 Today we're going to tackle the brain stem. This French paper mache, absolutely worthless model, could really confuse you and make it totally incomprehensible. And in addition, the key to all the numbers, of course in French, has been lost over the years. But we have a simpler way to look at the brain stem. And there are only three or four levels that you need to understand and associate with tracts and with cranial nerves. So let's throw away this old camel's foot and let's bring out the real brain stem. Here we have it. And we have all these cross sections that I've fished out of the bottom of the bucket. All of these brain sections. And right now they look like total mystery. Total mystery. By the end, you're going to be able to sort these out and know what part of the brain stem, this is the brain stem, they came from. So we have three regions to look at. The medulla as you see here in the mid-satrial section, the pons, and the short midbrain. And there are four cranial nerves associated with the medulla, four with the pons, and two with the midbrain. Three, four, five, six, seven, eight, nine, ten, eleven, twelve. Let's look at them and where they have their origin or termination in the brain stem. So now we're going to talk about the cranial nerves associated with each level of the brain stem. And we're also going to try to focus on what vessels supply that region of the brain stem. So here I have a brain stem that has been removed from the brain. This is the ventral surface, and this is the dorsal or superior surface. And the front end here is the midbrain. And back here is the connection with the spinal cord. But let's first look at this nice brain that has the cranial nerves, just to remind you because the cranial nerves are so difficult. Now we're not going to talk today about the olfactory bulb and nerve that come in because it comes into the cortex and not to the brain stem. Neither are we going to discuss the optic nerve, which also has connections to the thalamus in the cortex without going through the brain stem. Instead, we're going to focus on cranial nerves three, four, five, six, seven, eight, nine, ten, eleven, and twelve. This brain has almost all of them. It has a very nice third cranial nerve, ocular motor nerve that comes out between the superior cerebellar artery and the posterior cerebral artery. And there's one on each side. And these are very important for controlling your eye muscles and for the pupillary response and the rounding up of the lens for accommodation and for keeping your eyelid up so you can see out. That's cranial nerve three. Cranial nerve four comes around from the backside and you can see it nicely here. This is midbrain. And here is the only cranial nerve that comes out on the dorsal surface and it curves around to join up with the third nerve and going out into the orbit. But it is rinky-dink and small because it supplies only one muscle. Now the fifth, sixth, and seventh, and eighth nerves associated with the pons, here is the large fifth or trigeminal nerve. It is both sensory and motor. It's motor to the muscles of mastication and it's sensory from the head and the face. Cranial nerve six can be seen on either side. It's another nerve that goes to an ocular muscle, an extraocular muscle, here's the other one. The abducens nerve goes to the lateral rectus and when it's not working, you have problems with gaze to the lateral side of the nerve involved. And notice it's right on either side of this large basilar artery associated with the pons. Cranial nerve seven and eight are over in the cerebellopontine angle. I'm going to pull away this vessel. Here is cranial nerve seven and out here is cranial nerve eight. You can see them over on this side, cranial nerve seven and eight. And seven is very important for the muscles of facial expression. And if you have a paralysis due to a damage to this nerve, your face will look asymmetrical and certain reflexes will be interfered with. For example, the cranial nerve five, sensory from the cornea, has a reflex that sends axons out over the seventh nerve and this is the corneal reflex and so that would be interrupted and you wouldn't blink. The eighth nerve is the vestibular cochlear and it's for audition and balance. We're not going to focus on that very much now. And now you can see a nice group of fibers here. These axons are the ninth and tenth nerves for the glossopharyngeal and vagus. Very important for sensation from the pharynx, control of the autonomic nervous system and muscles of the pharynx and larynx. Problems with articulation or speech, dealing with the innervation of the larynx. Problems with the gag response are typically signs that the clinician sees when these cranial nerves, the glossopharyngeal, the vagus, nine and ten are involved. Eleven is the spinal accessory that supplies the sternocleidomastoid and trapezius and a little bit of it can be seen, I think, on one side here. Maybe it's dried up. Here it is. It comes up from the spinal cord, runs along and gathers branches from the medulla and exits through the jugular foramen together with nine and ten. We aren't going to focus on that very much either. And next to the vertebral arteries here, which supply the medulla, you have a little groove and in this little groove are some of the cranial nerves rootlets, actually, for twelve, the hypoglossal. Most of them have been torn off, but here are a few little remnants. So they would be coming out all along here in this groove between the pyramid and the olive. Also coming off of the vertebral artery, you can see if I pull it back and retract it. You can see the posterior inferior cerebellar artery. This supplies the side of the medulla and the inferior surface of the cerebellum. And if I lift up the basilar artery, you can see lots of little circumferential penetrating branches. So what we want to do is look at some of the landmarks that show us what the