Welcome to this tutorial on Upper Motor Neuronal Control. This topic, once again confronts the complexity of the brain. And we come now to consider how movement is controlled by the brain. Obviously a very complex topic. But one that we are beginning to understand with greater clarity. We're going to be describing circuitry in the brain that is determined by genetic plans, and laid down early in embryological development. But never the less, life experiences can have a significant impact on this circuitry. And this is a wonderful thing, because it allows us to have a flexible motor system, that can learn new skills and take on new challenges. And this of course is[UNKNOWN] to human performance, as well as to rehabilitation following impairment of motor control. So, this tutorial is going to be broken into several parts. we begin with the general overview of the motor system and the means by which upper motor nuerons govern lower neurons. And our focus, for the remaining portion of this part, will be on the primary motor cortex. So the learning objective for this first part is that I want you to be able to discuss the organization of the primary motor cortex, and its contributions to the control of volitional movement. I want you to be able to characterize the representation of the body in the primary motor cortex. And to compare that representation that we find in the precentral gyrus, with what we've already discussed in the postcenteral gyrus, which is where we find our primary somatic sensory cortex. And finally I want you to be able to think well about how movement is encoded in the primary motor cortex. So I want you to consider the phenomenon of population coding, as one means by which movement is, is governed in the cortex. Okay, so let me, once again, show you this schematic that lays out before us the organization of the motor system. And we've already talked some about our lower motor circuts, which includes the lower motor neurons themselves that ennervate skeletal muscles. But also local circut neurons that organize the output of these lower motor neurons. Those local circuts can Integrate signal reflexes. They can generate rythms and patterns, if their part of networks that we call central pattern generators. And I've already made the point to you that, its really these local circuits that are receiving this descending input, from higher centers in the brain. Although there is some access to lower motor neurons themselves. And that access is seemingly privileged for the parts of the spinal cord that govern the expression of skills, especially what we do with our hands and our fingers. But for the most part, what we're going to be concerned with today, is how upper motor centers interact with these lower circuits. That help organize the motor output of our motor neurons and the spinal chord in the brain stem for our cranial nerve motor nuclei. And what we're going to talk about is really two main stations. They are centers in the brain stem that provide for upper motor neuronal control, and centers in the motor cortex. So in this part of the tutorial, our focus will be here in the motor cortex. But we'll also spend some time talking about the brain stem, in subsequent parts. Now, just to help us understand really the relationship between upper and lower motor neurons. I'll, remind you of a phrase, a memorable phrase that, Charles Sherrington, great British physiologist, in the early part of the 20th century provided for us. He thought of these lower motor circuits as the final common pathway, for movement. So to me what this phrase, final common path, implies is that it's up to the lower motor circuits to actually implement the movement. And one thing that I'd like to share with you, is that what's really going on, especially at the level of motor cortex, is not micromanagement of movement, but rather the encoding of movement intention. And so this will be a theme that I hope to elaborate as we progress through this tutorial. So our focus again here is going to be on the motor cortex, and what we will discover there is really a mosaic of motor control areas. So this mosaic of motor control areas is found in the posterior part of the frontal lobe. So again, to re-orient you to the human brain, here's the central sulcus, so this then is the precentral gyrus. And the posterior part of the frontal lobe then would be from the central sulcus maybe anterior, including the precentral gyrus, and then the posterior part of our superior, our middle, and our inferior frontal gyri. So that's what we're talking about. That's what we mean by this motor cortex and that iswhere we will find it, in this posterior part of the frontal lobe. So these motor cortical areas are involved in the expression of what I like to think of as specific motor behaviors or skilled motor behaviors. So these are behaviors that we choose to perform, so there's an intentionality about it, or an aspect of will or volition. Also included in this motor cortex are areas within this broader mosaic, that are concerned with behaviors that well, perhaps are somewhat less volitional. And what I have in mind here, are the kinds of behaviors that relate to the expression of emotion. we can think of the extremes of fight or flight, but we can also think of the nuances