Hi, I'm Dr. Jim McDevitt at the Baylor College of Medicine, and I'm going to spend just a few minutes with you today talking about brainstem neuroanatomy in this brand new online Zoom format that we are becoming increasingly familiar with in these days. So I'm going to share my screen, and just to introduce this topic, this is really one of what I hope is sort of a trio of brainstem activities. One is getting the anatomy roughly in the right place, that's what we're going to talk about today. I think the second will be to think about intranuclear ophthalmoplegia and the wiring of the INO. And third, maybe eye findings. I think if you can do those three things well, you've got about 90 to 95% of brainstem anatomy really sort of nailed. So today we're going to talk about brainstem anatomy, and this is a trick that I didn't invent this, I found bits and pieces of it, and you all are probably familiar with some of this. I published it some time ago. But it served me well, I learned it many years ago, I started teaching it to residents and they seemed to like it, it seemed to resonate. So I hope you'll find it useful as well. So what we'll do is understand the basic anatomy of the brainstem and particularly where the major tracks are and where the nuclei are located. I'm going to teach you a mnemonic device to keep track of this. So to briefly review the surface anatomy, the brainstem has three components, the midbrain, the pons, and the medulla. Pons is the biggest part of the structure. You also notice the cerebellum that sits behind the brainstem is massive compared to the brainstem. And that's important as you think about the arterial supply. Remember how big that cerebellum is compared to the brainstem. It's important when you look at how the arteries are named, they're really named for the structure they serve and the cerebellum being the big man on campus gets most of the blood supply. But four major vessels, the posterior cerebral artery, the superior cerebellar artery, again, going to the cerebellum, but also to the upper pons, the anterior inferior cerebellar artery, again going to the cerebellum, but also to the pons, and the posterior inferior cerebellar artery going to the medulla. So if I superimpose the anatomy or superimpose those vessels over the actual anatomy, you see the posterior cerebral artery up in the region of the midbrain. You see the superior cerebellar and the anterior inferior cerebral cerebellar artery servicing the pons, and the pica, the posterior inferior cerebellar artery servicing the medulla. Now in fact, this is sort of like the spinal cord. Those are the major vessels, sort of like the greater auricular artery of the demchoids in T10, but there are also a number of little perforating arteries up and down the brainstem that are a substantial part of the blood supply. So that's the main blood supply of the brainstem. So before we get on with the exercise, I want you to do a pre-test, and what I'd like you to do is, and you're looking at me online, so you can do this, pause me, go and get a piece of paper and a pencil, if such a thing still exists in your office, and I want you to draw something. And what I'd like you to draw is a simple grid, and I'll draw it with you. And what this simple grid represents, and you can do this at home, again, you've paused and now you're back and you've got your piece of paper and a pencil, this represents the midbrain, the pons, and the medulla, three main sections of the brainstem. And furthermore, if you think about the midbrain and the pons and the medulla, we tend to think of them in terms of lateral structures and medial structures. So this being lateral, and this being medial. So this is what we're going to try and learn, is where do we put the different cranial nerve nuclei? And go ahead and you can pause me again. Now it's only 10 of them, there are 12 cranial nerve nuclei, but keep in mind that number one, the olfactory, that's not in the brainstem, so we can kind of cross that one off the list. And the optic nerve is also not in the brainstem. So one and two are a gimme. So three through 12, let's put them in the right place, and I want you to at least get them into the right anatomic location, midbrain, pons, and medulla, and put them in the right location, medial or lateral. So pause me right now, go ahead and fill that out, and then come back. I'm sure everyone followed those instructions to the T, so you've now been off filling in your nuclei with your piece of paper, and let me see if I can do this with you now. So I'm going to fill mine in, and let's see, I think I'm going to put three here, and I'll put four kind of right here, and then I'll put five out here laterally, and there's a little bit of a nuance to five that we'll talk about in a little bit, and I guess we'll put six over here medially, and maybe seven out here, and eight, and we'll put nine over here, nine, I'm sorry, that's eight, we'll put nine down here, 10, 11, and 12. So I think that's roughly right, and then we'll see how I sort of remembered that, that's sort of the goal of what we're going to do here today. That's our pre-test, we'll do that again when we're finished. So two relatively simple rules, one is the rule of five, the rule of five tells you whether it's in the midbrain, the pons and the medulla, there's a rule of 12, which will localize the nuclei laterally or medially, and then there's a rule of MS, which is how we're going to remember where the long tracks are, we didn't do that in our pre-test in the interest of time. So let's talk about the rule of five, again, these are sort of silly rules, but the rule of five is actually a rule of Roman numeral five, it's this V is the key, and the rule is that if the cranial nerve has a V in its name, then it's in the pons, very, very simple. So four through eight are all pontine nuclei. Now if you're going to split hairs, or split neurons in this case, four is right at the midbrain pontine junctions, that's one semi-exception to the rule, but it's certainly there in the upper pons, so four through eight, so that's the rule of five, really, really simple. Rule of 12, equally simple, you have to be able to do basic math, rule of 12 is that anything that is 12 or a factor of 12 is midline, so that's three, four, six, and 12 raw midline nuclei. And then if you put the rule of five and the rule of 12 together, knowing only those two bits of information, you can put everything in the appropriate quadrant, medial, lateral, and the three main tracks. And then think about how they act, do they act on the side where the nuclei is located, or do