 The neural layer ends at a margin of the radina called the aura serratum. The pigmented layer continues on the surface of the serratum body and it continues on the surface of the iris. Let's apply of the radina. You can enlarge these pictures to see. I put them very very small, but you can enlarge them. The two most important circulations are of plexus of blood vessels on the outer surface, which is called the coreocabularis. They supply the outer one third of the region. The neural layer is supplied by the central retinal artery, which is a branch of the ophthalmic artery. This is the branch of the ophthalmic artery. The ophthalmic artery gives us to the central retinal artery. I'm going to tell more about it. I'm just naming it. Here's drainage. The central retinal vein drains into the superior and inferior ophthalmic vein. Which drains into the cavernous sinus. So these are the few nuts and bolts. Layers of the radina. If you look up your board review book, it will tell you 10 layers. You'll say, oh my god, I have to memorize these 10 layers? No, you don't have to. We will make it simple. Basically, we will condense them into three most important layers. What are the three most important layers? Going from outer to inner. This is the radina. This is the vitreous humor. That means the light is coming from here. The cornea is here. Are we clear? The retina. The coroid is here. And the sclerida is outside. So this is the outer surface. This is the posterior and that's the anterior. Are we clear about the orientation? So going from outer to inner. We have three layers of neurons. I'm just condensing all the layers. An outer nuclear layer. An inner nuclear layer. And a ganglion excel layer. Ganglion layer. These are the three most important layers for us. Outer nuclear layer is composed of, the layer is also called the layer of rods and corns. Actually it's called outer nuclear layer because of the nuclei of the rods and corns. The inner nuclear layer is composed of bipolar neurons. And the ganglion layer is the final layer with axons of which constitute the optic neuron. Are we clear? And the inner nuclear layer synapse. This layer is called, this layer of synapse is called the outer plexiform layer. The word plexiform means layer of synapse. And where the inner nuclear layer synapses with the ganglion cell layer. That is called the inner plexiform layer. Now I need you to know something. There are many things which are in this picture. In fact one picture can tell more stories than a thousand words like they say. The retina starts, the pigment, the neural layer of the retina starts from here onwards. But this is also a part of the retina. Remember I told you the retina has got two layers. A neural layer and a pigmented layer. This is the pigmented layer of the retina. Can you see? This pigmented layer of the retina gets pigment from the coroid layer. This interface between the pigmented layer and the neural layer. This interface, I am saying interface because in actual life there is no layer of separation. However, you will see it in any other issue. I am going to tell you. The pigmented layer is derived from a different source and the neural layer is derived from a different source. So there is a potential space between these two layers. And what is the significance of that potential space? If you get a blunt trauma, retinal detachment occurs. It occurs between the neural layer and the pigmented layer. Contrary to what we may think, retinal detachment does not mean the retina is not getting detached from the coroid. Retinal detachment means the neural layer separates from the pigmented layer. The pigmented layer remains stuck to the coroid, the neural layer separates. And I have seen a case like that. This happened. This guy, he was participating in a rally. Going on the street and shouting slogans and all the rest of it. So the police fired water cannon to disperse the crowd. And the water cannon is, you know, the water has got a very strong blast. So the blast of water hit him in the eye. It so happened that he was in the front. Next day he came to the clinic, seeing the difficulty of vision. And when we did not thalmoscope, we found that he had undergone retinal detachment. So retinal detachment can occur from a blunt trauma on the coroid. And it occurs between these two layers, between the pigmented layers. You continue. A few quick words about the cells. Rods and corns. These are the first of the neurons in the outer nuclear layer. Rods are called rods because they are triangular. Rectangular shaped corns are called corns because they are, these are the ones which are the photosensitive receptors. They contain the photo receptors. They are the ones which respond to electromagnetic radiation, the physical part of the spectrum. They use glutamate as a