those, like spinotectal, spinomesencephalic, spinal hypothalamic, spinal reticular. There's so many of these darn things. But for right now, let's keep it with just this part, the ventral spinothalamic tract.

And the lateral spinothalamic tract, which is broken up into two different types of pathways, paleospinothalamic, which is the C, neospinothalamic, which is the A-delta. As they come upwards, they're going to give off certain types of structures. So this ventral spinothalamic tract, it's mainly going to go up to a specific nucleus within the thalamus. And that is called the ventral posterior lateral nucleus.

And there's another one called the ventral posterior inferior nucleus. But these crude touch and temperature sensations, these are mainly going to be going to a structure, a lot of them are going to be going to a structure in the thalamus, which is called the ventral posterior lateral, and there's another one called the ventral posterior inferior nucleus. Okay, you'll see as we talk that there's going to be a lot of collaterals.

Okay, now the C fibers, here's the important thing. The C fibers are really interesting. As they're moving upwards, some of them make it up to the thalamus, but about 85% of them terminate in this big orange structure here.

What is this big orange structure here called? This big orange structure right here is called the reticular formation. So this big orange structure here is called the reticular formation.

And it's a piece of gray matter that extends. throughout all parts of the brainstem, midbrain, pons, and medulla. So a good portion, how much of it again? About 85% of the fibers terminate in the reticular formation. Only about 15% of these fibers go up to the thalamus.

Here's the thing though, these fibers here, these red fibers which are coming from the C fibers, right, the slowpane, they go to specific nuclei Non-specific nuclei in the thalamus. In the thalamus, just a brief thing here, so I draw a little thalamus here. There's different parts of the thalamus, alright, so you have different parts, but this structure right here, this Y-shaped structure, it's called the intermedullary lamina, right? And there's a whole bunch of nuclei that are located within this lamina. Okay, they're called intralaminar nuclei.

And I have that pink representing that. So you're going to have some nuclei which are lodged in this lamina. And these nuclei here are called your intralaminar nuclei.

And these are really important. There's a bunch of them, but the main ones that we're going to be focusing on here, if you really like to know that, is the centro. median nucleus, and another one called the parafasiculus. Okay, so these are the two big... The ones that are actually a part of the intralaminar nuclei is the central media nucleus and the parafascicular, parafasciculus nucleus.

And again, they're nonspecific. What does that mean? From here, they do a lot of different things.

They go to different parts from this structure here. So from the intralaminar nuclei, they can actually kind of go out and supply different areas. They can supply the cortex, like the primary somatosensory cortex, but they go to two really important structures.

One of them, and I'm going to do it like this to represent its significance, is it can go to a part of the actual cortex, which is associated with the emotional aspect of pain. Okay? This right here, let's say that this structure here is called the cingulate gyrus.

There's another one that it could go to. There's another part here, which is kind of tucked deep within the temporal lobe, and that one is called... The anterior, this one's called the anterior insular cortex.

Okay, so some of these fibers can go out to different parts like the somatosensory cortex, the primary and the secondary association, the secondary association cortex. But two big ones is it goes to the cingulate gyrus and it goes to the anterior insular cortex. And the reason why this is important is because they play a role within the emotional aspects of pain.

Holy crap. Okay. Well, we're going to come back to the C fibers because there's a lot more that we got to talk about yet.

But let's go to this A delta. So these A delta fibers, they're also coming up, right? Some of them, very little of them give off some collaterals to the reticular formation. Most of them ascend upwards and go to this nucleus here within the thalamus. And again, what is that nucleus there called?

It's called the ventral posterior lateral. This is the main one. And some of them can go to the ventral posterior inferior nucleus. From there what happens? These fibers can radiate.

So we're taking with us fast pain and we're taking crude touch and pressure to this ventral posterior inferior and ventral posterior lateral. From here they're going to radiate. They're going to go up.

Here you have the internal capsule. And you have the anterior limb and the posterior limb. There's a part here of the posterior limb. Which is the posterior one third.

of the internal capsule, it can actually hop on that and then go to the corona radiata and supply large parts of the cortex. What is that part of the cortex that it's supplying? I'm going to abbreviate them, but it goes to what's called S1 and it goes to what's called S2, okay?

And these areas of the cerebral cortex, they're actually located on specific part of the brain. You know, you have what's called the central sulcus. So let's imagine here I draw a small little diagram.

Here's the central sulcus here. I'm going to draw it like this. Here's your central sulcus. Behind the central sulcus, you're going to have this gyrus here, which is called the postcentral gyrus.

The postcentral gyrus, there's another area just a little bit behind it, which is called the secondary, you know, secondary association cortex, right? The secondary somatosensory cortex. So this right here is S.

