 Well, this is supposed to be a model of the brainstem with the cranial nerves. Some people say it looks like a voluptuous dancing girl. Other people say it looks like a toy for their dog. However, the pharmaceutical companies maintain that this is how neurologists can learn their cranial nerves. There are 12 cranial nerves and it takes about 12 years to learn all 12 of them until it's automatic. I don't know why 12 nerves can be so complicated. But what we're going to try to do is to take some of the mystery out of this model and help you associate three things. The level of the brainstem, the cranial nerves that supply it, and its blood supply, and reflexes to be tested. So we have a medulla, we have a pons, we have a midbrain, and all of this can be ignored because this is part of the thalamus. So midbrain on the ventral surface by the cerebral peduncles, which we'll come to in another time, the pons on the ventral surface where the basilar artery was running, and the medulla where the vertebral artery was running. So here's the first little piece that you need to memorize. There are 12 cranial nerves. There are two cranial nerves associated with the short little midbrain, cranial nerves three and four. There are four cranial nerves associated with the pons, cranial nerves five, six, seven, and eight. And there are four cranial nerves associated with the medulla, nine, ten, eleven, and twelve. Blood supply to the medulla is the vertebral, blood supply to the pons is the basilar, and the blood supply to the midbrain is variable from the posterior cerebral to the superior cerebellar. If we turn it over, it gets more complicated than we need, so let's now look at the real brain. Cranial nerves one, the olfactory nerve, and two, the optic nerve, are not true cranial nerves. They are actually, embryologically, outgrowths of the brain. Their central nervous tissue, they will not regenerate, and while they are called cranial nerves one and two, they have their own system of connections and are not usually discussed when we talk about cranial nerves of the brain stem. So let's start with the midbrain. What you're looking at here is the midbrain. Cut. Before what we could see were these two bands of fibers on either side, the cerebral peduncles, they can be seen here and here. These cerebral peduncles, according to Dr. Renner, look like Mickey Mouse ears when seen in this position, but the midbrain with the ventricular aqueduct here and the substantial nigra and the cerebral peduncles, these is a classic appearance of the midbrain. Sometimes we look at it this way anatomically and sometimes we look at it this way radiologically. So the trick is eventually to be able to turn things around in your mind, but we're not going to worry about the day. We're just going to look at the cranial nerves associated with the midbrain. And the first one is the third cranial nerve. It's missing on the other side. It emerges between two vessels, the posterior cerebral artery at the top of the basilar and the superior cerebellar artery. It's very important. The third or ocular motor nerve for visual reflexes, pupillary constriction and the curvature of the lens of the eye for sharp accommodated vision. The other cranial nerve associated with this level, the midbrain is this fine, small, tenuous nerve called the trochlear nerve. This is the fourth cranial nerve. It's the only cranial nerve to come out on the dorsal surface of the midbrain. Inside the main brain it crosses, but we don't need to worry about that. And it supplies one rinky dink muscle called the superior oblique muscle. So this right trochlear nerve goes to the right superior oblique muscle. And clinically it's not particularly important. Whereas the third nerve, the ocular motor nerve, supplies all but two of the eye muscles and the eyelid. And it's important clinically because it's easy to recognize deficits associated with this cranial nerve. So cranial nerves three and four, ocular motor and trochlear, midbrain. Next we go to the pons. We've taken the vessels off so you can more clearly see the cranial nerves five, six, seven and eight. Let's start with five. Five is the trigeminal nerve. It's the largest of all the cranial nerves, has the most axons in it because our face, our lips, our nasal cavities, the inside of our mouth, highly innervated by the trigeminal nerve. It comes in right in the middle of the pons. So it's sensory from the face and other structures in the head. And it is motor to the muscles of mastication. You have four muscles for chewing the masseter, the temporalis and the teragoids. We don't need to remember that except that you can test those muscles. And if they're not working, you suspect this trigeminal nerve, nerve number five. Nerve number six has a lot in common with the trochlear nerve number four in that it goes to only one muscle, the lateral rectus muscle for moving your eye laterally. All right. And the seventh and eighth nerves come out here in this region here. See, there's quite a bit of space here called the cerebellopontine angle. All right. The cerebellopontine angle. There's a lot of cerebral spinal fluid in there. There's a fair amount of space. And in there are two very important cranial nerves. This is the fifth. Let me pull that aside. The more medial one is the seventh nerve. It's called the facial nerve. And it goes along with the eighth nerve or the auditory and vestibular nerve, the vestibulococlear nerve. The two of them travel together into the temporal bone, into the internal auditory or myotis, the seventh nerve goes to the muscles of facial expression. You're smiling muscles. You're wrinkling the forehead muscles. You're kissing muscles. You're whistling muscles. And the eighth nerve goes to the semicircular canals and to the cochlea for balance and for hearing. They can be seen very nicely. So we have five, six, seven and eight. Those are the cranial nerves associated with the pons. Next, we have the medulla. The medulla, you can see stretching from here to here. And it has cranial nerves, nine and 10. You can see all these little fascicles here. If I kind of separate them and pull them apart, these are cranial nerves, nine and 10, the glossopharyngeal and the vagus. I can't tell you which branches are going to which. The glossopharyngeal is sensory for taste. It's also motor to one muscle. And the vagus is also sensory and motor to the autonomic nervous system. I might mention that the seventh nerve also carries taste. So taste is carried in seven and in nine and a little bit of 10. But clinically, taste is not nearly as important as hearing and vestibular function and facial expression and sensory from the face and muscles, amastocation or eye muscles or the control of eye muscles. So that's nine and 10. What happens to 11? 11 is torn off of every single brain I have because it has a unique trajectory that comes up through the pharyngeal magnum. And it's called the spinal accessory because it has part of its origin in roots from the upper cervical spinal cord and others from the medulla. And then it takes a crazy course after coming up into the cranial vault. It goes back out and all that can best be seen in the following diagram, but this cranial nerve is not particularly critical and is easily diagnosed by a slumping shoulder because it supplies the sternocleidomastoid and trapezius. The 12th cranial nerve cannot be seen either. But it emerges in a groove in the medulla between the band of fibers here, which I'll call the pyramid, and this bulge here, which I'll call the olive. And in this groove along here, outcome the multiple little roots of the 12th cranial nerve or the hypoglossal nerve. And that too can best be seen on a diagram.