Ok then. Let's get back to our slide and make sure that you are comfortable and familiar with what we see here in this slide and that you know what the 12 cranial nerves are and the 10 that connect to the brain stem in particular for this lesson. Now notice that in this figure there's a color code and the color code is meant. To help you understand which nerves are purely sensory. Which are purely motor. And then, which nerves. Like, most of our spinal nerves have a mixture of sensory and motor axons within them. Well, that may seem, perhaps, surprising that there would be nerves that are mixed for sensation, and. Motor output but that is the case for our spinal nerves and it is the case, as you can see by the abundance of green nerves here, for several of the cranial nerves. So that suggests that there must be some complex relationship between the brain stem and the nerves. And often that is the case. So what I want to help you with next is to understand, how do these nerves connect up with the gray matter within the brain stem. That either are receiving incoming sensory signals or giving rise to axons that convey motor signals out to the effector tissues. Now in order to show you what those grey matter structures are. We're going to be using a dorsal view of the brain stem. And that's what we have here. So we have removed the cerebellum. So, the cerebellum, normally, would sit out here somewhere. And receive input via these massive fiber bundles called the cerebellar peduncle. So we've taken those away. And what we're looking at now is the fourth ventricle. This space down here between the brain stem and the cerebellum. So we're looking into the floor of the fourth ventricle now, so I hope you can appreciate the midbrain which is up in this region here and then the Pons. Between the midbrain and the medulla oblongata down below, okay? So next what we're going to see is a phantom view of the brain stem. With the cranial nerve nuclei that I want you to become familiar with. Illustrated. So this is something of a phantom view of the brain stem. We're looking through that dorsal surface, through the floor of the fourth ventricle as if we could see the gray matter themselves. And what we see is a complex array of nuclei that come in different sizes and different shapes. And for the purpose of making this illustration. Just slightly less complicated than it needs to be. We've organized it by color code, and by side. So on the left side of the figure, we have our sensory nuclei. And on the right side of the figure, we've illustrated the motor nuclei. Now of course the nervous system displays bilateral symmetry so on each side we would have both the sensory and the motor nuclei. So imagine just folding this illustration of nuclei one side on top of the other, so that all of these nuclei would exist on each side of the brain stem. And that's what we have when we look into the brain stem itself. Now, this illustration bears some resemblance to the dorsal side of the model that I showed you a moment ago. So let me show you that once again. So now we're looking at the dorsal side of the brain stem. Like the illustration, we've removed the cerebellum. We're looking into the floor of the fourth ventricle, and rather than making this a glass model it's a rubber model. So what we've done here is dissected away the tissue to reveal the shapes and the bulges that indicate the location of cranial nerve nuclei. So, I would encourage you to go into Sylvius and you have a view of this dorsal aspect of the model, with the nuclei identified and there's also a feature in Sylvius that allows you to rotate this brain stem around, so you can see these nuclei in various views. So I just wanted to show you what that looks like in the model and now we can get back to the figure and recognize some of these important cranial nerve nuclei. Now I do want you to make a pass through them all but for this view I'll just highlight a few that might catch your eye in particular. One of them. Is this long structure in blue that has this, this sort of bulbous region right in the middle. This is the trigeminal complex of the brainstem. It can be divided into several subnuclei. We'll spend a fiar amountof time talking about this particular complex of nuclei. It's all associated with the trigeminal nerve. And it's all associated with sensations derived from the face. But different parts of this nucleus receive different aspsects of facial sensation. For example, this long pointy region in the superior direction that's the mesencephalic trigeminal nucleus. There we find cells that are sensitive to the stretch. In our jaw muscles in our Temporomandibular Joint. So you can imagine that must be very important in helping to govern chewing behavior. Well, in the middle we have something called the principle sometimes called the chief nucleus of the Trigeminal nerve, and this is the part that receives signals about light touch. From the face. So if you ever have a gentle caress on you face this is due to activation of cells in the trigeminal nerve that synapse in this principle trigeminal nucleus. And then lastly, and perhaps least pleasantly, we have this long descending part of the trigeminal nuclear complex. This is called the spinal trigeminal nucleus, and that's the part of the nucleus that's concerned with pain and