This right here is S1. What I'm doing is, is we're taking a slice. We're taking a coronal section here.

And if I take a coronal section right here, we're going to see that we're focusing on all this part right here. This is all parietal lobe. Okay? So it's going to be supplying a large part of the parietal lobe and taking this actual information, these sensations, to different parts of the cortex. S1 and S2 mainly.

It's going to help us to become consciously aware and to understand. The concept of that pain, all right? Okay, so we got the information going to the cerebral cortex, primary somatosensory cortex, secondary somatosensory cortex.

We got branches off to the cingulate gyrus and the anterior insular cortex. Sweet, stinking deal. Now, we have to come back to this over here, the C-fibers.

Along the way, there's other components, right? Well, we're good with the C-fibers, but we have to talk about one more thing. Along the way... There's other fibers that are associated with this entire anterolateral system.

So if we talk about that, let me add these ones in. So in the anterolateral system, so again we're going to go ALS, okay? This is not amyotropic lateral sclerosis, we're talking about the anterolateral system.

There's different fibers, right? We're going to be carrying with us the lateral spinothalamic tract, the anterior spinothalamic tract, and there's some other ones, right? Another one is we're carrying with us what's called the Spino-Tactile Tractile.

We're also carrying with this anterior lateral system the spino-mesencephalic, which is really important, tract. These are just fibers that are part of this whole system here. And there's another one, and this one is actually going to be called the spino-hypothalamic tract.

And there's actually one more here. And this one is actually going to be called the spino-reticular tract. We've already kind of covered that, though.

These little branches, these collaterals that are coming off from the anterolateral system. But now we have to cover the spinal tectal and the spinal mesencephalic really quickly. So coming up through this, you have, let's do the spinal tectal first.

So where is the tectum? You know, if you look here, you take like a different type of section here of the brain. We take like a sagittal section.

We can look at the central nervous system like this. And on the back of the midbrain, you have these two little swellings. Okay, they call it the capora quadrigemina, which is made up of the superior colliculus and the inferior colliculus. We're going to take information to the superior colliculus specifically so that it can help to move our eyes or our head to wherever this actual stimulus is.

Maybe I stepped on a Lego or something like that, and I moved my head and my eyes towards that area. So now... Coming up through here, here's the midbrain, we have to give off collaterals.

And these collaterals that we're going to give off is going to go to this structure right here. Look at this structure right here. This structure right there, we're going to call this right here the superior colliculus. But it's a part of the tectum.

We're going to put the superior colliculus. So this tract right there, these fibers that are giving off these little collaterals, that are going to the superior colliculus, or the tectum, is called the spinal tectal fibers. Then we have some other ones.

We have the spinal mesencephalic. So you see these nuclei right here? They're in the pons.

This is called the parabrachial nucleus of the pons, right? So the parabrachial nucleus. This is going to give off some little collaterals, right? So we're going to give off these fibers, these collaterals that are going to come off to the parabrachial nucleus.

Why is this important? Remember I told you, pain is emotional. Sometimes it's really sad, you know? So what happens with these guys? These parabrachial fibers, they can send axons upwards.

Let's use this one for example, since it's kind of crowded over here. These guys can send axons upwards and they can go to a special structure located within the actual cerebral cortex. I'm sorry, within the cerebrum.

Deep within the cerebrum. It's a piece of gray matter deep within the cerebrum. There's a special structure up here. Let's do this one in this green right here. There's a special structure right here that's associated with the fear and the emotional aspects of pain, right?

And this is called the amygdala. Okay, this is called the amygdala. So we have a connection the parabrachial nucleus in the pons and the amygdala. And what is this little tract that's coming off here?

These fibers that are coming off to supply the parabrachial nucleus and another structure. That's going to be called the spinal mesencephalic tract. So two structures the spinal mesencephalic is going to supply. One is the parabrachial nucleus in the pons, which will go up to the amygdala, which is controlling the fear and the anger and the painful aspects within pain, right?

The other one we said is the superior colliculus. It's giving off fibers to that, which is called the spinotectal. Now, there's another structure here that this actual anterior lateral system is going to give to. It's right here. You see this right here?

In the midbrain, you have what's called the cerebral aqueduct. And then there's a piece of gray matter that wraps around it. That piece of gray matter that's wrapping around the cerebral aqueduct is called the periaqueductal.

It's actually, do it like this. Peri. PAG, periaqueductal gray matter.

Let's do that. Periaqueductal gray matter. It's a piece of gray matter surrounding the cerebral aqueduct.

