Okay. Now, I think we're ready to apply this embryological framework to understand the organization of the cranial nerve nuclei in the brainstem. So this is a chart you might want to have in front of you for a little while, so that you can begin to make sense of all of this new anatomical information that I'm sharing with you in this tutorial. So what this chart does is it organizes the brainstem according to embryological division, that is midbrain, pons, and medulla. And also, organizes across are various ways of breaking down motor outflow and sensation according to our embryological framework. So we have six columns in this chart. We have three to the left of center for motor control and three to the right for sensation. And for motor control, we recognize a somatic motor division, a branchial motor division, and a visceral motor division. So here is how I want you to think about this. I want you to think about each embryological division of interest for motor control and for sensation, and consider what's in the midbrain, what's in the pons, what's in the medulla? So, for example, let's think about the motor control for the muscles that are in the cranial region that are derived from cell mites. And it's fairly simple. It's the muscles that move the eyes in the orbit and the muscles that move the tongue. So the muscles that move the eyes are related to cranial nerves III, IV, and VI. Cranial nerves [inaudible] and IV are found in the midbrain. Cranial nerve VI attaches to the junction of the pons and the medulla, but the nucleus that grew out those axons is found in the caudal pons. The axons that move the tongue are derived from the hypoglossal nucleus, which is found in the rostral part of the medulla, so we have a hypoglossal nucleus. So one way to approach this challenge is to consider how the outside of the brainstem is related to what we find on the inside. So, again, taking our somatic motor nuclei as an example, we have the nerves that move the eyes, the oculomotor nerve, trochlear nerve, and the abducens nerve. Two nerves associated with the midbrain. One associated with the caudal pons. Up here in the midbrain, we have our oculomotor nucleus growing out these axons to move the eyes. The trochlear nucleus growing out the axons in the fourth nerve. And the abducens nucleus, growing out the axons in the abducens nerve. Now the muscles derived from the pharyngeal arches we call those branchiomeres or branchiomeric muscles and the motor nuclei that innervate those muscles are called the branchial motor nuclei. And, we just think about what are they. Well, they are the muscles of mastication, that is the muscles involved with chewing. They are innervated by a motor component of the trigeminal nerve. There's the muscles of facial expression that are innervated by the facial nerve. So, these axons are grown out by the facial motor nucleus so both the trigeminal motor nucleus and facial motor nucleus are going to be found in the pons. Now, a little less obvious, but important nevertheless, are the muscles that are deep within our neck region, around our larynx and our pharynx. Those muscles are controlled by components of cranial nerves IX and X. The glossopharyngeal nerve and the vagus nerve. And the axons that are governing these muscles deep in the larynx and the pharynx are grown from neurons in what's called the nucleus ambiguus for reasons that I think will be obvious to you shortly. And then finally, we have our spinal accessory nerve. The spinal accessory nerve innervates also muscles derive from the pharyngeal arches that are involved in turning the head and shrugging the shoulders. That would be the sternocleidomastoid muscle and part of the trapezius muscle. Now, let's move on, and think about our visceromotor elements. So, within, within our eyes, we have a smooth muscle in the iris that constricts the pupil. And that constrictor function is derived from parasympathetic neurons that send their axons out through the oculomotor nerve. And these parasympathetic preganglionic axons live in a small nucleus that sits right on top of the oculomotor complex. That nucleus is called the Edinger-Westphal nucleus. So that is our most superior visceral motor nucleus in the brainstem. Deep in the pons, and we won't see them here in this view from the outside, are the nuclei that are responsible for the secretion of saliva and also the secretion of tears from the lacrimal gland. So we have these salivatory nuclei that sit deep in the pons and they send axons out through cranial nerves VII and IX to their respective targets in the orbits, and in the, oral pharynx. Also in the junctional region near the pons and the medulla, we have the neurons that are associated with cranial nerve X. The vagus nerve that control the visceral that are found in the thorax and the upper, upper part of the abdomen. So these axons are grown out by the nucleus ambiguus and by the dorsal motor nucleus of vagus. Now notice that we've named the nucleus ambiguus twice, and I don't want you to be confused about why that is the case. There is some historical reason why we call two different sets of neurons that convey two different modalities and motor control part of the same nucleus. But they are not the same cells that supply the pharynx, larynx on the one canned for the branchial motor control. And the cardioinhibitory output that slows the heart via the vagus nerve on the visceral motor side. So, two different populations of neurons that sit within the same nuclear complex of the upper part of the medulla. Moving on now to the sensory side, we have our general sensory signals that are coming from the skin surfaces of our face and our head and some of the deeper structures associated with the muscles of our face. These are giving rise to signals about, like touch, about movement, about pain, and temperature sensitivity and all of this information is conveyed to the central nervous system across four cranial nerves, cranial nerves V, VII, IX, and X. And all this information is fed into that long trigeminal nuclear complex. And if I look again at the brainstem model, we see a long nucleus that seems to just snake its way down from the midbrain or the mesencephalon through the pons and into the medulla. That's our trigeminal nuclear complex. And so the mesencephalic portion of it, that is the midbrain portion of this, this is where we find our movement-sensitive somatic sensory signals related to the movements of muscles in our face, especially the movements or our chewing muscles and the movements of the temporal mandibular joint. The sensory signals that are derived from our skin surfaces pertaining to light touch and other aspects of mechanical sensation are fed into what's called the principal nucleus of the trigeminal, which is that large, bulbous structure in the pons. And these signals are derived mainly from the trigeminal nerve, but also a small contribution related to a little patch of skin in the external ear that's conveyed across cranial nerves VII, IX, and X. And then lastly, we have the spinal trigeminal nucleus. And this is the inferior or caudal extension of this nucleus that receives pain and temperature information. We'll talk