For this lab lesson, I want to show you the cranial nerves in a human brain specimen. So, there are 12 nerves that attach to the brain and the brain stem, and most of them will be visible in this specimen. but perhaps, we want to appeal to a model of the brain to show you one that often is lost in the autopsy room when we're removing the brain from the cadaver. but with that exception, let's begin with cranial nerve 1. Now, cranial nerve 1 is actually not present in the brain because it is a set of fine, fine axons that run from the nasal epithelium to this structure right here called the Olfactory bulb. And so, the Olfactory bulb is actually part of the brain, even though the bulb itself and its long connection that goes back to the posterior part of the frontal lobe looks very much like the nerve. It's really, it's not the nerve, it's part of the brain. It's derived in embryogenesis from the telencephalon. So, cranial nerve 1 would be those axons that run through the ethmoid bone through a region called the cribiform plate and make synaptic connections here with the Olfactory bulb. Cranial nerve 2 is the optic nerve, and that is a very prominent nerve here in the human brain. And you'll notice that there are, are two, in fact, there are two of each of these nerves. But, in this case, these two nerves seem to come together so this is the right optic nerve, cranial nerve 2, the left optic nerve. And where they joined, they form something of an x shape and that x shape is called the optic chiasm. So, just behind the optic chiasm is this stalk that extends from the hypothalamus down below the brain. Now, this might also look like a nerve, but it's not. This is the stalk of the pituitary called the infundibulum. And it's worth noting the proximity of this infundibulum to the optic chiasm. Unfortunately, one problem with the pituitary is that sometimes, it tends to grow tumors. And because the pituitary is encased in bone, should a term, tumor develop in the pituitary, the only place for it to grow is upward into the optic chiasm. So, sometimes, for that reason, visual field deficits are the first and earliest sign of a problem in the pituitary region. So, cranial nerve 2, the optic nerves that come together to form the optic chiasm. Now, in a different lesson, we'll talk about the blood vessels. Just behind what is a particular vessel called the posterior communicating artery, is a third cranial nerve. Here is the left third cranial nerve, and here's the right. This nerve is also called the oculomotor nerve. It supplies many of the axons that drive the movements of the eyes. Now, in order to see the fourth cranial nerve, we need to gently reflect away part of the cerebellum in brain stem so that we can peer deep down beside the pons and just below the temporal lobe. The fourth cranial nerve is called the trochlear nerve, and that's unusual for two reasons. One is that it's the only motor nerve that exits on the posterior or the dorsal side of the central nervous system. So, to see the trochlear nerve, we have to gently reflect away the cerebellum from the inferior surface of the temporal lobe. This fine little thread structure right here at the tip of my pointer, this is the trochlear nerve. The trochlear nerve exits on the dorsal aspect of the caudal midbrain, wraps its way around the brainstem and works its way towards the orbit, where this nerve is involved in controlling one particular muscle that moves the eye downward and inward. Now, the other unusual aspect of this trochlear nerve is that it's the only motor nerve that supplies muscle that is on the opposite side of the body from the source of this nerve. Which is to say, I'm holding at the tip of my wooden stick here, the right trochlear nerve. This trochlear nerve was grown out by motor neurons that sit on the left side of the brain stem in the trochlear motor nucleus of the caudal midbrain. So, that is cranial nerve 4. Cranial nerve 5 is a large nerve that penetrates right through the lateral aspect of the pons and this nerve has been cut fairly close to the pons so we don't have a long length to show you. But I think you can appreciate this tuft of nerve fibers that's right at the end of the pointer here that's penetrating down through the axons that connect the pons to the cerebellum. That's the fifth cranial nerve, the trigeminal nerve. Cranial nerve 6 is found right near the junction of the pons and the medulla oblongata. This, along with cranial nerves 3 and 4, is a nerve involved in moving the eyes. This is the abducens nerve. It's the most medial of three nerves that are present right along the junction of the pons and the medulla. So, that's cranial nerve 6. Cranial nerve 7 and 8 are out further laterally, still at the junction of the pons and medulla. Cranial nerve 7 is right here. This is called the facial nerve. It's one of our most complicated nerves and that it has both motor and sensory fibers associated with it that do a variety of functions that we'll describe in a different lesson. Now, just lateral to the facial nerve is cranial nerve 8, the vestibulocochlear nerve. And as