internal structure is. And I want to focus on those cranial nerves that are important for brainstem reflexes that you're going to use to tell you if the patient's problem is in the medulla, the pons, or the midbrain. So what cranial nerves you have to know are associated with each level, what vessel is associated with each level, and what reflexes you would test at each level. That's our goal. And also to recognize a classic section, cross-section, through the medulla, the pons, and the midbrain. We're going to look at that both grossly and in a stained section. Through the cerebellum and medulla, came loose a little bit. You can see the cerebellum much more easily than you can see the medulla. But here are characteristic structures associated with the medulla. This band of fibers on the bottom called the pyramid, which we're going to learn is the cortical spinal track for cortical control of motor activity. We're going to see this characteristic bulge here called the olive, which we don't need to discuss because it's not particularly important clinically. We can see an attachment here to the cerebellum called the cerebellar peduncle, which we'll ignore now since we're not learning about cerebellum. And my probe is trying to enter the very tip end of the fourth ventricle. The fourth ventricle dead ends at the end of the medulla. I can see the fourth ventricle very nicely. I can even see the roof of the fourth ventricle and a little bit of choreoplexus. And a little bit of choreoplexus here that's coming out to the side. This is that lateral aperture where the fourth ventricle communicates with the subarachnoid space. That's that space that has to be open on both sides as well as in the roof for the median and lateral apertures to move cerebral spinal fluid into the subarachnoid space and around the surface of the brain to the arachnoid granulations and the venous system. So very important that we don't plug up this little channel here. And there's a little bit of choreoplexus that sort of remains out there and produces a very small amount of cerebral spinal fluid. So this is a typical section of a medulla. So when you see these landmarks fourth ventricle with choreoplexus cranial nerves coming out beneath the attachment to the cerebellum, 9 and 10 or little filaments of the hypoglossal which are missing here. Cranial nerves 7 and 8 as you move toward the ponds. These pyramids on the bottom or the hypoglossal axons coming out in the groove here which are missing. Those are all clues that you are in the medulla. Now let's look at a stained cross section. I have here a myelin stained section through the medulla. Recall that black is the myelinated tracks and light or gray are the collections of cell bodies called nuclei. Here we can see the prominent convoluted inferior olivary nucleus. It's so obvious that we're not going to talk about it. Rather let's focus on some of our landmarks that we saw on the gross brainstem. We have on the ventral surface or the inferior surface or the anterior surface we have the pyramid. This is a large track of fibers descending to the brainstem and spinal cord. And up here laterally we have a large collection of axons that are headed to the cerebellum and are called the inferior cerebellar peduncle. We want to look at some of the more subtle features here. This pale area here this gray area here and here represent the hypoglossal nuclei. The hypoglossal nucleus sends its axons down to emerge. You can even see one here these little rootlets between the pyramid and the olive. So here we have the hypoglossal nerve cell bodies. Here's their course through the medulla and their exit here. So if this is the left side of the medulla these axons will be going to muscles of the genioglossus which help to protrude the tongue. If the left hypoglossal nucleus nerve axons inside the brainstem or axons after they leave the brainstem are damaged then the left genioglossus will not be innervated because these go out to the striated muscle of the tongue. These are what we call lower motor neurons. They go directly to striated muscle. Then we will have a person you ask them to stick out their tongue and this is their left side. Their tongue would deviate to the left because of the strong right genioglossus which is perfectly normal and it protrudes farther and therefore makes the tongue curve to the side and we always say to remember it, lick your wound. If the wound is on the left your tongue will deviate to the left so you can lick your wounds. Another group of nerves or nerve cell bodies I should say and nerves are down here in this pale region here we have something, it's a great name, the nucleus ambiguous and it's just ambiguous it's hard to see. I'm not sure I've ever seen it but it doesn't matter. Here are motor neurons that are sending axons out into the glossopharyngeal and vagus nerves for motor control of the pharynx and the larynx so the nucleus ambiguous is very important and think of the gag reflex you want to think of the gag reflex sensor information comes in, I'm going to use the same nerve here as if it were sensory comes in and relays in this area of the medulla and then axons enter neurons connect with this nucleus ambiguous and out comes another axon this time motor going out to the muscles of the pharynx and you would gag so sensation touching the back of the throat brings sensory information in connections and relays that we don't have to concern ourselves about and exits out back to the pharynx to cause contraction of the muscles so the gag reflex is a very good reflex to test in a person who is unconscious and you want to know about the status of the brainstem particularly of the medulla if the patient is conscious and you can ask them to stick out their tongue the hypoglossal is another cranial nerve so cranial nerves remember 9, 10, 11 and 12 are associated with this level of the medulla and so we have two easy tests to