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PROFESSOR: Shows a simplified picture of the optic tract or
one bundle of axons coming from the retina crossing over
to the opposite side of the brainstem.
And here are the major terminations in the geniculate
body, two parts, ventral and dorsal.
And this is your colliculus or optic tectum.
And I'm showing a couple of return projections here.
It's very typical of the optic tract termination zones to--
they interconnect with each other.
The major exception being that the geniculate body, even
though it gets projections from the other structures not
just from the retina, it doesn't project back to them.
It projects to the neurocortex only.
So this was a reconstruction showing all the
projections I could see.
And I just charted them on a surface view of the brain that
actually was reconstructed from a series of closely
spaced sections.
And then I showed you a photograph of the brainstem of
a hamster with the hemispheres removed.
I was identifying some things there.
And then I drew them in on this picture.
So you can use those to check yourself.
OK, now here's a picture that was--
I'm pretty sure I published this a long time ago.
I was doing work on the newborn hamster, so I'm
showing that brain here.
You can see how relatively undeveloped the
hemispheres are there.
The entire midbrain is exposed.
And then after this period, the hemisphere grows back.
In humans, it would cover completely the midbrain.
But here in the hamster, there's about a third of the
midbrain surface here of the superior colliculus is
uncovered, and most of the top of the interior
colliculus is uncovered.
And then you see the cerebellum covering the whole
rostral hindbrain.
Just some caudal hindbrain is left there.
The caudal part of the hindbrain is called the
medulla oblongata.
OK, so some more pictures with these hemispheres removed.
And then we look down on the brainstem to see what's there.
And I tilted it.
You can see by the grid in the background that I tilted it up
at a steep angle like this, which makes the surface of the
thalamus and of the cortex pretty flat.
OK, so now you can see--
let's see.
Did I identify--
yeah.
OK.
I did the same thing here.
There you see the corpus striatum.
There you see I've just left some cortex that I didn't
carve away.
They were behind the olfactory bulb.
This region in the middle where the issue is fused is
called the septum.
And in human, that's called the septum pellucidum It's
very, very thin.
But there are cells in it, and those cells receive
projections from the hippocampal formation as well
as from the hypothalamus.
And here you're looking down into the third ventricle.
Why could we see into the third ventricle?
Well, because the roof plate is very thin.
It usually tears off when you take the brain out.
So you don't see that thin roof plate.
The same thing happens down here in the hindbrain.
And here you can see the optic tract.
And here's a picture where I-- no, this
one's probably better.
The brain's tilted a little bit differently.
This one is actually the best.
Let's look at it without my pictures.
There you see the edges of the optic tract.
You see how much wider the surface is there.
You can see why this whole caudal thalamus was called the
optic thalamus by the old anatomists, because it's
pretty much enveloped by the optic tract.
There it comes over the geniculate bodies there.
And the lateral posterior nucleus here.
And then it becomes rare there, because now there's a
lot more cells mixed in with the fibers in
the pretectal area.
And in superior colliculus, the fibers travel below the
surface, so it looks a lot grayer.
So there I've made some identifications.
OK, now what's different here and here?
Look at the striatum, left and right striatum.
Now look at them here.
See, I've taken this structure away on the right side, and
I've be exposed the anterior commissure that's coming from
the olfactory bulb and crossing the midline and then
goes back to the olfactory bulb on the other side.
There's a posterior limb that I didn't expose, but it goes
through that deeper part of the corpus striatum and ends
up in the hemisphere out here.
In fact, in the human, that interconnects part of the
temporal lobes.
The rest of the interconnections between the
hemispheres, of course, are in the corpus callosum.
And here I've carved out some of the septum.
And you can see there's a stump of two
axon bundles there.
That's the fornix coming from the hippocampus.
The hippocampus, of course, is part of the hemisphere, and I
removed that along with the hemisphere.
But it's connected there by fiber bundles.
What about the internal capsule, all the fibers coming
between thalamus and the neocortex?
