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GERALD SCHNEIDER: All right.
Let's finish class 10, and then continue with the
hindbrain in class 11, which will get us into an
introduction to the midbrain, as well.
I just talked about the reflex channel in the hindbrain,
talking about the trigeminal input these fibers of the
trade gentlemen are here are traveling probably not a time
when we entered in our offline brand and the fibers of the
primary century axon to you in this position quite a ways
because they go all the way down to it structural part of
the spinal cord.
And we'll mention that again in a minute.
But I wanted to show you this level, because it's the level
of the facial motor nucleus.
It contains the motor neurons that control the eye blink.
And there are short connections there, simple
reflex connection showing--
you could call that a local reflex pathway for the
trigeminal system.
On this longitudinal picture, that section's right where I
show the arrow there.
There's the seventh nerve nucleus.
And here you see the trigeminal system.
It looks pretty complex just because the trigeminal nucleus
is so elongated.
But here are the primary sensory neurons.
I show a couple of them there--
pseudounipolar cells, just like dorsal ganglion cells at
the spinal level.
That trigeminal ganglion sits in the human brain and in most
mammalian brains underneath the brain, on either side of
the hypothalamus.
And notice that the nerve extending to the periphery has
three main branches, which is how it gets the name
trigeminal.
One branch goes to the mandible, another branch goes
to the maxilla and up to the lower eyelid.
The other branch, upper eyelid and forehead.
I've just enlarged it a bit here, showing those primary
sensory neurons in their terminations in what we call
the principal nucleus and the descending nucleus of five,
the fifth cranial nerve.
And the secondary sensory neurons in those nuclei give
rise to axons of the lemniscal channels.
So some of them do stay ipsilaterally in the
trigeminal reticular pathway.
I don't show those here.
I'm showing the more numerous ones in mammals, modern
mammals, including humans, axons that across the midline,
just like the spinothalamic tract.
We usually think of the ones from the descending nucleus as
more like the spinothalamic tract, because the functions
they serve, the ones from the principal nucleus, are more
like the medial lemniscus axons, carrying information
from the body's surface.
And they are sent to the ventral nucleus, of
ventrobasal nucleus of the thalamus.
That nucleus is often called ventrobasal in many animals
because of its position in the thalamus.
It's also called the ventral posterior nucleus, and we talk
of ventral posterior medial, ventral posterior lateral.
Ventral posterior medial is the part that gets the
trigeminal input, which is why I've shown it here on the more
medial side.
And this would be like a rodent, where at least 2/3 of
the nucleus is occupied probably more by the input
from the face, and only the lateral-most part gets input
from the body.
I wanted to say one more thing about this pathway.
If you look at that descending nucleus, we
see it's very elongated.
As I said, some of it is all the way down on the rostral
part of the spinal cord, that the dots there show the
separation of hindbrain and spinal cord.
And we now know that input to those, most caudal neurons in
the trigeminal nucleus, the descending part of the
trigeminal nucleus, are the ones responsible for pain
sensations.
Many of them are small axons.
So if you have a toothache, it's input to the uppermost
part of the spinal cord, but it comes in through the
trigeminal nerve.
So after all that hindbrain, I like to do something a little
easier here.
We're going to look at specializations again.
Some of these we've looked at before.
We've talked about the sensory pit in pit vipers--
detects infrared radiation, comes into the hindbrain, and
the rodent vibrissae.
I'm going to say a little more about that system, about the
brain representations there.
It's the first time seeing the space around the head, and of
course, very important for these rodents that live
underground in particular, because they can't make use of
vision there, so they are very dependent on all faction and
touch sensations, and much of that is done through the
vibrissae on the face.
We've already talked about taste
functions in some fishes.
We'll review those pictures at least.
We've talked about electrosensory abilities in
electric fish, like the mormyrids that have this
tremendous enlargement of the cerebellum.
But there's also cerebellar expansions in large animals
with highly developed manipulatory abilities.
We'll talk a little more about that and how that arises.
