Welcome to this tutorial on the internal anatomy of the brainstem. We indeed turn again to one of the most complicated parts of the nervous system, which brings to mind our first core concept in the field of neuroscience and that is the brain is the body's most complex organ. I have one learning objective for you today and that's to be able to identify the major subdivisions of the brainstem, as seen in representative transverse cross-sections. So in order to achieve this objective you're going to have to be familiar with what we discover when we look inside the brainstem in each of the three major divisions, the midbrain, the pons and the medulla. So here's the key in achieving this objective. I want you to mobilize your mental image of the external features of the brain stem, as we begin to look at our transverse cross-sections. And if you can do so You'll know what to expect to find in each of the subdivisions, so when we think about the external features, think about the form of each division of the brain stem. Think about the bulges, the furrows, the grooves, the columns that we might see from the outside and think about the cranial nerves and how they attach. That will be key in order to interpret what we see on the inside. It all begins with our mental image, so that's what we're working towards and in order to get there, I like for us to have a tour of the brain stem using Sylvius Four software. So, if you'd like, go ahead and open your version of Sylvius Four, if you have it, if you don't that's fine, you can follow along with me. Okay, I've just opened up Sylvius Four and I'm going to now click on. The brain stem cross sectional atlas. All structures. And this will open up now, our brain stem atlas. And this is an atlas of transverse sections through the brain stem. Where the sections are prepared in order to resemble a standard histological mylon stain. Which means white matter structures will appear dark, as if they've been stained with silver salts. And gray matter structures will be relatively unstained, so they will be light. So the convention is essentially opposite what the terms usually convey. So white matter is dark and gray matter is light. And in Sylvias, we have a window to the left that shows us our navigational views of the content including a list of thumbnails some slider tools that will help us navigate along and we can select and image to enlarge and as we mouse over features within the image we notice that a label appears and if we were to click on that structure. It becomes highlighted in the thumbnails as we can see over to the left, and a statement of structure or function appears in the text box to the right. So that's just a little bit about navigating in Sylvius. So let's go ahead and begin. We began at the top of this atlas or the most supeior section. Which is very near the junction of the thalamus and the midbrain. We recognizde elements of the thalamus here, all this gray matter alnog the dorsal and medial aspect. Just to the lateral wall of the third ventricle. Down below we have the mammillary bodies, which is part of the hypothalamus. So we are definitely in a portion of the posterior diencephalon, but we are also starting to get into the mid-brain. And we know that, because of these massive bundles of white matter that are beginning to take shape in the ventral lateral surface of the brain. That's where we expect to find the Cerebral peduncles. In fact, what we have here is the confluence of the internal capsule into the Cerebral peduncles. So, if I select the Cerebral peduncles, and then just go down one section We are clearly in the mid brain, and the cerebral peduncles are one of those key features that will allow you to be able to recognize this part of the brain. Another feature would be what's going on with the ventricular system, so we just saw the third ventricle opening up. In the region of the diencephalon. N the mid-brain we don't expect to see the third ventricle. Rather, we expect to see the narrow cerebral aqueduct, and indeed that's what we see here. So he presence of the peduncles, the presence of the cerebral aqueduct is confirmation that we are looking at the mid-brain. Well, let's think about again that outside to inside perspective. What else do we see from the outside when we think about the mid brain? Well, you should be thinking about a cranial nerve. Cranial nerve three, and indeed we have cranial nerve three here at the Oculomotor nerve. And so again another characteristic feature of the mid-brain, cranial nerve three is derived from a sematic motor nucleus, so long the dorsal midline of the take momentum of the brain stem. This is the Oculumotor nucleus, it also has a pair of sympathetic motor component from the Edinger Westphal nucleus. So those are the cranial nerve nuclei associated with this level of the