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PROFESSOR: OK, I've sent Andrew
homework questions for--
you'll have until Saturday to turn them in.
I mean, we're happy to take them on Friday, but most
people seem to want the extra time.
OK, sorry, I don't always remember for you people that
bring laptops to post the file.
I need to be reminded, I guess, more often, but I did
post it for today, a PDF file.
This one was already there, but the next
file you'll find online.
And then I add notes to it after the class.
OK, we had just been talking about the major components of
the visual system and where they came from and why they
arose in evolution.
I pointed out that the midbrain roof came to serve
two major functions.
Even though modern research on the superior colliculus has
always stressed orienting movements, in fact, perhaps
the more important function for the survival and passing
on of genes is its role in escape from predators.
So I covered that first.
So we've talked about predator avoidance, we've talked about
orienting, and now the third function of image formation,
identifying animals and objects.
And this, of course, is what we usually think of when you
think of vision.
First of all, what kind of visual identification can an
animal do with just these subcortical components of the
visual system?
That is, retinal projections to one of these structures in
the brainstem and then its outputs to the motor system
without involving the neocortex.
These are things we call innate releasing mechanisms,
where there is identification of objects, and it leads to
very specific responses.
I mentioned, for example, the role of the pretectum in
sidestepping a stationary barrier during locomotion.
It's been found in a couple of animals and probably could be
found in others.
It's not been a highly studied thing, but it's
built into the animal.
If he's locomoting and there's a vertical contour in front of
him, he will sidestep.
It's a barrier he's got to avoid.
The more well-known one is the one discovered at MIT in the
frog retina, and the retinal projection to the tectum in
the frog, the bug detector.
There are certain cells, one cell type in the retina, that
specifically starts firing when there's a little black
object moving around in one part of the visual field.
And because of the topographic projection, it goes to a
particular part of the optic tectum of the frog, and
usually, the frog doesn't immediately orient.
There is some kind of summation that has to happen.
And of course, there's modulators that could play a
role, too, in how quickly excitation builds up.
But let's assume the frog is hungry.
He will eventually--
if the object stimulates that part of the tectum adequately,
he will make an orienting movement.
And then once he makes the orienting movement, a
different part of the visual field is stimulated, and it
elicits then a flicking of the tongue to grab the fly.
So Jerry Lettvin many years ago called them--
Lettvin and his collaborators called those "bug detectors,"
just referring to the specificity of response of
these retinal ganglion cells that then sent their accents
to one specific layer in the optic tectum that, when
stimulated, would elicit the orienting movements.
So those are innate releasing mechanisms.
Now, of course, most of what we do when we're identifying
things visually are learned.
This required a lot of visual acuity, and it involved
something that became most important in evolution of the
visual system.
The more recent evolution of the visual system, that all
came after the invasion of the endbrain with visual pathways
that went to the corpus striatum and to the neocortex.
So we could give a couple of examples from animal behavior.
A hamster, when he's out looking for food, he will use
landmarks along the horizon.
Rabbits do this, too.
In fact, rabbits have increased acuity in the part
of the retina, increased density of cells and greater
visual acuity due to the projections to the brain for
cells that are responding to horizon.
So instead of a fovea, you have this streak across the
retina of high-density receptors and ganglion cells.
So the hamster learns to use visual landmarks to find
things, and he learns to find his way back home.
Yes, he goes out, loads up his cheek pouches--
doesn't eat, just loads up his cheek pouches-- and
finds his way back.
And he has to go out every day--
at least, the males do--
finding food and water sources.
Why do I say the males?
Aren't the females equally important?
Of course they are.
But the females are smarter.
They can steal food from the males.
Why do the males allow that?
Because of the odor of the female that
identifies her as a female.
And it's much more important to that male to have access to
that female for mating, so he doesn't attack her.
He will attack males.
And then, of course, I mention as a second example here that
many animals will learn to recognize specific
individuals.
I don't know how much of that occurs in these little
animals, in these little rodents.