of organizing our facial gestures and our posture in a way that communicates non-verbally. And perhaps somewhat unintentionally the status of our cognition as it pertains to our sense of well being, our sense of mood, our affect. So we'll see within this motor cortical mosaic divisions that are more concerned with the expression of emotion, than the expression of skilled behavior. So in addition to these motor cortical areas let me just say a word about these brain stem centers that we'll, we'll come to in a different part of this tutorial. These brain stem centers are organized aspects of movement, but I like to think of as setting the stage for the expression of skill. if you will, if we can work with this theatrical metaphor, the brain stem works behind the scenes. perhaps they're the stage managers and the theatrical production, so to speak. the brain stem centers are concerned with adjusting posture, with setting the gain of segmental reflexes. These activities are important so that we can express skill. Now thinking anatomically the skill that we expressed is often done with our distal extremities. Our hands and our feet. Where as the stage setting functions typically concern our posture, which involves our axial muscles and our proximal lean muscles. So one differentiation between the contributions of the motor cortex to movement and the brain stem. Have to do with the difference between the skilled behaviors that we express with our distal extremities, and also our oral facial structures. And those behaviors that set the stage for the expression of skill. Involving our trunk and our proximal limbs. And what I'm think about, primarilly, is posture. Now one important way to think about this differentiation between skill and posture, is to recall the way that the body is mapped in the ventral horn of the spinal cord. you will recall that there is a medial to lateral mapping of the body with the more proximal limb muscles mapped in columns of lower motor neurons, that are found near the medial part of the ventral horn. And the distal muscles are governed by lower motor neurons that are found at progressively more lateral positions, in this ventral horn. Now as we turn our attention to the way that upper motor neurons control these lower motor neurons. I would highlight that we see the same medial to lateral organization with respect to the white matter, that provides means for upper motor neurons to communicate with lower motor neurons. Out here in the lateral part of the spinal cord is where we find the lateral corticospinal tract. And this is mainly going to provide axons that are concerned with governing the control of these distal extremity muscles. So this huge input from motor cortex to spinal cord is primarily directed at the lateral part of the ventral horn. Now we also have a anterior or ventral and medial part of the white matter that is primarily receiving axions from the brain stem. So, these axons are terminating in the medial part of the ventral horn, where we have motor neuronal columns that are mainly concerned with posture, and proximal limb muscles. So, again this medial to lateral dichotomy is, is very important. We can associate this lateral corticospinal tract primarily with skill. And this medial group of axons, that includes components from the brain stem and a small component from the cortex, as mainly being concerned with posture and other elements that I'll simply call stage setting. And if you can remember my theatrical metaphore, then perhaps you will recall what is really going on here in these medial white matter systems. Setting the stage for the preformance of skill. Let's have a closer look now, at least in schematic form, at the way upper motor neurons relate to lower motor neurons. Now this is really a schematic view of the same principals that I just attempted to illistrate for you. notice that in the lower motor neurons, which are at this level. We've now schematically organized them into medial and lateral components. Okay? So we have the medial cell columns that are found in the medial part of the ventral horn, and then the more lateral cell columns that are found further out lateral. And so, that should remind you, that the medial columns are concerned with only axial and proximal limb muscles. While the lateral columns are mainly concerned with the distal limb muscles, through which we express skilled movement. Now if we look at the way, that the upper motor neurons and the cortex, and the brain stem relate to these columns of lower motor neurons. this figure is intended to emphasize the point that the upper motor neurons in cerebral cortex, are mainly concerned with the lower motor neurons in the lateral part of the ventral horn. And I want you to notice that in this figure, the anatomical relationship across the midline of the body. So the upper motor neurons on one side, let's call this the left side. Are going to govern the movements expressed via the contralateral side of the spinal cord. In this case, it would be the right side. And so that implies that there must be a pathway that crosses the midline. So we are going to spend a fair amount of time talking about this pathway. And this pathway is in fact that lateral corticospinal tract that I've mentioned just a moment