Anatomy and Physiology of the Eye

Anatomy and Physiology of the Eye.

As we go through this section, we're going to go through the superficial areas of the eye that are really important for you to know, and how light travels. Most of us when we look at the eye, we see a colored part which is the iris, a dark part in the middle which sometimes just looks like a dark spot but it's actually the pupil, it's an opening, and the sclera which is the white outer coating of the rest of the eye from the edge of the iris. When you think about the eye, it really is a lot like a camera. The iris in the eye that I mentioned is a little bit like the aperture in a camera. It lets light in and out depending on how much comes in. The pupil, that dark spot in the middle, gets bigger or smaller and lets light in that goes back and hits the retina.

The important thing about vision is it's more than just the eye. For years when we studied medical aspects of blindness and low vision, we mostly studied the eye. We didn't think too much about the brain at all other than just saying that's where it goes and it comes out an image. But now we're seeing there's much more interaction between the eye and the brain to interpret what light waves come into the eye and how that's interpreted into some kind of an image that a person understands. When we look at just the whole field of vision, we see there's three major components. There's the image, whatever it is that the light rays are bouncing off of. There's color, and then movement. If you think about what color most gets your attention, it's the color red. Yellow also is a big attention getter. The blues, greens aren't quite so pronounced in terms of getting your attention. Because of that, if you have certain color deficiencies that may interfere with your ability to be aware of certain things or if you have difficulty with contrast and color. Movement is something that's picked up very easily in the peripheral of your eye. Even the very far edges of your eye, if there's a little bit of movement, you can pick up that movement.

Now, this also presupposes that there's some kind of a light source that's going to be hitting whatever image it is that you would like to see. All of that then comes in to the eye and we'll go in a minute through the pathway that it takes through the eye. Then goes to the optic nerve and on to the occipital lobe of the brain, and also in some other areas of the brain. We used to think it was all in the occipital lobe but now we're seeing that about 80% of that is there, but the other 20% is around the other parts of the cortex of the brain. The camera that we mentioned earlier is actually a pretty complicated camera, more like a TV camera with a coaxial cable that goes back to the brain, so to speak, as the optic nerve. Little bit more complex than we used to understand in terms of all the various functions of that eye.

Let me tell you a little bit about the path that light takes into the eye. In your PowerPoints, there's an illustration of an eye if you want to follow along with that but I'll try and describe as much as I can so that you won't need to make reference to that.

The photograph that you have is the eye is cut in half. You're looking at the cornea on the left side and the optic nerve on the right side. It's as if the person were laying on their side and you're looking at it. When light first comes into the eye, it's coming in through the cornea. If you take your finger and you poke in your eye, your finger is resting on the cornea. It's a clear avascular tissue that is on the outside of the eye. Then that light moves in through what's known as the anterior chamber, the front chamber of the eye. The eye is about the size of a ping-pong ball for most people. It's usually round in shape like a ping-pong ball except where the cornea is and that bulges slightly. It isn't going to roll smoothly, so to speak, like a ping-pong ball.

That anterior chamber is the inside of that bulge part where the cornea is. Behind that, there is the iris and the opening of the pupil, and light moves through that opening and through the lens of the eye. The lens is - again, without blood vessels in it - avascular and it's in a little capsule that attaches it to all around the edges to something called ciliary bodies, and the ora serrata, and also the zonules of Zinn - those three areas we'll talk about them more in a minute.

Light then continues to travel past the iris into the posterior chamber of the eye. This makes up about two-thirds of the size of the eye. It's filled with a jelly-like substance known as the vitreous humor. That vitreous is something that's produced on a regular basis. It refreshes, it absorbs. If there's a leakage of blood in the eye, it will be absorbed into the vitreous and eventually that will be thinned out as the vitreous is reproduced. It is tacked around the inside of the eye to that tissue that lines the inside of the back part of the eye, which is known as the retina. You can have a vitreous detachment when that pulls away. You get little light flashes when that happens.

