Well welcome back, I think we're ready to continue to discuss the organization of upper motor neurons and focus on the premotor cortex. So my learning objective for you, is that I want you to be able to discuss the organization of the premotor cortex, and its contributions to the control of volitional movement. So, I'll remind you again that the premotor cortex is a region of the posterior frontal lobe, that sits just in front of the pre-central gyrus. it might include a bit of the anterior bank of the pre-central gyrus but it extends into the posterior part of the superior, middle and inferior frontal gyri. And it also extends onto the midline of the hemisphere. here for example is our paracentral lobule, so this is where we would find our primary motor cortex and then just anterior to it. In the medial bank of the superior frontal sulcus, and then in the banks of the cingulate sulcus, including the cingulate gyrus itself we find a medial extension of that pre-motor cortex. Illustrated here in this slide is the distribution of the premotor cortex. And a rhesus macaque brain, where the details of organization are much more clear. At least at this stage in our understanding. So where we find this more orangish color, that's our premotor cortex. Now there has been a fairly radical change in our thinking about the premotor cortex in recent years. some time ago we imagined some kind of hierarchical organization where the premotor cortex was primarily involved in planning movement. But the actual execution of the movement required signals that were derived from the primary motor cortex. So it was thought that the premotor cortex at a higher plane in some kind of hierarchy for motor control. Well, those concepts are, are beginning to unravel as we have the greater appreciation for what's actually encoded in the motor cortex. But we also, understand much better now that the premotor cortex itself, gives rise to descending projections to lower motor neurons. So the argument for premotor cortex, being conceptualized as a higher plane for motor control seems to be losing some of its attraction. I think the concept that is becoming much more viable, and frankly much more interesting and, and productive for understanding upper motor systems. Is the notion that this premotor cortex actually comprises of a mosaic of areas. That have some kind of modular organization, reflecting the encoding of ethologically important motor behaviors. Now it is helpful to find some way to simplify our understanding of this mosaic. And I would suggest a means to simplify would be to recognize a medial part of our premotor cortical mosaic in the lateral part. Where exactly one would make that division is not so clear. But perhaps one would, would roughly subdivide this premotor cortex at about the location of the superior frontal sulcus. So, our medial division of the premotor cortex, corresponds to what in the older literature was, described as a supplementary motor area. So for those of you that are looking at other resources its support of your learning, you, you may run into that phrase supplementary motor cortex. It's certainly is still used in both clinical and scientific discussion. But for our purposes, we'll simply consider that supplementary motor cortex or SMA for short, a division that is localized here to the medial part of the premotor cortex. Now included in that medial part of the premotor cortex, is a very interesting set of regions, here in the banks of the cingulate sulcus that will spend just little bit of time talking about. They seem to be involved in the expression of emotional behavior. Out on the dorsal lateral convexity of the hemisphere, we see other regions of this regional medial premotor cortex that are concerned with organizing bi-manual activities. And this implies that they have a rich set of colossal activities, as they do so bimanual coordination seems to be a function of this premotor cortex. There is also a division of this medial premotor cortex, that is especially concerned with governing voluntary saccadic eye movements, we call this region the frontal eye field. And this is a part of the premotor cortex that helps to orient our attention, and therefore what we're looking across the midline to some location in the contralateral visual hemifield. Now generally speaking, these medial parts of the premotor cortex seem to be especially involved with organizing movements that are self-initiated. Movements that are not necessarily triggered or directed by sensory cues. One example of what I mean by this, in the frontal eye field, would be turning our gaze to some object in the contralateral part of the visual field that we intend to look at. Perhaps there is a clock, over to your right and you want to look towards your right, to see what time it is. so, there wasn't necessarily an emerging stimulus that just grabbed our attention, rather there was an intentionality, about the shift of gaze and that intentionality was self initiated. Now contrast that if you will recall to the role of the superior colliculus in organizing eye movements. If you're outside and a bolt of lightning happened to strike somewhere off in the distance to one side. You may very well make a reflex movement of eyes and head posture in order to see that bolt of lightning while it persists. Well, that is a reflex of saccade that's coordinated at the subcortical level, through circuitry that involves the retina sending signals to the superior colliculus. And then the colliculus sending output that governs the movements of the eyes and the muscles that orient our head and our neck. We call that a reflex saccade. that wasn't self initiated in the same sense, as the desire to see what time it is. So