mid brain. But there's lots of other features here that's worth noting in the mid brain. There's a very large spherical nucleus in the mid brain that looks a little bit orange-ish. In a fresh specimen of the human brain that has been sectioned open. This is called the red nucleus, very prominent nucleus of the tegmentum in the mid-brain. Between the red nucleus and the cerebral peduncle is the substantia nigra, which is really two nuclei in one. There is a compact layer of cells on the dorsal margin of the nigra. And these are the famous dopamine neurons of the mid brain that project broadly to the fore brain including the basal ganglia, parts of the prefrontal cortex of the frontal lobe as well as the amygdala in the hippocampus Then there is a inferior part to the substantianagra, which is very much like the globas palettus in the basal ganglia. We'll say much more about all of this when we talk about the basal ganglia, but for now I would highlight that the substantiangra is found in the mid-brain. Okay, let's look at the dorsal part of the mid-brain, and again if we think about what we see from the outside when we look at the mid-brain you may recall that there are these four bumps that we see from the outside. They're called little hills, or colliculi, which means little hill. There are a pair of superior colliculi and a pair of inferior colliculi. We are at the level of the superior colliculus. The superior, of these little hills. One on either side of the mid-line. And just below the superior colliculi, around the cerebral aquaduct is a bit of gray matter, that we call the periaqueductal gray. All right, well, let's go down a section to a lower level of the mid-brain. And at this level, we are below the superior colliculus, and we are at the level where we Are beginning to develop the inferior colliculus. We're actually a little bit below that little hill, strategically, Because we want to see the nucleus of the trochlear nerve, which is found here along the dorsal midline of the tegumentum. Right in line, but inferior to where we saw the ocular motor nuclei. We still have a small cerebral aqueduct surrounded by a large periaqueductal gray, nucleus so that is consistent with what we saw in a more superior level. But now you'll note that the inferior part of the section looks quite a bit different. We still see something that resembles the cerebral penuncle, but now we're beginning to see some[UNKNOWN] of these axons in the ventral part. Of this section. In fact, what we're getting into is a set of nuclei that is defining the features of the pons. So this is very near the juction of the pons and the midbrain. We have nuclei in the base of the pons calls the pontine nuclei that give rise to axons to called the pontocerebellar fibers because they're going from the pons to the cerebellum. Well let's go ahead and section down into the pons. We'll actually skip this section. And go to the widest part of the pons, where the ponce is sending axons out into the cerebellum. And then we'll work our way, back up to those sections we just passed by. Well, we're in the pons. How do we know that? Well, because we see, these pontene nuclei defining much of this basal or ventral aspect of the pons giving rise to these, to this white matter, called the ponto cerebellal fibers. This accounts for that transverse orientation. Of axons we see in the pons that is bridging the long axis of the brain stem. "Pons" means bridge. And these pontocerebellar fibers give rise to these massive bundles of axons that enter the cerebellum, called the middle cerebellar peduncle. Now in the region of the tegmentum of the pons, we have some cranial nerve nuclei of note. We have the chief sensory nucleus of the trigeminal complex. Also called the principle sensory nucleus. And the trigeminal motor nucleus. That are connected up with the trigeminal nerve roots. Which we see here. Now, above the tegmentum of course, that cerebral aqueduct has now opened up into a large fourth ventricle. And so, lateral and dorsal to that foruth ventricle is the cerebellum. Now, you may be wondering about these dark bundles of white matter that we see here. In the lateral aspect, of the ventricles here. This is the superior cerebellar peduncle. Yet another major bundle of axons associated with the cerebellum. So we have the middle peduncle. By far, the largest of the three cerebellar peduncles. And the superior cerebellar peduncle. I'm going to select this structure. And then retrace our steps, and go up a few sections, and see what happens to this peduncle. This peduncle Is the means by which the cerebellum sends signals out of the cerebellum to reach our motor circuits in the thalamus and so these peduncles are leaving the cerebellum and as they do so they work their way into the tegmentum of the ponds. Where they cross the