neurotransmitter. Next. The bipolar neurons. Now the special, I told you that all the special senses have bipolar neurons. Remember? So they are called a pitfall process and a central process. The pitfall process synapses with the layer of rods and corns. The central process synapses with the ganglion cell layer. They also use glutamate as a neurotransmitter. Next. The ganglion cell layer. They also use glutamate. And their central, their pitfall process synapses with the central process of the rod. The bipolar neurons and the central process continues, collects together and forms the optic nerve which we shall see in the next slide. Next slide. There are two, three other types of cells which are briefly mentioned. Connecting adjacent layers of rods and corns, situated on the outer plexiform layer are small, small cells. They are small, small neurons. They are called horizontal cells. They use, and their job is to prevent excessive stimulus from coming through the rods and corns. So they exert natural inhibition to the rods and corns. So that excessive impulses do not reach the ganglion cell layer. Are these different from the K-troll, for example, K-troll? Yeah. Yeah, it's totally different. These are just called horizontal cells because they exert lateral inhibition. There's another similar type of cell which are located in the inner plexiform layer. What do these do? They receive impulses from the inner nuclear layer, layer of bipolar neurons. And they, in turn, send inhibitory impulses to the ganglion cells. And they also use GABA and LICEN as their neurotransmitter. They also prevent excessive impulses from reaching the ganglion cells. So these are the two in-between the natural neurons which are located in the region of the horizontal cells. And finally, there is a non-neural cell which is called the radial cells of Mueller. These red ones that you see here. These are something similar to glia. They are called the radial cells of Mueller, or radial Mueller cells, which go the full thickness of the retina. And they produce what is known as the outer limiting membrane and an inner limiting membrane. Just like the astrocytes give full processes and form an outer limiting membrane of the brain and an inner limiting membrane of the brain. Similarly, the glial cells here also produce an outer limiting membrane and an inner limiting membrane. And non-neural cells, their job is more supported. So these are some few other cells of the retina. There are a few special words. I'll just quickly summarize them. The optic nerve gets myelinated by oligodendrocytes after it is penetrated through the sclerar. So long as it is within the sclerar, it does not undergo myelination. Once it is penetrated through the sclerar, then it undergoes myelination. Why is oligodendrocyte involved in the myelination? Why not one cell? Because it is the central nervous system, right? Exactly. So that's an important point to remember. Oligodendrocyte is the one which is responsible for myelination. So you already guessed something. Can you tell me what you guessed? Remember when we talked about multiple sclerosis? We said it is a demyelination of central nervous system. That optic nerve is one of the structures which is predominantly affecting multiple sclerosis, the point I was going to remember. Because oligodendrocyte myelination is destroyed in multiple sclerosis. And because this is the central nervous system, this is the prime site of demyelination in multiple sclerosis also. Tight jumps etc. A few quick words about the rods and cones. Rods outnumber cones by several times. It's not written here, but I have written in my slide. Location. Rods are more common in the particular parts of the retina. Cones are more frequent in the macular region and the phobia centralis. Rods are more sensitive, so therefore they respond to dim light. They are called scotopic vision. Cones are more for acute vision and they work better in right light. That's why when you see something very closely, you see it in the center of your eye, you see it with your macular and your cones coming to action. Cones are for color vision. That's why it's also called photopic vision. Cones are for scotopic vision or dim light vision. Roads are played around, they have a pigment called rhodopsin. Cones have a pigment called photopsin and hyalopsin. You don't have to know the details, these are already complex. All derived from red. Rods require to be derived from A for their regeneration and they are responsible for dark adaptation. Let me give you an example. When you enter a cinema hall, late, when the movie has already started, what happens? You cannot see your way, isn't it? Because you are from outside, you came from outside your eyes already. The bright light has photo-bleached the rhodopsin. It takes about 20 minutes