Welcome back to Medical Neuroscience and welcome to this tutorial on eye movements. It's a beautiful day in central North Carolina. It's a bit breezy. So, if you hear the wind blowing in the background, maybe the wind chimes behind me, or maybe a little microphone pick up of the wind I'll hope you'll pardon that. I'll try to minimize that as best I can. But otherwise, I'm really pleased to be addressing you in such a beautiful day on the topic of eye movements. So, this topic pertains to two of our core concepts in the field of Neuroscience. Once again, the brain is the body's most complex organ. And, secondly, what we encounter again, is a discussion of circuitry. That circuity is largely genetically determined and it provides the foundation for function in the nervous system. And, for this tutorial, the function of interest is: how is it that we can manage to keep our eyes focused on a visual target of interest? Or, if the case may be, how might we switch our focus to some new target? Well, that gets us into our learning objectives for this session. What I want you to be able to do is to discuss the five major types of eye movements and indicate the function and purpose of each. I want you to be able to discuss the neural circuits that are responsible for making saccadic eye movements. And I want you to be able to discuss the roles of the relevant parts of the brain that are involved in governing these eye movements. And we'll focus on two brain regions in particular, the frontal eye fields, which is part of the premotor cortex and the posterior part of the frontal lobe, and the superior colliculus, which is on the dorsal aspect of the midbrain. Well, let's begin by talking in very broad terms about the goal of making eye movements. Well, the usual goal is to adjust fixation. Now remember from our studies of the retina and visual pathways that what we actually look at closely, with high visual acuity is a fairly small region of the visual world. In fact, if I were to extend my, my thumb at arm's length, the width of my thumb is roughly that portion of visual angle that's actually seen well by the macula and at the center of the macula, the fovea. So, the challenge in observing our visual world with detail is to maintain our eyes on a visual target. Even if that target is moving or if our head happens to move. So, eye movements adjust fixation when a visual target of interest happens to be moving, such as maybe watching a bird fly across your field of view. and eye movements help to adjust fixation when our head is moving. So, this is sort of the complementary scenario where we want to focus on a stationary object, let's say. And our head is moving, or maybe our entire body is moving. And so, we want to make minute adjustments in order to maintain fixation so that central one to two degrees or so visual angle continues to project on that part of the retina that's most sensitive for high visual acuity. Now, sometimes we actually want to break fixation. We want to acquire a new visual target. For example all of you probably have a clock somewhere nearby. go ahead and look at the clock and see what time it is. Well, that act of acquiring the view of your clock where ever it may be on your computer screen, a wristwatch strapped to your arm, or maybe on your phone that happens to be nearby, or maybe a clock on the wall. All that required some change in visual fixation. So, that's yet a very specific kind of eye movement that we want to try to understand and we want to be able to discuss the circuitry that's necessary for making these kinds of eye movements in order to acquire a new visual target. Okay. Well, how do eye movements occur? Well, in one sense, eye movements are remarkably simple but they also give us an exquisite opportunity to gain some insight as to how the nervous system governs not just eye movements but all kinds of movements. So, the way in which they're simple is that the eye movements are controlled by six pairs of extraocular muscles. And here they are illustrated in this figure. there are the superior rectus muscles. Which, elevate the eye. There are the inferior rectus muscles that depress the eye. There are lateral rectus muscles that abduct the eye. That's AB duct, the eye. Which means move the eye towards the lateral side. And then there are medial rectus muscles, that AD duct, that is adduct the eye, move the eye towards the mid-line. Okay, so, so, these four muscles work more or less in a Cartesian manner, pulling the eye along one standard axis of orientation or another. Well, in addition to the rectus muscles, we have two oblique muscles. There are superior oblique muscles that turn the eye downward and inward, and then there are inferior oblique muscles that rotate the eye outward and upward. Well, these six pairs of extraocular muscles are innervated by three pairs of cranial nerves. Two of the nerves do just one muscle each. And then the third nerve does the remaining four. So, let's begin from the caudal end of the brain stem. And I'll remind you that our abducens nucleus, the somatic motor nucleus, along the dorsal margin of the tegmentum near the mid-line of the caudal pons, grows out a axon from its motor neurons and that axon innervates lateral rectus muscles. So that's easy to remember, right? The abducens nucleus abducts, AB ducts, the eye, pulls the eye to the lateral side, via the action of the lateral rectus. OK. Well, next we have the trochlear nucleus. The trochlear nucleus, also somatic motor nucleus. It sits along the dorsal midline of the tegmentum. Now, the upper part of the pons, or the caudal part of the midbrain, sort of where those two embryonic divisions come together. Well, these axons do something quite remarkable. They grow their axons towards the dorsal margin of the brainstem, which is unlike any other motor neuron we have in our body. All the other motor axons exit the brain stem on the ventral aspect of the brain stem, or, or the spinal cord if we want to extend the discussion to our alpha motor neurons of the ventral horn. The trochlear nucleus, on the other hand, has these alpha motor neurons that exit the dorsal aspect of the brain stem. And then they do something equally remarkable. They grow an axon that crosses the midline. I hope you've understood, perhaps from some unit one sessions, that we've had together, that all of the alpha motor neurons, except for these trochlear motor neurons, supply ipsilateral muscles. Okay. So, the trochlear motor nucleus is a real oddball. Not only do the axons exit the dorsal aspect of the brain stem, but they supply contralateral muscle. Okay. So, the abducens nucleus innervates the lateral rectus, the trochlear nucleus innervates the contralateral superior oblique muscle. All the other pairs of extraocular muscles are supplied by our ocular motor nuclei. With the ocular motor nucleus on one side of the midbrain. Here's our ocular motor nucleus, supplies innervation to the inferior rectus, the inferior oblique, the medial rectus and the superior rectus all on the same side as the nerve. Okay. So, the third cranial nerve supplies ipsilateral extraocular muscles. Now I'll just remind you that there are a couple of other components of that third nerve I want you to keep in mind. One is the parasympathetic component, that supplies the constrictor muscles of the iris. Remember that's derived from the Edinger–Westphal nucleus, those axons also run in this third cranial nerve. And also in the third cranial nerve are the axons of other alpha motor neurons, that are involved in opening the eye. Especially when we want to do so forcefully. Okay? So, so, that's another, set of alpha motor neurons in addition to those that supply these four extraocular muscles in the orbit.