Light continues to go back. If it's in the center of your vision, it's going to go straight back into something called the macula. The macula is, again, avascular. You don't want any of those blood vessels getting in the way of light. So, it comes right back to the macula. The cells there goes through several different layers in the retina, back through something called the choroid and the sclera, down through into becomes part of the optic nerve, little nerve fibers. There are millions of them making up the optic nerve. That goes back into the major center part of the head in some ways that underneath the brain and divides.

There's actually a right and a left side of each eyeball in terms of where the nerves are attached. With your right eye, for example, you were seeing both the left and the right side through that eye. The same with the other eye, they meet and then go back on either side inside of your head to the optic nerve. One of the things that can happen if somebody has had a stroke, for example, they can lose the right side of their vision in each eye. It's not just that they're losing vision in their right eye, it's a little bit more complex than that. So, that's how light travels through.

We have a lot of different things that protect the outside of the eye. Nature wants it to be in good shape so you can get lots of use out of it. There's this bony orbit outside the eye that your eyebrow is the top part of that and your cheekbone is the bottom part and your nasal area there, all form a little protection for that eye. There are muscles that are attached over the top, underneath the bottom, one on either side, and then two that go diagonally over the top and the bottom of the eye. Those muscles help you to move either to look quickly at something or quickly away from something. They tend to, if all is well, work in tandem - both eyes together.

Your eyelids and, obviously, they close out light when you need to. They also are lined with something called a conjunctiva. That conjunctiva goes from the edge of your eyelid, all around the inside of your eye up to the edge of the cornea. It's a light tissue. If people have an infection in their eye, that tissue becomes very red as blood vessels engorged a little bit to put more nourishment into the tissue. The eyelashes are designed to keep out dust. It seems like sometime we get eyelashes in our eyes but that's not their intent. Then we also have tears. The tears wash away any kind of foreign substance that gets into our eyes. There's a certain enzyme in it called lysozyme that inhibits bacterial growth in the eye. There are lots of different things to protect the eye.

Now the structure of the eyeball itself, it's made up of three distinct layers. That's the sclera which is the thick, fibrous outer coat that gives resistance and durability of the eye. It gives it some flexibility. If it's struck, it will give a little bit but at the same time it works very hard to keep things from penetrating. It's very sturdy, very durable. The inside layer is inside, in terms of sandwich, because there are three layers. This is the one in the middle, it's the choroid. This is a very delicate vascular coat which supplies the nourishment blood to the inner layer of the eye which is the retina. Then the retina is that photosensitive layer that is very sensitive to light that translates light impulses into electric impulses which travel down the optic pathways to the brain.

Now the muscles of the eye - just kind of go back to that a second - they each have names, and I think it's helpful for you to learn these names. As I mentioned, there's one over the top called the superior rectus. There's one on the bottom called the inferior rectus. One on the nasal side of your eye called the medial rectus and one on the outside of your eye called the lateral rectus. Then across the top I mentioned there are two diagonal muscles, those are known as the superior oblique on the top and on the bottom is the inferior oblique. Those six muscles on each eye can help you to move your eyes in every direction you can imagine - just through those six muscles.

Now I mentioned also that you have tears but you have two distinct kinds of tears. The first is normal tears, and those serve as lubricant. They protect from bacterial infections like I mentioned before because they contain an enzyme called lysozyme. Then there's something called reflex tears. Those are mostly water and there are things that come in when-- I guess you need reinforcement for the first tears. It could be that you're having allergies and your eyes are watering. It could be that you're having some strong emotions and that, for some reason, the brain decides you need to wash your eyes when that happens. If there is irritation or dryness, those reflex tears may come into play.