To us, they all look alike, but in fact, they can be quite
different from each other if you get to know them.
And of course, they have other cues other than visual that
might identify them.
But of course, we are very specialized for identifying
individuals, not only by faces, but by movement
patterns, shape of the body, the nature of the--
and whether we do use odors or not, I don't know, but some
individuals can certainly do that, too, and many animals.
Perhaps that was the earlier means, the dominant means, for
individual identification in early mammalian evolution.
So I'm just posing a question here about why rather recently
in evolutionary time has this third function, visual
identification ability, evolved so much?
It's become the dominant system in our visual systems
and other primates.
Not all primates, but the apes and monkeys,
the old-world monkeys.
I think it happened once there was these high-acuity
topographic maps, especially the one in the neocortex.
Because once you have that, then by simply arranging the
projections, you can get different kinds of
specificity, response to vertical contours or
horizontal contours and so on and so forth, and build up
receptive field patterns and object specificity in the
response of neurons further down the line
in the visual networks.
And we'll be talking about the forebrain visual system a
little bit later, probably next time.
So let's talk about how this happened, the visual system
invades the endbrain.
I'm going to point out three major steps here.
The retina, remember, projected to the hypothalamus,
but those hypothalamic neurons, it wasn't a
topographic projection, and it didn't project to the
neocortex at all.
So it wasn't that projection.
The main projections came from the larger cell groups of the
tectum, also the pretectum.
There were some direct projections
early on to the thalamus.
We don't know why, but that projection remained very small
and not very significant for a long time in evolution, at
least from what we can judge by extant animals.
So I just want to point out these major types of
projections.
Both tectum and pretectum project to what we call the
old thalamus.
If I put a tracer in the deeper layers of the optic
tectum-- and we know those are multimodal layers, they
respond not only to visual but auditory and some of the
sensory input, also--
I can trace axons right into parts of the old thalamus.
We call them the intralaminar nuclei,
also the midline nuclei.
Mostly the intralaminar nuclei.
They also go directly to the subthalamus, which would be
another route to the striatum.
So they go to the old thalamus.
Then they also go from the tectum and pretectum into what
we think are newer parts of the thalamus, because they
become greatly enlarged in animals
with the large neocortex.
And then thirdly, of course, the more direct retinal
projections to the thalamus.
It became sort of a shortcut to the cortex.
The lateral thalamus already got visual input.
Why did it need a direct projection?
Couldn't it just use that retinal
projection to the tectum?
And that tectum became so elaborate with a nice
topographic map, and it projected to the thalamus.
But it's interesting.
That projection didn't have the type of topography, and
that slowed it down, too.
It took longer to get to the cortex.
So let's deal with each of those in turn, now.
The old thalamus, the intralaminar nuclei, where do
they project?
They have two projections into the endbrain.
They go directly to the striatum, but those axons
often continue right up into the cortex.
It's not a very precise projection.
We can trace these projections in modern mammals.
They've been done in various animals.
So they breach both of these major endbrain structures,
striatum and cortex.
They tend to be multisensory and widespread projections.
And then we have from the same sources there in the midbrain
and pretectum, they go into the neothalamus.
This in the visual system goes to the lateral thalamus,
lateral and lateral posterior parts, lateral and lateral
posterior nuclei.
This lateral posterior nucleus is the part that has ballooned
out so much in higher primates, including us, it
became the pulvinar.
Now, that doesn't mean the whole pulvinar gets this
retinal projection from the tectum.
It doesn't, OK, but part of it does.
And those nuclei project directly
to posterior neocortex.
And I think I talk a bit more about that
when we get to neocortex.
I don't want to introduce too many details and distract you
from the main points here right now.
Now, at least parts of those projections were unimodal.
If we record there, in those areas of cortex getting that
projection, they're just responding to vision.
And finally, the more direct retinal projections to the
thalamus, that's the one to the
lateral geniculate nucleus.
We also call it the lateral geniculate body, LGB.