What happens is some of these fibers, you have fibers from the spinal mesencephalic, right? Spinal mesencephalic fibers can give off collaterals to the periaqueductal gray matter. Why is this important?

We'll talk about this in another video, but the periaqueductal gray matter controls your descending pain modulation pathways. They can control the pain. It has different descending fibers that can go down and release norepinephrine and serotonin that can help to modulate the pain by releasing different types of encaphalins, endorphins, and dynorphins.

We'll talk about that after. Okay, so we've covered that now. One more.

You know, right up here, you're going to have another structure, and that structure is called the hypothalamus. Let's just say that as we're going up here, as we're going up, we give off some collaterals here from this anterolateral system to another structure right here. And let's say that this structure here we're talking about is the hypothalamus. So this right here is called the hypothalamus. So that's called the hypo...

Thalamus. So let's say that you get kicked in the nads, right? What happens?

It can cause you to maybe become nauseous. It can cause you to increase your heart rate, maybe change your respiratory rate in depth. Why?

Because these collaterals that are coming off and going to the hypothalamus, the spinal hypothalamic fibers, the hypothalamus controls our autonomic nervous system. So these Spinal hypothalamic fibers that are coming off here and supplying the hypothalamus controls our autonomic responses to pain. Changes in our blood pressure, changes in our heart rate, changes in our respiration, right? So different things like that. Or think about a woman giving birth.

It's going to do what? It could actually trigger oxytocin release, and he controls our hormones, right? Or even the suckling reflex. So the hypothalamus controls a lot of the aspects, the autonomic effects of pain.

Okay, one last thing, guys, and we are done, is this last part here. We didn't finish up on this. We said that 85% of the fibers, from the 85% of these C fibers, go to where? The reticular formation.

The reticular formation is so, so, so important. What is the function of your reticular formation? It controls the alerting, the arousal, the wakefulness of the entire central nervous system. So whenever you're awake, Alright, someone screams, right? Our reticular formation can sift through all the non-specific, not really important information and help us to become aware of the most important, the most recognized situation or whatever sensation it might be.

Whenever you go to sleep though, what happens to your reticular formation? It shuts off. And think about this.

Let's think about me saying something out loud. And the impulses get to travel down the vestibulocochlear nerve, right? You don't hear that sound when you're in deep, deep sleep. Why?

Because your reticular formation is not alerting your central nervous system of that. Or if you touch somebody and they're in really, really deep sleep and they don't wake up, does that mean that they're not able to pick up those touch sensations and send it up to the central nervous system? No. It just means that their reticular formation is not going to be alerting the cerebral cortex of that touch. That's the beauty of the reticular formation.

So in certain types of pain, like think about this. It's really simplistic. If you think about it and you have arthritis or you have a lot of achy pain, right? that dull or burning pain. What's it doing to your reticular formation?

Activating it. A lot of them, 85% of the fibers. Well, what is the reticular formation?

What did we just say it does? It pretty much sends these fibers upwards, okay? And these fibers go to these intralaminar nuclei.

The intralaminar nuclei, what do they do? They supply the primary somatosensory cortex. the secondary somatosensory cortex and it's going to go to main areas which is the anterior insular cortex and the cingulate gyrus but the big thing with this is is the reticular formation is alerting almost your entire cerebral cortex why is that important think about it when you're trying to sleep but you're in pain because of arthritis or achy or dull pain why is that it's because it's alerting your reticular formation your reticular formation is telling your entire cerebral cortex Hey, I'm in pain.

So that's one of the crazy things about this, okay? One last thing I want to talk about, guys. So remember how we said that with these C-fibers, right? They come over to the dorsal regangulum. They cross over to the contralateral side and then ascend, right?

As this lateral spinothalamic tract, but particularly the paleospinothalamic pathway of it. Well, just remember that it's not always contralateral. Some of the fibers, some of them, not all of them, but some of them can also stay.

ipsilateral. It can stay ipsilateral and then give off collaterals to the reticular formation, right, and some of them can actually reach the intralaminar nuclei and then go to different parts of the actual cortex. So it's really important to remember that these C fibers can not only go contralateral but they can also go ipsilateral via, this is called the spinal reticular fibers, right, it's the ones that are going to the reticular formation.

85% of them, very little, maybe 15% of them. goes up to the interlaminar nuclei. Ningeners, we covered so much information.

If you guys stuck in throughout the whole video, I can't say thank you enough. I hope you guys really enjoyed. I hope you learned something.

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All right, Ningeners, as always, until next time.

Transcript for:Ascending Tracts | Spinothalamic Tract

Transcript for:
Ascending Tracts | Spinothalamic Tract