The normal tears of the eye are made up of three different layers. The first layer is mucin which is kind of a mucus like substance - sounds like it the name - that's produced by the goblet cells that help to adhere the tears to the eye and are produced deep inside the eyelids. This keeps the tears from just falling out of your eye, that extra attachment there. The middle layer which is the widest layer or the mixed up most of the substance of the tears is about 90% of it is water, and this is produced in the lacrimal glands which are located underneath your eyebrow. The third layer are just fatty oils, and they are on the outside. They're called lipids. They're there to prevent evaporation of the tears and they are produced by the meibomian glands at the edge of the eyelids. There's three different layers in the eyes of the tears of the eye, and those tears can be in or out of proportion in terms of how they're produced. For example, if the fatty oils/lipids are not being produced sufficiently, it may be the tears evaporate very quickly off the eye. Or if there's not enough water, your eyes can be very dry. In fact, dryness is what would happen if any of the three were not being produced sufficiently.

The cornea, again, is that transparent, avascular - without blood vessels - tissue at the front of the eye and it has five distinct layers. Its major function is to give the initial bending of light waves to come back and focus in the back of the eye. It's also surrounded by a vascular layer known as the limbus which nourishes the cornea. It should be smooth and clear. It should have around the same arching shape all the way across the front as the rest of the eye does because it's a little bit of a bulge to the rest of the eye, but still it should have about the same arch curve. The five layers: first of all is the epithelium, which is about five or six layers of cells. Next comes Bowman's layer, which is just one thin layer of cells. Then the stroma, which makes up about 90% of this thickness of the cornea. Descemet's layer which is another clear membrane, and then the endothelium which is also one layer.

I mentioned the limbus which is around the cornea and gives it nourishment but at the same time, there are some stem cells existing in that limbus. What they do is they regenerate the cornea constantly. One of the things that has been discovered in recent years is that if a doctor is going to transplant a cornea, typically they have like a little cookie cutter. If you'll excuse the expression, but they kind of cut out the cornea with one cookie cutter, use the same cookie cutter to cut out another replacement cornea, and put it back in and stitch it in. That has been fairly successful but sometimes, for some reason, just didn't work. The main reason it didn't work is they discovered that if they cut out the stem cells, that cornea would not stay healthy. Now, they've learned that the positioning is much more exact in terms of making sure those stem cells there to continue to regenerate corneal tissue.

Now the next layer after that I mentioned is the anterior chamber in the eye after you moved back from the cornea and it's filled with something called the aqueous humor. That aqueous humor is secreted by the ciliary bodies which are actually in the posterior chamber between the iris and the lens, but they secrete on the anterior side. It doesn't actually go into the posterior chamber but it kind of just oozes right into the front of the eye. It flows from where it’s produced through the opening of the pupil into the anterior chamber, and then flows through something called the trabecular meshwork, and exits the eye through something called the canal of Schlemm, then flows through the aqueous veins into the surface of the eye where it functions in part as a lubricant. It helps with some of that washing at the front of the eye.

The two things that I want to mention after that are the iris and the pupil. The iris is that colored section of the eye. It acts as the aperture allowing in more or less light. It is attached to the choroid which if you remember, was the center, the sandwich layer in the eye. You have the choroid all around the posterior chamber of the eye in the center layer and when it comes up around the front, it's attached to the iris. I mentioned that because if you get an infection, it covers all the way around the inside of the eye - if you have an infection on the choroid or the iris. All that tissue gets involved. Then the pupil which is in the center of your eye, which is the aperture opening that lets light in.

The lens is in a small capsule. It's bi-convex which means it's curved outward on both sides, avascular, it's colorless supposed to be if it's healthy, and is almost clear. It's suspended from these little ciliary bodies. Remember the ciliary bodies produce the aqueous humor, they also are holding the lens in place.There are little tiny fibers amidst the ciliary bodies that are known as the zonules of Zinn, and those are the major attachment guy wires, so to speak, for the lens.

When the brain sends a message down, you're looking at something and it wants to focus on whatever it is you're looking at. The zonules of Zinn are what would either pull the lens a little flatter or relaxed so it's a little bit thicker to make up that final fine-tuning focus that the cornea hasn't done. The cornea begins it, the lens is what finishes up. It's about 65% water and about 35% protein. Depending on the curvature of the cornea, it'll help to focus at just the right spot so that you can see the clearest that you need to see.