It's actually just the dorsal part of the lateral
geniculate, because the ventral part is subthalamus.
It's a subthalamic component of the geniculate body.
And that's the cortex that gets that projection that we
call primary visual cortex, or V1 to a physiologist, or Area
17 to a neuroanatomist that follows a scheme invented by a
man named Brodmann.
So it's got a lot of different names.
So if I just say "primary visual cortex," I mean that
cortex where the main visual input comes from the lateral
geniculate nucleus, that nucleus in the thalamus that
gets a direct projection from the retina.
So what were the advantages of all this?
First of all, we know the striatum, probably the main
reason it was getting this extrasensory input was because
of its plasticity.
It already existed, the more ventral parts, what's still
there of the more ventral parts, it got olfactory input,
and the synapses could change, the animal could learn.
Feedback had developed from the ventral midbrain that gave
that system the information that something good had
happened or something bad had happened.
So the synapses could change and lead to habit formation.
So why, then, did it go also to neocortex?
We know that there are animals where for solving a visual
problem, discriminating one pattern from another, the
primary visual cortex isn't actually essential, isn't the
most important.
But that projection from the tectum's more important.
Animals like the tree shrew from Malaya.
A lot of people don't know this, but I know the studies
that were done probably 25 years ago now at Duke
University.
So what did the visual cortex add in those animals?
It added acuity.
That is, they got better at responding to details.
And they also got better at discriminating things that
where was relative movement in the field, especially if the
animal's moving his own head.
Then later, of course, cognitive functions evolved
that made use of these projections to the endbrain.
It provided a visual route that came by way of visual
cortex and then through other cortical
connections into the amygdala.
It didn't reach it directly.
It had to go through these cortical structures.
But that carried object information that was useful
for learning avoidance and approach responses.
It also provided a visual route to the hippocampal
formation for learning and remembering place information.
And there's still details about that that aren't
actually that well studied, even though there's so much
attention to both amygdala and hippocampus.
These transcortical routes, I think, are still worthy of a
lot more study.
And as an anatomist, I can see these deficiencies and like to
point them out.
Some of you, perhaps, may do research on these
areas in the future.
OK, one more thing.
Speaking of adaptations, why did this all happen?
It's that that high-acuity projection to the visual
cortex became very important for depth discrimination.
Using a particular kind of depth cue that had developed a
bit in the midbrain, but it wasn't specialized as the
cortex was for this.
Binocular vision, using the slight difference in what's
seen by the right eye and the left eye, that's called
binocular disparity, when specific neurons in the cortex
develop the ability to respond specifically differently when
the disparity was wide or narrow so depth could be
judged just by the binocular disparity.
And certainly as acuity improved, cortex became very
dominant for depth vision.
It's not that we didn't have it.
There's a lot of redundancy in the system.
But that's a particularly important thing for an
animal-- for example, a monkey in a tree that's feeding from
objects, and he has to reach for them.
This precise depth vision is very important.
Not very important for escaping from predators.
If you have to have binocular acuity to escape from a
predator, I'm afraid it would be too late.
He'd be too close to you.
So a hamster doesn't have very good depth vision at all.
He can't tell the difference between a little spot I put
about this far from his head and a hawk way up in the sky.
He responds about the same.
But it doesn't matter.
He gets away.
So this function evolved for other kinds of things,
approach responses that require some kind of precise
movement, like reaching movements.
So now we have these various compounds.
And don't worry.
We're going to go over them several times in different
ways so they'll get in your mind.
They expanded and specialized different ways
in different animals.
For example, the tectum became really
elaborate in some animals.
It really is a cortex of the midbrain.
How do you define a cortex?
We mean a structure at the surface of the brain that's a
layered structure, layers of cells.
And of course, layers of axons coming in and terminating on
them, as well.
And I'll show you more examples of this layered
tectum in a minute.
It varied greatly in different species depending on just how
dependent they were on the kind of vision, the orienting
movements and escape movements that that structure
controlled.
So there was an evolution of great size differences.