What's happened to them here?
They should be here too.
Well, if this were a human brain, it would form a very
large bundle of axons in the middle here separating the
more medial part, caudate nucleus, and the more lateral
part, the putamen.
But in these rodents, the internal capsule is a series
of little bundles that coarse up through
the caudate and putamen.
So the caudate and putamen aren't really separate in
these little rodents, except there's a very medial part
here that you don't see those axons.
We often call these after a guy named
Wilson that named them.
We call them Wilson's pencil bundles, because they're about
the size of little pencils coursing
through the corpus striatum.
And it shows that caudate and putamen are the same kind of
cells, same kind of organization.
But they were named separately in human and other large
animals where the fibers are gathered together coming up
from the thalamus into the cortex.
OK.
So that's why I didn't name the internal
capsule in these pictures.
OK.
Now here it's the same brain.
All I've done is change the angle of the lighting.
What I wanted to show was that you can get boundaries that,
for a beginner, are very hard to see in some of these.
But look at here.
Very clear line there.
That's the line between the geniculate bodies laterally
and then more medially, the lateral posterior nucleus.
And we can follow that right down to the medial geniculate
body right there.
And then you can see the line between the lateral posterior
nucleus and the pretectum also very clear and the line
between pretectum and superior colliculus.
Actually, that line is here.
Everything between those two lines is the pretectal area.
So you can make out a lot in brain dissections, especially
when you're used to--
I learned a lot of anatomy just from looking at
cross-sections, and I was constantly trying to put the
3D picture together in my mind.
But if you're working with brain dissections along with
your study of sections, you can gain a 3D picture.
I know it's very hard for some people, and they can't do it
as well, but everybody has their own way of learning this
kind of stuff.
And don't be too frustrated with yourself if you have
difficulties.
You just have to find your way of trying to remember some of
it at least to get the broad pictures in mind.
OK, now in the embryo, I just want to show you some
differences, because in the human embryo in development,
it's the same even though the course of that optic tract
becomes more distorted because of the growth of structures
related to the cortex that is so huge in the human.
But look at it here in the baby.
The axons don't have so far to travel.
Of course, this is a tiny little thing in the embryo.
I didn't mark the age.
I think this is 11 days after conception.
But you can follow--
especially for the caudal edge of the optic tract, they're
pretty straight from the chiasm right to the midbrain,
almost no bend at all.
And then they go across the tectum.
Here the rostral edge, it's also pretty straight because
there's not much of an internal [? caps ?]
yet.
Look at the telencephalon.
It's just starting to bloom out.
You see it's very early in development.
So it's as this telencephalon grows, becomes the cerebral
hemisphere, it's going to end up folding back over most of
that superior colliculus.
And in humans, much more.
It will be way back to here in some cases in
the much larger brain.
But even in this very young animal, you can make out
boundaries and sections of the two geniculate bodies.
And certainly the superior colliculus is--
this line is a little harder to define in the newborn.
The axons are growing over the surface more
than they are later.
And notice the bend in the neuraxis here.
Here's spinal cord.
Then you see this bend, cervical
flexure, the pontine flexure.
Here's the pons down here.
So the boundary between hindbrain and
midbrain goes like this.
You see?
This would be the the isthmus area.
And then the midbrain area is everything in between those.
And then you would have epithalamus up there,
thalamus, or dorsal thalamus, ventral thalamus, and then
hypothalamus.
So there's two things then about the baby.
Everything's smaller, so axons don't have to grow very far.
And then the relative growth of things is very different,
because the more recently evolved things
tend to develop later.
And those are the things grow the most.
And as they grow, they distort the picture.
Remember what happens to the geniculate body in the monkey.
It starts out on the side.
It gets pushed over, folds the geniculate over, and rotates
it back so it's sort of a bump on the caudal end of the
thalamus in human and monkey.
OK, we'll talk a little more about species differences with
the focus on the tectum here.
And we saw these pictures before.
