 So it's time to discuss the voluntary control of eye movements. Eye movements are a little bit different from the other motor cortical areas we've already discussed, which was the pre-central gyrus here in front of the central sulcus. This was our primary motor cortex in the frontal lobe, as opposed to sensory cortex in the parietal lobe. And if this is the right hemisphere, which it is, there is an area up in this region here called the frontal eye fields. And the frontal eye fields are where the command, so to speak, comes from to move your eyes. If this is the right hemisphere, using this functional area here, moves my eyes to the left. So right hemisphere controls horizontal gaze to the left. Left hemisphere controls horizontal gaze to the right. So it's the same principle of the cortex controlling the opposite side of the body. However, we have one problem, and that is moving your eyes together conjugately requires using cranial nerves on both sides. And that is what complicates our story a little bit. Today's subject is the control of eye movements by higher cortical centers. But to understand the control, we have to understand the eyes and how they move. So I have this nice animation from the University of Toronto. And we have here an eye sitting, staring at you. It is the left eye, and it's in its primary position, staring straight ahead. And over here on this side we have the muscles, and we're looking down on them. We're looking as if we were lying in the skull, looking down from the bottom of the frontal lobe. So this would be the superior rectus muscle. So now let's see how the eye movements go. We have six cardinal positions of gaze where you test eye movements. First we move medially, and that's using the medial rectus muscle. And when we have our eyes in this position closest to the nose, we look up using the inferior oblique, and we look down using the superior oblique. Similarly, we can look laterally. When we look laterally, we use the lateral rectus when our eyes are staring forward. And we add the superior rectus to look up, and the inferior rectus to look down. So this is the left eye. Now, when we stare straight ahead and look at these six muscles, the inferior middle superior on this side, and the superior rectus, lateral rectus, inferior rectus on this side, I want you to remember what cranial nerves innervate these muscles. So the big cranial nerve for eye movements is the ocular motor. And the ocular motor innervates the medial rectus, the inferior rectus, the superior rectus, and the inferior oblique. So it's the biggest of the cranial nerves. The rinky dink superior oblique muscle, which is so hard to understand. It says superior, but it moves the eye inferior, is innervated by the trochlear. And it's not that important clinically, so we're going to ignore it today. The other cranial nerve that is innervates only one muscle is the abducens, which innervates the lateral rectus. So today's conversation is going to be between the innervation of the medial rectus by three, cranial nerve three, ocular motor, and the lateral rectus by cranial nerve six. The conjugate horizontal gaze involves the coordination of cranial nerve three and cranial nerve six. One is on one side, the other is on the other side. So now let's move into the brain stem and look at where these cranial nerves enter the brain. Here we are looking at the ventral surface of a very nice brain. My probe is pressing in the region of the chiasm, and you can see the two optic nerves. And very nice demonstration here of the arachnoid membrane as it covers the system beneath it. And if I pull a little bit on the arachnoid, I can expose the very nice large ocular motor nerve. It's the largest of the cranial nerves to the eye muscles because it goes to four of the muscles and not just to one. And I'm looking here for the one on the other side, and I think as often happens it's been pulled off during removal of the brain from the cranium. Notice that it's coming out on the side here just in front of the cerebral peduncles. Here's the cerebral peduncle, our landmark for midbrain. So third nerve, associate with midbrain. Now here is our big pawns with the basilar artery on it. And then I'll pull down on the vertebral artery on one side. And on either side of the basilar artery we can see two very small delicate nerves. These are the two abducens or cranial nerve number six. Notice it emerges right at the junction between the medulla and the pawns. And it's going to run forward into the cavernous sinus along with the third nerve and out into the orbit. So control of six and three requires descending fibers from the cortex to go down to the pawns. And we're going to look at that in more detail. I want to show you the pathway for controlling lateral gaze. This is voluntary lateral gaze. And so when we use both eyes and we want to look to the left side, we have to use our right frontal lobe, the right frontal eye field. Gaze to the left is right cortex. Gaze to the right, controlled by the left cortex. Very similar to our motor control of our voluntary muscles. Right hemisphere controls left side of the body. Now we have a complication with the conjugate eye movements. That is because the cranial nerves are on opposite sides and cranial nerves don't cross. So we have to have a command or a control center. And that center is called the paramedian pontine reticular formation. It's the pink center, which I like to call the center for horizontal gaze. This center and there might mention there is also a vertical gaze center and we're not going to talk about that. But we need to coordinate the cranial nerve number six, the abducens, that goes out. And we need to control the cranial nerve three, the ocular motor that goes out on the opposite side. And so we have neurons in the abducens nucleus, these green neurons, that cross to the opposite side and ascend in a pathway called the medial longitudinal fasciculus, which terminates in the ocular motor nucleus. And there the motor neurons from that nucleus go out to the medial rectus muscle. And so we have coordinated conjugate eye movements at the same speed and in the same direction. And it allows us to have binocular stereoscopic vision and to track things. So it's a very useful pathway. So