Because it results in a distortion of the hind brain
that can make it difficult to see that organization of
hindbrain when you're in the rostral part, say, in the
human brain.
So if you're going to medical school and you have to deal
with the human brain, you should know what the logic of
that change is.
You can actually still see the same organization that I'm
talking about in the human embryo, or in the rodent
brands that we usually use for our laboratory studies.
I have a question here about corresponding brain
specialization.
If any of you can find something on the web and make
it available to me, then I'll give it to the whole class if
it makes sense.
I'm happy to give you some extra credit.
You can replace a missed this quiz, for example, with this
kind of extra credit.
I have not, so far, found anything, but I've not spent a
lot of time looking, either.
The question is, what about brain representations of the
infrared detection ability of the sensory pit on the face?
Let's look at the trigeminal of moles and rodents.
We mentioned rodents before.
We'll go for that again.
But first moles.
The general idea here that I want you to understand is the
correlation between sensory acuity and the amount of brain
tissue involved in the sensory processing.
It's well established for visual acuity in monkeys and
humans, where we have the greatest visual acuity, we
have the most cortex per piece of the visual field.
If you plot the amount of cortex per degree of visual
field, it's greatest in the foveal region.
Well, the same is true for touch sensation.
So the amount of cortex we have representing our
fingertips or our tongue or our lips is much greater in
relative terms than the amount of cortex representing other
parts of the body.
So if you take--
here's a star-nosed mole.
And the star-nosed mole has this very interesting mouth
and has highly developed touch sensations.
And here's a picture of the mole at the top, so you see
about how big it is.
In this picture, they've re-drawn it, they've distorted
it to show you the amount the brain, of cortical tissue,
where the input from the various parts of the body go.
And you can see it's greatly enlarged with the snout.
So this is a molunculus, you could say.
It doesn't represent anything in reality except just to show
you relative amounts of brain tissue, representing.
So if you see a homunculus, you know it's a homunculus if
you see a person standing there with enormous lips and
tongue and fingertips, and tiny little everything else.
Yes, that includes the sexual organs, because we don't have
great acuity in those organs.
And the same is true for the skin of our back, back of the
hand much less than front of the hand, and so forth.
So here we talked about, the rodent as having specialized
sensory ability too.
The mystacial vibrissae.
And this part of the snout of the animal, where the whiskers
are located, is enervated by the maxillary branch of the
trigeminal nerve.
We talk about the ophthalmic branch, maxillary branch, and
mandibular branch.
So these infraorbital nerves come in below the orbits that
hold the eyeball.
The infraorbital branch of that one component of the
trigeminal nerve enervates the vibrissae, and each vibrissae
gets its own innervation.
And if we look up in the cortex, remember we see those
specialized little rings of cells that have sort of a, if
you just look at where the cell bodies are most
concentrated, they have a barrel shape, one barrel per
whisker up on the cortex.
So in this picture here, you see a diagram of it.
If you look at the surface of the cortex, they have an
arrangement like that.
Well, that was the first discovery about the
representation of the whiskers in the rodent.
And then later that was discovered by Van der Loos and
Woolsey, a couple of neuroscientists, one in
Switzerland and one the USA that were working together.
But then they later found, you could find the same kinds of
little bunches of cells--
I should say the same kinds-- the little bunches of cells
for each whisker that you could separate from others,
just with a [UNINTELLIGIBLE] stain.
So in the thalamus, they called them the barreloids,
and in the hindbrain, they called them the the
barrellettes, just to keep the cuteness going here.
And this is from a study done at MIT.
Some of you know [UNINTELLIGIBLE]
who's involved in the lab teaching here.
She was one of the scientists doing that work.
One of the studies was done in the Tonegawa lab because of
the work on receptors that they did.
Now, what did they do here?
They injected the thalamus with a dye that's fluorescent.
It's called DiI, because it has such a long name, nobody
can remember it, so we just call it DiI.
And it will actually diffuse through the axon.