for the rhodopsin to regenerate. So that is the period of what is known as dark adaptation. Let me give you a story, an interesting story. During the Second World War, the Royal Air Force phallics, they had to take off at a moment's notice, even in the night or any time. And those days they didn't have all those laser sights and all those things like we had today, laser guided missiles. They had to do everything by vision and everything is dark. So even the few minutes it takes for them to be dark adapted was very dangerous. They can be shot down, especially when they are on a bombing raid in Germany or something. In order to keep their eyes dark adapted throughout, they used to be asked to wear red glasses. So if you wear red glasses, your eyes will be dark adapted. So therefore they would have to wait for the eyes to get adapted. So this is the practical example of dark adaptation. And we will see later on that the rods are responsible for the vision, which we had mentioned earlier in the cortex chapter, the magnocellular slices. And the cones are responsible for the phallic cellular loss system. Remember what was the function of the bar ocular loss system? Colour. And this thing, you reach your patterns and face recognition. The magnocellular stress system was responsible for motion, depth and spatial perception. So rods and cones will play a role now, yes. Is that what that is? Okay. Excess pigmentation, pigments of the rods is very strong. Is it commonly adapted from the eyes? Yes. I think they shave him cartilage in the endowel. Maybe he's got some special level of logistics, cones. There are three types of cones. LM and SN are not going to be details. L cones for red, M cones for green and S cones are for blue, but those are not required for you. Maybe he's got some special level cones, yes. Is there a reason you're using the red glasses at the post time? Yeah, so what happens is it keeps your eye constantly dark adapted. Why don't you wear sunglasses? No, they found that it works better. Dark adapted. Sun glasses actually just filter out the ultraviolet radiation and they filter out the intensity of the light. But it does not really keep your eyes dark adapted. But red glasses was too used to keep them dark. Especially if you read a book called The Dampusters, there's a movie also. Dampusters. They were specially trained pilots who were supposed to fly very low and actually blow out the dam in Germany. And they had to do it by vision. They had to fly just 60 feet above. And they were especially kept dark adapted throughout during the training courses and they had to do it in the night. So all these things are mentioned there. They didn't blow out the dam in Germany. Okay, so a few clinical correlations I mentioned here. Night blindness is due to vitamin A deficiency. Deficiency of that is called mentalopia. It can be due to many other conditions also. And there are some situations. Genetic conditions where there is genetic deficiency of the bones. And that is called day blindness or hemorrhobia. Okay. Rental physiology can be done on your own. Just the point to remember is the brighter the light, less will be the neurotransmitter released. Contrary to what we may think. The brighter the light, less will be the light. Because bright light causes hyperpolarization. It closes the sodium channels. So therefore constant efflux of potassium will lead to hyperpolarization. So less light, more neurotransmitter and vice versa. The mechanism is very complex. We are not expected to go into the details. In fact, if you were to read up, it's very complex how vitamin A is used. But the net result is less light, more neurotransmitter and vice versa. And the process of conversion of electromagnetic energy to the chemical energy is called retinal transduction. And this also requires vitamin A, retinol. Now let's come to a few important points about the retina itself. The word fundus means the ophthalmoscopic appearance of the retina. In your anatomy you have heard the word fundus, isn't it? How many fundus do you know? I'm trying to remember the point. How many fundus do you know? These should be able to tell me at least three fundus from your anatomy. Yes, good. Stomach. Fundus of the stomach. Second? Come on, come on, come on. Oh, thank you. Kidney. Kidney fundus. Kidney fundus. Really, really, I'm just thinking about it. What about the fundus of the uterus? What about the fundus of the gallbladder? You are supposed to teach me all these things. I'm not supposed to teach you all that thing. Okay, why are they called fundus? That means there must be common meaning for the word fundus? Yes or no? No, fundus actually means the most distal or the dependent part when you approach it from one particular direction. So in the uterus, you approach it from the cervix