The huge tectum in tree shrews and squirrels and birds and
fishes, relatively much smaller
tectum in rats and humans.
And then we'll look at lamination patterns that
differ a lot too.
Here's the picture I showed you earlier.
Remember, these aren't to scale.
The human is relatively larger.
The size differences between rodent and human is larger
than what I'm showing here.
Just pay attention to the relative size of the superior
colliculus there and the cerebral peduncle.
So here's the huge peduncle in human, corresponding to the
huge hemisphere and relatively smaller superior colliculus,
but still a lot of input from retina and even more from
cortex in human.
And here's the tree shrew.
And squirrel is very similar.
Very large superior colliculus and
relatively small peduncles.
I mean, the squirrel and the tree shrew have as much cortex
or maybe a little more than the rodent or at least
equivalent to a rodent.
So these bundles would be pretty similar in size.
But their midbrain tectum--
they have a lot more midbrain vision if we put it in
functional terms.
OK.
So here's that human stain for myelinated fibers, just to
show you the cerebral peduncle.
And does anybody know what that bundle would be?
These are coming from the spinal cord and also the
hindbrain carrying some metasensory fibers.
Do you remember what it's called?
Down here, it's the medial meniscus.
Up here it's spinothalamic and spinotectal.
And there's another bundle out at the edge there.
That's coming from the inferior colliculus.
That's the lateral--
sorry, the breaking of the inferior colliculus carrying
auditory fibers towards the thalamus.
Now here the midbrain section, so about the same level as
this section.
It's from a blowfish.
And now you can really see why the tectum originally in these
animals was the roof of the ventricle.
Where's the ventricle here?
It's just in there.
But in the embryo, in fact, it did extent out.
But as the brain grew, the ventricle didn't grow.
So there's the blowfish.
And you can see the cells are mostly
there near the ventricle.
So what's all this up here?
Well, it's dendrites of those deeper-lying cells and all the
axons coming and terminating on them.
And if you take a little piece of this--
just take a piece of this and look at it in a little more
detail-- you can see the cell bodies at the bottom.
So it's a cortex.
Just like in the neocortex of a mammal, you have these cells
with dendrites going up towards the surface.
The difference would be that in the neocortex,
we have more layers.
But notice that there are layers here
if you look at inputs.
There's the visual input terminating in the upper band
there in the tectum.
There's my sensory input terminating in the
deeper-lying band.
And that pattern is retained even in mammals.
In the optic tectum, you have some other sensory fibers
coming and terminating in a deeper layer.
The auditory fibers tend to terminate in the middle there.
OK?
OK, now here is the [? telias. ?]
[? Telias ?]
include animals that have evolved further than in the
ray-finned fishes.
And you get much more elaborate optic tectum.
So there's the wide ventricle.
You can see more cell layers there.
There still are some deeper-lying cells, but many
cell layers.
The auditory input tends to come in just below the
ventricle in this structure.
Torus semicircularis is the term usually used in
comparative anatomy for that auditory region.
But now let's look at Golgi studies of that examination.
If you just take a piece of this now and look at cells,
you see this kind of thing.
It looks a lot like a cortex.
There are cells like this in our neocortex.
There are pyramidal-type neurons that you see here with
elaborate dendritic trees.
Some dendritic trees are laminated in the way they
branch, like this one.
And the axons of cells--
here a cell that terminates mainly within the tectum, but
in one layer.
So you can see its distribution there.
All right.
This just shows an even more elaborate cellular pattern in
the iguana, a lizard.
And you can see they've named these--
S always means stratum when you're looking at a laminated
structure like this.
And G means griseum for gray.
A means album for white for fibers.
And the S there, this one means stratum, griseum et
album superficialis.
So it's got both cells and fibers, and it's more
superficial in location.
There's another one like that down here, but it's got a P at
the end because it's deep.
It stands for profundum.
Now the mammal here, you can make out-- this is a rat.
And you can make out layers.