temperature. So if you ever, burn your tongue, burn your mouth on a, on a hot cup of tea or a hot bowl of soup or if you've ever had dental pain or some other source of pain in your face, you've activated this spinal trigeminal nucleus of your brain stem. And I'd to me just point out one nucleus on the motor side, just to wet your appetite for our studies of the motor system that we'll have in a few weeks. That's the facial motor nucleus, we find it right here. So the facial motor nucleus is a collection of motor neurons that innervate the muscles of the face that convey expression. So, whenever you smile, whenever you greet someone, whenever you laugh or, or cry or frown, or express some other human emotion. You're activating the outflow of your facial motor nuclei that are painting on your face the emotion within. Well, if you're feeling pretty overwhelmed right now, then join myself and the hundreds of thousands of students around the world in the last 100 years. That have faced the challenge of understanding the complexity of the cranial nerve nuclei. So how can we possibly simplify this challenge for you? Well, I'm going to suggest one way to simplify it, and that is to use an embryological framework to help you understand this assortment of cranial nerve nuclei. Now in this figure we notice that there are subdivisions of our color code where the various shades of red are indicating our different kinds of motor nuclei and the shades of blue are indicating the sensory nuclei. And what these terms refer to are the embryological origins of the tissues that are connected up with these nuclei in the brain stem. So there are muscles that are derived from the embryological somites so we call the axons that connect to those. Muscles, somatic motor axons. And the nuclei that grew those axons, somatic motor nuclei. And in the head and the neck region of the embryo, we have something called the pharyngeal arches. And from the pharyngeal arches are derived a variety of tissues. Including some of the muscles of the, cranial region. And, deep within the neck. And the motor neurons that innervate these derivatives of the pharyngeal arches that become muscles are called branchial motor neurons, giving rise to branchial motor axons. And so we can recognize the distinct set of nuclei in the brain stem that are connected to these muscles derived from the pharyngeal arches rather than the somites. So it's useful to distinguish the somatic motor from the branchial motor, and we'll do that as we look inside the brain stem. And finally there are smooth muscle cells, of course cardiac muscle tissue and glandular tissue that is innervated by axons that are controlled by the central nervous system. And that out flow from the CNS comes from our visceral motor axons. And those visceral motor neurons that grow those axons are found in the brain stem and also found in the spinal cord. And we'll get to the spinal cord in a later session, but for now I want to show you the brain stem nuclei that. Throughout those visceral motor axons. Now on the sensory side we can also recognize different distinctions. There are, is a general sensory system derived from our skin surfaces, our muscles, our joints, structures in the peripheral parts of the body, so to speak. And then there are special sensory systems, or special sense organs. Such as our, our eye, our ear, our tongue, our nose. So those are systems that are especially dedicated to processing just a, a fine modality of information derived from our environment. And then finally there are visceral sensory axons that feed the central nervous system with information derived from our visceral organs. Well as this color code makes explicit these different cell modalities for motor control and for sensation connect with different nuclei within the brain stem. And, in the tutorial notes, and here in front of you now is a really beautiful drawing made by, my friend and colleague Dr. Nell Cant here in the department of neurobiology at Duke University. And it's a simplified way of explaining how one can relate the devleopment of the neural tube, to the more complicated mature forms that we see in the spinal cord and in the brainstem. Well, we'll talk some about brain development in a few weeks. But for now, I would just emphasize that the nervous system is derived from the walls of a tube. And that's what we have here drawn simplistically in the upper left. So imagine this as a cross section through the caudal part of the developing embryo where the nervous system is going to form the spinal cord. And I think as you may know already, the dorsal part of that spinal cord is related to sensation. Sensory signals derive from the peripheral parts of the body, or the superficial parts of the body. And then sensory signals derive from the visceral, the more interior organs within the body. That's all found in the dorsal aspect of the developing neural tube that will form the spinal cord. The ventral part on the other hand is motor. And again there's a systematic organization where the motor neurons that. Innervate the tissues derived from somites, that is our stride and skeletal muscles. They are innervated by cells that sit in the more inferior part, sorry the more ventral part of this developing neural tube, whereas the motor neurons that are going to supply motor signals. To those