Last time we talked about the very basic anatomy of the eye. It was a lot of material. It's too much. For you, I will sum up.
Light and images come through the cornea and hit the lens. The lens flips the image backward. The light continues to the back of the eye, often to your area of best focus called the fovea, where it filters through the layers of cells to hit your rods and cones. The rods and cones send signals to bipolar cells, which send signals to ganglion cells. The axons of the ganglion cells mass up at the back of eye and head off as your OPTIC NERVE!
Got it? Good. 🙂 If you want more, please head over to Monday's post!
So, an image comes in. It will get flipped BACKWARD. It will hit the first cells LAST (the rods and cones are actually buried beneath several other layers). From here, we will go along to the back of the brain, and on the way the information will get flipped upside down. And then our brain processes it, and everything's all right.
So, the image has come through. We're past the lens, and everything is backward.
One more thing to remember from last time, remember the macula, that yellowish area containing the fovea, where you have the most cones? The macula and fovea may be small, but because they have very high cone densities, they will be over-represented the whole way down the optic pathway. That fovea, you know when they have a big section of tract they're trying to make up for something. 🙂
So anyway, once the ganglion axons form up and get myelinated, they head off as the optic nerves, one from each eye. And then, they run into each other at a place called the optic chiasm.
You can see in the picture above where the two nerves cross, and in fact this is totally visible on a fresh brain, it makes a great anatomy landmark. But the thing is, the nerves don't cross completely. They partially decussate. This means that all the fibers from the part of the eye closest to the NOSE cross, while the fibers from the part of the eye closest to your ears DON'T. It looks like this:
Put another way, you can see there the green fibers from the right half of each eye go to the right, while the purple fibers from the left half of each eye go to the left. This seems really odd, but it's actually extremely important for depth perception, as the brain has to compare the views from both eyes to get an accurate sense of how far away something is.
From the partial decussation, the two tracts (the left containing information from the left half of both eyes, and the right containing information from the right half of both eyes) will curve around and in toward the thalamus, where they will make their first synapse in the lateral geniculate nucleus. This is one of my favorite thalamic nuclei. It's so cute, all hanging off the butt of the thalamus like a little testicle! It has a partner, the medial geniculate, and with the two of them together, well. 🙂
Side Note: You only even see an image of one half of the thalamus at a time, but remember that's located just off the midine in both halves of your brain. So you have TWO lateral geniculate nuclei hanging off the butt of your thalami.
The lateral geniculate nucleus may be tiny, but there's a lot going on. The lateral geniculate has six major cell layers, and each of these layers sees something different. They all see a given point in the visual field, but layers 1, 4, and 6 only see information from the contralateral (the opposite side) eye. This means that layers 1, 4, and 6 of your RIGHT lateral geniculate will receive information from your left eye, while layers 2, 3, and 5 receive ipsilateral (the same side) information, so they would receive information from your RIGHT eye going to the RIGHT lateral geniculate.
Layers 1, 4, 6: Contralateral
Layes 2, 3, 5: Ipsilateral
What's really crazy is that layers 1 and 2 are more sensitive to movement and contrast, and are made mostly of large cells. We call layers 1 and 2 magnocellular layers. Layers 3 through 6 are more sensitive to color and form, and the nerve synapses on smaller cells, so we call these the parvocellular layers. But remember, the organization is retained, so layers 1 and 2 are more sensitive to movement and contrast, but they are still seeing the SAME bits of the visual fields as their fellow areas in 3-6.
After the information coming from the optic nerve is transferred at the lateral geniculate nuclei, the next set of axons project through the internal capsule, a tract of white matter that carries large amounts of information to and from the cortex to lower parts of the brain. Once they come out of the internal capsule, the axons from what are called the optic radiations. These are surprisingly beautiful branches that head off toward the back of the brain to your occipital lobe, where primary visual processing takes place. Check 'em out.
Anyway, you can see the optic radiations spreading out (the front of the brain is that the top of the picture and we're looking from the bottom), and heading to the occipital lobe, specifically to the primary visual cortex which is on either side of a divide in the brain called the calcarine sulcus. And here's where the final flip takes place. Things have come in from the outside world. The lens flipped them backward. The information partially crossed at the optic chiasm, but don't worry, the information is still backward. It went into the lateral geniculate nucleus in the same pattern. But in the optic radiations, though the order is maintained, the BOTTOM parts of the visual field head to the TOP portion of your occipital lobe, while the TOP part of your visual field heads to the bottom of the visual cortex. So now, we are backward, and we are UPSIDE DOWN.
I'm hoping that picture is big enough for you to see the patterns, but on the right is your visual field, while on the left is the primary visual cortex on either side of the calcarine sulcus (this is all very inside your brain, along the midline at the back. If you could stick a finger in the back of your skull at the center and wiggle it around about two inches in, about there). You can see that the top parts of the visual field end up at the bottom of the visual cortex, while the bottom parts end up at the top. The outer parts of your visual field also end up the furthest in.
From there, the visual information goes through primary processing, and off to secondary processing in other parts of the brain, causing you to eventually identify the thing you saw as a lolcat.
So. The image came in. The lens flipped it backward and it hit the rods and cones. The rods and cones sent it to the basal cells, the basal cells sent it to the ganglion cells. The ganglion cells form your optic nerve and head back, where they partially cross at the optic chiasm (the right side of the visual field goes to the right, the left side goes to the left). We're still backward.
Then the image information (still backward) heads to the lateral geniculate nucleus of the thalamus. There, the information striates out by type (form and color vs contrast and motion), but we're still organized the same way, and the image is still backward. Then the next axons fan out into the optic radiations, and here the backward information is flipped upside down, with the top part of the visual field going to the bottom, and vice versa. Final result: primary visual cortex sees that lolcat backward AND upside down. But don't worry, the secondary processing will make all of that come out in the wash.
And that, my friends was the optic nerve! On Monday, it's onward and backward in the brain to the OCULOMOTOR NERVE!!!