elliptic. The most distal is the fundus. Stomach, when they first examined it, they saw it from the pyloric end. So the most distal part was the fundus which is under the dome of the reactor. Gallbladder, you see it from the cystic gut side. So therefore, the prediction below the liver is the fundus of the gallbladder. Here also, why is this called the fundus? Because you are looking at the eye from the eye disc or from the pupil. The most distal dependent part is the retina. That's why this is called the fundus. Fundus of the eye. Fundoscopic appearance. Optaloscopic appearance. Okay, so this is the meaning of the word just so that you understand why it is called the fundus. What are the landmarks? These are the ones you want to know. Because we are going to mention all these things again in our FCM, but we are going to be asked these questions. Normal appearance. Nobody is going to ask you the abnormalities of the fundus in this basic sciences. But you will be asked the normal. What are the landmarks? The first landmark that you see here, here, here and here is the macula. Macula is the most central, absolute geometric center of the eye. Geometric center. Approximate eyeballs, eyes 24 millimeters. Exactly the posterior pole is the macula. It used to be called macula lutea. Lutea means yellow because the first appearance of the macula was seen in a special type of light which was green from red. And therefore it appeared yellow. But nowadays the word lutea is dropped. Nowadays we just say macula. So this is the central part of the eye. Absolute geometric center. What is the appearance of the macula? First, it is brighter red in color than the rest of the antenna. Why is it brighter red? Because here the macula is in. Therefore the capillary vessels shine through. So this is the appearance of the macula. Brighter red. In the center of the macula, you have got an area which is called the obiac centralis which is the center of the highest visual activity. The center of the macula, absolute center of the macula is called the obiac centralis. Next landmark. For the eye, we don't use the word medial or lateral. We use the word nasal or temporal. We use the word nasal. That means medial or temporal means lateral. Please note very carefully. The macula is this absolute geometric center. A little nasal to that. You have the optic disc. You have the optic disc. The optic disc is approximately 2 to 2.5 disc diameter. 3 to 4 millimeters nasal to the macula. Or 3 to 4 millimeters nasal to the macula. This is the side which is creamy or yellowish in color. Point number 1. 2. It is called a shallow depression. Which is referred to as the physiological cup. It is called shallow depression which occupies less than half the diameter of the disc. Point number 3. This is the area where all the axons of the ganglion cells converge and form the optic nerve. So this area does not have any rods and bolts. This is the area of blind spot, the physiological blind spot. Are we clear? Next, it is called fine blood vessels which are known as the disc vessels. It is not shown here. Next point. The temporal margin of the disc. That is the lateral margin of the disc. Is more clearly demarcated than the nasal margin. This is how you will determine which eye it is. The temporal margin is more clearly demarcated than the nasal margin. And the final point. From the center of the disc, you will see 4 pairs of blood vessels emerging. 2 superior to inferior pairs. They are respectively the superior and inferior temporal and nasal retinal artery and retinal veins. So these are all the sadient points about the optic disc. Next. The appearance of the retina in general. The appearance of retina in general. Again, it is pale pink in color. But it is lighter than the macular. Why is it pale pink in color? Why is the macular red in color? Again, remember I told you the outer surface of the retina is supplied by the coriocardial plexus. It is the plexus of blood vessels which shines through. That is why when you take pictures of each other. And if the light happens to catch you right in the pupil at that moment of time, do you get that red eye? Can you see the red eye here? I put a picture here. Can you see the red eyes? This has been taken from one of you. I mean not you. Some free magic. This is because of the coriocardial plexus. And it is this which gives the red appearance to the retina and the macular. The coriocardial plexus. That is why now we have the red eye reduction, red eye flash and all those things. Okay. And finally a few words about the retinal plexus. I told you there are four pairs. From superior to inferior. How do we distinguish the artery from the vein? The artery is always thinner than the vein. The artery has got a brighter light reflex because of the horoscopic light. It reflects more. So