SCIENCE 101: Cranial Nerve II: The Optic Nerve, part 1, aka, the EYE

...or rather, the visual system. Cause you can't really talk about the optic nerve unless you talk about the rest of the eye along with it.

We humans rely pretty heavily on vision as a species. At least, being able to see is a lot more important to our daily lives than, say, being able to smell. But the visual system is, in many ways, surprisingly simple. In many OTHER ways, it's confusing as all get out. I will do my best. :)

So, I'm going to start with this: an image is going to come in. It will get flipped BACKWARD. It will hit the first cells LAST. It will then go along to the back of the brain, and on the way it will get flipped upside down. And then our brain processes it, and everything's all right. It's opposite day, my friends!!!

Got it? Good.

Let's go!

We're going to start with the anatomy of the eye. That picture I put up there was small, but here's a nice big one:

So, the front of the eye is to the right. The big, thick black layer that makes up the whole outer edge there is called the sclera. It's dyed in this photo, but in real life this is what makes up the "whites" of your eyes (it doesn't have to be white though, horses, for example, have black sclera). It's a protective layer that also helps form the shape of the eye, and is made of strong connective tissue. At the VERY front of the eye on the right, you can see a little thin layer over the sclera. That's your cornea, a transparent part covering your iris and pupil.

The tan disk to the middle right of the eye there is the lens. This is where the light comes in. The wavy bits of tissue on either side of it is your iris. When you put those wavy bits around the lens, and mentally tilt it so it faces front, you get this:

Those little wavy bits form the colored portion of the iris, and the black spot in the middle is your lens. There's a gap in the iris to allow light through direction to the lens, and that gap is what makes up the pupil.

Side Note: You probably noticed that there's space between the lens and the cornea and sclera. This area is called the anterior chamber, and is generally filled with aqueous humor (don't you love that there's something in your body called aqueous humor!?). This is a liquid secreted by an epithelium on the side of the eyes, in that little chunk of tissue behind the iris. The fluid flows up in between the iris and the lens and into the anterior chamber space, and then gets recycled through a little hole to the front side of the iris called the Canal of Schlemm (another great phrase of science!). This is enough to keep that area of your eye nice and lubricated, but a problem with the circulation of this aqueous humor can cause pressure to build up, which can cause damage to the optic nerve, and is what we call Glaucoma. (End Side note)

I'm sure you've noticed by now that inner ring in the first picture. In a normal eye, this ring, the retina, is pressed hard up against the sclera by the vitreous fluid that fills your eye and keeps the whole thing inflated (hehe, vitreous fluid. Don't all the words in the visual system sound so good and Galen-esque?). But this eye obviously isn't alive anymore, and with the drying out of the vitreous fluid, the retina has pulled away from the sclera. The final thing that you need to see here is that big bundle to the left. That, my dears, is the nice big chunk of optic nerve, heading off to the brain. It's also your BLIND SPOT. We'll get to that in a minute.

So, light is going to come in, hit the lens, and head to the back of the eye. Because the lens is a curved shape, the image that you're seeing will be flipped, and sent in backward. When it gets through to the back of the eye and hits the retina, it's going to hit your rods and cones. The light activation of these cells is what heads down to our optic nerves and gets processed, allowing us to see.

(yeh, yeh, I know it's large. It's good for the details)

There are five types of cells in your retina that are going to form the inputs to what will become your optic nerve. The first major types are the famous rods and cones, which lie up against your retinal pigmented epithelium. Rods are good in low-light situations (so good they can sense a SINGLE PHOTON!!), but they can only see in black and white, because they only have one type of light-sensitive pigment. They are the pink cells in the picture above that are...rod shaped at the tip. Like you might expect. Cones, on the other hand, are cone shaped (duh), and require brighter light to function. They can also work faster than rods, and contain three different types of light-sensitive pigment (for red, green, and blue), which, when they overlap, all you to see all the colors in the visual spectrum. The heads of each of these cells are filled with disks, containing the light sensitive molecules (in rods they are called rhodopsins, in cones they are called photopsins).
Rods and cones actually signal pretty constantly, and their signaling is INHIBITED when photons activate rhodopsins and photopsins. In this case, activation is actually inhibition (as with everything else in the visual system, it's opposite day!). The horizontal cells collected information across rods and cones and modulate that information and the rods and cones themselves, while the bipolar cells take information from only a few rods and cones per cell, and send it toward amacrine cells and ganglion cells. Amacrine cells are much like horizontal cells, and modulate the information coming in, but the final informational load passes to the ganglion cells. These cells have axons which form a nice big mass we like to call your optic nerve.

But here's the thing about this whole arrangement. You'd THINK that the light hits the rods and cones first. And it DOES...but only because the other cells involved aren't sensitive to light at all. Look at that picture and look at the bottom. Light comes in from the BOTTOM, from where the ganglion cells are. It hits the actual light sensitive cells...last (if you ever needed any proof that evolution is really random...).

There's another thing about this. In the big picture I put up there, you'll notice that your optic nerve, when fully formed at the back of the really FAT. And when all those axons join together and go heading off toward the brain, there's a space where there are no rods or cones at all. This is your blind spot. You might think to yourself "HAH, I don't have a blind spot!". Yes, you do. But your brain is SO AWESOME that it can fill in this blind spot in your visual field with whatever it thinks should go there, and along with information coming in from your other eye, you'll never know you missed anything.

Side Note: There's a way to test your blind spot here. Try it! It's FREAKY!

Near your blind spot, there's a 1.5mm area or so where a bunch of cells have a kind of a yellowish look. This is called the macula lutea (yellow spot, so creative, we scientists), and in the middle of it is a part of the eye called the fovea. We may have a blind spot, but this makes up for it. Here, the outer layers of the retina peel away, and your rods and cones see the world almost directly (well, as directly as you can when you're at the opposite end from where the light's coming from). This area is VERY highly concentrated in cones, and is the area where you have the highest visual acuity. When you shift your eyes to focus on something, you concentrate the light coming in on your fovea.

(I know it's huge, but I promise it's worth it)

Above you can see the macula lutea. The vitreous is the inside of your eye, filled with fluid. You can see the outer layers of the retina peeling away (the ganglion cells and all which will form your optic nerve), allowing the rods and cones (which are in the pale layer) a much more personal view of the light coming in.

So now, the light is in, the image is flipped, and we've hit the first cells LAST. Next time it is off to the brain, where things will get more mixed up, even as they stay exactly the same.

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