It will even diffuse in a dead animal.
But in most cases, they do it in the live animal, because
it's also taken up by cells and transported, just like
[UNINTELLIGIBLE]
or proteins in the cell that can be
labeled with amino acids.
This was done with DiI, and you can see in our frontal
section, the axons in this low-power view are terminating
in these little tufts, because they terminate in the middle
of the barrels.
So surrounding each of those tufts of axons of the cells,
the main concentrations of cells in the barrel field.
If you make a tangential section--
so you cut it like this in a flattened piece of cortex,
through the barrel field--
then you get this picture, and you can see the representation
very well for each whisker, and you can make out the
different rows of whiskers you can see on the face.
And they correspond exactly to the whiskers.
And this kind of study is done a lot in order to study, for
example, the distortions in the cortex that happen with,
say, if you remove a whisker very early, what happens to
the representation.
Studies like that.
But you can see it even with stains that are a lot less
work, because you don't have to label the axons at all.
Here's acetylcholinesterase, and you still
see the same pattern.
Here's one with Cytochrome c, and you can still
make out the patterns.
So it's been a useful system for studying the distribution
of the axons themselves in the branch.
So it goes from the whiskers to the barrellettes, the
barreloids to the barrels, and in every case, we have this
specialized architecture.
There are many examples of size increases and changes in
architectural details within the different
systems of the hindbrain.
We've now talked about the cerebellum of electric fish.
We've mentioned the cerebellum of mammals, and we're going to
talk more about humans in just a few minutes.
And we just talked about the
somatosensory system of rodents.
And earlier, we had talked about the taste cell groups in
the hindbrain of some of the rays and fishes.
In every case, when you have this relative enlargement of
particular cell groups generally corresponding to
functional demands, we call it mosaic evolution.
That's in contrast to what's called concerted evolution,
where you have the growth of the whole branch together.
Every structure enlarges.
And you have both kinds of things happening in evolution.
You have increases between everything, together or
proportionately, we should say.
That's concerted evolution.
And then you also have mosaic evolution, and we'll see more
examples of mosaic evolution.
I have stressed so far in the class mosaic evolution, but I
did mention, with increasing body size, you have increasing
brain size, as well.
And in Streeter's book, he deals with each of these
properties of brain size changes and evolution.
And these changes lead to various distortions, and we've
talked about some of those already.
Let's just review the fish, and then we'll go right on to
the cerebellum in a few minutes.
Remember C. Judson Herrick and our favorite little fish, the
buffalo fish, with this enormous bulge in his
hindbrain because of the specialized palatal organ he
has for feeding?
It has specialized taste organs, and also has some
movement ability involved in filtering out the edible
particles from the water.
The catfish is similar, except it's smaller, and he also has
a facial lobe, because the facial nerve has a
distribution over the entire body surface.
So it's a cranial nerve, because it enters in from the
head, but because of the specialized case function on
the body surface, it's increased its distribution to
most of the body.
So just continuing with that, emphasizing here the
distortions.
And then we'll talk about midbrain and forebrain
evolution, and the organization of the midbrain.
So we want to go to this part of the outline.
We know the cerebellum is very large in mammals, particularly
the mammals with the most coordinated movement ability.
So it's very large in humans, especially when it plays a
special role in the control of distal
movement, distal muscles.
And that enlargement of the cerebellum all results from
the development of the cerebellum by cell migrations
from the alar plate region.
They migrate into the roof plate area, that becomes the
cerebellum.
There's also a migration from that same region even down
into basal plate regions, to cell groups that give rise to
inputs to the cerebellum.
So that's another kind of distortion.
But it's all related to the evolution of the cerebellum.
This is where it's happening.
Here's a picture of the top view the embryonic brain,
embryonic mammalian brain.
It's happening right there in the alar plate in the rostral
part of the hind brain.
We call that region the rhombic lip.
The rhombic lip is simply part of the alar plate in the
rostral hindbrain.
And here's what's happening.