It's very large in some teleosts, teleost fishes.
It's very large in most birds.
And it's very large in some mammals, but not all.
There's very visual mammals, like primates.
Only a few of them have this enlarged midbrain.
Why?
Because the cortex became more important.
So as far as this dorsal thalamus and neocortical
projection area, this first visual cortex, the V1 or the
visual cortex, all mammals have it.
Even the animals we think of as the most primitive, like
the European hedgehog or the opossum or the hamster, they
resemble early mammals more.
They're not mammal-like reptiles, of course.
Those have disappeared.
But all of them have visual cortex.
And what happened, then, is with evolution, especially in
the large primates, more representations of the visual
field-- the visual cortex didn't keep getting bigger and
bigger and bigger.
It got bigger for the parts with higher acuity, the foveal
region, but it didn't just keep expanding.
So the more important vision was, the bigger the visual
cortex got.
Instead, what happened was other representations of the
visual field developed.
And the bigger the cortex got, the more
representations there are.
And we'll look at that again next time.
Then, of course, the retina evolved in various ways.
There's a tremendous range of sensitivity.
Acuity depended on the density of receptors
and ganglion cells.
Great variation in color detection.
And the Striedter book on Page 261 has a nice discussion of
some of that in mammals.
Other components of the subcortical visual system,
even though there are specializations, there are
variations in pretectal size, LGB, the ventral lateral
geniculate body size, they're a lot less--
there's a lot greater similarity from one
mammal to the other.
And that projection to the hypothalamus doesn't change
much at all in different animals.
So let's just focus now on the retinal projections, because
the pattern we find there in the hamster or the rat or the
mouse you find in all the mammals.
So I'll point out what the differences actually are, and
there are real size differences, as
I've already mentioned.
And in fact, this same pattern you can find in other
vertebrates, as well.
Just about all the vertebrates have the same pattern.
But of course, the differences in relative size and the
arrangement of cells is even greater when you get outside
the mammalian group.
So here I've just put it on the Shmoo brain, so you can--
here's the Shmoo brain that you're probably
familiar with by now.
There's the retina, and I just show a--
take that as a group of ganglion cells in the retina.
They're not the primary sensory neurons.
There are receptors that are neural, the rods and the
cones, and then there's the bipolar cells.
And there are some cells that interconnect in a lateral
fashion within the retina.
And then you get to the retinal ganglion cells.
Actually, the ganglion cells are where the
light passes first.
I know that's a little strange, the light reaches the
receptors after going through all these other cells.
But those are the cells.
They're transparent.
They don't distort things very much.
Those axons then go down what we call the optic nerve.
Why is the optic nerve not really a nerve?
Because it's a tract of the central nervous system.
Remember, the retina is an outgrowth of the diencephalon.
So this is really a part of the diencephalon.
It's pushed out towards the surface.
It happens early in evolution.
And when that tissue reaches near the surface, it induces
the formation of the lens right in there, in the cornea.
So right near where those axons first reach the brain,
right above where many of them cross over--
and remember, early on, they were both crossed and
uncrossed--
they terminate in the hypothalamus.
That's the suprachiasmatic nucleus.
And then this is the main optic tract.
It goes right up the side of the diencephalon and then
turns caudally and reaches the midbrain tectum or superior
colliculus.
This is sort of an offshoot that's called the accessory
optic tract.
It's not where most of the axons are going, but it has
specialized functions.
And there's several nuclei of that system, the accessory
optic tract system.
The main optic tract, the majority of these axons are
going right over the cerebral peduncles, and they reach the
subthalamus there--
that's the ventral nucleus, the lateral
geniculate body, LGB--
and then the dorsal nucleus.
Then here's the LP, the lateral posterior nucleus, and
there are a few terminations there.
I don't show them here because I'm showing where the dense
projections are.
There are some projections there in LP, very few.
And then pretectal area, where there's an organized
projection.
And then the superior colliculus, which becomes the
largest structure that the tract
terminates in in many animals.