Sometimes, the cornea isn't as cooperative as we'd like in terms of its initial focusing ability, and so you may have a light ray that'll come through. It'll meet right on the macula on it which is in the back part of the retina, just right where you want it to focus perfectly. But then again, sometimes it'll focus slightly in front or slightly in back. If that occurs, a person is either near-sighted or far-sighted. The lens does most of it but it needs the cooperation of the cornea to get the initial hit in. The reason the cornea may not be shaped exactly as it needs to be could be the eye is a little bit elongated rather than round or it could be a little narrower than it is high, and that causes the focal range to be off too.

The posterior chamber of the eye which is the back two-thirds of the eyeball structure is filled again with vitreous humor. Sometimes, there'll be a little flaking of retinal tissue or even some of the ciliary bodies may be a little flaking that cause floaters to appear in your vision. Those are normally absorbed into the body and go away without too much problem. There also can be flashes that are caused by a slight detachment of vitreous, or a more serious flash that can happen if you have retinal tissue that detaches. Again, the aqueous humor is that clear fluid in the anterior or the front part of the eye, and it's produced by the ciliary bodies. And the vitreous humor is that clear gelatinous or it's a little more like jello fluid in the posterior chamber of the eye and it's tacked around the retina.

The retina is the next layer we'll be discussing, and it makes up the inside of the posterior chamber of the eye. If you remember, there are three layers: the inside layer being the choroid, the outside being the sclera, the inside is the retina. Retinal tissue is basically brain tissue. It needs just as much nutrition and oxygen to stay healthy as brain does. When a physician is doing an examination, they dilate somebody's eyes so that the iris moves back and they can look directly in. That's known as a fundus view. A lot of times when you see photographs that are photographs of the retina and the back of the eye, that's what it is. It's a fundus view. It's a thin, semi-transparent, multi-layered sheet of neural tissue. It appears pink when it's healthy. Part of the pink is in one of the layers of the retina and much of the pinkness comes from the layer underneath that called the choroid, which is what's moving blood vessels through the eye.

Retina is actually made up of nine layers. For our purposes, we're really going to be concentrating on three of them, two more than the others. I want to make sure that you're aware of all the layers. First is called - and this would be from the outside to the inner layer - is the Bruch's membrane. The Bruch's membrane is basically the suction that holds the retina to the choroid. The next layer is the retinal pigment epithelium known as the RPE, and a lot happens. It's kind of the magic layer. That's where those photoreceptor cells translate somehow into electrical impulses that go to the brain and that are RPE layer. Then the photoreceptor cells known as the rods and the cones which take light and begin that process of changing into an electrical impulse. Then you have the fourth layer which is the external limiting membrane, then the outer nuclear layer, the inner plexiform layer, the ganglion cell layers, the nerve fiber layer, and the internal limiting membrane. Again, the only three that you really need to be responsible for are those first three - Bruch's membrane, retinal pigment epithelium, and the photoreceptor cells.

Now, there's certain areas of the retina also that I like us to focus on a little bit. There's six basic areas. The macula which when you looked at the fundus view of the eye and you looked in, it's a little darker pink than everything else but it doesn't have any blood vessels in it. The RPE, the retinal pigment epithelium, has concentrated more pigment in that area. And part of that is because of the functions that take place on the macula. There's also the arteries or the veins of the eye, and those come out of a yellow area which is on the nasal sort of part of the eye. It looks very yellow in appearance, and it got blood vessels and veins coming out of it. That's the optic nerve. What you're looking at is the optic disc which is the attachment layer of the optic nerve. Then if you look very closely at the macula, there's a center on the macula which is even more-- a little bit more pink and that's called the fovea centralis, or sometime just the fovea. Then the rods and the cones which you really can't notice, but they're-- when you're looking into the eye but they're all over the inside layer, and the pigment epithelium cells.