ganglia that innervate the visceral organs. They come from this more intermediate region right in here. Now that's the plan for the developing neural tube in that region that forms the spinal cord, and then when we actually look at what we see in the adult Central nervous system. We see a spinal cord with this characteristic butterfly shape of gray matter in the middle, and we have a dorsal horn where we find our sensory neurons, and we find a ventral horn. Which is filled up with neurons that are affiliated with motor output. With this large anterior part of the ventral horn, where we find our somatic motor neurons. Well, here's another way to conceptualize this. And I really ant you, to do all of this me. You might even want to stand up. If you're comfortable doing that, in a position to do this. So I want you to take your two hands and I want you to put them together in such a way as they resemble the illustration that we have here in the upper left. Okay. Let me clear our annotations so you can see that clearly. So what I want you to do is I want you to make with your hands this figure. Okay, so here we go. We're, we have just made a cross-section of the developing neural tube. Now, I want us to note how our hands resemble the distribution of these neurons related to sensation and motor control. We have our fingers up on the dorsal aspect of this transverse section. The fingers would represent our somatic sensory neurons whereas the palms of our hands are sitting in a more anterior or ventral position. The palms of our hands would represent our somatic motor neurons. But we also have. These knuckles right. Near the junction of our palms and our digits. Sot he knuckles would represent the visceral elements. With the visceral sensory elements right on the top of our knuckles. And the visceral motor elements just on the anterior or ventral aspect of the knuckles relative to this cross-section. Okay. So we've got somatic sensation, dorsal, somatic motor control, ventral, and then along the ridges formed by our knuckles we have visceral sensory and visceral motor. All right. Now let's take this conceptualization up into the brainstem. So what happens in the brainstem is that the ventricular system expands. In the spinal cord we have a central canal, a central space for the, flow through the spinal fluid, but in the brain stem we have a large fourth ventricle So essentially, this face expands in the following way. The sensory elements open up to form the floor of the fourth ventricle. With the digits now still representing somatic sensation, but now they are lateral, underneath the floor of the fourth ventricle. And where would we find our somatic motor cells? Well, we'd find them in the position of my palms. Near the midline, in the dorsal part of the tegmentum. Just underneath the fourth ventricle. Now, notice where my knuckles are. My knuckles are somewhere between the somatic motor. Region and the somatic sensory region. And that's where we find our visceral sensory and our visceral motor nodes. So again, a very simple neuro tube opens up to form a fourth ventricle with the sensory systems moving far lateral. All right, now that simple picture hopefully. Will stay with you. Now, if you did that with me, hopefully the significance will stay with you, and you'll have a picture of the developing brainstem. Where our somatic sensory elements, and we'll also add now our special sensory elements as we get into the cranial region, are to be found in a far lateral position. And just medial to those, we would find our visceral sensory. And special sensory systems relater to the visceral, specifically our sense of taste. So these sensory elements are lateral. Now more medial, we find the motor elements right along the. Midline of the tegmentum underneath the ventricle, we would find our somatic motor cells. And just between the somatic motor and the sensory cells, we'll find our visceral motor cells. Now, when it comes to the cranial region, there are different kinds of muscles. Right? So in addition to the smooth muscles and the striated muscles. There are striated muscles that are derived from somites and those that are derived from the branchiolmeric arches or the pharyngeal arches of the developing embryo. These include many of the structures around the cranial region and into the neck region and there are different motor neuorns that are growing out axons to innervate either the striated, somatic somite derived muscle. Or the striated muscles derived from the branchiomeres and those that are derived from somites are receiving input from our somatic motor cells. Which are found again right along this dorsal midline underneath the floor of the fourth ventricle. The motor neurons that supply innervation to the derivatives of the branchiomeres have migrated into an intermediate position. So these motor neurons initially sat very near our somatic motor neurons. But they migrate into a more intermediate somewhat more anterior and lateral position and as they migrate their growing axon is dragged down with it. So consequently we have motor neurons, such as those that supply the muscles of facial expression. That sit in this intermediate position and grow axons that loop around in this fashion. And in this case what I'm describing for you is the facial motor nucleus in the facial nerve.
