Welcome to this tutorial on Upper Motor Neuronal Control. And what I really want to focus on in this session is the anatomical organization of the corticospinal and corticobulbar pathways. This topic relates to several of our core concepts in the field of neuroscience. I hope this subject will allow you to break down some of the complexity of the human brain. Nevertheless, I'm sure you appreciate by now that the brain is indeed the body's most complex organ. Now the pathways we're going to be discussing today are genetically determined. They provide a foundation for upper motor neuronal control of lower motor circuits. So it's very important that we understand their organization and anatomical distribution, but that's not to say that even these pathways can't be changed by life experiences. Indeed the strength of the connections between our upper motor neuronal pathways and lower motor circuits are subject to experience-dependent modification. And we think that this might be an important site of neuroplasticity, as we acquire new motor skills. Indeed, many neural systems are changed with practice and with the acquisition of skill, but let's not forget that even these pathways that we're going to be talking about today may themselves be modified by experience. So, I have several objectives for you in this session. I want you to be able to characterize the organization of the corticospinal pathway from the cortex to lower motor circuits in the spinal cord. I want you to be able to do the same thing for the corticobulbar pathway. Only as we'll come to define this pathway, this pathway terminates in the brain stem, not in the spinal cord. I want you to be able to recognize components of the corticospinal and corticobulbar pathways as we look at the brain from the outside, but also as we look at the brain in cross sections. Through the brainstem and the spinal cord and I even want you to be able to look into sections through the hemispheres and know where elements of these pathways will be located. Well one way to help you consolidate the information that I hope for you to learn in this tutorial is to draw, to make a sketch. that sketch could be on a white board, a chalkboard, a piece of paper, it could be made with a stick in the dirt at your feet, but what I would say about sketching is that the bigger the better. So I want you to be able to sketch the corticospinal and corticobulbar pathways. From cortex to lower motor circuits. And you might even want to include in your sketches the lower motor neurons themselves and their connection to the muscle fibers that they innervate. Okay, well let me give you a broad overview of our pathways of upper motor neurons to lower motor neurons, and this is a, a figure that you've already seen in conjunction with our studies of upper motor neurons. And this pathway that you see here, illustrates the way that circuits of upper motor neurons in the cortex and in the brain stem connect with lower motor circuits in the spinal cord. The main pathway that we're going to be concerned about today is the one that relates upper motor neurons in the posterior part of the frontal lobe, that is the motor cortex, to lower motor neurons that are found in the lateral part of the ventral horn of the spinal cord. Now there are other kinds of motor neurons that are involved in governing the output and spinal cord. Namely three major sources of motor neurons in the brain stem, probably most important would be the reticular formation of the brain stem, it supplies projections that terminate bilaterally, in the medial parts of the ventral horn of the spinal cord. There's also descending projections from the vestibular nuclei of the brain stem, also distributed to the medial parts of the ventral horn of the spinal cord. An thirdly there are projections from the superior colliculus to the upper part of the cervical cord. Although, in humans, we think that this effect of the superior colliculus on the orientation movements of the head and neck may actually be mediated by the reticular spinal connections. Well, so this is the broad view of our perspective of the distribution of upper motor neurons to lower motor neurons. And I think it's very helpful for you to keep in mind this medial to lateral somatotopy that we find in the spinal cord. So when we talk about our lateral corticospinal fibers that are found here in the lateral white matter of the spinal cord. I want you to relate function to this anatomical location. And appreciate that these axons are terminating among the lower motor neurons that are concerned with the skilled movements of the distal extremities. Meanwhile those axons that we find in the anterior and medial part of the spinal cord, are giving rise to projections that typically terminate bilaterally on the medial part of the ventral horn. So these would terminations on columns of cells that are concerned more with posture. And with the gross movements of the limbs via the actions of proximal limb muscles. So I want you to be able to associate these two different anatomical locations with two kinds of functions. The lateral column of the spinal cord is concerned with the expression of skill, [SOUND] whereas these