its name implies, this nerve is involved in both vestibular funtion, related to our sense of balance, and auditory function, related to our sense of hearing. Nerve 8 sits in the lateral margin of this junction between the cerebellum, the pons, and the medulla. Just below nerve 8 are nerves 9 and 10. Nerve 9 is called the glossopharyngeal nerve, and here it is right here. It sits right below nerve 8. And just below the glossopharyngeal nerve are the nerve root [UNKNOWN] of the vagus nerve, which is cranial nerve 10. And we see a collection of these nerves that are emerging from the lateral margin of the upper medulla. Cranial nerve 10. Now, this particular specimen has been cut through the upper part in the medulla. And in the medulla is where we would find the nerve roots of our last two cranial nerves, nerve 11 and 12. So, to show you those nerves, I'm going to appeal to a model of the brainstem that I have, that will give you a sense of how those nerves relate to the medulla. In order to see cranial nerves 11 and 12, I'm going to appeal to the brainstem model, that I have here in my left hand. And I'm holding the brainstem model next to the actual brain, so that you can get oriented and recognize the divisions of the brainstem that are depicted here in this model. Near the middle, we have the pons, above it, you can see the cerebral peduncles which are a hallmark of the midbrain and then down below the pons is the medulla oblongata, which in the model, is depicted to extend into the spinal cord. So, let me put the human brain to the side and we can focus our attention on the medulla in the model so that we can observe cranial nerves 11 and 12. Cranial nerve 11 is this long flanking structure that emerges from the lateral side of the upper cervical spinal cord and the caudal part of the medulla. This is called the spinal accessory nerve. It's an usual nerve, in that, it includes contributions throughout the upper cervical spinal cord. And it enters the cranium through the foramen magnum only to loop back out through the jugular foramen. So, it may be somewhat questionable whether we should think of this as a cranial nerve, but the fact that it does enter the cranium is good enough reason to include it in our standard means of accounting for 12 cranial nerves. So, cranial nerve 11, the spinal accessory nerve. Cranial nerve 12, unfortunately, was not present in our brain specimen because there are tiny rootlets that often tear in the process of removing the brain from the skull, which was the case in our specimen. But in the model, cranial nerve 12 is represented by this series of rootlets that emerge right next to the olive which is this lateral bulge of the upper part of the medulla. And the medullary pyramid, this long column that runs along the midline of the ventral medulla. So, the proximity of the nerve rootlets of the hypoglossal nerve to the medullary pyramid is worth remembering. The medullary pyramids contain axons of the corticospinal tract allowing the motor centers in the forebrain to communicate with the spinal cord. So, right next to it are the rootlets of the 12th cranial nerve, the hypoglossal nerve, which is the nerve that allows us to protrude our tongue. This nerve innervates the intrinsic muscles of the tongue. Unfortunately, some patients have strokes or other kinds of brain injuries that impact this region in the medulla. And in such patients, there is a very predictable set of neurological signs and symptoms. Because of the corticospinal fibers in the medullary pyramid, those patients are weak on the opposite side of the body. But because of involvement of the hypoglossal nerve routes, those patients also have problems extruding their tongue. And specifically, the tongue will deviate towards the side of the injury because of the architecture of the fibers of the intrinsic muscles of the tongue. So, a deviating tongue and weakness on one side of the body are hallmarks of injury to this upper ventral medulla where the roots of the hypoglossal nerve penetrate right lateral to the medullary pyramid. So, this model allows us to account for all 12 cranial nerves. And just to show you one more time, we have the olfactory nerve connecting to the olfactory ball, that nerve is really in the skull. And then, nerve number 2 is the optic nerve. Nerve number 3, the oculomotor nerves, that lead just to the medial side of the cerebral peduncles. Nerve number 4 wraps around from the dorsal aspect of the caudal midbrain, that's the trochlear nerve. Nerve number 5 is the trigeminal nerve that penetrates right through these transverse fibers of the pons. And then at the junction of the pons and the medulla, we have three pairs of nerves. The abducens, the facial, and the vestibulocochlear nerves. Nerves 6, 7, and 8. And then, just below cranial nerve 8, we have the glossopharynageal nerve, nerve number 9. Just below that are the rootlets of the vagus nerve, nerve number 10. And then, coming up from the cervical spinal cord, is the spinal accessory nerve. And then lastly, the hypoglossal nerve, right between the olive and the medullary pyramid.
