The nervous and endocrine systems
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Structure of the nervous system
In this video I'm going to introduce the structure of the nervous system and the nervous system is divided into two main structural parts. The first is called the central nervous system. And the second is called the peripheral nervous system. Central and peripheral, and both of those are themselves divided into two main parts. The central nervous system is made up mostly of the brain which is in the head. So I'll just color that in here in magenta. And the spinal cord which is in the spine. So I'll just color that in in blue. That's this long thin tube-like structure that goes down the spine. Now the brain is divided into a bunch of different parts. And I'll just mention some of the big ones for this kind of introduction talk. Here we're looking at the brain from the left side. All of this part on the top, that has several different colors here, is called the cerebrum. Cerebrum, which is the biggest part of the brain and the part on top. Now if we look down from the top, like in this drawing here, you can't see the parts underneath. You can only see the cerebrum and you see that it's divided into a left and a right half, and we call those halves the cerebral, or cerebral hemispheres. Cerebral hemispheres, like half a sphere. And there's a left and a right cerebral hemisphere. Now on this picture over here, we're actually looking at the middle part of the brain, so it's like we've cut down the brain this way, and separated the left and the right cerebral hemisphere. So here we're looking at the left cerebral hemisphere, but we're looking at it from this direction. And we see that all of this stuff on the top, is the cerebrum or one of the cerebral hemispheres, and then the part on the bottom, which is smaller, we divide into two other parts. So first let me just draw an outline around this part right here, and this is the part that hooks onto the spinal cord down here. And this part is called the brain stem. Let me write that out, brain stem, which is all of this part right here that connects the cerebrum to the spinal cord that they've cut off down here on this picture. And then the brain stem itself is divided into three smaller parts. The very top part of the brain stem that connects to the cerebrum, is called the mid brain, mid brain. And the middle part that's just below the mid brain we call the pons, P-O-N-S, for pons. Under the pons, and the part that actually connects to the spinal cord, is called the medulla, or medulla, or sometimes people use a longer name of medulla oblongata. And then the last part, but not the least part of the brain, is this big part in the back. It's behind the brain stem and connected to the brain stem. And this, we call the cerebellum. Cerebellum. Now sometimes brain structures are referred to by the names of the structure that they develop from in the embryo. So here's a picture of the human embryo and it's developing it's brain here. And this very front part is called the fore brain or it has a longer name of prosencephalon. This part behind the fore brain is called the mid brain and it also has a longer name called the mesencephalon. And then the part behind the mid brain is called the hind brain, hind brain, and it also has a longer name of the rhombencephalon. So the fore brain is going to become the cerebrum. The mid brain is going to become the mid brain. Just this part of the brain stem, up top. And then the hind brain will become the rest of the brain. The pons, the medulla, and the cerebellum. So just in case you hear people referring to structures in the brain by these names, that's where those names come from. They're from the developing nervous system. Here's a drawing of the spinal cord. This kind of long tube that runs down the spine. And there's a number of structures coming out of the spinal cord that I'll talk about next. So those are the parts that make up most of the central nervous system, and everything that's not in the central nervous system we call the peripheral nervous system. And the central nervous system is called that because it's kind of in the center of the body and then the peripheral nervous system is called that because it's going to go out all over the rest of the body. The peripheral nervous system consists of two types of structures. The first are called nerves. Let me just underline this nerve right here. And these are the long stringy structures that are going to go all over the body. And nerves carry the axons of neurons. The second main structure of the peripheral nervous system are called ganglion. Ganglion is singular and ganglia is plural. And ganglia are these lumps that are attached to nerves and they contain the somas of neurons. Now let me just draw that a little differently over on this picture of the spinal cord that have these nerves coming out of it. So here's one of these lumps, one of these ganglia that contains the somas of some of the neurons in the peripheral nervous system and some of these axons traveling through these nerves are going to be carrying information in to the central nervous system from the periphery. So they're going to bring information in this way from out here in the periphery. And when they do that, we call those afferent neurons, afferent neurons carry information in to the central nervous system. Now other neurons are going to have axons that carry information in the opposite direction. So they're going to carry information away from the central nervous system out into the periphery. And neurons whose axons carry information away from the central nervous system we call efferent neurons. Efferent neurons. Now there are lots of these nerves that are going all over the body. And you can divide them up in a few different ways. But we usually start by dividing them into the cranial nerves which are nerves that exit the skull or the cranium. So these nerves primarily come out of the brain and they're passing through the skull on their way between the central nervous system and the periphery or the peripheral nerves can be spinal nerves. And they're coming out of the spinal cord and passing through the spine on their way between the central nervous system and the periphery. And I'll just draw a few of these, but there's actually lots of these. They're paired on both sides of the body. And there are 12 pairs of cranial nerves and 31 pairs of the spinal nerves. Now if we go back to our drawings of the spinal cord here, we can see that the spinal nerves actually form from these two parts which are called spinal nerve roots and there's a root in the front and there's a root in the back. And in this drawing we're looking from the back. So here's the root in the back that have the ganglia and the way these spinal nerve roots work are that the afferent neurons bringing information into the central nervous system travel through the spinal nerve roots in the back and the efferent neurons that are carrying information away from the central nervous system travel in the spinal nerve roots in the front and then they come together in the spinal nerves, so we call those mixed nerves because they have a mix of afferent and efferent neurons usually. Now as any of these nerves travel from their proximal origin, proximal just meaning close to the center of the body, toward their distal ends, the word distal just meaning far away from the center of the body, or you can think of the word distant, all the nerves are going to branch repeatedly, so they're going to branch, and then they'll branch, and then they'll branch again, and they'll just keep branching into tinier and tinier branches, because they have a long way to go and have to spread all the way out all the way through the body, and these proximal parts of the nerves are big nerves that we can see with the naked eye. But once you get to these distal nerves, after they've branched a bunch of times, actually become microscopic, and they're little microscopic nerves that go all over the body connecting the entire body back to the central nervous system. And this is true for almost all of the cranial and the spinal nerves. Oh, I almost forgot here, I have this other drawing to show the cranial nerves. Here's a drawing of the brain looking up from the bottom, and all of these long stingy looking things coming out of the brain are cranial nerves. They're going to pass through the skull on their way from the brain out into the periphery. I won't draw them all in here, but there are a bunch of these cranial nerves that are going to pass through the skull. So that's a brief kind of overview of the structure of the nervous system and there's a lot more to it of course. But I just want to give you an introduction here. And we'll get into some more of the details in later videos.
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