these are the two important differences between the artery. The artery is thinner than the vein. It has got a brighter light reflex and it is less red than the vein. It appears less red, lighter red than the vein. So these are some important differences. So whichever is the thinner one is the artery, whichever is the thicker one which is the vein. Yes, the vein is thicker. The artery is thinner. In the retina. So I am talking about the retina. Okay. So by looking at this, can you tell me which eye it is? Eye. Four is written there. How do you say it is left eye? So always look for the temporal margin. Whichever is the part that is more sharply demarcated that is the temporal side. So if this is the temporal side that means this is lateral, right? So the person is looking at you, this is lateral. So it is a left eye. So that is how you relate. Another way to determine which side is this or which side is this. The temporal blood vessels will be, because remember this is nasal. So the temporal blood vessels will be longer. That is another way to determine. Because optic disc is situated more nasal. So the temporal blood vessels will be more curvy. Temporal blood vessels will be more curvy. There is also another way to determine which side it is. But you will be asked the normal findings. You will not be expected to do different diatoms in your biology, and you will be expected to see this during your FCM also, but in the examples you will be asked. That's why I mentioned all these things. So these are the some points about the retina itself. I told you about the retina histology, the types of cells, a few quick words about the rods and cones, the retina physiology, and the fundus opiates. Now let's start with the visual pathway as a whole. And every step of the way, just like you have to know every step of the way of the lateral endothelium and just by the thalamic tract. Similarly here also, you have to know every step of the way. So let's first give you a quick overview. I'll give you first overview. It's a four layer, four order neuron change. First point. Three of those neurons are already in the retina itself. Yes? The layer of rods and cones, the inner nuclear layer that we have bipolar cells, the ganglia cells. The fourth layer of neuron will be the thalamus. So it's a four neuron change. First point. So here is the retina, which is already going to three layers we mentioned. And we said that the axons of the ganglia cells constitute optic nerve. So this is optic nerve. What are they? They have nothing but the axons of the ganglia cells. Optic nerve. I'm just first nailing the structure, then we will go into the details of each. Optic hyacinth. Optic tract. Optic tract. LGV. Lactogenic lead body. Thalamus. Now you will see the details of LGV. Optic radiation. Oxopilipornix. Cuneous gyrus lingui gyrus. You will see the importance of why we were hammering our heads over. Cuneous and lingui. Cuneous and lingui. They will have lots and lots of things to say for us. So this is the full pathway. Now let's take them step by step. This is another picture to show you quickly, just a part of the optic tract and then what happens later on. This gives you a diagrammatic representation right up to the cortex. We will be using this similar picture again and again. Okay, let's start with optic nerve. We have already seen optic nerve is formed by the union of all the axons of the ganglia cells. Location of the optic nerve. Location of the optic nerve. It passes through the optic for M. Where? Here. This is where optic nerve emerges. So after it passes through the orbit, the optic nerve emerges through the optic for M. This is an MRI picture to show you the location of the optic nerve inside the orbit. Incidentally, this is also a picture of the left CN6 palsy, but we will not bother about that. I put this picture. This is also the location of the optic nerve within the optic in the orbit. That will do. This is a diagrammatic representation of the same thing. Some special features about the optic nerve. Because it's a central nervous system tract, it has got all the three layers of the dura. I want you to note that point. That means it's got three layers of the meningitis, dura, arachnoid, pyramid. It has also got a covering of CSF. This is an important point. It has got lots of clinical significance. One of the features of meningitis is optic otophobia. The patient has got difficulty in looking at light. Why? Because these meninges get inflamed, and therefore they really adopt a nerve. Why do you get papillodema when there's increased intracranial pressure? Now think and answer. It's mentioned in this slide here. Why do you get papillodema when there's increased intracranial pressure? I'm going to talk about it in the clinical part, but I want you to understand why