. Welcome to today's tutorial on the cranial nerve nuclei. This topic pertains to one of our foundational core concepts in the field of neuroscience. And that is, the brain is the body's most complex organ. And we turn our attention in this lesson to one of the most complex regions of that organ, the brain stem. So my learning objectives for you in this tutorial are for you to be able to identiry the major subdivisions of the brain stem. And the spinal cord. Although our focus will be on the brain stem, primarily. And I want you to be able to do so in representative transverse cross sections. That, is, not just from looking at the outside, but from looking at the inside. And I want you to be able to have a discussion. Maybe with a family member or a friend or a classmate. And I want you to discuss the relationship between the cranial nerves, that I trust you have some familiarity with now, and the nuclei that we find within the brain stem. And so, what I'm referring to are the clusters of cells that receive sensory information from the cranial nerves or the clusters of motor neurons that grow out the axons that run through cranial nerves on their way to muscle. Okay. So let me just remind you of the basic organization of our sensory and motor systems. Beginning with our sensory system. So obviously we have information coming from the outside world and information coming from within our own body. That is feed into the central nervous system via our special sensory cells that we find in our special sensory organs like the eye, and the ear, and the nose, and the tongue. But we also have this vast sensory surface, our skin, as well as sensory cells that are growing out receptor endings in our muscles, in our joints, in our tendons. And all of this information informs the brain about the internal state of the body and the movements of the body through this environment. Well, all of this information is conveyed via axons that run in nerves. And those nerves are spinal nerves and cranial nerves. So these nerves are grown from cells. And those cells typically reside in ganglia, which are small clusters of cells just outside of the central nervous system. Now, the special sensory organs are a bit different, and we'll talk about them in due course. So when we focus on the cranial nerves now I want you to just have in mind a nerve which is very much like one of our spinal nerves. Except these nerves are attached to the brain stem. Alright, so these are sensory components that are fed up into higher levels of processing through pathways that we'll come to study one at a time over the next couple of weeks. So, that's the sensory side. There's also a motor component to most of nerves and most of our cranial nerves. And that motor component is concerned with activating skeletal muscles, and activating muscles associated with the viscera. That is, smooth muscles, the muscles of the heart, cardiac muscle, as well as the secretory cells in our glands. These effector elements are engaged by axons that grow out of hte central nervous system. And for skeletal muscle, for example, those axons are derived from motor neurons, that reside in the ventral horn of the spinal cord. And in a collection of nuclei in the brain stem. Now, the activation of smooth muscle, cardiac muscle, glandular tissue is just a bit more complicated. That gets us into a division of the motor system that we call the visceral motor system, or sometimes it's called the autonomic nervous system. And here, we have. A chain of neurons that connect the central nervous system to the effector cells that, do the contraction, be it in the heart or in the blood vessels or in the, viscera. So we'll talk about that topic in, a few weeks from now. And hopefully, the organization of that autonomic nervous system will be clear. For now, I just want you to get the big picture that the motor output is derived from, from two principle kinds of motor systems, a visceral motor system and a somatic motor system. And as we'll see, the somatic motor system can be further derived in terms of the embryological origins of the tissue that. Is innervated. Now what we're leaving out from this diagram of course, is the wonderful integration of sensation, and the production of commands from motor output that takes place at higher levels of processing. Namely in the cerebral cortex and in circuits that run through sub cortical structures like the basal ganglia, the thalamus and even the cerebullum. Well, all of the sensory motor integration will be the topic of forthcoming tutorials so, so hang tight on that. Alright so, with that as the background lets now focus in on the cranial nerves themselves. And the brain stem. So, I will remind you of what the surface features of the brain stem look like. First beginning with the basic subdivisions of the brain stem. The brain stem is conventionally divided into a mid brain, pons, and the medulla oblangata. All of which sits between the forebrain, which would include the Thalamus and parts of the basal ganglia. Which is basically what we have from about this region on in the anterior direction and the superior direction, and then below the madela we have the spinal cord. So the brain stem is an intermediate part of the central nervous system between forebrain and spinal