now we are going to willfully look to the left using our right frontal eye field. And axons of these neurons are going to descend through the internal capsule. And if I move over to the sagittal surface, we can imagine them traveling down past the thalamus in through the midbrain and then crossing over to the caudal pons on the opposite side. So eye movement control involves structures all the way from the cortex to the pons. So lesions of these areas can give you problems with eye movements just like cranial nerve lesions can give you problems with eye movements. So here we have a coronal section and we can imagine that in our right frontal eye field, we'll just pretend, we have axons coming down through the internal capsule and on down into the midbrain. Here we have a classic picture of the midbrain. And the structures we're talking about today are the ocular motor nucleus which is just below the aqueduct here. Here is our cerebral peduncle and the other one. The cranial nerves have been torn off of this section. But our fibers are coming down in this region heading for the pons. Here we are in the pons and by now the fibers that we were following down here on the right side have crossed over to the left side. And right in this general region here, right beneath this bump is the PPRF or horizontal gaze center. And right more or less underneath this bump here is the abducens nucleus. So the two are very, very close together. And so it's at this point then that we have to form that tract called the medial longitudinal fasciculus to run back up and coordinate the pons with the midbrain for moving eyes to the left. So here is a mid-sagittal section of the brain and this is the medulla. Here is the pons and just the beginning here of the midbrain. Here is the inferior and superior colliculus and our fourth ventricle. So our third nucleus is approximately here and our sixth nucleus is approximately here. So the MLF is going to be running right along here. And let me show you that in a nicer dissection from which I have a photograph. This photograph is similar to the specimen you just saw with the medulla, the pons, the midbrain, the thalamus and the ventricular system. At the level of the superior colliculus is where we're going to find the third nucleus. And can you see this band of white, kind of a shiny white line that's coming down like this? This is the medial longitudinal fasciculus on the right side in this case going between the pons and the midbrain. So I want to explain a little more clearly the MLF, medial longitudinal fasciculus. Here we have three sequential slices through the superior colliculus, the inferior colliculus and the pons. So you have to imagine these approximately an inch apart in distance. And we're going to make this green line represent the tract, the medial longitudinal or MLF. And it's going to run from the level down here of the abducens nucleus all the way up to the level of the ocular motor. And we're going to ignore the trochlear. So we have a tract running up, a nice myelinated, thick myelinated tract on either side. And something unique about this tract is that in disease such as multiple sclerosis there is a propensity to demyelinate. And one of the classic areas of demyelination is this tract or MLF. Another area that often demyelinates is the optic nerve. So when you see a patient you need to think about whether or not this patient might have damage to the MLF, either from demyelination or other disease. And usually accompanying this is an oscillation of the eye that does move. The one that goes to the lateral rectus has a condition often called nystagmus, which is an oscillation of the eye. So there is a specific name given to this condition. It's called internuclear ophthalmoplegia, internuclear ophthalmoplegia, I-N-O. Why is it called I-N-O? Because it's between the nuclei, internuclear, between the nucleus of the abducin and the nucleus of the ocular motor. And ophthalmo means eye and plegea means paralysis. So the side that the MLF is going to the third nucleus on, the medial rectus doesn't get innervated. And therefore the lateral rectus moves over, the medial rectus doesn't, and it's like all the electricity went to the eye that moves over and it oscillates with nystagmus. And that's an I-N-O. So just as lesions or damage to the midbrain and pons can interrupt the coordination of eye movements in the horizontal direction, so can damage to the frontal lobe in the area of the frontal eye fields. And there are two conditions. First of all, let's assume you have a vascular lesion. Some branch of the middle cerebral artery is occluded and this area for eye movements is damaged and doesn't work. What happens is that the frontal eye field of the left hemisphere is still working. And so what happens is the eyes of this patient would deviate to the side of the lesion, to the right side, because the action of the left frontal eye field is unopposed. Now let's take the opposite. What if you had a tumor here, for example, or some kind of scar tissue that set up an epileptic situation and you had an epileptic electrical discharge of this area. So the neurons in this area are firing abnormally. And what would you see? Since this is the right hemisphere, during the seizure the patient's eyes would deviate to the left. So too much energy, so to speak, action drives the eyes to the opposite side. Damage to this area, at least transitorily, leads to looking at the lesion. So we want to conclude our conversation about horizontal eye movements and say that there is also a center for vertical eye movements. And since vertical eye movements just involve the third nerve, and since the third nerve is associated with the midbrain, there is a midbrain center for vertical gaze, which we have not talked about. But it coordinates the eye muscles on both sides for looking up, just similarly to the horizontal gaze center. But it doesn't use the MLF to coordinate it. It all happens on either side at the level of the ocular motor nucleus. So we have quite a number of areas, cortex, internal capsule, midbrain, pons, MLF. All of these structures can lead to abnormal eye movements. And it's your job to figure out where the lesion is by looking for the other signs and symptoms that accompany it.