Well, let's talk just a little bit more about how information flows through the cortical network. So I've already mentioned that our thalamus sends inputs up to layer 4 providing the main source of drive to this network of cells that populate the cerebral cortex. There's also a small input to layer 6. but the primary one that seems to have the greatest physiological impact is destined for layer 4. Well, from there, cells in layer 4 interconnect with one another. But they also send major inputs to the upper layers of the cerebral cortex, layers 2 and layer 3. And once information gets into those upper layers of cortex, it can be broadly distributed, so information can go in the horizontal direction from one region of layer 3, for example, to a neighboring region of layer 3. information can be projected through the corpus callosum to the corresponding cortical area in the opposite hemisphere. So there can be long projections that come out of theses neurons in layer 3 that enter the white matter, and eventually, the corpus callosum. one can also find local connections from these cells to nearby neurons within layer 2 and layer 3, as well as projections that will drive the activity of neurons down below in what we call our infragranular layers, those layers below the granular layer, layer 4, in layers 5 and 6. Well, those infragranular layers can give rise to descending projections. Layer 5 cells can project to a variety of structures in more inferior parts of the nervous system including the basal ganglia the brainstem, and even the spinal cord for certain regions of the cerebral cortex. Cells in layer 6 project back to the thalamus, they also provide a return input to layer 4, where this cortical circuit got started. Well, what I've been describing for you is the flow of information through the cortical microcircuit. And we believe that this basic flow of information, this basic circuit structure exists across the entire cerebral cortex with some modifications, depending upon the different cortical regions in question. But this basic flow of information of starting in layer 4, progressing to upper cortical layers, then down to lower cortical layers is a common theme of cortical processing. Well, the circuitry performs a variety of unctions. We're only beginning to scratch the surface to truly understand what those functions are, but I want to give you a sense of what the function is of this columnar circuit that we find in the radial dimension of the cerebral cortex. We call this the canonical microcircuit of the cerebral cortex. Well, I'm going to suggest an acronym that makes sense in this part of the world, and I hope it might make sense wherever you are. Well, if you know anything about Duke University, you know that our Athletic Department is pretty important to the life of this university. And one sport in particular seems to dominate, and that's basketball. Well, for those of you familiar with how basketball works in this part of the United States, you're familiar with three letters, ACC. But you know that as the name of the conference within which our Duke basketball teams compete, that is the Atlantic Coast Conference, the ACC. Well, I'm going to suggest that we hijack those letters and use it to remember some of the basic functions of this canonical microcircuit in the cerebral cortex. I would suggest that the ACC of the cortical microcircuit is amplification, computation, and communication. Let me tell you what I mean. Let's consider first, amplification. So, as I've already described for you, the thalamus provides the principle input that modulates activity in cortical networks. That activity goes into the middle layers of the cerebral cortex, layer 4, and from there, it's amplified. So, one purpose of this network of radial connections is to amplify this signal, and once that signal is amplified, then new computations are possible. So the networks that we find largely emanating out of our upper layers, our supragranular layers can take that amplified signal, and now, compute new properties that were not carried with that signal as it first entered the cortex. We'll see some examples of that when we get into our sensory systems. And then, also, from this part of the cortex as well as from deeper layers, this information can be communicated broadly. And, that communication can happen from one cortical column to the next, from one cortical area to another, and even from cortical areas to more distant regions that lie below the level of the cerebral cortex. So amplification, computation, communication, the ACC of the cortex, hopefully, you'll be able to remember that. Okay, now I want to finally turn to the question of the organization across the extent of the cerebral cortex, and this allows us to return to what we looked at earlier, and that is the cellular architecture of the cerebral cortex. This is what we refer to as cortical cytoarchitecture, cyto referring, referring to cells, and architecture referring to the arrangement of those cells. So I would just remind you that neural tissue is comprised of a variety of different kinds of cells big cells, small cells, from a physiological point of view excitatory neurons, inhibitory neurons, those that fire in various kinds of patterns. just a tremendously rich diversity of cell types are found in the cerebral cortex and these cell types are organized in columns, but those columns have different compositions in different parts of the cerebral mantle. We see some of them illustrated here in this slide. So, for example, if we look in the visual part of the brain, back here in the occipital lobe, we see an arrangement of cortex quite similar to what you saw earlier in this tutorial. In fact, those histological slides were taken from the visual cortex. And, if we moved a little bit more anterior into the motor cortex, we'd see something that looks a little bit similar, but some things would really stand out. One thing that stands out is the presence of these really large cells in layer 5. These are called bet cells and they are the largest neurons that we have in the cerebral cortex. But there are a little bit more subtle differences that you can appreciate once you, have a bit of experience looking at the cerebral cortex. You'll notice that while, while the granular layer 4 is very prominent in our sensory cortex, it's largely diminished in its thickness, as well as the density of cells in our motor cortex. That's because the function of the motor cortex is largely to provide output rather than to receive input. As we progress towards the more medial aspects of the brain and the more ventral part of that cerebral mantle, we encounter a region of tissue that is wrapped into the medial edge of the temporal lobe. And this tissue is perhaps the most simple of all the regions that we find in the cerebral network. the anatomists have chosen not to recognize six layers as we do in what we call the neocortex or that more phylogenetically modern part of the cerebral mantle. And what we find is an older part, a paleocortex and even and archicortex. These are terms that we think reflects the evolutionary history of the cerebral mantle in the mammalian brain. Well, around the turn of the 19th to the 20th century European anatomists began to develop a variety of ways of studying the cellular architecture of the cerebral cortex. And one individual really stands out among many seminal contributors of the time his name was Korbinian Brodmann. So Brodmann was a German scientist who studied the cytoarchitecture of the cerebral cortex. And he proposed a scheme for recognizing divisions of the cerebral cortex that is still with us today and we call these Brodmann's areas. That's what's illustrated here in this color-coded representation of the cerebral mantle with all of these numbers in place. So Brodmann was a fascinating figure and it's a really interesting time in the development of modern neuroscience. So, if anyone's interested, I would invite them to read a little bit more about Brodmann. And for the rest of you you will invariably encounter his nomenclature as we talk about different parts of the cerebral mantle. So what Brodmann did was he looked under the microscope at a Nissl stain and he recognized that different parts of the cerebral cortex have a different appearance. Very much like what is shown here in this slide that I just showed you. And so Brodmann proposed, I think, a rather bold idea and that is, that these subtle architectural differences that he recognized in the cerebral cortex had functional consequences and his scheme has largely ushered in the modern era of neuron science where we come to, recognize that different parts of the cerebral mantle are specialized for certain kinds of function. Now, we can get a little bit carried away with this conceptualization of the cerebral cortex and have an erroneous view that cortical areas function in isolation. That would be a mistake because different parts of the cortex maintain, very specific interconnections with surrounding cortical areas such that the cortex operates as an integrated network. But nevertheless, there are networks nested within networks and the recognition of Brodmann's areas allows us to talk about those networks in ways that allows us to interpret our data. That we're generating with, let's say functional magnetic resonance imaging studies as well as the more invasive methods that can be done in animal model experiments where were looking at cell to cell patterns of, to the function. Well I've hoped you enjoyed this brief survey of cortical structure and function. We will see application of these principles as we continue to explore our sensory and motor systems. And hopefully, this concept of the ACC of the cortical microcircuit will be a useful way for you to understand what exactly does the cerebral cortex do in neural processing.