Let's just talk about two animals that you're familiar
with and say, well, how big are these two main structures
that people always talk about when they talk about vision,
the geniculate body that goes to the cortex and colliculus
in the midbrain?
I actually measured this once with a group of graduate
students in this department, and I was trying
to teach them histology.
And I wanted to teach them some quantitative methods of
analyzing data, so we measured the size of the terminal
region in the geniculate body and the size of the terminal
region in the tectum-- not the whole superior colliculus,
just where the retina terminates, the
retinal axons terminate.
Same for the geniculate body.
Now, for a hamster, the terminal region in the
midbrain was three times the size of the geniculate body.
Not quite the same as I've shown it on this picture,
which is the more generalized picture.
Then we did the same thing in a monkey.
What would you expect?
The geniculate body was 10 times the size of the superior
colliculus, just the retinal terminal zone.
And that would be similar in human.
And that is typical for the kind of variations you see
among mammals.
There's great variations.
This tectum, the colliculus termination zone being three
times the size of the geniculate body, that's not
much compared to, say, the tree shrew or the squirrel,
where I haven't measured it, but the difference would be
much greater because the tectum is so
large in those animals.
So now this is what we'll do, follow the course of those
axons and deal with their topography.
I'm going to go through the layout of the tract.
You'll see a little more about species differences, some of
the details.
And that's especially important because of the
differences you see in human and monkey and apes.
Then we'll deal with the lamination of the midbrain
tectum, and then we'll talk about topography.
I want you to understand something about how it forms.
I won't ask you to remember details.
These are the two views we're going to use
to look at the tract.
I will show you enlargements of these.
This is a frontal section or a coronal section.
So there's the ventricle.
That's the third ventricle.
The diencephalon is heart-shaped, larger at the
top, smaller at the bottom, where we have hypothalamus,
then subthalamus, then this is all thalamus.
And then the pretectal area at the top, that's part of the
epithalamus.
And I've named those major components here.
And then the other thing, here's the optic tract.
This is all optic tract.
And if we went a little bit in front of that, we would see
the tract crossing at the ventral midline, at the
hypothalamus.
Now we're a little bit behind that, and it's
going up the side.
And we can see it going all the way up to
the epithalamus there.
Now let's take a knife, and we'll start right in front of
this level, right at the optic chiasm, and we're
going to cut through.
And then we're going to move our knife all the way up and
just follow the course of the axons all the
way into the midbrain.
And so we're going to get a section, and then we're going
to stretch it all out.
And you would get a section like this.
So this isn't a sagittal section or a horizontal
section a frontal section.
It's a section through the tract to show what
the axons are doing.
So here we are at the optic chasm.
And the axons are leaving the tract and terminating.
Let's look at that and deal with these one at a time.
First a little more about this, and then we'll come back
to that other section.
A section that looks like this you would find in a rodent.
You also find it in an embryonic human tween-brain.
Now, I've looked at this.
There are publications in the Journal of Comparative
Neurology that discuss the embryonic human brain.
And you look at it, and, yeah, it doesn't look exactly like a
rodent, but the basic configuration of the cell
groups is pretty much the same.
If you know the rodent, you know what that embryonic brain
looks like.
And so we name these structures.
Here's hypothalamus.
This is the subthalamus, in between hypothalamus and the
dorsal thalamus.
I've named the major components on the right.
And that's where the dorsal lateral geniculate body is,
right out by the edge, with the optic tract going mainly
over the surface.
Some of the axons are going through it, as well.
Then you get to the rest of the lateral nucleus.
The geniculate body's part of the lateral nucleus, but the
rest of the lateral nucleus here at this level is called
the lateral posterior nucleus.
Now, that's LP.
Down here, it's the ventral nucleus, where those
somatosensory axons are coming in.
And then the pretectal area is part of the
epithalamus up there.
Now, these four major components here are basically
the diencephalic neuromeres.
Remember the rhombomeres, these segments of the
hindbrain, and then there's the mesomere, the midbrain.
