Well, here we are in Sylvius. And I've jumped into our photographic atlas, looking at sectional anatomy. And we're looking at a section roughly through the middle of the brain in the coronal plane. And what I want to draw your attention to, is the presence of the internal capsule. So, there's a bit of white matter that's making its way through the middle of the hemisphere, separating the thalamus and the caudate nucleus on the medial side, from the putamen and the globus pallidus on the lateral side. And this provides a means for the axons of pyramidal cells that are sitting up here in the motor cortex, to grow an axon and have it enter the middle of the white matter of the hemisphere that allows this axon to grow down and through into the brain stem and on through the brain stem. Okay, so there is the location of our internal capsule. So, this is where we would find the axons of our corticospinal, and our corticobulbar pathway. Okay, so lets section on through, and now we see the appearance of the midbrain. And we know that of course, because we have this beautiful dark substance here in the midbrain. This is the substantia nigra parts compacted. The dark substance reflects the accumulation of a pigment called neuromelanin by neurons that synthesize biogenic amino acids. And in this case, this is the location of our dopamine cells. So it's a convenient visual landmark, that tells us we've entered the midbrain. Well, just inferior and lateral to our dark substance, our substantia nigra, we find the cerebral peduncle. So here's our cerebral peduncle. So now the axons of the corticospinal and corticobulbar tracks have made it down here into the cerebral peduncles. We can follow these cerebral peduncles even further into the brain stem. And we notice that now those peduncles have entered the pons. So we know that this is the pons, because we have this broad base of the pons that's giving rise to these peduncles that are spreading out into the cerebellum. So, these now are the cerebellar peduncles out in this direction. So our corticospinal and corticobulbar fibers are running down through the base of the pons in this location. Well really to, to carry on this story, we need a closer look at the brain stem. So, I'm going to leave this photographic atlas of the whole brain, and enter our brain stem cross sectional atlas. I think we can make this just a little bit larger. And what I'd like to do is to highlight, again, the location of the structures of interest. So we know that this is the cerebral peduncle, which is conveying the axons of the corticospinal, corticobulbar pathways. They tend to be found near the middle of the peduncle here. With the corticospinal fibers being just a bit more lateral to the corticobulbar fibers. And meanwhile, the rest of this cerebral peduncle, is comprised of axons that are conveying signals from the cortex to the pons. I think we see this system even better in the next section. So now we have this nice, compact cerebral peduncle. Now, if you have your tutorial notes in front of you, you notice that I added a little bit of quantitative detail here. I said that in each cerebral peduncle, there's about 20 million axons. Whereas in each medullary pyramid, there's only about one million axons. So, these numbers give you a sense that the corticospinal tract is really only a minor component of the overall output from the cerebral cortex that is distributed down through the hemispheres, and into the brain stem and below. really most of this projection is going to influence the pons, and its connection to the cerebellum. Well, we can follow this on through, into the, upper part of the pons. So here is the upper part of the pons and, and notice what's happening. This cerebral peduncle is defasciculating. And so, we can look for these axons here. these are our corticobulbar, corticospinal axons. but also included here would be some corticopontine axons, and the defasciculation allows those corticopontine axons to access these nuclei that are found here in the base of the pons. And those nuclei give rise to these pontocerebellar fibers which provide input then out here to the middle cerebellar peduncle. Which we'll see grow in dimensions as we get near the center of the pons. So, here's our middle cerebellar peduncle receiving the input from these pontocerebellar fibers that are derived from the pontine nuclei. But notice that as we've passed from the upper part of the pons into now the middle pons we're bringing back together again our compact corticobulbar and corticospinal fibers together with what corticopontine fibers remain. And then as we pass on even further down into the pons we will eventually lose most of the corticopontine fibers, of course, because they're terminating there in the pons. And what we have left is going to be primarily corticobulbar and corticospinal fibers. Now, along this journey from cortex through the upper part of the brain stem, we've been giving off collaterals from the corticobulbar pathway that will terminate among the appropriate motor nuclei. And well, we have in an important motor nucleus that's found right here in the caudal part of the pons. This is the facial motor nucleus. So we imagine that there have been some axons that are part of this corticobulbar projection, that are running through this basal region of the pons that are going to make their terminations here in the facial motor nucleus. Specifically, to the contralateral side. So this is one of those nuclei that have a contralateral bias, at least with respect to those lower motor neurons that are concerned with the lower part of the face. And again, I will refer you to our tutorial on the governance of the facial muscles for more detail there. There's our facial motor nucleus. If we go down even further into the brain stem, we encounter another important motor nucleus of the brain stem, the hypoglossal nucleus. So, now at this level most of what we have for corticobulbar and corticospinal fibers is brought together in this nice compact bundle at the base of the medulla, called