now itself. Because, yes, because the central retinal artery and central retinal vein, they pierce through the layers of dura, they go through the subacnoid space, they enter the optic nerve, and then they come out on the retinal surface. So when there's increased intracranial pressure, it compresses the central retinal vein first. Yes? Because veins is thin, won't it? Artery is coughing blood, vein is not draining the blood, so there is eating of it. Makes sense? That is why. Similarly, I told you just now, the optic nerve, when it is passing through the optic foremen, here it's a complete amount of pulmonary artery. It is situated very close to the ethoid and the spinoid sinuses. And the bone here is very thin, ethoidal bone is very thin. So when a person gets ethoidal or spinoidal sinusitis, it produces information of the optic nerve causing infective optic neuritis. I'm using the word infective optic neuritis. Why? Because there is something called demyelinating optic neuritis. Infective optic neuritis. Well, there's information of the optic nerve because of ethoidal and spinoidal sinusitis, as opposed to demyelinating. Now, I mentioned the compression of optic nerve produces optic atrophy, and we just now saw osteotelitis improve. And finally, blood supply is supplied by the central rectal artery. Next structure is optic hyacinth. Location of the optic hyacinth. This is the location of the optic hyacinth. There's a shallow sulcus. This is called the chiasmatic sulcus. This is where the optic hyacinth is located. Important relationships of the optic hyacinth. You have to know important relationships. This is a sedetal view to show you. Anteriorly, it is related to the lamina terminalis. Posteriorly, it is related to the infundibilum and the pediatric restock. I need you to note this relationship down. It's very important. The optic hyacinth is located exactly at the junction between the anterior wall and the floor of the third ventricle. Optic hyacinth is located at the junction between the floor and the anterior wall of the third ventricle. Optic hyacinth has got a small recess of the third ventricle which is known as the hyacinthic recess. Optic hyacinth is made by CSF from outside also. This is called the chiasmatic cistern. So optic hyacinth is made by CSF from inside as well as outside. Chiasmatic recess inside and chiasmatic cistern outside. Laterally, the optic hyacinth is related to... Please take a good look at this picture because these are all clinically important. Laterally, the optic hyacinth is related to a curved portion of the internal carotid artery. And that is called the carotid siphon. The carotid siphon is not shown in this picture. It will be located here. It is shown in that picture. Laterally, the optic hyacinth is related to the carotid siphon. Just note these relationships. Antiretriminal is posterior. In fundibilum, the beauty stock. Laterally, internal carotid artery. Next, what happens in the optic hyacinth? In the optic hyacinth, take a good look. Look at the blue and the pink fibres. Very important. 60% of the fibres coming from the optic nerve, they cross the optic hyacinth. Which fibres? The menial fibres. The nasal fibres. Sorry, I should use the word nasal. So take a look at this pale blue here. It is crossing over to the opposite side. Yes? Look at the pink here. It is crossing over to the opposite side. Nasal fibres. They cross over. Temporal fibres remain on the same side. I need you to understand that point. Temporal fibres remain on the same side. Nasal fibres cross. So once they have crossed, I need you to notice something. Look at the color coding. This color coding is not random. It has been done with a purpose. This pale blue is the nasal fibres from the region. Yes? And on this side, he has shown the pink fibres. Again, the nasal fibres. So this nasal fibres has crossed over to this side. And this nasal fibres crossed over to this side. So in the optic tract, what happens? The color coding becomes same. Yes or no? In the optic tract, the color coding has become same. Pink, pink, pink and blue. It has done the significance. And I'll tell you about the significance just now. So important point, nasal fibres cross. Temporal fibres do not cross. We will see an important clinical correlation because of the arteries which are sitting on either side. I told you the crowded side. I'll tell you it's written there, but I won't talk about it now. Optic tract. Optic tract starts from the posterior lateral aspect of the chiasma. Optic tract starts from the posterior lateral aspect of the optic chiasma. And ends in the lateral chain. The types of fibres in each optic tract. Now please follow me very closely. Each optic tract, again, each optic tract receives fibres from that side retina. Did you understand my point? So this