And between those two, there's often--
we call a little division there the isthmus.
Well, when you get in front of the midbrain segment, you get
to the epithalamus.
So why are they arranged like this?
Here is the embryonic neural tube.
Here's the spinal cord down here.
And there's the rhombomeres.
There's the isthmus.
There's the midbrain.
And then they go from caudal to rostral--
one, two, three, four--
and some people would name a couple more there at the
tween-brain level.
It depends on what your criteria are, but
there's four main ones.
And if you cut it as in the way the red line is there, you
make a frontal section because of that bend.
Up on the top, you're going to get the first segment, and at
the bottom, you're going to get the fourth.
So here's the first one--
one, two, three, four.
So this is really the caudalmost in the embryonic
neural tube, and this is the rostralmost, the hypothalamus
in the embryonic neural tube.
Now we're going to follow the optic fibers again.
Remember what I did.
I'm just cutting into the tract, and I'm trying not to
cut any of the fibers.
I'm just going to go parallel to them to see what they do.
We always have the tract there at the surface.
And there were some axons leaving right
at the optic chiasm.
Those go into the suprachiasmatic nucleus, SCM,
or sometimes SCH, the suprachiasmatic nucleus.
What do those projections do?
They synchronize their circadian activity rhythm,
which I mentioned last time.
Then there are a few other terminations in the
hypothalamus.
You need a very sensitive tracer to see them, and we
don't know what they do.
I can guess at what they do, but it's not known for sure.
It wouldn't be that hard to figure it out, but nobody's
seen that as a big enough problem.
I think it's actually a very interesting problem, but--
then they get to the subthalamic region.
And the first structure they reach there that they have
heavy terminations in, after the suprachiasmatic nucleus,
is this ventral nucleus, the lateral geniculate body, which
is really just the lateral edge of the subthalamus.
And it gets a nice, orderly projection.
And there we have axons coming in and terminating.
And then they get to the dorsal nucleus of the lateral
geniculate, generally much larger, and they penetrate
into the nucleus, as you see there.
But they don't go to one part of the nucleus there.
Why is that?
Because that's where axons from the other eye that didn't
cross at the chiasm are terminating.
That's the area of the ipsilateral projection.
And then they continue.
They go over and through the lateral posterior nucleus.
There's a few terminations there, as I've
shown in the picture.
And then they get to the pretectal area, and there's a
couple of different cell groups where they terminate.
There's some deeper-lying cell groups
where they don't terminate.
And they're still on the surface.
And then they come into the colliculus.
And suddenly, now they're not on the surface anymore.
They turn up.
Most of them terminate.
A few of them do terminate right among the
tract fibers there.
Most of them are turning up.
That isn't what happened when they first developed.
They came right over the surface.
And I've looked at this in the development of the tract in
the hamster.
They come over the surface, but then the
cells are still migrating.
And the cells migrate up through the axons and end up
mostly above.
And then these terminal arbors are forming, and they form
mostly on the cells that end up above more superficially.
So it looks like the midbrain tectum there has this-- in
mammals, anyway, has this gray cap, the [UNINTELLIGIBLE],
just for Latin buffs.
And that's where the retinal fibers terminate in mammals.
Most species, other vertebrates like fish and
birds, they stay on the surface and then penetrate to
different layers of the tectum.
Now, let's say a little more about this geniculate body.
This is for a rodent, and I'm just showing two layers, an
ipsilateral layer there that's smaller, and the rest of them
constitute a contralateral layer of termination.
Well, there are other animals where instead of just two
layers, there's four layers.
And the axons originating in the contralateral eye will
still terminate in the outer layer here.
Then they'll skip a layer and terminate some more in the
third layer.
And the ipsilateral fibers, then, will terminate
in the other two.
When you get to monkey and human, there's six layers, but
it's the same idea.
There's just more layers.
Here is the geniculate body of a monkey, very
similar to a human.
I looked there to make sure this was monkey and not human.