But in fact, what I would need here--
and I haven't taken pictures with my own camera to show the
beautiful lamination you see in these mammals if you have a
fiber picture.
But if we had fiber stains, we would see this beautiful
pattern of fibers of a commissure here.
And then we would see--
let's see if I can get my pen to work if I
don't use it too fast.
You would see bundles of axons in this middle layer here.
We won't draw the whole thing.
And it's mixed with large cells that give rise to some
of those tectospinal tract fibers.
And then up above here, we would see a layer also
traveling at right angles to the section here.
These are the optic tract fibers
coming in from the retina.
And then in the mammal, they would also come from the
visual areas of the cortex.
So it's a very clear laminate pattern, even clearer than the
fiber stain.
But if you've got a good eye and a very good Nissl stain,
you can make out these layers pretty well in
the cell stain also.
OK.
So now we get to this issue, topographic organization.
We'll look at a goldfish and then a rodent.
And it would be very similar in all the rodents, in fact,
all the mammals.
But I have these goldfish pictures that can tell us
something about how they go about doing the study.
Most of you already know this.
But there's a picture where the brain is a dissected brain
with the eyes left.
So the entire CNS of the brain is there,
including the retina.
And there you see the optic nerves.
They're mostly crossed.
And then you see them coming in.
And their main termination is in the tectum, mostly in the
opposite side.
Now if you take recording electrodes, you come down from
the surface, and you plunk your recording electrodes into
the tectum at various points and then go
advance in a row here--
sorry.
Here we are.
Here's the more medial tectum, so they're going right across
here, one, two, three, four.
You can map the visual fields.
And this would be upper visual field.
One, two, three, four.
They're just mapping the center of the receptive field
of those neurons.
And then you move the electrode a little more
laterally, and you go down on the visual field
and record the row.
So you can map the orderly topography of the retina on
the tectum.
And that corresponds exactly to the way the axons are
growing in and terminating.
And this picture is in the Purves and Lichtman book.
OK.
So now here's the baby rodent showing the side view and this
stretched-out view of the whole optic tract.
And did I put--
no, I didn't put any large ones here, but I've noted here
that as it comes up the side of the 'tweenbrain there, the
axons form a topographic map of one axis.
The axons from the superior retina go to the caudal edge.
Those from the inferior retina go to the rostral edge.
So by the time they cross over the geniculate bodies, they're
already topographically ordered in that one axis.
OK?
So the upper visual field is located in the rostral part,
the lower visual field in the caudal part.
And then it bends caudally, so that defines the topography
within the tectum.
Superior retina laterally.
Inferior retina medially.
Everybody gets mixed up about it, because sometimes we talk
about the visual field because we're interested in function.
Sometimes we're talking about the retina, because we're
anatomists.
And it's only when you work with it a lot that it becomes
sort of automatic.
So then here's that stretched-out view.
And I've shown the inferior retina axons at the top here.
So those are the more rostral ones as it comes up the side
of the diencephalon.
And here's the caudal of the optic tract.
This would be the lateral tectum here, the medial tectum
here because of the bend in the middle there.
And I've noted that the other axis of the visual field,
actually, the axons are just mixed in here.
And there's no real topography in the
suprachiasmatic nucleus.
But once you get to the geniculate bodies, you get a
nice topography.
The nasal retina terminates first and then the temple
retina as you cross the nucleus.
So you get a complete topographic map there.
The same thing starts all over in the dorsal nucleus.
Nasal retina first, then temporal retina.
And then you get to the pretectum,
and it repeats again.
Nasal retina, then temporal retina.
And when you get the superior colliculus, it's different.
It reverses.
Temporal retina first, then nasal retina.
So the business end of the visual field that is right in
front of the animal is in the rostral tectum.
But here in the geniculate body, it's in the upper part
of the geniculate.
And now you can see why we call this the accessory optic
tract, because it behaves totally differently from the
main optic tract.
It's these three little cell groups.
And some animals have only one, but in
most, you have three.
