The Visual Pathway: Neuroanatomy Video Lab - Brain Dissections

1400477392sstensaas.txt >> I have here a schematic of a hemisection of the eye.

And it's nice and simple.

So you can imagine a light coming from where my arrow is and passing through the cornea where the light rays are bent to a certain degree and then through the aqueous fluid that occupies this anterior chamber.

Next through the lens where the light rays are bent even more.

And back through the jelly-like vitreous to end up, if you are looking and focusing on something right here in this pit called the fovea, in the back of the eyeball.

The fovea is part of the retina, which is this dark red structure here.

And if we look at the retina in more detail, we can look at a cartoon that illustrates the different types of cells found in the retina although they're not distributed in exactly this fashion.

This is a composite view.

So here comes our light.

It's coming through, and it has to move through a layer of cells here and another layer of cells here and the cell bodies of the receptors to finally reach the outer part of these receptors where the photopigment is that is important in the initiation of the phototransduction process.

Wait, you say, light is coming in here and has to go all the way through these layers in order to begin the process?

That's right.

It's just inside out from how a logical person would have designed it for the best visual acuity.

But, be that as it may, let's now go backwards from the photo transduction moment back to cells here in this intermediate bipolar layer processing information to another layer of cells, ganglion cells, of different sizes and shapes doing other types of visual processing.

And finally to the axons.

The axons coming from these individual ganglion cells and forming a nerve fiber layer on the inner surface of the retina.

These axons are not myelinated, so that light can pass through them more easily, but only when they come to the surface and amass here at the level of the optic nerve where they're going to exit do they pass through.

And at this point these little dotted lines represent the myelinated axons.

The myelin allows the nerve impulse, which is now generated, to be propagated all the way to the thalamus.

So we have axons in the optic nerve.

But notice, we have something else.

We have a red retinal artery coming in and splaying out over the surface of the retina.

And we have this blue retinal vein exiting through the optic nerves.

So we have retinal vessels going and coming.

And they're not floating out here in the vitreous.

They're plastered up against the surface.

So now let's look at a diagram that will show us in more detail the foveal area and the optic axis.

This hemisection of the eye is turned 90 degrees from the last one.

So light now is coming from above here.

And we want to focus on the visual axis.

So light rays coming in get bent.

And you'll notice if it's coming from the left side it gets bent a little bit and ends up to the right side of the optic axis.

And that's the way that we want to think about light falling on the retina.

This pit here is the fovea that we have this dip in the retina, which is for acute vision.

It's acute vision because it has a thinner surface for the light to penetrate.

It doesn't have all those cell layers.

The cells have been displaced laterally.

So light falls without as much disturbance upon those receptor cells, cones in this case, in the fovea.

And the rods for peripheral vision are concentrated around the outside.

So think of the foveal region for cones and the peripheral region for the rods.

High acuity, color, low acuity, low light, and dispersed light.

So now this area on either side of the fovea is the macula.

Macula means spot.

And lutea means yellow.

So we have this yellow spot.

But when you look at it through the ophthalmoscope it really doesn't look very yellow.

Now let's look through the ophthalmoscope and see what both the optic nerve or disk would look like and what the fovea looks like.

This is the image you would see if you were to take your ophthalmoscope and look through the right eye of a patient with skin that has very little pigment in it.

So we have the two most prominent areas, the optic nerve, very, very easy to see, and notice you can see those retinal vessels, the artery and the veins coming through the optic nerve head, and the axons from the ganglion cells would be passing back through this space.

So this is the optic disk also called the optic nerve head, also called the optic papilla.

And over here, this is the area of the fovea.

There's supposed to be a yellow spot, but I personally can never see it.

What I see is a blush.

And the retina is thinner particularly in this pit.

This is the axis upon which the collimated, most focused light rays fall.

This is the area of acute vision.

This is cone vision.

Color vision.

And this is the area which degenerates in macular degeneration.

And now you can understand why macular degeneration is such a huge problem because it is the type of acute vision that you use for reading and focusing all of the time.

Now let's look at what it would look like if we looked at both eyes and compared them and put them side by side.

Now we have a picture with the fundoscopic view from both eyes.

So you have to imagine the nose in the middle.

And this is the right eye.

And this is the left eye of the individual.

Note that the optic disk and the fovea, the optic disk and the fovea.

And the fovea is always to the side of the optic disk.

In other words, the optic disk is more nasal when the eye is looked at through the ophthalmoscope.

And you can see all these little retinal vessels just beautifully.

And this is the area of the fovea.

So this is the macula lutea, but it's not very yellow.

And then here is the deeper pit of the fovea, which is just a fraction of a millimeter.

And we also can see that this retina is different from the last one in that it looks browner.

And that is because this is from a person with more pigmented skin.

We have here a person who is looking out at their visual world, and we're hovering looking down from above.

And we put up this blue wedge, which shows what the left eye sees when the right eye is closed.

And we put up a red wedge, which shows what the right eye sees with the left eye closed.