Here I have the embryonic hindbrain in the middle, shown
in black, and with the arrows I'm showing the movement, the
migration of cells.
These are the embryonic cells that haven't formed
mature axons yet.
They're still migrating, and they're migrating into the
roof plate and forming the cerebellar cortex, mostly, but
they're also migrating down into the basal plate region
and forming the pre-cerebellar cell groups, and that
includes the pons.
So you can think of the cerebellum and pons as sort of
this late addition to the hindbrain that distorts the
original organization.
Now remember, the original organization was very much
like spinal cords.
If you learn that original organization first, which is
summarized here in the diagram, then you can begin to
understand why the adult hindbrain looks like does
because of the cell migrations that have occurred late in
development.
Down at the bottom, I'm not showing cell migrations.
I'm showing axons that originate in the pons that
travel up into the cerebellar cortex.
They come from the opposite side of the cerebellum,
opposite side of the pons, to reach the cerebellar cortex.
In many inputs, that is the main way the cortex, our vast
cerebral cortex, talks to the cerebellum--
through connections there in the pons.
And here's the [UNINTELLIGIBLE]
brain.
It's just showing an example of the cortical
spinal tract here--
two cells shown with their descending axons.
And I'm showing most of them connecting there in the pons.
And I also show the pontine cell there that sends out to
the cerebellar cortex.
Just summarized in that picture.
OK.
So a picture of the human hindbrain where the cerebellum
has become so huge that this part here--
can you see my spot there?
I'm just drawing a line around the part that would correspond
to this, the original cross section there, the hindbrain.
It's still there.
Here's the ventricle, the fourth ventricle.
And look at what's happened to roof plate.
It's become enormous, and it's become so enormous, it would
extend way out here.
It would be above the ceiling, way out here someplace.
It's a lot of tissue.
It's become huge.
So it's not only the mormyrids that have developed this
enlarged cerebellum.
The primates, the higher primates, especially apes and
humans, have also.
I should say the hominids, because all of them have large
cerebellum.
And here is the pons, down here.
You don't see the cell, but this is all pontine gray, and
it has a lot of axons going to it.
Some of the axons are going like that because they're
coming from cells here and going up like that.
We're just seeing the myelinated fiber stain.
And what are these huge bundles seen in cross-section?
What would you guess?
Well, cortical spinal.
Cortical spinal tract also goes to the hindbrain, so many
of them will terminate [INAUDIBLE].
But most of them will go all the way down the spinal cord,
especially to control the distal muscles, but
controlling, basically projecting to all levels of
the cortex.
So we've cut those in cross-section there.
And they will come together at the ventromedial part of the
cross-section, and we get caudal to that from the
parameter tract.
So why do we have a midbrain?
Let's just go a little bit more rostrally.
We already know we've got the hindbrain
controlling vital functions.
It controls breathing, it controls the vascular system.
It has [? a role ?] in temperature regulation.
So we get pretty good stability, the internal
environment, we have reflexes that keep us upright, control
our balance, the distributor input comes in there, many
distributor reflexes are all hindbrain reflexes.
We can do a lot at these lower levels.
We would be missing, though, some very basic things going
on the pituitary and hypothalamus, which also
evolved very early.
But why do we need that midbrain sitting in between
this crucial structure of the hindbrain and the forebrain?
The forebrain with this visual and olfactory inputs, and its
hypothalamus and pituitary?
Well, we can take a look at Amphioxus, to see what it's
got, and begin to get some ideas about function.
We know it may look, at first blush, like it just has a
neural tube, like a spinal cord, but in fact, we can
identify structures that indicate it's got a hindbrain,
a pretty tiny midbrain, and some elements of the
forebrain, though it doesn't have any hemispheres, so it
doesn't look like a forebrain in more advanced creatures.
And it's the gene expression studies that have given the
strongest evidence for that, we can positively use those
methods to identify between brain and brain regions.
We see at least two inputs above the midbrain.
