Okay, well now with that bit of an overview of the relationship between upper motor neurons and lower motor neurons, let's focus in on the motor cortex. Again we're going to be considering the posterior part of the frontal lobe which will include the precentral gyrus, and then the posterior part of our three long parallel gyri that we identified in our laboratory lessons. OK, our superior, our middle, and our inferior frontal gyri. So, from both an anatomical, and a functional perspective, we can broadly conceptualize the motor cortex as being divided into two main regions. There's a primary motor cortex, which is found in the anterior bank of the central sulcus, including much of the crest of the precentral gyrus. and then just in front of that primary motor cortex, we find a premotor cortex. And these areas extend into the midline hemispheric surface. So we can recognize the primary motor cortex is occupying a large portion of that paracentral lobule, if you remember that anatomical term. Paracentral lobule refers to this gyral formation that forms the medial or the medial terminus of the central sulcus. Just in front of the paracentral lobule then is were we find the medial extension of our premotor cortex. We will spend just a little bit of time talking about an interesting region right about there that's concerned with the expression of emotion in the face. Now each of these divisions of the motor cortex, the primary motor cortex and the premotor cortex, give rise to these descending projections to lower motor neuronal circuits and the spinal cord as well as the brainstem. Collectively these divisions of the motor cortex receive input from the thalamus as do all regions of the cerebral cortex. We can identify two nuclei at the thalamus that are primarily responsible for projections to the motor cortex. They are the ventral anterior nucleus and the ventral lateral nucleus. Sometimes, we refer to them together as the VA-VL complex of the thalamus. In addition to input from the thalamus, these divisions of the motor cortex receive important projections from other parts of the cerebral cortex. And one important source of projection comes from the somatic sensory cortex, which sits just posterior to the central sulcus. There are projections from the somatic sensory cortex in to this motor cortex, both the primary and the pre-motor areas. In addition, there are projections from the more posterior parts of the parietal lobe that are processing information about, visual spatial relations recall that where pathway, that, was processed and elaborated in the posterior part of parietal lobe. This part of the parietal cortex, likewise, sends inputs that are relevant to the guidance of movement. So the remainder of this part of the tutorial I'd like to focus on the structuring function of the primary motor cortex. And this is really a fantastic part of the human brain. if we prepare histological slices as, as I did some years ago with my colleagues here at Duke. we can really, recognize the fantastic appearance of this part of the brain. As it's viewed under the microscope with a standard histological stain like initial stain. And a few features really stand out. one feature, which is highlighted in this slide, is the presence of these very large neurons that we find in layer five of the primary motor cortex, which corresponds in Brodmann's nomenclature to area four. So here in layer five we find these very large neurons called Betz cells. In fact, they are the largest neurons in the entire cerebral cortex and they contribute to the corticospinal tract. So generally speaking, layer five in the motor cortex is what gives rise to projections, to the spinal cord and to the brain stem. So we call these connections again, the corticospinal and the corticobulbar tracts. They come from layer five. Now, just a word about these Betz cells. perhaps when they were first described and recognized, it was tempting to think that these cells are the sole origin of the corticobulbar and corticalspinal pathways. But we know that there just aren't enough Betz cells to account for the number of axons we see descending to the brain stem in the spinal cord. So, really these Betz cells account for a small percentage of the overall projection from cortex down to lower motor circuits. however they, they seem to have some privileged access to those lower motor neurons. I mentioned that most of the connections from cortex to lower motor circuits are actually targeting the interneurons, not the alpha motor neurons themselves. Well it seems that these betz cells are among those cortical cells that have monosynaptic connections to alpha motor neurons. So they do seem to be given privileged access to govern the output of our lower motor neurons. So you may be wondering, why do we call this Brodmann's area four, primary motor cortex? Well, it's primary in the following sense. When motor cortex was first characterized with micro electrode stimulation, it was discovered that as one approaches the posterior frontal lobe with a micro electrode and then stimulate at different sites. It was possible to elicit various kinds of movement. We'll talk more about that in a few minutes. And the thresholds for stimulation that produced movement got lower and lower and lower as the micro electrode was advanced towards the central sulcus. And the cortex in the anterior bank of the central sulcus was found to have the lowest threshold for producing a movement when stimulated with an electrical current. So this region was defined as being primary for that reason. Now, it was, clear from the very earliest stages of these kinds of experiments that one could elicit movements for more anterior regions. So this posterior frontal lobe was considered the location of the motor cortex but there was a clear distinction in threshold. and so for that reason this primary motor cortex was identified with the precentral gyrus. There's another sense in which this Brodmann's area four can be considered to be primary cortex, and that is because it's there in the primary motor cortex where we seem to be especially concerned with the movements around our personal space. For example, bringing the hand to the mouth or bringing an object close to the visual mid line, so that we can inspect with our special sensory