test for cranial nerve function and remember cranial nerves never cross these hypoglossal neurons will always come out on this side these hypoglossal neurons will always come out on that side cranial nerves do not cross and once they leave the brainstem so that's all I want to tell you right now to remember and we'll go back to the brainstem alright so now we're ready to leave the level of the medulla of the brainstem and go to the pons we call the pons extends from here to here from the end of the pyramids to the beginning of the cerebral peduncles pons means bridge and you can see that it is like a bridge holding on the cerebellum to the brainstem so this big bridge across here has the large fifth cranial nerve right in the middle of it it's torn on this side and we're going to cut it and look through so here is our fourth ventricle and right beneath here would be the nucleus of the abducens with its axons coming down to go out to the lateral rectus muscle so if this area was not functioning you would have double vision when looking to this side nice nice demonstration of the trigeminal nerve coming in it's burrowing through this middle pontine group of fibers and it's coming up to bring sensation for example from the cornea and connecting with other neurons here for the facial nerve that would cause the blink reflex all of this is cerebellum all of this is cerebellum all of this is cerebellum this is the fourth ventricle and the sixth and seventh nuclei as well as the fifth cranial nerve you want to associate with this level facial sensation lateral eye movements facial control blinking the blink reflex five in seven out is a very important reflex because again it's unilateral same side same action don't need consciousness this is a myelin stain section through the pons our landmarks are these criss-crossing fibers down here and our fourth ventricle and the big band of white fibers connecting with the cerebellum this region in here is the region of interest and this general region this pale region remember pale means cell bodies means nuclei this is the nucleus for the abducens nerve and we can actually see some of the axons coming down remember they emerged on the ventral surface so this would be important for lateral gaze this would be lateral gaze to this side so if this was damaged the nucleus the inside the brain stem projections of the axons or the nerve traumatized inside the skull any of those would produce double vision when looking to this side because this lateral rectus muscle would not be innervated but this one on the medial side the medial rectus would be working and you would have double vision another nucleus is this nucleus here this is the facial nucleus these are the muscles for my medic expression and they take a funny sort of jog up and around and they come out here remember right next to the auditory nerve so if this nerve is damaged it is close to the other nerves in here so sometimes you have a problem with both cranial nerves 5 trigeminal and 7 facial and they come out here in the cerebellum-pontine angle we get a classic section of the midbrain with the aqueduct the bump on the top the black substantia nigra a typical midbrain structure sometimes you can see a circular nucleus here and these big cerebral peduncles which we will learn are very important for the downward travel of the axons from the cortex for willed voluntary movement the cortical spinal track runs in this region here so this is pawns and midbrain and at that level we should see here are those bumps remember cranial nerves 3 coming out down here between the cerebral peduncles and cranial nerve 4 coming out on the dorsal side right here and curving around this silly nerve is the only cranial nerve that crosses but the crossing occurs inside the brainstem so once this nerve comes out it goes to the superior oblique on the same side as the nerve there would be another one on this side and this is the level of the midbrain this is the typical midbrain features you have the two bumps of the tectum here you have the cerebral aqueduct here with the gray matter around it you have your cerebral peduncle on either side this was the black substantia nigra that we saw it's pale here because cell bodies are pale myelin is dark and here you can see a nucleus in the midline here and here with axons trickling down they're visible on this side not on this side this is the third cranial nerve the important oculomotor nerve and remember associate the pupillary light reflex with this level of the brainstem the afferent or the incoming sensory side is the optic nerve which we didn't talk about but the constriction of the pupil by constricting the smooth muscle is controlled by autonomic fibers traveling in this cranial nerve the rounding up of the lens for looking at objects up close so that you have clear sharp vision is also controlled by autonomic fibers in this nerve and lifting the eyelid and all of the ocular muscles extraocular muscles except the inferior oblique no, excuse me except the superior oblique and the lateral rectus are controlled by neurons in this region so midbrain is a very short piece of brainstem but a very important piece of brainstem so to recap there are three brainstem levels you need to remember medulla, pons midbrain there are three reflexes that you can test in an unconscious patient at each level the gag the corneal and the pupillary cranial nerves that are motor emerge from each of these levels cranial nerves supply structures on the same side of the brain they never cross branches of the vertebral artery supply the medulla branches of the basilar artery supply the pons and the blood supply to the midbrain is more variable from the top of the basilar artery the ventricular system associated with these levels the medulla the fourth ventricle for the pons and the aqueduct for the midbrain this concludes our brief but far too long discussion of the brainstem