medial systems are more concerned with the maintenance and adjustment of posture. Okay. Well, let's talk now about our corticospinal and corticobulbar pathways. And to begin, we need to consider once again the organization of the motor cortex. So, recall that the motor cortex is found in the posterior part of the frontal lobe, and we can divide it into a posterior primary motor cortex that occupies the anterior bank of the central sulcus and the precentral gyrus. And then just anterior to that primary motor cortex we find the premotor cortex. Which begins at the interior part of the precentral gyrus and continues on into the posterior part of those three long parallel gyri that come to meet the precentral gyrus almost at a right angle. The superior, the middle, and the inferior frontal gyri. Okay, so that's what we have here. In our premotor cortex. While both the primary motor cortex and the premotor cortex give rise to descending projections that contribute to the corticospinal and corticobulbar pathways. And to understand the difference one needs to be reminded a bit of the somatotopy. That we find in the precentral gyrus, and that extends into the premotor cortex as well. Now, recall that the somatotopy in the motor cortex is a gross reflection of the somatotopy that we find in the somatic sensory cortex, across the central sulcus and the post central gyrus. But the detail is quite different. Rather than there being a faithful representation in a point-by-point fashion of the contralateral body surface, what we see in the motor cortex is a map of movement intention. And that intention is roughly somatotopically organized in the following way. With movements that we intend to make involving our lower extremity being represented here in the paracentral lobule on the medial face of the hemisphere, and just out a bit into the dorsal medial margin of the hemisphere. Well, as we progress in a lateral and inferior direction, we move from the lower extremity up through the trunk and into a large expansive region near the center of the motor cortex that is concerned with the movements that we intend to make with our arms and our hands. So there's a large upper extremity representation near the middle of the precentral gyrus and extending into premotor cortex. While just inferior to that representation of that upper extremity, in this lower third, approximately, of the motor cortex. We find a large representation of the contralateral face and specifically the lower part of the face. Now, I want you to learn more about the way the upper motor neurons of the cortex control the movements of the face in a separate tutorial that I've prepared called, Upper Motor Neuronal Control of Facial Expressions. So, I will refer you there for more detail about how the face is governed by, not only this lateral inferior precentral gyrus, but also a fairly newly discovered region, right about here, in the banks of the cingulate sulcus that we call the cingulate motor area. And it will be important for you to understand how these two parts of our motor cortex relate to the divisions of the facial motor nucleus that govern the upper and lower parts of the face. So, please see this other tutorial for a fuller expla, explanation of this aspect of the connection between the motor cortex and the facial motor nuclei. Well, for the rest of our time, I'd like for us to focus then on the organization of these descending pathways, principally the corticospinal pathway. I'll say a little bit about the corticobulbar pathway, but I'll refer you mainly to this additional tutorial for some deeper explanation of the connections between cortex and those relevant cranial nerve motor nuclei. All right, so let's use this figure from our text book, which I suspect is really quite small on the screen that you're viewing this on, but I'll try to use it nevertheless, and refer you to the additional figure that we have in the tutorial notes. And then I want us to jump into Sylvius and really find the elements of this pathway and cross-section as they are displayed for you in the tutorial notes. Well, let's focus on the pathway shown in red. This is the cortical spinal pathway. So, what we're illustrating, for example, will be the descending projections. Of a motor cortex neuron that might be found near the middle of the precentral gyrus. So we imagine this as a neuron that's concerned with the movements of the contralateral arm and hand. So this neuron grows an axon that runs through the white matter of the hemisphere and it enters the posterior limb of the internal capsule. This is the means that allows this axon to pass through the hemisphere between the basal ganglia and thalamus, and then eventually down into the brain stem. And these axons are component of this massive stalk of white matter that we find on the ventrolateral surface of the midbrain. Called the cerebral peduncle. So these axons pass through the cerebral peduncle, and then they enter the pons. Now, along the way these axons might sprout collaterals that supply additional structures that they may pass. for example, there may be a branch that supplies the red nucleus, which is a large nucleus of the midbrain. There may be axons