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MITOCW | MIT9 14S09 Lec36-Mp3 MITOCW | MIT9_14S09_lec36-mp3 The following content is provided under a Creative Commons license. Your support will help MIT OpenCourseWare continue to offer high quality educational resources for free. To make a donation or view additional materials from hundreds of MIT courses, visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: So this class, we will talk about some of the cell types in the neocortex and the way they're connected. We'll talk about different regions of neocortex, that is actually after it had experienced, in evolution, a lot of expansion, and we're just going to deal with mammalian cortex in small animals, like rodents, and human being. And then we'll talk a little bit about some of the major fibers in and out of the membrain. We've talked about those already, but I want to review them to make sure that they're in your mind. And then we'll get, at least, to an introduction of thalamocortical connections, which again, you've had some because when we talked about visual system, we talked about the genicular body rejection in the visual area, primary visual cortex, the auditory cortex. We talked about medial geniculate body, somatosensory has been talked about several times. We talked about the ventral nucleus. All right. So what are the two most commonly encountered classes in the cortical cells? The most characteristic cell type in the neocortex is the pyramidal cell, and this is what they look like. This is Brodel's simplification of the pyramidal cell that shows their major characteristics, a cell body with a sort of pyramidal shape in an apical dendrite that goes up towards the surface, and many of these cells send their apical dendrite all the way up to the surface where they arborize right up in layer one. And this doesn't show the fine ramifications of that dendritic arbor that you really see in these cells. I think the next slide will show some of those. And you see the largest ones are in layer five. Layer five is the layer where the large neurons project out of the cortex to subcortical regions. The longest one's going to the spinal cord from the motor cortex and from somasensory cortex, as well. 1 In layer three, also fairly large pyramidal cells. They're projecting mainly to the opposite side, the neocortex, and they're smaller pyramidal cells, mostly in layer two, that project transcortically, to the same hemisphere. And it's interesting that layers two and three, that give rise to these, what we can call, association connections, connect one part of the neocortex with another part, had expanded tremendously in higher animals. And here's a layer six pyramidal cell. Some of them have dendrites that go all the way up to the surface. Some of the don't. And here's the other major cell type, stellate cells. We'll see some cartoons of it, too, but these are actual drawings of Golgi stain. Here's a pyramidal cell here. Here's a couple of stellate cells. Now, what you're seeing there is dendrites radiate in all directions, just like a motor neuron. But then you also see a somewhat thinner axon, and you can see that the axons and the stellate cells tend to arborize near the cell body, but not always in the same column. Often they're terminating in adjacent columns, but their major connections are actually within the same column, and we'll see some other pictures like this one. Here's different types of stellate cells that Brodel drew these cartoons of to show you the variety of, not just dendritic arbors, but especially the axonal arbors. They're quite specialized. Like for example, this one, in layer four, that has axons that go up all within the column, terminating along this line, and then there's a small arbor way up in layer one. Here's another one with a totally different kind of arbor. It's not primarily within the column, but it's an adjacent columns. We can be pretty sure this is an inhibitory interneuron. It's terminating on cell bodies of adjacent cells in adjacent columns, and you can see the little arbor for each cell body. And here you see an interneuron in layer five, but it's a stellate cell so it's right among all those big pyramidal cells. It's probably an inhibitory interneuron there. OK now, when we talk about the major morphological characteristics of neocortex, one would be they've got these pre-pyramidal cells. That's certainly a morphological 2 characteristic, but the other characteristic that you would want to point out if you were asked about major morphological characteristics in the neocortex would be the layers, of course. The dorsal cortex of many species is not six layered. It's many fewer layers. We've seen pictures of the-- I think, last time, I probably showed a frog cortex right at the end when we were looking at the different axon types coming in, and they were very few layers. But this is the way the layers are usually numbered in name, we saw this picture once before, you see in the fiber stain here that there's actually more layers if you want to subdivide it more. And often in the cell stain, for example, layer six has often at least three clear, very different, layers. And in primary sensory areas, layer four has often got at least three sublayers. And the molecular layer here is often layered in different fibers, like here you see a lot of fibers in the upper part of layer one. But there's actually other fibers terminating in deeper parts, and we call them the multiform layer, the deepest layer. These are considered the oldest layers, most similar to the layers in non-mammals, layers five and six. So the large pyramidal cells in layer five and then the multiform layer, which has pyramidal cells, stellate cells, a few [UNINTELLIGIBLE] cells, and other cell types. And then the granular layer. Sometimes we just call that the granular layer, but more properly, it's the internal granular layer because there is another layer of small cells, often small pyramidal cells, but also stellate cells all in layer two. And then Layer three is the external pyramidal layer, which is why the layer five here, with the biggest cells itself, is called the internal pyramidal cell layer. OK, so you'll notice here in the fiber stain, we see these radial fascicles. Radial means it's perpendicular to the surface of the brain, which is up here, we call the pial surface. Down here, below the white matter, would be the ventricular surface. And often the layer of white matter here is much thicker. This would go down much further before you get to the ventricle. Depends, of course, on the species how much white matter they show. The larger animals, larger brains, and more white 3 matter. One of the reasons is the larger the brain, the fibers are thicker because they're traveling further. They've got more mile in their larger axons, so just for speed, you get larger axons. But there's also a lot more of it with the expansion of the cortex. OK so I'm asking a question here. What are the radial fascicles? Well, we just saw picture of them. They run up and down and within the columns, or usually within the columns. And then I ask what are three major groups of cortical output axons found in these fascicles? Well, why don't we start with just the input? So let's start a little further back. OK, here comes a fiber from the thalamus, and it's going up and terminating mainly, say, in layer four within a column, also up in layer one, with a few terminals in layer six, perhaps a few in the other layers as well. Well, that hybrid is one of those fibers in a radial fascicle, but it's an input. It's coming this way. What other kind of fibers are coming in to a column like that? Somebody? What have we said? Well, they come from other parts of the hemisphere. They're transcortical axons. So they would be coming in, too, and terminating mostly up above layer four. And then there's callosal axons, but the callosum doesn't go to all areas. It goes to most areas. For example, it doesn't interconnect the part of the cortex representing my hand, here, and fingertips doesn't get callosal projection. Most of the visual cortex doesn't get a callosal projection, with one exception, the cells that represent the midline because you have some receptive fields that cross from one half of the visual field to the other. So to make that work, you've got to have callosal axons because the right half of the visual field represented in the left hemisphere, and the left half of the visual field's represented in the right hemisphere. So now in these radial fascicles, the output axons. So here's a layer five cell, and let's draw in red the output axons. OK, so there it goes, and it's going out to the corpus striatum, to the optic tectum, or to other areas. 4 [PHONE RINGS] PROFESSOR: Sorry. [PHONE STOPS RINGING] PROFESSOR: That's somebody who should know I'm teaching, now. Right. That's one type of output axon.
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