We see a light-sensitive pigmented structure in the
forebrain region that corresponds, in some respects,
to the vertebrate eye.
Although this is an invertebrate
animal, it is a chordate.
The nerves, we don't know for sure their function, but they
have a resemblance to forebrain nerves in
vertebrates.
It's just, there haven't been enough who study the function
of this animal to be sure, but it looks like it might
correspond to an olfactory input into [UNINTELLIGIBLE]
innervation of the skin at the very anterior [UNINTELLIGIBLE]
the animal, like what we call the terminal nerve in the
human embryo.
And there is evidence of secretory cells.
Even though the functions haven't been studied, they
have the characteristics, histologically,
of secretory cells.
So they seem to have cells that function as a
neuropituitary and surrounding cells, presumably like
hypothalamus.
So what about the midbrain now?
The midbrain was, of course, a kind of rostral extension of
the hindbrain, but it did something very important.
It gave an interface between the visual and olfactory
inputs of the forebrain to that hindbrain and cord, more
caudal, to enable these new inputs.
Of course, they might not have been very new.
They occurred, actually, very, very early.
But it gave them some kind of control over the fixed action
patterns like locomotion, like orienting, emotional
expressions, even--
I doubt if Amphioxus has any emotional expressions, but
does have some ability to orient
his locomotor abilities.
We know that the midbrain receives inputs fairly
directly from both visual and olfactory structures of the
forebrain, and it also gets inputs from our caudal
structures, including the cerebellum and
including the viscera.
I should add that.
In fact, I just changed that in the book chapter, because I
want to emphasize that the fact the midbrain, perhaps the
most primitive part of the midbrain, is the central gray
and ventral tegmental areas, which are more related to the
limbic system.
Those are very, very early, and it gets visceral input,
and also heavy input from the hypothalamus.
So let's talk about these now-- the forebrain with its
two inputs.
What was primitive vision like?
Early role of the optic input, which came into the
diencephalon, or tween-brain.
The most primitive connection seems to be the one that we
now know controls daily cycles of activity by synchronizing
the circadian rhythm.
It trains endogenous clock.
There's actually two different structures involved in that,
if you look at many different vertebrates.
One is the pineal eye, which we don't have, and many
mammals don't have, but it's present in amphibia, some
fish, some reptiles.
We also get retinal input directly to the hypothalamus,
and I'm just noting here that the retina seems to develop as
an outpouch, just like the whole hemisphere
develops that way.
Well, the rat with the smaller outpouching has developed from
the neural tube in the tween-brain region, and the
part of the tween-brain it comes from the
hypothalamic region.
And we know the control of sleep-waking at the highest
level is hypothalamic, but also epithalamic.
These are the regions connected with the pineal eye,
that primitive eye in vertebrates.
And that probably occurred before there were vertebrates,
and before there was any real imaging ability.
Like the Amphioxus has no imaging ability there.
It's just pigmented cells that respond to light.
And many of our cyclic motivational states are
influenced by that clock mechanism that's regulated by
the light-dark cycle--
forging, feeding, drinking, nesting
activity, and so forth.
Now what about olfaction?
Also a very important controller
over behavioral states.
Detect sexual and individual identity.
Those are the functions that where major in influencing the
evolution of the amygdala.
We're going to talk more about that later on the class.
And what olfaction allowed--
I see two major types of functions that had major
influence on how the entire endbrain developed.
One is, it can discriminate what's good to consume and bad
to consume.
Well, that sounds like taste, right?
Yes, but this was another type of input that allowed animals
to make that discrimination before they actually had to
touch it with their tongues, because olfaction was a
distant sense.
Originally molecules passed through the water, but then
when animals were no longer in water, they'd come through the
air, and at a distance, they could--
it's like you can smell food, and you can detect whether
it's good or bad.
In fact, the olfactory effects are so powerful, we sometimes
just forget about it.
My wife got sick at a restaurant in Porter Square
not too long ago, and we were walking by
there the other day--
we probably got within about 50 feet of that restaurant.