The macula is that area of the retina that's responsible for your central detailed vision. There's no blood vessels passing through the macula, and its nourishment comes from the RPE, the retinal pigment epithelium. It appears, again, darker pink in color. The center of that which is most concentrating in terms of the pigment is also known as the fovea. The veins and arteries that come into the eye, again they come out of the optic disk or the optic nerve. And you have the central retinal vein, the branch retinal veins, central retinal artery, branch retinal arteries and when occluded, the retina turns kind of pale and yellowish. If any of those veins aren't working right, they're not bringing oxygen in, things get very pale and yellowish very quickly. Again, the arteries are going to be bringing blood flow into the eye and the veins are going to be taking the blood out of the eye.

The optic nerve consists of over a million axons from the nerve fiber layer at the retina. 80% consists of visual fibers and axons that are going to terminate in the occipital lobe of the brain and the rest are going to be on the other part of the cortex of the brain. It has been thought not to be able to regenerate if it's broken. If somebody severs the optic nerve for some reason, it's always been thought that that would never be able to regenerate, partly because of all the millions of cells that have to make contact and where to become a whole again. But now there's been some research done at Schepens Eye Institute in Boston, and they are in clinical trials looking at regenerating optic nerves in rats. We don't know how far out in the future it may be that that optic nerve tissue will be able to regenerate but we'll look for that in the future.

Now again I mentioned the optic disc, which is on the nasal side of the macula and appears yellowish in tint, and then the optic nerve head where ganglion cells known as axons exit the eye to form the optic nerve. It does cause a slight break in our field known as a vision called as scotoma but the brain covers it up. So, it isn't like when you are looking, you see a couple of blind spots - one for each eye where the optic nerve is - but what instead, the brain just kind of feels in the blank there. Well, 99.5% of the time it fills it in pretty accurately, but occasionally it may miss something. I'm convinced that it has to do with some of those parking lot bumping road vendors because they're in your blind spot that you are not aware of.

Anyway, moving on, the next thing is the retinal pigment epithelium. Again, that's a single layer of hexagonal cells that are packed with pigment granules that give the retina its pinkish color. It supplies blood for the outer two-thirds of the retina, re-synthesizes photo sensitive pigments, and it trims the outer segments of the rods and the cones. It keeps the retina attached in combination with the Bruch's membrane causing the retina to stay attached to the choroid. I mentioned to you that that's where the place, a kind of magic happens. If there's swelling or you hear the term macular edema where somebody has some swelling in the macula, a lot of times that edema is actually happening in the RPE. If it happens there, you're will have a big distortion vision because it's going to delay the function or somehow get in the way of the function.

The photoreceptor cells transfer light and images to nerves that convert images into electrical impulses. They are sent to the occipital lobe of the brain and translated into what we see. There's two kinds: rods and cones. The cones are concentrated in the macula. About 99% of them appear in the macula and the fovea, center of the eye, and they're responsible for our fine detailed vision. They're responsible for our color vision. They also require quite a bit of light in order to work effectively. And in fact, for somebody in their mid-60's, they need about 10 times as much light as somebody in their teenagers in order to read the same thing. One of the things that made them so effective is that they're bundled one to one with the nerve ending. You have one cone cell, one nerve ending that follows all the way down through the optic nerve to the brain.

On the other hand, the rods which are another type of the photo receptor cells, they don't require nearly as much light to work. If you get up in the middle of the night to go to the bathroom, that your rods are wouldn't working because you don't have as much light. But, they don't give you as quite clear vision. In fact, they don't give you too much color vision at all. They are responsible for your peripheral vision, your side vision that allows you to travel and move and walk around comfortably. But one of the reasons they don't have the same kind of details, they're bundled in groups of 30, 40 or even a hundred photoreceptor cells/rods attached to one nerve cell that then goes back through the optic nerve to the brain. And again, they don't require much light. People have a problem with their photoreceptor cells, they often have difficulty seeing at night and they have difficulty with their side vision. They may trip or they miss things, they spill drinks, those kind of things. If somebody has difficulty with their cones, their central vision, they have difficulty reading, recognizing faces, same details.