They're the little cell groups that receive axons of these
offshoots that branch away from the tract, and they form
these separate bundles in the superior fasciculus, the
middle fasciculus, and an inferior fasciculus.
And these just refer to medial, lateral, and dorsal
terminal nuclei.
And they have a different kind of response than any other
cells that we see of optic tract projections, because
they're responding to movements of the entire visual
field, the kind of movement you get when the animal is
falling or his head is rotating.
We should say the eyes are moving with respect to the--
he's not fixating steadily on something.
So here's the other view.
And of course you can add the topography just as we did in
the other picture.
Nasal and temporal retina.
Nasal and temporal again.
And notice here for the temporal retina, that's part
that responds to the visual field right in front of you.
That's where you get binocular vision.
And that's where you get termination of the two eyes
separately.
So the contralateral eye is terminating near the tract and
these two-layered geniculate bodies of the rodents and the
ipsilateral isthmus terminating here.
And then with more layers, it's just the same pattern for
the additional layers.
And I pointed out the pattern for
humans and monkeys earlier.
We don't ask you to memorize that pattern.
So this, we've talked a bit about that, a
little bit about that.
So now I want to get to this topic, the
roots to the endbrain.
And I probably put some slides here we don't need.
Well, I did mention this before that with sensitive
stands, we've seen a few pathways that weren't well
known before the more sensitive methods including
the amount of methods were developed.
One is sparse projections to hypothalamus that are many
places in addition to the suprachiasmatic.
You can even follow a few axons all the way to the
amygdala, but not very many.
And you also see projections into parts of the thalamus,
particularly the LP that are outside the geniculate body.
So there's the picture we looked at.
So now the optic radiations, that term refers to the axons
that carry visual information from the thalamus to the
neocortex to the primary visual cortex.
And when we talk about secondary optic radiations, if
you hear that term, it only means the fibers coming from
that cortex back to the brainstem.
They go in both directions.
In fact, the lateral geniculate body that sends the
axons through the optic radiations up to the visual
cortex, the visual cortex sends axons back to the
geniculate body by the secondary optic radiations.
But then they have more there that go all the way to the
tectum and other structures in the midbrain.
Now when you learn neuroanatomy, in medical
school, often they only talk about one of these
projections, the retina to geniculate
body to striate cortex.
And then as an aside, they'll mention that there's some
reflexes controlled by another projection, retina to superior
colliculus.
And that's it.
But we're interested in evolution, how did it get
there, and it was much more complicated than that.
We're rather specialized animals for
vision as are the monkeys.
When we talked about midbrain, I mentioned the connections of
the midbrain, that they go from the
membrane into the thalamus.
Those were the connections to the forebrain.
So that's one of them.
Here are the multiple roots.
And this was known from electrophysiological studies
even before there were microelectrodes, people just
putting electrodes into the forebrain and recording evoked
potentials.
And then they would carve out various things, and they would
see which components disappeared.
And they can take out the whole geniculate body and
still get projections to the endbrain.
But the two that we always hear about--
this is number one for the primates.
It's not even the biggest for many mammals.
And it certainly isn't of much importance at all in many
non-mammals.
But for the primates, it's very important.
Retina to geniculate body directly, and that projects to
the occipital neocortex to what we call the striate
cortex or area 17 or V1 if you're a physiologist
And then the second one we always talk about is this one,
retina to the optic tectum or superior colliculus.
And it goes to the lateral thalamus, to this part of the
lateral thalamus we call the LP, which is part of the
pulvinar in the larger animals.
And that has a projection to posterior neocortex.
It goes to layer four of cortex near the striate
cortex, but it's not V1.
It's mainly V2.
Now it actually projects to V1 also, but that's because there
are cells there in the LP that have much more widespread
projections.
And when we talk about neocortex, I will talk about
that, because those projections are similar to
projections you'll get in much more primitive animals and
non-mammals But they're not the ones that are talked about
by physiologists who study this region.
Now look at all the other projections.