the medullary pyramid. So, for those axons that are destined to innervate the contralateral hypoglossal nucleus, we imagine that some of the these axons in here might cross the midline. And then make their terminations there, in the contralateral hypoglossal nucleus. This is why, for example, we can direct our tongue out against one side of the cheek or the other, because of this contralateral bias. And our projections from the appropriate part of the precentral gyrus to this hypoglossal nucleus. Well, nevertheless, here we are in the medulla. And most of what we see here, in the anterior midline of the medulla, will be the corticospinal fibers. We've passed through most of our cranial nerve motor nuclei targets. So the corticobulbar pathway is nearly depleted. And, that would certainly be so if we section down just a little bit further, until we are clearly past our hypoglossal nucleus. The only cranial nerve motor nucleus left would be our spinal accessory nucleus. so with the exception of the projections to the spinal accessory nucleus, what we have in the medullary pyramid is almost entirely the corticospinal fibers. Now, let's watch what happens to this medullary pyramid, as we approach the junction with the spinal cord. What we see is that there's gradually a folding over of this pathway. Like the folding of the arms, one on top of the other. So what's happening here is that there are axons that have been passing through the medullary pyramid on one side that are going to grow across the midline, in this fashion. So, they're actually going to course in a somewhat dorsal direction, and thereby end up in the lateral column of white matter of the spinal cord. So, once we get into the spinal cord, the axons that are present in one lateral corticospinal tract are derived from the contralateral motor cortex. Okay, so here's one question that I'd like for you to consider. And I'll give you opportunity elsewhere to think about this again. Imagine I draw your attention to the axons that are present in, well let's call this the left lateral corticospinal tract. And I were to ask you question like, well, where are the cell bodies that grew the axons in this left lateral cortical spinal tract? Okay? What would you say? Well, I would hope that you would understand this pathway that we've just talked through and recognize that these axons were grown by pyramidal neurons that have their home in the contralateral motor cortex. Now, we're talking about the lateral corticospinal tract here, so one could even be more specific and say, well, in at least the center or the medial part of the motor cortex. Not the most lateral part, because that would be related to the brain stem and the corticobulbar pathway. The corticospinal pathway would be from the middle part of the motor cortex, and then on medial through the paracentral lobule. Okay, so there's our lateral corticospinal tract, and we can follow it all the way through the spinal cord. And it extends the entire length. Although quantitatively the largest portion of the axons of the lateral cortical spinal tract are terminating in the cervical segments of the spinal cord. Indicating a strong bias of this pathway to govern what we do with the distal upper extremities. Okay, now there remains one small component to consider. And this is something that we call the anterior or ventral cortical spinal track. Let's just back up to the medullary pyramids for a moment. Now here's our medullary pyramid, and as we've just described, most of the axons are going to cross over the midline and end up on the contralateral, lateral column of white matter in the spinal cord. But not all. Some of the axons will remain, and they are just going to project straight on through this region of a decussation. And they will remain on the ipsilateral side of the spinal cord. So those axons might be present here, and then they will continue on down. And once we are into the spinal cord, they end up in a medial position on the anterior side of the spinal cord. And so these are axons that are derived from the ipsilateral motor cortex. And when they make their terminations as they pass on through the spinal cord, here we are down in the lumbar enlargement. These axons tend to project bilaterally, to the medial columns of lower motor neurons that are found in the ventral horns. And this tells us that this anterior or ventral corticospinal tract is concerned with what we do with our proximal limbs and our axial muscles. So it's concerned with posture, and with just the gross movements of our arms and our legs. Well, I would like to conclude this tutorial at this point. I would encourage you to do this kind of exercise on your own, especially if you have Sylvius. imagine what a cross section through the corticospinal or corticobulbar fibers might look like, where they are to be found in that section, and then what structures surround them. And then you can begin to interact with your own sketches, or perhaps your own models that you may wish to make, and imagine, okay, in my drawing, what would happen if we made a cross section through, let's say, the midbrain? Where would we find our descending corticospinal and corticobulbar fiber? So, this exercise of making visible your anatomical knowledge I think is quite valuable, because eventually I do want you to become familiar with the appearance of the human brain. Not just in gross specimens as we see it from the outside. But also through histological sections, such as what we find here before us in our brain stem cross sectional atlas. Okay, so the best thing you can do is to draw and to discuss. and to join in the conversation that we have in our global learning community here in medical neuroscience. So be looking for a peer assessment that will encourage you to engage in just this very kind of activity. So, you'll be hearing about that on the website, and otherwise I hope you enjoy your studies of the corticospinal and corticobulbar pathway, as you make visible your understanding of this important pathway in the human brain.