cord. It's a very complicated place. And it includes most of what we call the hind brain. From embryological development of the nervous system. So the fore brain sits the anterior and superior. The hind brain is the brain stem, and the spinal cord is inferior or cottal to the brain stem. Okay, now let's look at the ventral surface of the brain stem and just quickly review the cranial nerves. And they're laid out for you in this figure from the textbook that we refer to. I would also refer you to my tutorial where I actually showed you the surface of the human brain stem in the lab. And I also have with me today the same model of the brain stem that we used. And, I think, rather than going through this figure, I'll just show you quickly this model. And if you feel like you're good on the brain stem and the superficial aspects of it, feel free to skip over this part. But otherwise let's just take a few minutes and review the surface features of the brain stem. Well, I trust that you all can see this, and this brain stem is also produced for you and fully annotated and movable within Sylvius software. So if you have access to Sylvius, you might want to get that out and follow along, and click along with me. Alright, so what we're looking at is the ventral surface of the brain stem. This resembles the drawing that we had in the previous slide in the upper right. So, from about this level on up is actually forebrain, so what we're looking at here, these large egg, egg-shaped structures, that's actually part of the basal ganglia. Okay, and the hypothalamus and the thalamus on the dorsal aspect. Which I'll show you more in just a moment. So the brain stem would be this region right in here. So we have the mid brain, pons, the medulla oblongata. And just below we have spinal cord. And I want you to notice the cranial nerves that are visible here in this model. Well, we've been talking mainly about the brain stem, so we're concerned with cranial nerves 3 through 12. Cranial nerve one, the olfactory nerve, is not present in this model, and it doesn't attach to the brain stem. Neither does cranial nerve two, the optic nerve. However for completeness, we do have the optic nerve here in this brain stem model. And the nice thing about this model is that you can follow the optic nerve to the chiasm and then we can turn the model around and note its progression from the optic tract right to the posterior. Pole of the thalamus, a region called the lateral geniculate nucleus that we'll talk about in a later session. Well, that's cranial nerve two. As I said, it does not attach to the brain stem so we won't be talking about it further in this tutorial, but let's find the rest of the cranial nerves in this model. So, cranial nerve three This right here. That's the oculomotor nerve, it's the nerve that emerges from the space right between these massive stalks that define the ventral surface of the midbrain. These stalks are the cerebral peduncles. The space between them is the interpeduncular fossa, and Cranial nerve three, the ocular motor nerve, emerges right in that fossa. Okay, cranial nerve 4 is a bit unusual. As you may recall, it's the trochlear nerve. It's unusual because it emerges from the dorsal aspect of the brain stem. It's the only motor ax, the only motor nerve to do so. All the other motor nerves exit the ventral. And lateral regions of the central nervous system. But here's the trochlear nerve. You can see it's a fine little thread that wraps around the posterior lateral aspect of the quarter mid-brain. And then joins up to make its way to the orbit, with the other ocular motor nerves. So trochlear nerve, nerve 4, nerve 5, is this very large protruding nerve, very obvious nerve. It plunges right through the fibers of the pons, the transverse fibers of the pons, so it's a very easy nerve to recognize in a whole brain specimen or in our illustrations. So that's the trigeminal nerve, nerve 5. Now next we come to the junction of the pons and the medulla, and along that junction are three pairs of cranial nerves. From medial to lateral, they are the abducens nerve, nerve 6, the facial nerve, which is the seventh cranial nerve, and then the vestibulocochlear nerve. The eighth cranial nerve. So 6, 7, and 8. If you notice where cranial nerve 8 is, just below it are the nerve roots, nerve rootlets of 9 and 10. So we have the nerve rootlets of the glossopharyngeal nerve, nerve 9. The rootlets of the vagus nerve just below the glossopharyngeal nerve. So the vagus nerve is cranial nerve 10. Cranial nerve 11, is a series of rootlets. Which, in the model, it looks like these wings that come off on either side of the, Lateral medulla, but really there's a series of rootlets that join up to form the spinal accessory nerve, cranial nerve 11. This nerve then enters the foramen magnum, and then loops back out through the jugular foramen. So its source is actually in the upper part of the cervical spinal cord, but we consider it a cranial nerve nevertheless because it does enter the cranium. And then finally nerve 12, the hypoglossal nerve are these fine little rootlets here that emerge between the medullary pyramid along the mid line and the olive. A bulging structure in the ventral lateral aspect of the medulla. So there's the hypoglossal nerve, nerve 12.