they act on the opposite side? And the cranial nerves are pretty easy, they all act ipsilaterally, with the exception of the abducens nerve, with cranial nerve four, which crosses, and when eventually we talk about intranuclear ophthalmoplegia, that will make a little more sense why the abducens nerve has to cross. So again, really, really simple rules. The rule of MS, let's talk about the tracks now, which are also pretty simple, and we'll start with the medial tracks, that's in the rule of MS, M is for medial, and the trick here is that all the major longitudinal tracks of the brainstem that are on the medial side of the brain, well, happily, they all start with the letter M, doesn't that make it easy? So the first is the motor track, and I guess I apologize, that's actually the corticospinal track, but my mnemonic doesn't work if it's corticospinal track, but it is the motor track, so it starts with M. The medial meniscus, vibratory sense, positional sense, and the medial longitudinal fasciculus, which helps to coordinate eye movements, and again, is important in the development of an intranuclear ophthalmoplegia. So three tracks, a little bit more complicated to remember which act ipsilaterally and which act contralaterally. I don't have a great way to remember that, the way I do is I think about the normal parts of the typical neurologic examination, and those routine parts of the neurologic examination, they cross. The sort of parts of the neuro exam that are out there that you don't do routinely, they tend to work ipsilaterally. So the motor track, for example, you wouldn't do a neurologic exam without doing a motor exam, that nuclei crosses. The medial meniscus, you probably shouldn't do a neurologic examination without testing position sense or vibratory sense, that crosses. The medial longitudinal fasciculus, MLF, you could do a neuro exam and skip that part, that's more of a specialty part of the exam. So that's the rule of M, motor, medial meniscus, medial longitudinal fasciculus. The rule of S, and again, sorry to keep cheating, but S is, it'd be great if they were L's, but S is for side. So rather than lateral, we're going to talk about side nuclei, and there are three of them as well. The spinothalamic track, responsible for pain and temperature sense, the sympathetic track, and the spinocerebellar track. And again, do they act ipsilaterally or contralaterally? Well, you would certainly check pain on a pin sensation on a neuro exam. So that's the spinothalamic track. You may not check sympathetic function, that's ipsilateral. Technically you probably should check cerebellar function, but that's ipsilateral. So again, not a great way to remember that, but roughly that gets you in the ballpark. So six tracks, three medial, three lateral, or side. So as you start thinking about how all those things work together, let's work through a couple of clinical examples. We see a patient. That patient has an eye that is down and deviated laterally, down and out. And on the opposite side of the eye finding, they have a hemiparesis of their face, arm, and leg. Well, knowing what we know now, we can think about the nuclei and the tracks. Well, down and out, that's a third nerve palsy. You're probably going to have a blown pupil as well. Where is that? That's up here in the medial aspect of the midbrain. We've got a hemiparesis, face, arm, and leg. That's what? It's a motor track. The motor is medial, that's a medial track. And that acts ipsilaterally. So we're going to have a lesion that is a third nerve on the side of the lesion affecting the motor track, which is then going to cross. Where is that? It is in the medial midbrain. And this is the creatively and aptly named medial midbrain syndrome. Another example, we've got a patient with nystagmus, vertigo, and nausea. Well, instantly, you would say, well, that must be cranial nerve eight. That's easy. They have facial paralysis. So involvement of the facial nerve, cranial nerve seven, has a V in it, incidentally. They also have decreased facial sensation. The trigeminal nerve also has a V in it. I neglected to mention before the unusual thing about cranial nerve five, the trigeminal nerve. The motor section of five, the muscles of mastication, is solidly in the pons. There's actually a separate nuclei, the sensory nucleus, which is reflected in this picture. You can see it, VS. That is a long, hot dog-shaped nucleus that actually extends down into the medulla. So it's pontine and medullary. But in this case, decreased facial sensation, lateral pontine. So you've got three nuclei involved in the lateral pons. And what else do we have in this particular syndrome? We've got decreased pain and temperature sense. Well, what is that? That's the spinothalamic tract. Spinothalamic, S, is a side tract. So this is going to be also lateral. We've got nuclei laterally that are in the pons. We've got a tract involved laterally. So what is this? It is lateral inferior pontine syndrome. Again, pretty straightforward. And we'll do just one more. In this case, we've got paralysis of half of the tongue, so deviation of the tongue, and paralysis of the arm and leg, and decreased proprioception. So what is the tongue motor movement? 12, cranial nerve 12, which acts ipsilaterally. Paralysis of the arm and leg is going to be, again, the motor tract, which is medial. And decreased proprioception is the medial meniscus, which is also an M, medial, also on the medial part of the brainstem. So we've got a nuclei involved in the medulla. We've got some tracts that are both medial. And this is medial medullary syndrome. So again, pretty straightforward. When I'm seeing patients, I draw that grid sometimes. I put the cranial nerves down. I put little Xs through them as I'm working through it if it's a complicated patient. And if you do this routinely, it gets you where you need to be about 90% of the time. It's a really good way of having a decent knowledge of brainstem anatomy. So that is our pretest. This is actually a post-test. I'd encourage you, again, we're not going to do it now, but draw your grid. Test yourself. Make sure you get everything in the right place. Put your tracts in the right place. And further, I would encourage you to sit down with somebody else, teach them the trick. You'll find you muddle it up a couple of times before you get it right. But I think if you practice a little bit, it'll serve you well, and you will be much more confident in your ability to manage cranial nerve nuclei. Thanks for your attention, and stay well.

brainstem neuroanatomy

midbrain

pons

medulla

cranial nerve nuclei