Now, how can we relate that to this and to this?
Here are the axons out on the surface of the brain.
And we know those axons are terminating like this.
They're penetrating in mostly from the surface.
Where are those axons here?
They're down here.
This is the surface of the brain.
I'm following the surface here.
Here the tissue is torn.
So this is the surface.
What's happened?
Let me see.
I need my right hand for this.
Imagine you're looking at a cross-section now.
And so here's the thalamus.
And here's that geniculate body.
The surface of the tract is out here.
And now the thalamus next to the geniculate body is getting
bigger and bigger and bigger.
What's it doing?
It's pushing this like this.
It pushes it clear over.
And then not only that, it gets bigger up in front, too,
so it rotates it.
So the geniculate body is bent over and pushed to the back of
the thalamus.
It's one of the caudal two bumps or geniculate bodies,
the knee-like bodies, at the back of the
human and monkey thalamus.
That's what's happened.
So the axons then are terminating--
if they're from the contralateral eye, they're
terminating there in layer one, in layer
three, and layer five.
And the ones from the ipsilateral eye are
terminating in the other layers.
Now, let's go back to a rodent or embryonic
human would be similar.
And just take one group of axons and deal just with the
bigger projections here, the two geniculate bodies and the
superior colliculus.
And I'm just going to point out that the colliculus cells
there in the superficial gray themselves have a projection
to the LP nucleus there and the ventral part of the
lateral geniculate.
There is a smaller projection, also, to the dorsal nucleus.
These structures tend to interconnect with each other.
The only exception is that the dorsal geniculate doesn't
reciprocate the connections it gets from the other
structures.
It projects just to the cortex.
Now, if I take a hamster and I label all the axons in the
tract, here they come from the chiasm.
And I'm just charting all of them together.
And we can follow the two.
Here's the caudal edge of the tract.
Here's the rostral edge of the tract.
And here they're covering the two geniculate bodies.
That's their actual size in a side view.
Here they are in the pretectal area.
Here they are in the superior colliculus.
And there are the little offshoots.
So there's some terminations at the base of
the midbrain here.
There's some more here, and there's some more way up
there, way at the edge of the pretectum.
Here's the actual brain.
In this one, I've taken the--
if I blow it up a little bit, I just want you to see, now,
if you can make out anything here.
I've taken away the cortex.
So what's this?
Olfactory bulb.
There's the lateral olfactory tract.
Then I eliminated the rest of the hemisphere there.
There's the optic tract, white because of
the myelinated fibers.
The optic chiasm's right down there at
the base of the brain.
So what's right there at the bottom of the brain behind it?
Hypothalamus.
That's hypothalamus.
There's where the pituitary would connect, but I've torn
it off when I took the brain out.
There's the mammillary body, at the very back of the
hypothalamus.
There come the axons.
They're going all the way up here, and they
end up in this structure.
That's the superior colliculus.
There's the inferior colliculus behind it.
So what's this big structure in front of the optic tract?
Corpus striatum.
There's the lateral olfactory tract.
That hemisphere would fit in here and would normally cover
all of this stuff.
This would be the olfactory cortex down here.
This would be the neocortex up here.
I've taken all that away.
The cerebellum would be in here.
What's right under the cerebellum there?
There's a nerve coming right under there, if you imagine
that cerebellum isn't there.
And there's the stump, and it's going into a nucleus
right there.
That's the cochlear nucleus, and this is the eighth nerve.
The pons is right under there, and there's the big bundles of
fibers that connect to the cerebellum.
But I've taken the cerebellum off in our dissection.
So if we do a sheep brain dissection, which I think we
should do fairly soon, for those of you who want to, we
can remove the cerebellum like that and
expose the whole hindbrain.
And then you'll see the colliculi in front of it.
And then it looks like we're out of time, so the next time,
we'll look at a few more of these brains.
I just want you to get used to looking at a few real brains
before we do dissections, and then we'll go back.
We'll be talking about these same structures in the visual
system a little more next time.
See you then.