She started to smell something related to--
I didn't even notice it, but she had gotten sick.
She started retching, just from the smell.
I mean, olfaction is a powerful controller of our
behavior even now.
That's called a poison bait effect.
Not many of you know about it.
We talk about that a bit in 920.
But they also played a role--
and this is probably even just as important in evolution of
the endbrain.
Discriminating what's a good place and a bad place.
This had a spatial function.
We don't know so much about how that occurred, but I've
seen in the hamster olfactory projections directly to part
of the hippocampus.
And when we talk about hippocampus, I'll
mention this again.
And it's probably the most primitive part of the
hippocampus.
The hippocampus is no longer considered an olfactory
structure at all, but at least in some animals, like a
hamster, it still gets olfactory input, and I think
it's what, very early, played a major role in why the
hippocampus became so important.
Of course, for both of those kinds of function, learning is
very important.
So just like they influenced a lot of that learning to happen
in what we call the corpus striatum.
In fact, the amygdala was really an
outpost of the striatum.
But it also happens in the hippocampal
formation, as we know.
Well, knowing something is good to consume or bad to
consume, or something's a good place or a bad place to be,
you need some control of actions.
And the basic actions that you want to control, first of all,
are approach and avoidance.
So how are you going to get to the
approach-avoidance mechanism?
That requires a link in midbrain.
And in the midbrain, the caudal midbrain where you
[UNINTELLIGIBLE], physiologists have discovered
areas which, when stimulated, cause locomotion.
And depending on how you stimulate it, you can get
various forms of locomotion--
calm locomotion, frantic locomotion, and so forth.
We call it the midbrain locomotive area.
And it's very difficult to study neuroanatomically, but
we do know how to find it now anatomically, though its main
definition has been through the electrophysiology.
But then we also have orienting functions.
Animals get to input to the midbrain--
the midbrain tectum, in this case, will turn towards or
away from something.
And when they turn away from it, they will generally also
flee, so it activates the midbrain
locomotor area, as well.
That happens through the midbrain tectum.
So I mentioned escape first because it probably was more
important with evolution, but orienting towards food was
very important, very early, too.
Towards novelty, because it enabled an animal to figure
out whether something was good or bad, or perhaps more
important, whether it was dangerous.
And we mentioned this hypothesis before, about this
priority that escape behavior receives in evolution was
probably the major factor that guided the evolution across
projections, because escape mechanisms had already evolved
in the somatosensory system were present in the hindbrain,
and the fastest way to get to them, the most rapid route
from the visual world to the mechanisms for turning away,
required a cross-projection to the midbrain in order to
engage those already-evolved pathways for escape behavior.
But then later, with the evolution of better imaging
ability and usefulness of these orienting mechanisms for
turning towards things, then a pathway for orienting towards
objects also evolved, and that led to a cross-descending
projection.
And that's something we'll talk about further when we
talk about detailed visual systems.
OK.
So here's the midbrain on our diagram.
This is all forebrain up here, with a tween-brain behind the
hemispheres.
And below the hemispheres there, we talk about basal
forebrain going all the way up to the end there.
And I don't show olfactory bulbs separately--
they would be underneath there.
So here's then the midbrain, there's the hindbrain and
spinal cord.
So let's say a little bit more now about how inputs come into
the midbrain and how the outputs go out.
We call them correlation centers, and we've mentioned
the midbrain locomotor area for approach and avoidance
locomotion.
I've mentioned the central gray and ventral tegmental
area in terms of visceral sensory inputs.
They also get many other inputs.
It's a major controller of moods.
Stimulating there can elicit attack and defense movements
in, say, cats and rats are the main animals that have been
studied for that.
But we also know the moods of pain and pleasure are
represented in these structures, and that's played
a major role in studies of reward-based learning.
Because whenever we feel something pleasurable, we seem
to activate cells in ventral tegmental area, and when we
feel pain, especially somatosensory pain, but I
think other kinds of pain, too, we activate cells in the
central gray.