that terminate among the pontine nuclei here at the base of the pons. There may be axons that pass through the core of the brain stem and terminate in the reticular formation. Which is going to receive information from the motor cortex about the intention of movement. Well, in order to actually express the movement these neurons have to make their way through the brain stem and into the spinal cord. And here's what happens as these axon pass through the medulla and enter the caudal part of the medulla, where the spinal cord medullary junction is found. The vast majority of these axons will cross the midline in a structure that we call the pyramidal decussation. We can think of this maybe as, as a folding of the arms, where one medullary pyramid crosses over the other. And consequently, the bulk of the axons that were present in one medullary pyramid are now found in the opposite side of the white matter of the spinal cord. It's at that point that we call this the lateral corticospinal track. So, roughly 90 to 95% of the axons. That are present in one medullary pyramid will cross the midline and end up in the lateral column of white matter of the spinal cord on the opposite side of the midline. The remaining 5 to 10% of the axons or so, remain on the same side of the spinal cord. And they are present in this anterior and medial white matter. From there they tend to branch and give rise to bilateral connections, to that medial part of the ventral horn that controls the more proximal muscles of the limbs. So this is a picture of our descending corticospinal pathway. And I hope you're not too distracted by these collaterals, to structures in the brain stem, this will serve the purpose of informing important circuits in the brain stem about the intention of movement. Okay, now let's turn our attention to the corticobulbar pathway, which is shown here in this yellow colored axon. Now notice where this axon begins. It grows out of a neuron in the inferior and lateral part of the precentral gyrus. That's where we would expect representation of the facial region. In our motor cortex. So this axon, then, runs through the white matter of the hemisphere, just as we saw for the corticospinal track axon, through the posterior limb of the internal capsule, into the brain stem, where it runs through the cerebral peduncle. And perhaps along the way, it too might give rise to branches that terminate in structures like the red nucleus. Now, as this axon passes into the brain stem, it will be directing its projections, its terminal fields, primarily towards one or another. Of the cranial nerve motor nuclei. Now in this illustration I think just for artistic simplicity the branches are shown only to ipsilateral structures. But as is detailed, both in the tutorial notes and in the separate video tutorial that I have for you. On the control of the facial muscles, we know that these collaterals are actually bilaterally distributed. And for some structures, including the trigeminal motor nucleus, there may actually be a contralateral bias in that distribution. We know that's certainly true for the lower motor neurons that are governing the lower part of the face. That is, the lower motor neurons in the facial motor nucleus. But for the most part, we can think of these corticobulbar pathways as bilateral. The only issue is the degree of contralateral bias. Okay, well, these axons are distributed only into the brain stem, they're corticobulbar. This term bulbar refers to the brain stem, and many authors choose to restrict it even further to projections from cortex to the motor nuclei of the cranial nerves. Not just all brain stem projections, but specifically those brain stem targets. I think we can compromise somewhat and consider the corticobulbar pathway to be the means by which the motor cortex motivates the output of our cranial nerve motor nuclei. Well, just one additional quantitative point that, that might give you some perspective on the projections from the cortex to the brain stem. the dominant projection, really, from the cerebral cortex to the brain stem, really is not corticobulbar in the sense that I'm using this term here in this tutorial. There are actually corticopontine fibers. So these are the projections that arise from all over the cerebral cortex of one hemisphere and terminate among the pontine nuclei that are present in the base of the pons. These cells, in turn, project across the midline into the cerebellar hemisphere, which is sitting out here somewhere. So if you review your understanding of the cerebellum and the cerebral pathways, you'll recall this connection from cortex to base of the pons, and then from base of the pons to the contralateral cerebellum. This quantitatively is actually the largest component of axons in the cerebral peduncle, the minor contributions are actually the corticospinal and the corticobulbar pathway. Okay, well let's return then to our focus on the anatomy of this corticospinal and corticobulbar pathway. And I'd like for us to look at some cross sections in Sylvius, and identify the locations of where we would find these axons if we were to look through histological sections of the brain stem and spinal cord.