Now we open both eyes, and we see both the red and the blue wedges.

The central area between 60 degrees is seen by both eyes.

This is how we get stereoscopic or binocular depth perception.

To orient you to the visual system, I have this little animation, which let's walk through as an overview of what we're going to see later in other diagrams and gross specimens.

This represents the visual field.

All right.

We call it visual field of what each eye sees when the other eye is closed.

So this is the right eye visual field.

And this is the visual field of the left eye.

Now there's another term that I want you to avoid using, and that is retinal field.

That is where the image falls on the retina, because what you test in a patient is what they see as if you were standing behind the person just as we saw with the wedges.

Now, the back of the eye has the retina with the ganglion cells.

And these ganglion cells are connected with other cells, the receptors, which we'll talk about later, but what you need to understand is what part of the visual field falls on each retina?

The visual field is described in quadrants.

There is the central macular vision for each eye.

There is the upper half, the superior half, and the inferior half.

And there is the right or temporal half and the left or nasal half of this eye.

So we have nasal because it's toward the nose.

We have temporal because it's on the outside.

We have superior because it's above the horizon and inferior.

So we have all these quadrants or hemispheres of the visual field.

We use terms superior, inferior, temporal, nasal, peripheral, central, or macular.

And we describe each visual field for each eye in that way.

Now, the ganglion cells, because they are stimulated by light rays in the opposite half of the visual world and are bent on to the retina by the cornea and the lens, the retinal fields are just the opposite of the visual fields.

So to avoid confusion, we only talk about the visual fields.

Now the ganglion cells are the cells that send their axons back to the thalamus.

Now, let's look at the left eye here.

The left eye has two halves, and those that are nearest to the nose are going to cross after they come back through the optic nerve.

This is the optic nerve.

And there is at the chiasm, the optic chiasm, a partial decussation, crossing, some of those axons cross to the other side, and some of those axons do not cross to the other side.

The result of this is that the information that is coming from the left half of the visual world, the left visual field of each eye, falling here on the retina is crossing and not crossing to end up in the right thalamus.

Left half of the visual world is going to the right thalamus to a nucleus called the lateral geniculate.

From that nucleus come axons called the optic radiations.

They're similar to the internal capsule, the back part of the internal capsule.

And those axons come back and terminate in the visual cortex, in striate cortex along the calcarine fissure.

And this represents the upper bank of the calcarine fissure, and this is the lower bank of the calcarine fissure.

And there is a retinotopic projection such as the lower part of your visual world is on the upper part, and the upper part of your visual world is on the lower part.

You ought to be used to things being backward from what you expect by now.

The end result of all of this is that the left half of the visual world of each eye is now in the right thalamus and the right cortex.

Very similar to sensory information, Sensory information from the left side of your body ended up in the right postcentral gyrus.

Visual information from the left half of your visual world is going to end up in the right visual cortex.

Now before we go to the gross specimens, let's look at this model.

And we can see here in the left orbit we can see the optic nerve in the back of the orbit.

Now let's see what happens as the nerve comes in through the optic canal.

I am going to turn this.

And now we're looking at the inside of the cranium.

And here we have our two optic nerves, the right and the left.

And behind here we have the chiasm, the optic chiasm.

This is where some of the axons are going to cross to each side.

This is our partial decussation.

And behind the chiasm, these are called the optic tracts.

The right and left optic tract.

The left optic tract is looking at the right half of each visual field.

And the right optic tract is looking at the left half of the visual field of each eye.

Now, notice here some relationships.

This is the internal carotid artery.

It sits right here, right in the notch between the optic nerve and the optic tract.

And if you recall some anatomy, underneath the chiasm, not visible here, is the pituitary gland.

So remember that pressure from a tumor of the pituitary gland can push up onto the optic chiasm and disrupt some of that visual information that's coming in or pressure from a dilatation of the internal carotid artery could press on some of the axons in the lateral area.

So now, let's move on and look at the gross specimen.

So here we have the ventral surface of the brain, and to orient you we have the olfactory bulbs and tracts coming in, cranial nerve I.

And here is our cranial nerve II, our optic nerve.

And behind it we have the chiasm, the partial crossing or decussation.

And behind that, the optic tract on each side.

This small artifactual hole is where the pituitary gland was attached.

And the stalk has been torn off leaving us a nice view of the ventral surface of the hypothalamus.

Now, each optic tract on either side comes around the cerebral peduncle on either side.

So the optic tract wraps around that side and that side.

And notice it is in close proximity to the posterior cerebral artery.

The tract is going to relay in the thalamus, in the lateral geniculate, and the posterior cerebral artery is going to supply our visual or occipital cortex.

All right.

So now we're going to pretend that we're making coronal sections through this beautiful brain.

I don't want to disturb it, and we're going to make and create three slices, three coronal or frontal sections.

And we're going to look at the most rostral one right here.

This section is through the optic chiasm right here.

This is the third ventricle.

And here's where the pituitary stock was attached.

And out here we have the chiasm beginning to form the optic tract.