And then we mentioned the superior colliculus or the
optic tectum, it's called.
And they're used often synonymously, but they're not
completely synonymous, because the colliculus includes more
than optic, more than visual structures.
Because the layers below the visual ones are auditory and
somatosensory.
And we know it has outputs of two major types--
one, turning towards something, and the other,
running away from it.
And then the caudal part of the midbrain surface, also
part of the tectum, is an auditory structure.
It's mainly a structure for relaying auditory information,
but some of that information goes into the superior
colliculus in the deeper layers.
And then finally we have a structure involved in this
kind of movement, grasping movement.
Control of limbs, but primarily the distal muscles.
So there we can see these three major systems for
control of multipurpose action patterns.
So they were of basic importance for a number of
different fixed action patterns and for learned
action patterns.
Locomotion to the midbrain locomotor area, orienting from
the tectum--
and that is the tectospinal tract--
and then the rubrospinal tract controlling
the grasping movements.
So these are the functions below, these are the
structures.
Let's just look at them in sections.
We talked briefly already about the
brain locomotor area.
It's very ancient in its origins and difficult to
define anatomically.
I just want to point out that it does get inputs, known
inputs from the endbrain region.
Some of them come directly from the corpus striatum.
In humans, it was for a long time not looked at as very
important, because it seemed relatively small, compared to
other outputs.
But remember, this is in a highly evolved creature where
the endbrain cortex becomes so dominant.
But in animals that don't have those hemispheres, they still
have these other structures.
That was probably more important for controlling
locomotor functions.
The major pathways probably didn't go directly to the
locomotor area in the midbrain.
They went to a structure in the hypothalamus that's
sometimes called the hypothalamic motor area, a
locomotor area that it then projects to
the midbrain structure.
So still the forebrain structures, including the
hypothalamus, are using the midbrain as their major link
to the output system of the spinal cord and hindbrain.
And then the other two, the orienting towards something or
away from it, reaching and grasping, are [UNINTELLIGIBLE]
these other structures.
And we can see all of these in a section of the midbrain.
And this is where we'll end today.
Look at this cross-section.
This is the way an embryonic human would look.
The human would rapidly develop huge peduncles here at
the bottom, and we'll show the changes that happen with the
development of the larger hemispheres, but this is the
way it still looks in the mice and rats and hamsters we use
in laboratory.
So here at the top, you have axons coming from the eye,
from the opposite eye, primarily.
And the axons are terminating throughout
this uppermost layer.
And from the deep layer here, you have these cells, and
there's many of them.
They're all through there, like that.
And they give rise to axons that go out like this.
I just show one of them there.
And they descend right it in that position.
That's the tectospinal tract.
Even though we call it tectospinal for its longest
axon, so many of them go into the hindbrain.
And then here's the red nucleus, called red because of
its pinkish color in humans.
It's highly vascularized.
And that also, the caudal part has many large neurons that
gives rise to axons that, likewise, cross over.
They go to a more lateral position here and descend.
That's the rubrospinal tract.
So orienting towards things, through the techtospinal.
Grasping movements from the rubrospinal tract.
And then just caudal to this, in this location, we have the
midbrain locomotor area controlling
the locomotor movements.
So we'll look at some of these long axons.
One of the difficult parts about hindbrain and midbrain
is you also have to deal not only with what's unique to the
structure, but also all the axons going through.
So we'll look at those a little bit next time, before
we continue talking about the forebrain.
Most of you have probably seen it, or at least some of you
have, but I did post homework, or Andrew did
[UNINTELLIGIBLE].
Because we're going to try to do a little more homework this
year and less in-class quizzes.
I will announce a quiz in advance when we're
going to have one.
I know you often work with each other.
I think it's actually very good idea in this class to
meet with other people and form study groups, because
there's a lot of study questions, and if you share
the work, I know it's a lot of work going
over all this stuff.
You're welcome to do that.
But when you write out the answers for these things, turn
in the homework, please do it individually.