The subthalamus is a projection to the ventral
lateral geniculate body, the least studied of the
projections of the optic tract.
There is a root into the striatum, and there's even a
few fibers that make it all the way to the cortex.
A lot of those projections to the striatum go by the rest of
the subthalamus.
The lateral geniculate body there, the ventral part
projects to the rest of the subthalamus.
And from there, you get right up to the striatum.
That's another root into the endbrain.
And then both the optic tectum and the pretectum separately
project to those thalamic structures we called the older
thalamus, the midline and intralaminar nuclei.
Those are the cell groups that will survive a huge cortical
lesion that would cause the geniculate body, the ventral
nucleus, the LP, all those-- the whole lateral thalamus
would just shrivel up if you took out the neocortex.
But you would get a lot of survival in these old thalamic
cell groups, because they have a lot of axons
going to the striatum.
So they survive.
A cell generally won't survive without its endings, but they
have a lot of projections to the striatum.
So they'll survive perfectly well without a neocortex.
So each of those, optic tectum and pretectum, are getting
input from the retina, and they project to both striatum
and neocortex, usually by the same axons that terminate--
I will show you again.
I showed it once, but I will show it again when talk about
striatum, axons that go to the striatum and then have
branches growing into the neocortex.
And then both the tectum and the pretectum here have roots
into the lateral thalamus, not only this one
from the optic tectum.
There's one from the pretectum too, and it goes more
rostrally to the lateral dorsal nucleus.
I'm surprised there hasn't been more attention to that by
physiologists, but they don't study anatomy, and that's why
there hasn't been.
Why do I think it's so interesting?
Because the cortex that that one projects to happens to be
the cortex that in small animals projects to the
frontal eye fields.
It controls eye movements.
And all kinds of people are studying the frontal eye
fields, but they don't seem to know about this projection.
So here is a picture of these two major roots in phylogeny.
And even in this well-known comparative anatomy book by
Butler and Hodos, they don't talk about most of those
projections that I was pointing out.
But here's the mammal.
Here's the geniculate body.
And here's the LP or the inferior pulvinar.
And here's the retina, which projects directly there to the
geniculate and indirectly to the LP.
This is colliculus.
That exact same pathway that I just drew you find in diapsid
reptiles, crocodiles, other lizards.
Birds are actually diapsids too, and it's
very similar in birds.
The only thing that's different
is some of the names.
You call this equivalent to LP.
You call it nucleus rotondus, because it's a round nucleus,
pretty easy to define in the thalamus of these animals.
So you can define it in birds and these reptiles.
But then the thalamic nucleus, in some non-mammals they call
a geniculate body.
Others they don't.
Here in the diapsids, they call it the
dorsolateral optic nucleus.
And in birds, they call it nucleus opticus principalis
thalami, the principle optic nucleus of the thalamus.
So it gets different names.
But it's very equivalent.
And then look where these project in these animals.
We know these.
The geniculate body projects to the striate cortex here.
And the LP projects to extrastriate
as I described before.
Now here in reptiles and birds-- let's look
at it first in birds--
that equivalent to the geniculate body projects to
what looks like a cortex there.
But usually you can't see the ventricle in the bird.
That's called the Wulst.
And in some birds like owls, it's very well developed and
very complex.
And then the rotondus projects not to cortex, but the whole
endbrain is developed differently in these birds.
It projects to ectostriatum, which develops from the dorsal
ventricular ridge area of the embryo.
It's part of the neostriatum.
And in the turtle or the crocodile, it goes to this
dorsal ventricular ridge tissue, which is equivalent to
ectostriatum.
So they have two visual projections.
But in mammals, they're both to the neocortex.
In these other animals, some of it's to
a cortex-like structure.
In fact, it's called dorsolateral cortex in the
turtle and the crocodile.
It's called the Wulst in birds because of the lack of the
ventricle there.
But the rest of it goes to a dorsal ventricular ridge
derivative.
And that would be equivalent, we think, to
the amygdyla of mammals.