now lets turn our attention to the lateral parts of the premotor cortex. Here what we find is a mosaic [UNKNOWN] that are involved in organizing movements, that are often guided by sensory information. Or at least with the interactions that we might have with the world around us, and that would include social interactions as well. For example, we find areas that exist here in the inferior part of this pre-motor cortex, that are especially concerned with social communication. Now for us and the human brain we can recognize one region that is of great interest, and it's found in the posterior part of the inferior frontal gyrus. And this is a region that goes by the name Broca's area. So, Broca's area is part of this premotor cortex, that organizes the vocal motor apparatus for the production of speech. They are very likely nearby regions also part of this premotor mosaic, that is involved in the production of language in written form. Either through the act of writing with a writing implement, or, of course now, we can touch on keyboards and touchscreens. And that is a motor act, that is involved in communicating thought. And semantic content through symbolic representation, and that seems to be one function of these more lateral and inferior divisions of this premotor mosaic. Also in this same region of the pre-motor cortex, we have some really fascinating neurons that have captured world wide attention in recent years. These are neurons that are concerned not just with the expression of movement, that we make with our own body, but also encoding the intentions of movements that we see other individuals perform. And because of this mirror like capacity of encoding our own movements, and observe movements, such neurons with this behavior are called mirror motor neurons. Mirror motor neurons were discovered quite accidentally, when recordings were being made in this lateral premotor cortex, while monkeys were engaged in visually guided reaching behaviors to pick up a small object. And what was observed, is that when the monkey simply sat passively while the experimenters reloaded the apparatus by executing a very similar type of hand movement. The neurons in the premotor cortex in the monkey's brain, responded at the sight of the human hand performing the very similar act that the monkey himself was about to execute. This is illustrated here, in this figure from our textbook. what we find is a recording from one such neuron in the lateral part of the premotor cortex. And we're looking at raster plots, such as what we saw previously, so each row is an individual trial. And each little tick mark represents an action potential. What was really surprising at the time was that the very same neuron, was seen to increase its firing rate when a very similar act was observed in a different agent. In this case the hand of the experimenter. Well as you might imagine this was a really shocking discovery that neurons in the motor cortex might represent movement observed in another person. So, quite a lot of work has gone on in the last two decades or so to really explore, the encoding of movement and tension, that is observed here in this lateral premotor cortex. what is discovered is that these neurons seem to care about the agent itself. if a tool is used for example, to preform a particular movement goal, then this neuron might not be concerned with that particular behavior. In this example, if a food pellet is retrieved with a pair of pliers this neuron doesn't seem to fire. whereas the natural movement that the monkey makes, or is observed by a human hand will elicit a robust set of discharges in this premotor neuron. very interestingly, if the monkey is prevented from actually observing the contact of the hand with the food pellet, that is the consummation of the motor act. by occluding that view with some kind of a screen, we still see activation of this so-called mirror motor neuron in the lateral premotor cortex. It's as if what this neuron is encoding, is the goal of the behavior, or the intention of the behavior, which in this case, is executing your precision grip to retrieve a pellet. Even when that act is not observed, there's still activity in this premotor neuron. So the discovery of these mirror motor neurons has really led to, I think, a really remarkable hypothesis. And that is that, the lateral part of the premotor cortex is involved in encoding the intention of movement. And that movement intention might provide a foundation for social cognition, because these neurons are concerned not just with my intentions, but also with your intentions. When we observe the activities of others we might actually represent that behavior within our motor system. And that might give us a sort of knowledge or understanding of the intention of that motor act. And this is a very active and frankly quite controversial domain of cognitive neuroscience at the moment, so we don't quite know how this is going to all play out. But, I do suspect that this is an important discovery, that is going to inform our understanding of social cognition. Perhaps even the basis for disorders that lead to impairments in social cognition such as some developmental disorders that result in the autism spectrum of disorders. This also provides some intriguing possibilities for intervention. If, in such neurodevelopmental disorders of cognition, there may be an underlying problem with the motor system. Then perhaps there is a movement-based intervention, that might help to restructure connections in this part of the brain that could produce some functional benefit to the patient. Well, that's total speculation on my part at this point. But I think it's an interesting idea that's worthy of some study.