A lot of times you've heard the rumor that you're supposed to eat like carrots in order to keep your eyes healthy. Well, there's more than just a wives' tale on that. Carotene which is an element found in both vitamins A and vitamins B triggers a chemical reaction of the rods. There's two different kinds: alpha and beta carotene. It's found in things like carrots, yams, sweet potatoes, broccoli, asparagus, kale, peaches, spinach, tomatoes, and winter squash. All those times that your mom said, 'Eat your vegetables, it's good for your eyes. Particularly eat your carrots, they're good for your eyes,' and in fact, she was right. As I mentioned to you before, the retina requires a lot of nutrition. In many of the eye diseases that we see affecting the retina now, there's more emphasis being placed on diet and getting the right nutrition to the eye in order to help combat some of the progress of those diseases.

And then the last thing I want to mention is the - or almost the last thing - is the occipital lobe of the brain. That's the processing center for the brain. It's on the visual-- it contains most but not all of visual cortex. I mentioned to you that the rods do not see color. Based on what I described to you, you would think that you would have lots of color concentrated in the center part of your vision but then out in the periphery, things would look more gray or more black and white. But, yet, we still see color in the periphery of our vision. There's two reasons for that. There are few cone cells, just a very few. They're kind of sprinkled out there in the periphery, and then-- but they require a lot of lights so they're not going to function very well unless they get light. At night when you get up, you don't see the vivid colors.

The other thing is the brain fills it all in. We're seeing that more and more the activity of the brain involving of what we see in terms of what we used to think of things happening in the eye are actually happening in the brain. There's even some theorist who are thinking that there may be a particular cell that helps you see something. For example, a red pickup truck might be in one cell of the brain. If you see that truck often enough and it always looks the same, it gets into the memory of the cortex of the brain. You just need a little bit of information to recognize the truck because it's all there in your brain. Oliver Sacks who wrote the book "The Man Who Mistook His Wife for a Hat" has been doing some research in this area, looking and seeing what kind of memory maybe attached to even just one cell. That's still in an experimental stages but it's interesting.

The uveal tract, again, is that inside layer made of ciliary bodies, the iris, and the choroid. It divides the outside from the inside of the eye. It also divides the front anterior from the posterior section of the eye. And I mentioned if you get an infection, it can affect the whole eye if you get some part of that becomes infected. Again, going back to how the brain functions. As small children, when you see a baby, they'll pick up something. They touch it, they feel it, they put it in their mouth, they smell it, they experience it to its totality, so to speak, and part of that is building a taxonomy in the brain to sort out what you see.

I have a couple of slides. One is the slide that says bear or dog. They're two very large, furry, dark looking creatures. They both have long noses. One has floppy ears, one has smaller ears. If you were to describe them, other than the fact one has a collar around its neck, you might describe them in very similar ways unless you are particularly astute at being able to describe things. But your brain immediately knows which one is the bear and which one is the dog. It has sorted based on all the very characteristics.

Now, there's another slide that says bear or dog again. In this case, they're spotted. One is white, has dark ears, dark eyes, fur around the eyes, dark paws and legs. The other one does too. It has dark eyes and dark ears. But, one is the panda bear and the other one is a St. Bernard puppy. They look very similar but the brain has been able to sort them out so that a person looks immediately knows which one is which. It's the miracle of vision. The eye sees, the brain functions properly, in all kinds of sorting and understanding of what's out there visually happens.

Now one of the things that we do know is some people who get vision at later ages, and the most well- known right now is Mike May who got his vision back in his 40s, he wasn't able to interpret what he saw. It isn't like you just see it and automatically you can sort, it's something that the brain slowly does as you develop. And apparently, it's much more active in that very early years of life and the cortex of the brain is more flexible in being able to take information in and it’s beginning to sort things out. So, vision is a miracle. There will be a number of references and websites that you might want to check out. Thank you very much.