is the left optic tract, yes? It receives fibres from the left, left retina. Are we clear? I need you to understand this point. Each optic tract receives fibres from that side of retina. I didn't say nasal or temporal. Nasal or temporal. I said that side. So right optic tract receives fibres from the right, right retina. Clear? The left optic tract receives fibres from the left, left retina. Are we clear with this much point? So what have you guessed? Each optic tract serves the opposite half of the binocular visual field. Yes or no? This is the most important point to be carried for. Because each optic tract receives fibres from that half of each retina. Each optic tract receives fibres from that half of each retina. Each optic tract serves the opposite half of the binocular visual field. I need you to get this point absolutely clear. Because the entire crux of the visual pathway lesions will be dependent on this single point. So this optic tract serves this half and this half of the binocular visual field advisers. Did everybody understand this? Who has not understood this survey? I want this to be absolutely clear. Okay. Some more important relationships of the optic tract. The optic tract, as you can see, this is a cut section of which part of the brain? Can anybody hazard a guess? Which part of the brain? Which part of the brain? Brain stem. Wonderful. So this is a section of the midbrain, right? So these two things that you see here, these are the crux cerebri, the cerebral peduncles. That is what is shown here in another view here. The optic tract winds around the cerebral peduncle of the midbrain. Yes, you want to ask something? Yeah, about what you said for each optic tract serves for the opposite half of the binocular visual field. Because it receives fibers from that side of retina. I'm going to go into it again, just after I finish the pathway. But right now itself, I wanted to be with me and follow what I'm saying. Look at the color coding. Pink, pink, yes? But they're from two different... That's why I use the word from the same half of each retina, did I use that word? Each optic tract receives fibers from the same half of each retina. What is the meaning of this phrase? The left optic tract receives fibers from the left half of the left eye and the left half of the right eye. Makes sense or no? The right optic tract receives fibers from the right half of the right eye and the right half of the left eye. That's what I meant by it receives fibers from the same side of each retina. I spelled it out okay much more elaborately, but I need you to understand that point. That is the reason why he has given the color coding. The optic tract then winds around the central pinhole. When it is winding around the central pinhole, take a good look. It is closely related to this artery here, which is called the anterior coroidal artery. It is a branch of the internal carotid. It is an anterior carotid artery, which is a branch of the internal carotid. And this internal carotid artery supplies the optic tract. And again, there's a very important clinical correlation, which we shall see later on on Monaco's syndrome, but I won't talk about it now. It is related to the anterior carotid artery. This optic tract, it projects to the lateral geniculate body. Optic tract projects to the lateral geniculate body. And if you remember, when we were talking in talk one, we talked about something called transneuronal degeneration. Remember, can you define it for me? What is the meaning of the phrase transneuronal degeneration? Exactly. And I use this example. If I were to do a transaction or some injury to the optic tract, there is transneuronal degeneration of the lateral geniculate. This is an example I use on Instagram. Okay. Next point. Optic tract, I told you, ends in the lateral geniculate body. It is shown here. Let me use this picture. Optic tract ends in the lateral geniculate body. 90% of the fibres sent to the lateral geniculate body percentage of fibres do not go to the lateral geniculate body. They go to four other places. What are those four other places? Can you see this picture here? Some fibres are branching away. Some fibres are branching away from the optic tract. Now, some set of fibres go to the pre-tectal nucleus. Do we remember the pre-tectal nucleus? They are situated just into the superior colliculus nucleus in the membrane. What do they serve? They serve the buprenerial nitric acid. I'm going to tell you the pathway of the nitric acid at the end of this chapter. Second, some set of fibres go to the superior colliculus acid. What do they serve? The visual body reflex. Which also I'm going to tell you at the end of this chapter. Third, some set of fibres go to a special nucleus in the hypothalamus which we have not seen yet. That is called the