much more about both of these two preceding divisions of the trigeminal system as we get into a discussion of mechanosensation and pain and temperature sensitivity. Okay. Well, this leads us then, with a brief consideration of our special sensory systems, and then, our visceral sensory systems to our special sensory systems that are connected to the brainstem. Are those related to our sense of hearing or auditory system and our sense of balance being derived from the vestibular system deep within the petrous temporal bone. So both of these two special sensory systems are sending signals to the brainstems via the eighth cranial nerve. And those signals, diverge once they enter the brainstem into different sets of nuclei. There's a set of nuclei related to hearing, called the cochlear nuclei, and then, a set of nuclei related to our sense of balance, equilibrium. Basically, the accelerations of our head and the static positions of our head. That's the vestibular nuclear complex of the brainstem. Both of these nuclei are found near the junction of the pons and medulla. And finally, we have our visceral sensory signals that enter the brainstem via cranial nerves VII, IX, and X. We can divide them into special visceral sensory, meaning, basically our sense of taste, and that sense innervates the rostral part of this nucleus of the solitary tract. The other aspect of visceral sensation is more of a general sense of visceral sensation that is conveyed to the same nucleus, but to a caudal division of it. So the nucleus of the solitary tract is all about visceral sensation. I'll just put this table in front of you again which runs through the cranial nerves and gives you a sense of where the nuclei are to be found. And what they are as they relate to these cranial nerves and also says a word about how one might think about testing the function of these, these nerves. So you might want to take a moment at some other point, come back to this table and maybe try to do some of these tests on a friend or a family member. They're pretty easy to do, they don't really require any special equipment and if you just think about the actions of our muscle systems and the activities of our sensory systems, I think you can very readily figure out how a clinician might go about testing the integrity of the function of each of these cranial nerves. And, if you want to see how a clinician might do this, I would refer you to an excellent website that is, maintained by a fantastic neurologist and author, Dr. Hal Blumenfeld. That website is neuroexam.com, so check it out. You'll see lots of cranial nerves, being tested and the standard way the neurologist would do it, so, I would encourage you to have a look at that, neuroexam.com. Okay. Finally, we are ready to actually look at into the brainstem and find these cranial nerve nuclei. So, we can do that by looking at some cross sections and the figure in front of you here shows cross sections through various levels of the brainstem, beginning at the top of the brainstem with the mid brain. So, in a section through the midbrain, we should be thinking about, okay, what are the nerves that connect to the midbrain again? Remember, nerve III, nerve IV. So nerve [inaudible] is in a more superior position. And that's what we find in this first slice. We find the oculomotor nucleus and the Edinger-Westphal nucleus, both growing axons out to the third nerve. Now, not illustrated here because we'd have to be right at this level, would be a view of the truncular motor nucleus, but we'll see that in a few minutes. Now, below into the pons, at two different levels of the pons we first find the principle trigeminal nucleus. And the trigeminal motor nucleus sitting right next to one another at the level at which the trigeminal nerve, actually, sends its axons into the brainstem. So that's the level at which we have this slice number two. Slice number three is just below it, and the purpose of catching this slice is to show you the facial motor nucleus, which as a brachial merric motor nucleus, it is just a little bit off of the dorsal aspect of the tegmentum. Because of the anterior or ventral migration of these motor neurons in development. So that's where we find our facial motor nucleus. We also have some nuclei associated with sensation here, the spinal division of the trigeminal nucleus. In red here, we have the somatic motor nucleus, the abducens nucleus. And then lastly, the vestibular nuclei that we're beginning to see as we get into the caudal part of the pons. Now, we go just a little bit further down near the junction of the pons and the medulla. We find additional vestibular nuclei in about the same position, but we also see the cochlear nuclei, which seem to sort of cap the dorsal lateral aspect of the upper medulla. We're continuing to progress through the caudal medulla. So we would expect to see the spinal nucleus of the trigeminal complex, which we have here in this blue shape. It's extending all the way down through the medulla, and in fact, will seem to merge with the upper part of the spinal cord as the spinal nucleus of the trigeminal complex merges with the dorsal form of the spinal cord. Well, there are a number of other nuclei to note here in the tegmentum of the medulla, all in that dorsal region. And again as we, as we look at this, I want you to recognize that the embryological framework helps us to interpret this. So right along the dorsal midline, we're going to expect to find a somatic motor nucleus, that's were we find the hypoglossal nucleus. Now just to the lateral side of that somatic motor nucleus. We might expect to find a visceral motor nucleus and we do. We find the dorsal motor nucleus of vagus. Alright, now just lateral to our motor nuclei, we should expect to get into sensory nuclei. And indeed we do. First we find the nucleus of the solitary tract, which is found right there in purple shade and just lateral to it we find our special sensory nuclei, the vestibular nuclei. So, again, motor nuclei are medial, sensory nuclei are lateral. And then, finally, in the far lateral region, that's where we find our trigeminal system. Okay, and finally down in to the section number six that we see here in this illustration, we have the caudal part of the medulla getting close to merge with the upper part of the spinal cord. And we essentially see the same set of cranial nerve nuclei just in a slightly different positions. And some are reduced in size, with the exception of the spinal trigerminal nucleus that continues to be a prominent feature of the medulla throughout its entire link. Okay, so, what I like to do with you next is to get into Sylvius and show you what these nuclei actually look like in histological sections taken through the brainstem. So, if you're ready for a break, this would be a good time to pause and open up Sylvius and, get ready to, perhaps, go back and forth between the remaining segment of this tutorial. And Sylvius, should you have it. If you don't have Sylvius, that's fine. Just refer back to my tutorial notes, as well these images, and this movie file, and follow along as I give you a tour of the brainstem through our histological atlas that we have in Sylvius.