So here is the temporal lobe on either side.

And the third ventricle, the very front of it, you can see a bit of the anterior commissure and the two lateral ventricles.

And here are branches coming off of the middle cerebral artery.

Now, let's go to the next section.

This section is back a little bit farther.

We have the level of the thalamus.

This is the central part of the thalamus.

This is the third ventricle and hypothalamus down in this region with the mammillary bodies.

And out here to the side on either side, this area if you look at it carefully up close, this is the optic tract that's coming back.

Remember we are more caudal than the last section.

So here's the optic tract coming back, and it's looking for the lateral geniculate where it is going to relay or synapse.

So this section is really far back.

This is the level of the pineal gland and the superior colliculus and the aqueduct.

So this is midbrain.

We're at the very back end of the thalamus, a region called the pulvinar, which isn't important for our discussion.

And then right out here to the side, below the pulvinar, are two geniculate bodies, a lateral one and a medial one.

The lateral one is for vision.

And we're going to learn that the medial one is for audition.

So the way I remember that is lateral light, lateral geniculate, medial music.

So we're just going to focus on the lateral, and then out here come the axons from the lateral geniculate out into the back part of the internal capsule, and they're going to continue posteriorly, caudally, toward the occipital pole.

So next we're going to look at an axial or a horizontal section as if we had cut through in this direction.

So just rotate this like this, and then we're going to put the brain down, and we're going to lift up an axial or a horizontal section.

So orienting you, this continues to be the frontal lobe.

This is the occipital lobe.

This is the ventricular system, the frontal horn, and the occipital horn of the ventricles.

And now, if we come in close, we can see kind of a white band of fibers.

Visual Acuity Those are called the optic radiations.

They are the axons coming from the lateral geniculate, which was approximately here but down a little deeper, coming from the lateral geniculate on each side, and going back to the visual cortex.

They're going back to the visual cortex in an arranged fashion.

We call that retinotopic.

Remember, that if this is the right hemisphere, these are going to be axons representing the left half of the visual world of each eye.

And if this is the left hemisphere, then these are representing information coming from the right half of the visual field of each eye.

So now let's look at a mid-sagittal section of the brain, and look at the organization of the visual cortex.

So now we have very nice right hemisphere.

And we can see the occipital lobe going back like this.

And this sulcus is the calcarine sulcus or the calcarine fissure.

And the cortex on the upper part of the bank of the fissure is representing the lower part of your visual world.

And the part on the lower bank is representing the upper part of your visual world.

Yes. I know.

It's just backwards.

And this area that's more toward the rostral pole is the peripheral part of your vision.

And the part that's back here, particularly around the pole here, is the macular or fovea area.

So there is a greater area of cortex devoted to your central and important foveal vision than there is to the periphery.

And it's just the same as in the somatic cortex.

You had a bigger hand and face area than you had a foot or leg area.

So we have a retinotopic projection with the macula, and then the periphery coming along the bank here.

So this occipital pole back here is your foveal or macular vision.

This is the important part.

And all of this area is supplied by the posterior cerebral artery.

So as humans, we are very visual people.

And this occipital lobe is almost entirely devoted to visual processing.

Now, we've only been talking about the primary visual cortex, but surrounding it sort of in concentric circles are secondary or association visual areas.

There are probably several maps of the retina onto the cortex, but they're doing more sophisticated processing than is going on in the occipital pole.

And in general, we like to think about information in the association areas as going in two directions.

There is this dorsal stream going up toward the parietal cortex, and a ventral stream going down toward the temporal cortex.

And some people say that the dorsal stream going to the parietal area is trying to help you with location of things in space, where things are.

For example, if you are doing visual guidance and trying to coordinate a hand movement whereas the ventral stream is more involved with object recognition and knowing what it is including things such as faces or naming of objects, which occurs when you have the information coming

down into this ventral region toward the temporal lobe.

So there are a few take home messages.

The most important one is that the right hemisphere is looking at the left half of the visual world of each eye.

You just have to remember that.

And the other thing is that most of the time you're using your central vision.

You're using cone vision.

You're using daylight vision.

You're doing color vision.

All right?

And the periphery is looking at night vision or rod vision.

It's important now to realize that the visual pathway extends all the way from the eyeball, which was right under the orbital cortex here back through the optic nerve, the optic tract, through the thalamus all the way back to the occipital pole.

So there are many places in the brain where you can have visual defects or visual lesions, which will give you changes in the visual field testing.

So there are various arteries.

We already looked at the retinal arteries supplying the retina, but there are other arteries supplying the chiasm, the tract.

The middle cerebral artery is important in supplying some of the optic radiations here.

And we've seen the posterior cerebral artery that supplies the posterior region.

So anterior circulation and posterior circulation problems in your vascular system can give you a visual defect.

So the visual system is a very sensitive way of analyzing the brain.

And don't forget that the eye is the only place where you can look into your brain.

Right? The eye, looking at the back of the eyeball you're looking at central nervous system.

You are looking into the brain of another person.