Which then raises the question, well, why does this
go to cortex then if this is like the amygdyla?
And all I can say is, well, there is a projection from the
lateral posterior nucleus to the amygdyla, but it's not as
well developed as this projection to the cortex.
This is just a picture of a turtle and another diapsid
reptile there.
There's the dorsal ventricular ridge area, and there's the
part that's visual.
You see it's below the ventricle.
This cortex up here would be cortex that also gets visual
input coming from that other cell group that gets direct
retinal projections.
And this is a fairly similar arrangement in other lizards.
Now we're just going to go to mammals and take a section
through the caudal hemisphere in this case of a raccoon.
And to see this well, we really have to blow it up,
because we want to see the architecture of those cells.
And why it's called striate?
This fissure, very deep.
We have it in humans too and in monkeys.
This is the calcarine fissure.
And you can see that most of that cortex here that's visual
is buried in that fissure.
How can I see that's visual?
Well, look at this dense cell layer here that I'm following
with my pen.
That's layer four.
That's where the granule cell layer, the small cell layer
is, and that's where most of the thalamic fibers of what we
call the neothalamic cell groups like the geniculate
body, the lateral nucleus, the ventral nucleus are going.
And then look at the sudden change when
you get right here.
That layer is becoming more complex.
And notice that there's a white band above
it and below it.
That's why we call it the striate cortex.
It's got white stripes in it.
When you do a dissection, you can see those.
And it was named for the way it looks in the dissection of
a human brain.
And it's an elaborate cortex.
This part of the cortex is more elaborate than this part.
That's because this is the binocular part here.
This is the monocular part here.
But it continues to have these characteristics all the way
around to here and then suddenly changes.
Now we're outside of striate.
So we can see the boundaries very clearly, there and there.
That is area 17.
And it doesn't take lengthy training in cytoarchitecture
of cortex to be able to see these.
Some areas the differences are more subtle.
It takes more microscopic views, more attention to cell
details in order to separate different areas.
But for striate cortex, it's relatively easy, as it is,
say, for the motor cortex, which has no layer for granule
cells to speak of and huge pyramidal cells that give rise
to the output.
So I've mentioned what the optic radiations are.
We're going to look at their course in large-brained
mammals, define again the secondary optic radiation, the
fibers coming back.
And then we're going to talk about the older and newer
visual areas.
I've been talking about striate cortex and also the
area just next to the striate cortex that gets the LP
projection that gets tectal input.
These are relatively old areas.
They've been around the longest.
But in more advanced animals-- advanced I say because they
seem to be in animals that have appeared
more recently in evolution.
We don't have much time here.
I just want to show you a couple things.
This is a seven-week human brain, a horizontal section.
So you see the cerebellum in the back, hemisphere there.
And there's thalamus.
And this is striatum in here.
Well, look.
There's not much that's mylenated yet.
It's so early in development that most of the
myelin hasn't formed.
Myelin tends to form after all the axons have completely
grown, all the neurons have been born.
And then the myelinazation occurs later.
And that's when the axons grow more and become larger in size
and so forth.
Well, the geniculostriate pathway is
very early to mylenate.
There's the geniculate body, caudal lateral
thalamus right there.
And there is the early myelinating optic radiations
coming out of the lateral thalamus, bending around and
then caudally and to the occipital
cortex medially there.
Notice that there's another pathway
that's mylenating here.
That's the auditory projections.
And there's another pathway that's myelinating here.
That's more sensory.
And that brain--
if you look at this one, this is the newborn, 40 weeks.
This is 10 weeks.
This picture is at seven weeks.
So the brain is less than a tenth of full size.
It's very tiny.
And yet all these--
look how complex it is already at 10 weeks.
Just smaller.
Just a lot smaller.
And you can look at this online.
The striate cortex is marked with the reddish
color in all of them.
The midbrain is marked in green, the hindbrain in blue.
So we'll talk about temporalization of the
hemisphere, something that happens later in
development, next time.