systems what we happen to be holding in our hands. This kind of movements seems to be well represented and encoded in the activities of neurons in the primary motor cortex. So in this sense, the primary motor cortex can be considered primary because it has the lowest threshold for the elicitation of movement, and it seems to be concerned with those movements that are primarily concerning our bodies and the space immediately in front of our bodies. Now, let's talk some about the way the body is mapped out in the motor cortex. So, we can recognize a, a kind of body mapping, or somatotopy, in the motor cortex. And generally speaking, along the length of the central sulcus in the precentral gyrus, there is, in fact a kind of somatotopy. And this somatotopy bears some resemblance to the body mapping that we saw in the post central gyrus, such that here on the paracentral lobule we have representation of the lower extremity and as we progress towards that S-shaped band near the center of the central sulcus we move up the lower extremity through the trunk and, and to the upper extremity. And near the central part of the central sulcus where that S-shape bend is found, that's where we see representation of the movements that we perform with our arm and our hand. And then just lateral and inferior to that region just as with the central sulcus tends to straighten out we find representation of the face in the inferior one fourth or one third or so of the precentral gyrus. So this organization from lower extremity through trunk to upper extremity and then to face, is something of a mirror image of what we found in the postcentral gyrus. However, there is an important sense in which it's not a true mirror reflection of the fine detailed mapping of the body that we find in the postcentral gyrus. What we find is a much less precise mapping of the body. In fact we call this fractured somatotopy. Let me give you some examples of what I mean by fractured somatotopy. Here again is this basic layout of the somatotopy in the pre-central gyrus from lower extremity, to upper extremity, to face. And if we simply focus in, on the region of the upper extremity I can illustrate for you why the somatotopic map in the pre-central gyrus, is not a mirror reflection in detail of the map that we find in the post-central gyrus. So there's a number of senses in which this body map is fractured. one sense that we can find, in overlapping regions of the pre-central gyrus, representation of extensor movement and flexor movement. One might think that these would be very different kinds of movements that might have differential columnar representation in the motor cortex. well that may not be the case. There seems to be an overlap in where we can find sites that when stimulated would produce extensor activity or flexor activity. Another sense in which this body map is not a mirrored reflection of the, smooth and continuous mapping of body part that we find in the post-central gyrus, is that we can find multiple representations of the same body region. So, for example one could probe an area of the pre-central gyrus and discover that there is a, collection of cells that when stimulated move the first digit. Well one might find a different location probably not far away but a different location nevertheless that likewise would stimulate and produce movement in that same body part. So we can have multiple representations in addition to overlapping representations of different types of movement. And lastly, if we look within this region that represents the upper extremity, what we can find is an interdigitation of sites that, when activated, can move, for example, the wrist. And that might be right next to a site that, when stimulated, can move a digit. And, the, those regions might be adjacent to a shoulder,or perhaps elbow flexion or extension sites. So, there seems to be no strict, internal somatotopy within this general region that represents the arm. And it's for these reasons that I elected not to illustrate the somatotopic representation of movement in the precentral gyrus with a Homunculus-style cartoon, because I think that would be misleading. So you might get the impression that there is a detailed somatotopic mapping of the body in the precentral gyrus. And that really would, would be misleading, I think, because of these various features of motor representation. What we find is more a general representation of body region that roughly corresponds to the detailed map we find in the post central gyrus. but it is fundamentally different and it's internal organization. And I think this is quite interesting because, what this arrangement allows for is the organization of functional ensembles of neurons for the performance of particularly meaningful behavior. the circuitry seems to be dynamic and flexible and plastic and that circuitry may provide us with a neural substrate for the acquisition, and storage, of motor skill. So, if this is the means by the motor cortex is represented, it makes some sense that we would see some reflection of this in the connections between motor cortex and lower motor neurons. And, in, indeed, we do. If one were to trace, for example, the distribution of a single axon in the cortical spinal tract, what you might find is that that axon terminates in multiple columns of lower motor neurons in the ventral horn. That might be consistent with a single location in the motor cortex being concerned with multiple kinds of movements. Also, if one were to look physiological, one might find that a single action potential or a single spike in a cortical spinal axon, might activate, different muscles in the forelimb. So these two points sort of go together if a single axon can innervate multiple columns of motor neurons, it makes some sense then that one would associate the activity of that single axon with movement of multiple muscles. So these anatomical features and these physiological features at the unitary level I think are a reflection of what we find at the levels of maps in the pre central gyrus. So if all of this sounds very complex and very confusing I can assure it is and we're still trying to understand what is mapped in the primary motor cortex. So I hope that's a question that Is raised in your mind in this point in our discussions.
