Biological basis of behavior: Nervous system
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Voiceover: In this video, I'm going to talk about the brainstem. Brainstem. We briefly discussed the brainstem when we were talking about the structure of the nervous system as a whole. Recall that the brainstem is right around here, and if we look at our larger drawing of the central nervous system, this is the brainstem right here. The brainstem connects, basically, all the parts of the nervous system together. The brainstem connects the cerebrum on top and it connects the spinal cord below, and it connects the cerebellum, which is behind the brainstem. The brainstem also connects most of the cranial nerves. I'll just write that over here. Cranial nerves. Here's a different image of the brain, looking at it from below. Here's there cerebrum. Here's the cerebellum, and here's the brainstem. All these little stringy things coming out are the cranial nerves, and most of them are coming out of the brainstem. We'll come back to that in a minute. Basically, because the brainstem is connecting all these others of the nervous system together, its anatomy is complex. There's a lot going on in the brainstem, even though it's a fairly small structure. It's just this little guy right here, but it does a lot of things. Let's take a look at some sections, to look inside the brainstem. Recall that this top part up here is called the midbrain, this middle part is called the pons, and this lower part is called the medulla. Now let's take some sections. Let's pretend we're cutting through the midbrain right here. We're going to look at it from the top. Over here this would be a section of the midbrain. Midbrain. When we're looking down on it from the top, it's got these big parts that stick out the front of it, and some little bumps on the back. Don't worry about the names of any of those right now. Then, if we take a section through the pons, so if we cut through the pons like this, and we look down at the pons from the top, this would be a view of a section of the pons. Let me just write that. Pons. It's got this really big part that sticks out the front. Then let's take a section here through the medulla. Let's cut through the medulla and look at it from the top. That would be this guy right here. This is the medulla. "Meduh-la" or "Medoo-la." I hear people say it both ways. The medulla has different shape. Each one of these three main divisions of the brainstem has a little bit of a different shape. The inside of the brainstem has some similarities to the spinal cord, particularly down low in the medulla, where it's connecting to the spinal cord right here, and similarly to the spinal cord, most of the grey matter of the brainstem is on the inside and most of the white matter of the brainstem is on the outside. It's a lot more mixed together than it is on the spinal cord. There's white and grey matter more scattered about, although most of the grey matter's on the inside and most of the white matter's on the outside. Much of the brainstem grey matter are just these distributed neurons that aren't in nice discrete little places or bundles, and we call this the reticular formation of the brainstem. Just all these neuron somas scattered about to a lot of the grey matter inside the brainstem. Let me just write that over here. The reticular formation. The reticular formation of the brainstem. This turns out to be a really important structure. The reticular formation of the brainstem plays a big role in lots of autonomic functions. Autonomic. Plays a big role in controlling things like circulation, respiration and digestion. Some of our critical organ system functions. In addition to that, participating in a lot of these lower functions of the nervous system, the reticular formation sends lots of axons projecting up to the cerebrum up here, and plays a major role in lots of the higher functions of the nervous system as well, including cognition, emotion and consciousness. A lot of the white matter that's passing through the brainstem is actually connecting the different parts of the nervous system, so for example, on these drawings over here, which are views from the front, cutting through the nervous system, like I've done here, and what these drawings are showing are what we often call the long tracts. Long tracts. We call them that because they are collections of axons traveling a long distance through the central nervous system, often connecting the cerebrum up top down to the spinal cord, and they're just passing through the brainstem on their way to or from the spinal cord or the cerebrum, and there are two big categories of long tracts that are really important passing through the brainstem. Let me write two little marks here. The first play a big role in the motor functions of the nervous system. These are actually the upper motor neurons. Here in this drawing, what they've shown is upper motor neurons starting way up here in the cerebrum and then passing down through the brainstem, and mostly crossing over to the other side of the spinal cord, where they're going to innervate lower motor neurons, and here what they're showing in this drawing are somatosensory long tracts. Somatosensory. There are different kinds of somatosensory tracts carrying information to the nervous system, but what this one's showing is somatosensory information coming up the spinal cord, crossing, and going up the brainstem, and then going to the cerebrum on the other side from the body. The brainstem is playing host to these long motor and somatosensory tracts that are carrying information up and down between the cerebrum and the spinal cord. As if all of that wasn't enough functions for the brainstem to be doing, most of the cranial nerves, as I mentioned, are attached to the brainstem, and the cranial nerves are doing all sorts of different things. There are 12 pairs of these cranial nerves, and most of them are attached to the brainstem. Let me just draw a few on my drawing over here. Here's a much better drawing, where they're showing the brainstem, and all of these little nerves coming out here are cranial nerves, and there are 12 pairs of them, most of which come out of the brainstem. The cranial nerves perform all sorts of functions, including motor functions, sensory functions, and a number of different kinds of senses in addition to somatosensory functions. There's things like hearing and taste and a number of automatic functions, like reflexes and autonomic functions. Automatic functions. The cranial nerves are related to a lot of the grey matter inside of the brainstem, because in addition to the reticular formation, this number of neuron somas that are scattered around, there are collections of neuron somas that are nuclei, that are discrete nuclei, that have been named, and the cranial nerves are often carrying information away from these nuclei or into these nuclei, depending on what functions they are performing. For example, in this illustration over here, we're looking at the brainstem from the left side. This is the front and this is down, and what they're showing are these nuclei have neuron somas and then axons are leaving the brainstem through cranial nerves to perform motor functions. In this drawing they're showing sensory information coming in through cranial nerves to the brainstem, to different nuclei in different parts of the brainstem. These cranial nerves are mostly performing these functions in the head and the neck. Let me just mark that in on our person over here. There are a few cranial nerves that actually travel from the brainstem all the way down into the body and perform functions in the trunk and in the limbs. Some examples of cranial nerve functions include sensation of the face and movements of the eyes, face, jaw and throat, in addition to influencing organs like the heart and the intestines. There's a lot going on with the brainstem, and I think I'll stop here. In later videos we can spend some more time going into some of the details of the different structures and the different functions of the brainstem. I just want to introduce it and give you a feel for all the different things it does.