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MITOCW | MIT9 14S09 Lec16-Mp3 MITOCW | MIT9_14S09_lec16-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: OK, today let's continue with the motor system. The quiz for this week will be posted. Just a few questions. One of them might require you to do a little bit on the web, but it should be pretty easy for you to finish it by-- I'll accept them through Saturday. We were talking about hierarchical control of local motor behavior. And I pointed out how local motor pattern controllers and initiators in the hypothalamus or sub- thalamus-- they call it the mid-brain-- get a lot of input from the end-brain, as I show here in blue. And I just want to point out that this picture might lead you to believe that locomotion is always originating in the end-brain. Show it being initiated by olfactory inputs, other kinds of sensory inputs that come in through the older pathways through the old thalamus and striatum. And the newer pathways reaching the neocortex. But I note here that some inputs from the visual system, they actually reach that area, the mid-brain locomotor area more directly without going through the striatum. They come through a part of the [? pretectural ?] [? legion ?]. And it's very likely that other systems do the same, but we know-- remember those lesion experiments I talked about some time ago? Animals with decerebrations where the whole end-brain was removed? They didn't initiate much locomotion at all. You'd have to push them, nudge them in various ways to get them to locomote. But of course that might be partly an artifact of that huge brain lesion you're making. So I think there's some initiation of locomotion without too much above the mid- brain. It's been-- it's not been studied as much as these pathways shown here in the far left. 1 So this is the pathway from olfactory bulbs that goes through the ventral striatum, that then goes to this area, the caudal hypothalamus. We call that, that's the region of the hypothalamic locomotor area. And then, to this region of the caudal mid- brain, the mid-brain locomotor area. Just to show you on that diagram I've been using, of the ancestral brain, where these pathways are found. And have been found since very early evolution. But we know the generation of the actual pattern of locomotion, control of those movements, is spinal and hindbrain. It depends on [? propria ?] spinal input, from muscles and joints. It depends on tactal inputs from the limbs. So I want to talk now a little bit about these circuits that influence locomotion in various ways. We know its execution can be modulated by reflex inputs to the spinal cord directly, from the field. They come in through the dorsal roots and use propria spinal connections to reach motor neurons. But I want to say now more about these hindbrain mechanisms, coming through to the stibula nuceli, and the cerebellum. Let's just take a look at what they're like. The stibula nuclei get input through the eighth nerve. So the eighth nerve carries both auditory and vestibular input from the inner ear. The stibula nuclei are actually a collection of four different nuclei. One of them has very, very large cells. It's called Deiters' nucleus. This nucleus gets direct input from the vestibular nerve. It has direct projections to the spinal cord. So it's just one synapse, the vestibular system can reach the spinal motor mechanisms. We call that the vestibular spinal track. It comes from Deiters' nucleus. We also get pretty direct input to the motor neurons controlling eye muscles. These follow the MLF, the media longitudinal fasciculus, it's called. And you'll see that in the diagrams here. Directly connects vestibular nuclei and the ocularmotor nuclei. So a lot of the stabilization of our eyes as we move our head-- when I do this, my eyes stay pretty much in the same position, looking out at the class. Part of that's visual, of course. I'm maintaining fixation. But even if I close my eyes and if you kept track of the position of my eyeballs, you'd see pretty much the same thing 2 happening. It's under vestibular control, not just visual control. So these are the [? Nauder ?] pictures, where he shows the brainstem of a human as if it were transparent. And he's showing the secondary sensory cell groups and the groups of motor neurons. And I've shown the vestibular nuclei there with the blue stippling. So you can see its location there, on the rostral hindbrain. These are the cerebellar peduncles. So that's where all the fibers come to connect the cerebellum to the brainstem. All these fibers in this really big bundle here in the side come from the pons. Hearing input from the cortex up to the cerebellum. That's been removed. The region there of the cochlear nucleus and the vestibular nuclei, you wouldn't see all if the cerebellum were there. So the cerebellum has to be removed to get this kind of picture. So now let's look at the part of the cerebellum that's closely related to the vestibular system. The vestibulocerebellum. It's probably the most ancient part of the cerebellum, and it's somewhat different in its connections from the rest of the cerebellum. These parts of the cerebellum have these peculiar names, the flocculus and the nodulus. They get direct input from the vestibular nerve. And they influence muscles of the body axis, as does this medialmost most of the deep nuclei of the cerebellum, where all the other outputs of the cerebellum come from. They come from deep nuclei. They don't come from the cortex of the cerebellum. So I just want to show you, here's a diagrammatic top view of a cerebellum. Probably based on the cerebellum of a cat, or a rat. And in the dark red there, and over here in the red dots, you see the vestibulocerebellum. or So that's the part that's getting the direct input there, on the sagittal picture there, it's tucked under in the back. The other structure's located medially here in the lighter, in the pink dots, we refer to as the spinocerebellum, because it's dominated by input coming from the spinal cord. Whereas the hemisphere's the largest part-- in the large animals, anyway-- shown 3 on the small black dots, is getting its input through the pons coming from [UNINTELLIGIBLE]. So they call that the cerebrocerebellum here. It's because of that cerebellum has expanded so much in recent evolution, because with the expansion of the hemispheres the cerebellum expands along with it. But primarily in the hemispheres. And in this picture you see the diagram here of the primary sensory neurons. Out here in the inner ear. And here comes the axon into the hindbrain, through the eighth nerve. And note that some of them go directly to that floccular, nodular cortex of the cerebellum without any synapse. So they're secondary sensory neurons of the vestibular system in that this part of the cerebellum here. There's also secondary sensory cells there in the vestibular nuclei that project up to that region of the cerebellum. So you get both types of inputs to these vestibular parts of the cerebellum. And then here we see those regions of the cerebellum projecting to the vestibular nuclei, just bypassing the vestigial nucleus. Bypassing the deep nuclei. So in these respects, the vestibular parts of the cerebellum are different from all the rest of the cerebellum. Primary sensory neurons reaching them from the vestibular system, and output cells coming directly from that cortex to the vestibular nuclei of the hindbrain, and not going to the deep nuclei here in the cerebellum. And then those vestibular nuclei, as we mentioned, have direct projections to the spinal cord. There's a diagram of it. Calls it the vestibulospinal pathway, or tract. And then here, axons of that medial longitudinal fasciculus that are connecting to the third, fourth, and sixth nerve nuclei controlling the eye muscles. This is much too complicated at this point. I just want to show you here, this diagram of the cerebellar cortex. Which is taken from what a frontal section through the cerebellum would look like. And they've shown three deep nuclei there. That's where most of the output, with the exception of those vestibular system projections, these deep nuclei receive the output of the cortex of the cerebellum. And then they project to the rest of the brain. And this medialmost one not only connects with the 4 vestibular nuclei, but connects with the spinal cord also. Like the vestibular nucleus does. So does this vestigial nucleus. It go straight down to the cord of cerebellospinal tract. It's often called the vestibulospinal tract. Not to confuse you, just that's the way neuroanatomists name it. They name if after the source and the termination. The other projection of the cerebellum, we'll mention bit by bit as we go along. We're not going to put any great emphasis on the cerebellum except to point out the cerebellum has enlarged as the hemispheres have enlarged.
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