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Voiceover: In this video, I'm gonna talk about gray and white matter. In the central nervous system, which is mostly the brain and the spinal cord, there are areas called "gray matter" gray matter, that contain most of the "neuron somas". So most of the neuron somas in the central nervous system are found in the gray matter. And then there are other areas called "white matter". And the white matter of the central nervous system, is most of the "myelinated axons". Myelinated, so axons that are wrapped in myelin. Myelinated axons. And these are actually kinda funny names for these parts of the central nervous system because they're really not gray or white. They're all kind of different tan colors, in life. But when the tissue is prepared certain ways after being removed from the body, it can have a grayish appearance or a more whitish appearance. And so these are the names that have stuck for these general areas, the gray matter and the white matter. Now, the distribution of gray matter and white matter is a little different between the spinal cord and the brain. If we look first at the spinal cord here, and here we have a really nice illustration of some different sections of the spinal cord. So right over here. What these drawings represent are different levels of the spinal cord. Like if we made little sections of it. Like if we were cutting a loaf of bread and kind of cut from the top down to the bottom. And then we're looking down at these sections from the top, so that we're kind of looking down this way at the different sections over here in these drawings. And what we see in the spinal cord is that most of the gray matter of the spinal cord is on the inside, this kind of "H" shape or this butterfly shape of gray matter. And the white matter of the spinal cord is mostly on the outside, all this stuff outside of this "H" shape of gray matter. And it's a little hard to see on these drawings, so let me just take, like this one, for example and let me just draw it here, just a little bigger. So I'll just draw kind of an oval like we're looking down at the top of a section of the spinal cord. And on the inside is gonna be the gray matter, that's gonna be in this kind of "H" shape or this butterfly shape. Let me just draw the back part, here. And all of this stuff would be gray matter. So, it would be lots of neuron somas that would give it a different color from the white matter on the outside. Let me just draw in a little bit of white right here on the outside, because that's how the spinal cord is set up, with most of it's myelinated axons forming white matter on the outside. Except for this little bit here where the gray matter goes to the very back of the spinal cord. Now, for the brain, it's a little different than it is for the spinal cord. So let's take a look at the brain and make it a little bigger with this drawing over here. And on this drawing, we're gonna be looking at the brain from the left side. Let me just write that up here. So this is the brain from the left side, like we're looking over this way at the brain, with the cerebrum on top and the cerebellum back here, and then the brain stem, we just see a little bit right here. And then this drawing is gonna be of kinda the inside of the brain, like if we cut along the brain kinda like this, through a bunch of the cerebrum and the brain stem. And we're kind of looking at it from the front. So we're looking at it this way, after cutting it through here, so that we can look at the inside of the brain tissue. Now, where the spinal cord had gray matter mostly on the inside, the brain actually has gray matter mostly on the outside. So here you can see this layer of gray matter that's going all the way around the outside of the cerebrum here. And this layer of gray matter on the outside of most of the brain, is called "cortex". Cortex. And this cortex covering the surface of the cerebrum is called the cerebral cortex. And the cortex covering the cerebellum down here, is called the cerebellar cortex. And that's all gray matter on the outside of those parts of the brain that contain most of the neuron somas. I'll just draw some little circles here to represent neuron somas. Although, they're actually much smaller than this, if we did it to scale. And also, kind of the reverse of the spinal cord, where most of the white matter was on the outside of the spinal cord, most of the white matter of the brain is on the inside. So this lighter colored stuff here, under the cerebral cortex, this is all white matter deep inside the cerebrum and white matter down here in the brain stem. Now, there are some other areas deep in the brain that are gray matter, that contain lot's of neuron somas, like this right here. And instead of calling these areas "cortex", like the gray matter on the outside, the gray matter that's kinda deep in the brain, we call "nuclei". One would be a "nucleus" and multiple is "nuclei". And there are multiple nuclei deep in the brain, these areas of gray matter. Like here are several right here, and here's another one right here. Now, in the white matter of the central nervous system, are collections of axons that are traveling together to different areas. I'll just draw a dash line to show that these axons are gonna keep going. But there are multiple neuron-axons that are kind of traveling together. They're starting in a similar area and they're heading to a similar area somewhere else in the central nervous system. And collections of axons traveling together in the central nervous system, we call "tracts". Tracts. So this would be one tract, right here. And a tract could have many, many axons in it carrying, often a very similar kind of information from one part of the central nervous system to another part of the central nervous system. The central nervous system has a huge number of neurons in it, doing lots of different functions. In addition to neurons involved in motor, sensory, and automatic functions, like we talked about with the peripheral nervous system, the central nervous system also has lots of neurons participating in the higher functions of the nervous system. That is, participating in cognition, emotion and consciousnesses. And that's particularly in the cerebral cortex, all this gray matter on the outside of the cerebrum. And certain other parts of the brain are very involved in those higher functions of the nervous system, as well.