ago. Okay? So, the lateral components of our open motor neuron or pathways in the spinal cord relate to the lateral part of the ventral horn, which is concerned with skill. Now, let's talk about the upper motor neurons of the brain stem. So, these neurons are sending axons down through the white matter of the spinal cord, but mainly in this anterior and medial part of the white matter of the spinal cord. And these axons govern the output of the circuits that we find in the medial part of the ventral horn, that are mainly concerned with axial and proximal limb muscles. And with respect to sightedness, I want you to know that this input is distributed to both sides of the spinal cord. There are some exceptions where the inputs are primarily to one side or the other. But for the most part we can generalize and say that our brain stem pathways give rise to bilateral projections, to the medial part of the ventral horn. In contrast to the inputs from the motor cortex, which are primarily directed across the midline to the counter lateral side of the spinal cord, and specifically to the lateral part of that ventral horn. So what I've tried to outline for you here is this concept of parallel path ways. So we've talked about this quite a bit in our sensory systems and now we encounter it in the motor systems. Where there are descending inputs from different grey matter structures ,that are running in parallel and targeting different parts of our lower motor circuitry that we find in the spinal cord. Now, there's an even larger dimension of parallel pathway for motor control. So here in this slide we have illustration of how our upper motor neurons in the cortex and the brain stem collaborate, in order to express those movements that arise from our volition. And so we have again these lateral components that are mainly derived from the cortex. And they're concerned with the fine control, or the skilled behavior that we express with our distil extremities. And then there're medial components, that are arising primarilly in the brain stem and they're concerned with adjusting posture. As well as governing the more proximal limb muscles, that are necessary in order to express behavior with the distal extremities. Now, there are lateral and medial pathways. The lateral pathways are primarily associated with a major projection from the cortex to the spinal cord that we call the lateral corticospinal tract. And this is a collection of axons that, as it runs through the brain stem, and specifically in the medulla, forms a structure that we call the medullary pyramid. So for this reason, we call this pathway the pyramidal pathway. [SOUND]. Sometimes we speak of a pyramidal motor system, what we have in mind is the connection between the motor cortex, and the spinal cord that runs through the medullary pyramid. So the pyramidal motor system is a kind of short hand, to refer to this aspect of volitional motor control that links the cortex with the lower motor circuits of the spinal cord. Now there's another term I'd like to introduce, and that's corticobulbar. this is a bit of an odd term, I know. but what it refers to, are projections from the cortex to the brain stem. This term "bulbar" often is used to refer to the brain stem. perhaps because of the way that the pons gives the brain stem the appearance of a, of a bulb, perhaps. but never the less, the point here is that cortical bulbar is to be contrasted with corticospinal. Corticospinal obviously refers to projections from the cortex to the spinal cord. Cortical bulbar refers to projections for the cortex to the brain stem. And for some authors, it means specifically the projections to the cranial nerve nuclei, that are expressing motor behavior. So those would be our somatic motor nuclei and our branchial motor nuclei. There are massive projections from the cortex to the brain stems that are not so much concerned with the providing upper motor neural control. We'll talk about those separately when we consider the cerebellum. So in the subsequent part of this tutorial, I'd like to spend just a little bit of time illustrating for you an emotional motor system. So what I have in mind here, are projections that are coming from what we call the limbic centers in the fore brain, which are primarily in this ventral medial fore brain. and some other structures including including the hypothalamus and the amygdala that provide descending projections. Now I'm going to stretch the medial and lateral concept just a bit. But I think it's possible to apply the same concept of lateral and medial descending pathways that we see very clearly in the control of voluntary movement, to the organization of emotional expression. So what we see in our lateral pathways, are pathways that allow for the expression of specific emotional behaviors. And what I might have in mind here as examples, would be motor behaviors that characteristically define a particular emotion be it a facial expression for example, or a posture. meanwhile, there are medial pathways that set the stage. They're concerned with setting the game, for example. modulating the threshold that which we express that specific emotional behavior. Be it our tendancy to laugh, to cry, or to become angry, and to express that emotion within the muscles of the face.