suprachiasmatic nucleus. And what does it serve? It serves the circadian rhythm, menial-medical institution. A and L, circadian rhythm. And fourth, some set of fibres go to the medial geniculate body in the thalamus. We will see the medial geniculate body in the auditory pathway. And it serves the audio-visual reflex. So you see the auditory acid, some fibres do not go to the lateral geniculate body. They go to four other places. What are they? Pre-tectal nucleus or buprenerial nitric reflex. Superior colliculus for the visual body reflex. Suprachiasmatic nucleus for circadian rhythm metatonic. And medial geniculate body for audio-visual reflex. So these set of fibres which are branching away from the optic tract, they are serving various visual reflexes. The reason I put this picture here is just to remind you, we talked of something called the brachium of the superior colliculus or the superior brachium and brachium of the inferior colliculus, the inferior brachium. The fibres which go to the pre-tectal nucleus and the superior colliculus, they go through the superior brachium. They go through the superior brachium. That's why I put this picture here just to remind you and correlate with what you have seen in the brainstem. So this is the optic tract. And finally the optic, the lateral geniculate body. This is the picture of the thalamus. You will see the details of thalamus in the chapter at the end of this vlog. At the posterior end of the thalamus, where you see this green color here, this is the posterior enlarged end of the thalamus and I think I did mention it during the lab session, this posterior end of the thalamus is called the pulvinar of the thalamus. I'm not talking of the thalamus now, I'm just going to give you the relationship. If you look under the pulvinar of the thalamus, you'll find two bent swellings. The medial one is called the medial geniculate body and the lateral one is called the lateral geniculate body. The medial geniculate body is concerned with visual, the auditory pathway, the lateral geniculate body is the place where the optic tract synapses. Why is it called the geniculate body? Because I told you genomics are bent. These structures, they are bent like the knee, that's why they are called the geniculate body. This is again another picture to show you, the thalamus, and you can see the medial and the lateral geniculate body. This is the lateral geniculate body. The lateral geniculate body is the place where 90% of the optic tract synapses. The structure of the lateral geniculate body. The lateral geniculate body is shaped, this whole thing is the lateral geniculate body. Can you see its curve? This is the ventral part, this is the torso part. The lateral geniculate body has got six layers of cells, numbered from below our, from ventral to torso, as layer 1, 2, 3, 4, 5, 6. There's one and two. They are phylogenetically the older cells, the older cells. They are called the magnocellular layers. Layers 3, 4, 5, 6 are phylogenetically recent and more advanced. They are called the parbocellular layers. So you're wondering, yes or no? One and two serve the magnocellular stripes. 3, 4, 5, 6 serve the parbocellular blots. In between each of these layers of cells, there's a little bit of white matter, which is called the neuropil. Neuropil by definition is the white matter, in between gray matter. And these neuropil layers also contain a few scattered smaller cells, which are known as the corneocellular layers, but they are not important worlds. Most important are the magnocellular and the parbocellular. Layers 2, 3, and 5 receive impulses from the same eye. Layers 1, 4, and 6 receive impulses from the opposite eye. Layers 2, 3, and 5 receive impulses from the same eye, and 1, 4, 6 from the opposite eye. The magnocellular layer and the parbocellular layer, they all project to layer 4 of the visual cortex. And this projection constitutes what is known as the stria of Gennari. Go back to the Chapman Cortex, you will find you can see the stria of Gennari. The parbocellular blots layer is responsible for color vision, visual equity, and background recognition. The magnocellular stripe system is concerned with motion, depth, and spatial perception. I'm repeating again. You don't have to know the details of all the pathways. Just know this much, which I told you. So these are few selling points about the primary and the lateral geniculate body. Let's apply the lateral geniculate body. It is supplied by the anterior coroidal artery and the posterior cerebral artery. The lateral geniculate body is supplied by the anterior coroidal artery and the posterior cerebral artery. I think we will stop now, because the optic radiation is a little complicated and I'm going to start the video tomorrow when all of you are a little fresh. So we will stop here.