mid-line, so we see a crossing of this peduncle called a decussation in the upper part of the pons, and that's really the primary feature I wanted to highlight in the sections between. The level of the trochlear nucleus and the trigeminal nerve. So let's go back to the section that we were just viewing and highlight a few other features. We notice in this basal region. While many of the axons running through have defesiculated, we're beginning to see a re-condensation of axons into a tight bundle, this will emerge on the other side of the Pons as an important pathway called the medulary pyramid, which contains among other axons, the cortical spinal tract. So we'll say much about that pathway in this course. All right, so let's go down one section, which is the level of the cottle part. Of the pons and, I'll deselect the superior peduncle here for a moment, and show you some other features that are present in the tegmentum of the pons. Again, starting with cranial nerve nuclei. So now we have a different division of our trigeminal nuclear complex. This is called the spinal nucleus of the trigeminal complex. This is part of the trigeminal nucleus that extends from the level of the principle or chief sensory nucleus all the way down to the merging of the caudal medulla with the upper cervial spinal cord. So there's a nucleus, and then just lateral to the nucleus is a tract, called the spinal trigeminal tract. We have other important cranial nerve nuclei here. We have the facial motor nucleus in the colu pons, which gives rise to axons that sweep across The tegmentum, and loop around an important grey matter structure, just below the pornter/g here. And these axons then arc back out and exit the ventral lateral aspect of the pons. What they loop around is the Abducens nucleus. So here's the Abducens nucleus, a somatic motor nucleus along the dorsal mid line of the tegmentum, It gives rise to the axons of the abducens nerve that exit near the junction of the pons in the medulla. We've accounted for the abducens nerve as it exits near the mid line of the pons. We've accounted for the facial nerve that exits a bit more laterally. The third of these pairs of nerves at this junctional region Is the vestibular cochlear nerve. And at this level, we're beginning to get into some of the nuclei that are associated with that nerve. This is one of the vestibular nuclei that we have in this region of the. Pons here is another one. Because there are multiple nuclei here, receiving incoming vestibular signals. We call this the vestibular nuclear complex. All right. Lt's move now out of the pons and into the upper part of the medella. Here again we expect to find vestibular nuclei. We have yet an additional vestibular nucleus here, part of that nuclear complex, but we also have the auditory component of the eighth nerve which becuse we didn't achieve an optimal section through it in sylvias we elected not to label it. But we see a little bit of it here. It's this grey matter that forms this rind around the outside of the dorsal-lateral aspect of the medulla. And what it's surrounding is a, is a large mass of white matter, that I'll get to in a second. So this grey matter that we see out here , this is part of the Cochlear nuclear complex. So there are multiple division of this complex. We can call them collectively the cochlear nuclei. So we've got our vestibular nuclei and then our cochlear nuclei associated with the eighth nerve. Well this big massive bundle of white matter. This is the inferior cerebellar peduncle, one of the three major stalks that attaches the cerebellum to the brain stem. This is inferior, and it attaches from below, mainly conveying signals from the spinal cord but also signals from an important nucleus of the medulla that I'll mention in just a moment. Well, we also have elements of the trigeminal system here, here is our spinal trigeminal tract and our spinal trigeminal nucleus. We'll see this all the way through the medulla. Now, in the more ventral apsects of the medulla, those fibers that were penetrating through the basal region of the pons and coalescing back into a nice compact bundle of white matter, have now formed the medullary pyramids. And at this level, these axons are almost entirely the corticospinal tract. That is the pathway by which the cortex governs the motor systems of the spinal cord. Just above the medullary pyramids is really a, quite a lovely nucleus. It's called the inferior olivary nucleus. And this is the nucleus that gives rise to a bulge In the ventral lateral aspect of the medulla that we call the olive. So you may recall from our surface views of the medulla, seeing the columns or the medullary pyramid. And just lateral to it is the inferior olive. Or the olive for short. That's what we call it from the outside when we're looking at a cross section as we are here. We recognize this as the inferior olivary nucleus. Well, let's move on. Now you may remember between the olive and the medulary pyramid exits the axions of the 12th nerve, the hypoglosal nerve. We can see the hypoglossal nerve roots here as they pass between the pyramid and the olive and their source is the hypoglossal nucleus. Another somatic motorn nucleus that we find on the dorsal mid line of the tegmentum in the medulla. Now, just lateral to that nucleus we have the dorsal motor nucleus of vagus. And right next the that nucleus is a sensory nucleus, the nucleus of the solitary tract. And that's grey matter that surrounds this solitary compact bundle of white matter. Called the solitary tract. The nucleus of the solitary tract is a visceral sensory nucleus. The dorsal motor nucleus, a vagus is a parasympathetic visceral motor outflow nucleus. So, notice again the organization that we have here. Somatic. Motor, visceral motor, visceral sensory and then we have special sensory and general sensory. Our special sensroy at this level are still vestibula nuclei and then our general sensory is still the spinal trigeminal nucleus in the spinal trigeminal tract. Now, I also should point out one important white matter pathway that we'll spend a fair amount of time on when we talk about our mechanosensory system. And it is a medial ribbon of white matter, that sits right on top of the medullary pyramids, its called the medial lemniscus. So I'm going to select the medial lemniscus, so I can show you how this structure, this medial ribbon of white matter, runs all the way through the brain stem. I haven't highlighted it until this section, because we probably see it best in the middle to lower part of the medulla. But indeed we can track it all the way up from the caudal medulla to the thalamus. And we will do so when we study our mechanosensory pathways. For this structure, conveys Our sense of touch from the body below the face, from the brain stem to the thalamus. Okay. Let's go now to the caudal part of the medulla and highlight a few structures of interest. Now, what we find is still a spinal trigeminal nucleus in tract. But now, in the dorsal part of this section, we've lost the fourth ventricle entirely, and what we have are a set of gray matter nuclei that are bounded by white matter pathways. And these nuclei are called the dorsal column nuclei because they sit right on top of the dorsal columns of white matter that rise up from the spinal cord. And there are two of them. There's a medial tract called the gracile tract, which ends in a gracile nucleus. And then there's a more lateral tract called the cuneate tract that ends in the cuneate nucleus. Now, these nuclei, these dorsal column nuclei, they grow the axons that form the medial meniscus. So they grow axons that sweep around through the tegmentum of the brain stem, called the internal arcurate fibers. And they form the medial limniscus. The medial limniscus axons derive from the contralateral, or the opposite side dorsal column nuclei. So there's a crossing. There's a decussation here. There's one other important decussation to note. At this level of the medulla. And it's a decusation of the medullary pyramids. Notice how one pyramid is starting to stream up and over the mid line on top of the other pyramid. If we go down one final section, we'll see this crossing more obviously. This is where the cortical spinal tract crosses the mid-line. Actually as we'll seen in a few weeks about 90% to 95% of the axons cross some remain on the same side, but the vast majority of them cross and here's where the do so, in the caudal medulla. Well, that completes our brief tour of the internal anatomy of the brain stem. We'll have the remainder of the course to reinforce this anatomy and come back to it. We'll do so when we talk about our long pathways. For somatic sensation and motor control. And then we'll apply this knowlege to consider the localization of injury in clinical cases whrer there has been some focal damage. And what will be importatn to emphasize at that point in your learning in the regional anatomy. That is, where do various systems come together in small places in the nervous system? And the brain stem is one such important place, where even very small injuries of nerve tissue can give rise to widespread and disparate neurological signs and systems. So, I look forward to having that level of discussion with you in a few weeks. But for now, take your time. And go through the brain stem sections and identify the principal grey matter and white matter structures that are highlighted for you in the tutorial notes, including those that I've highlighted here in this video. And, use this knowledge, we'll apply this, we'll discuss it as we go along. So, thanks for your attention, and I'll see you next time.
