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PROFESSOR: Before we start, I wanted to tell you that I
noticed when I've been uploading the chapters of my
book, I haven't always been uploading the pictures at the
same time, which means that you had to
find them in the slides.
Of course, they're there.
Sometimes I've changed them a little bit for the book.
But that makes it a little harder to read the book.
Please remind me, if I do that.
I was always downloading both in the early going.
But more recently I hadn't, until I noticed it.
So now that's corrected.
But please let me know if you notice anything like that.
You can let either me or the TA know.
We were talking about outputs of the midbrain.
And this is where we ended.
I talked about three major output
systems of the midbrain.
Remember, the midbrain was the link between endbrain
structures and well, the whole forebrain
and control of movements.
And initially I said three major output systems.
So I can see a fourth major thing also.
So I'll bring all that up today.
The first one I talked about was the midbrain locomotor
area, a system defined physiologically.
Just because the anatomical substrate is a little more
difficult to draw lines around, although we can make
out the neurons there of a couple of systems.
But it's really the physiology that's defined it initially.
And that's the most important, because that is function.
And we're really talking about the functional organization of
the midbrain here.
It's much easier to define the systems for controlling
orienting movements and controlling movements of
grabbing things.
And that's what I'm showing you here, the neurons in the
deep layers of the optic tectum or superior colliculis,
which is a lot more than just visual.
They originate in these large neurons in the
deeper layers here.
They send axons across the midline into
the tectospinal tract.
Again, named for the longest axons, like many tracts in the
CNS are named.
Many of them end in the hindbrain.
And then the rubrospinal tract.
The red nucleus actually contains smaller cells
rostrally and large cells caudally.
But the descending output axons originate from
the large cell part.
And those axons also cross over.
In this case, crossing into the rubrospinal tract.
Now, we can find a number of sensory systems in and passing
through the midbrain.
Those auditory axons from the caudal end of the midbrain,
the inferior colliculus, can be found out here at the
lateral edge.
You'll see it later in a section of the human brain,
right at the edge.
And then the retinal axons are coming in, in
this layer, the tectum.
So that's two major sensory systems.
And then the somatosensory system can be found in this
line of axons, which we'll again see very clearly in the
human brain.
Trigimenal lemniscus in here.
Then the medial lemniscus axons just above that
structure we call the substantia nigra.
Then you can follow that band of axons to the spinothalamic
tract, which are beginning to mix in with the medial
lemniscus there.
But then those axons include many that are actually
terminating here in the deep layers of the superior
colliculus.
Those are sensory systems.
We've omitted a major midbrain system important in control of
actions and for telencephalic control of actions, the
central gray.
It's often called the periaqueductal gray, PAG.
I usually call it the CGA, central gray area.
We're talking about this area here, all of this.
I put it red, because it's related to the visceral
function too and to what we call the limbic system.
And then this triangular area at the bottom here, the
ventral tegmental area.
Perhaps a better way to outline it here is
motivational systems that it's associated with.
So let's instead, just striations, just note here
that if we put an electrode in there and stimulate, animals
don't like it.
We say it's negatively reinforcing.
That just means that they will work to turn it off.
They don't like to be stimulated there.
Whereas down here in the ventral part, it's rewarding
to be stimulated there.
Animals will work to turn the stimulation on.
Those are experiments that we call intracranial
self-stimulation experiments.
It has been done in humans too.
But most of the work has been done in the rat.
But the kinds of motivational states that it controls are
also marked.
For example, the basic ones that have been mapped out for
central gray, besides of course human studies have
focused its role in pain perception.
If we look at action patterns in animals, it's associated
with defensive actions, aggressive actions in
predatory animals, and also sexual behavior.
You can find different groups of cells involved in all three
of those kinds of behaviors in the central gray area.
And many of them are also represented here, especially
sexual behavior and feeding behavior, at least the reward
aspects of those functions.
And here, I've just named them for you on this slide.
So in this I just summarize aspects of the superior
colliculus here.
But I've added here, we've mentioned these other things,
but note here the major modulators coming from the
corpus striatum by way of this structure, the substantia
nigra, which is closely connected to the striata.
And then various diffuse projection systems that we
will talk more about after we talk about motor control.
The diffuse projection systems use catecholamines, or more
broadly, the monoamines, serotonin,
norepinephrine, dopamine.
And there's also pretty broadly projecting systems
using other transmitters, acetylcholine, for example.
And still others that originate in the hypothalamus.
And then we talked about the inferior colliculus and
superior colliculus for visual and auditory processing.
And then the multi-modal regions, deep layers of the
superior colliculus, but also the reticular formation and
central gray get different modes of input.
What does the central gray get that's different from the
other modalities?
It gets another kind of modality input.
And that's visceral.
It gets ascending visceral inputs.
That comes in here, the central gray.
That's in addition to inputs coming say for example from
the superior colliculus carrying other types of
modalities into the central gray.
And the red nucleus related to the motor system.
Well, let's just say a little bit now about species
comparisons.
This you could call an exercise in topology because
there's really huge differences in the midbrain,
depending on specialized functions.
We saw this briefly before when we talked about the
expanded auditory system in animals that use echolocation.
This is human.
So get in mind the human midbrain.
This is the superior colliculus.
It's the visual layer up there, that looks clear in
this myelin stain.
There the central gray stands out also because of the lack
of myelinated axons.
They're only lightly myelinated or unmyelinated.
You can see in the center, the aqueduct, the ventricle.
Much of the ventricle tegmental area here also is
unmyelinated, although the lateral part has quite a few
myelinated axons.
There's those auditory fibers way out at the edge.
And then here's the band of somatosensory axons
going in this band.
But there's another kind of axon ascending there that
accounts for a lot of those myelinated axons in humans, in
addition to all the somatosensory.
And those are coming from the cerebellum.
The cerebellum projects into the thalamus rather heavily.
And it's a route that gets to motor cortex that way.
But what are all these?
The most obvious thing about the human midbrain is this
huge bundle of myelinated axons at the base.
We call them the cerebral peduncles.
They're little feet at the base in the midbrain.
You remember what those are?
What's so big in the human brain that they must be coming
from, that's projecting to all levels of the CNS?
The neocortex.
These are coming from the neocortex.
They're going, especially in the middle here, to the pons.
So they're going to the cerebellum by way of the pons.
And they're going to the hindbrain and the spinal cord,
controlling movements.
Those are the cerebral peduncles.
Now, look at them in other species.
There's my redrawing of the human.
And here's a rodent.
Of course, the rodent brain would be relatively much
smaller than this, if we put them all on the same scale.
But you can see the superior colliculus is relatively
speaking, a little bit bigger.
The cerebral peduncles are a lot smaller.
Why?
Because the neocortex is much smaller in area.
So many fewer neurons giving rise to those descending axons
come from the neocortex.
But look at the tree shew.
Compare it to either of these.
What is different?
Well, we call these the optical lobes in the tree
shrew, the superior colliculus.
The reason it's so big is because the
visual input is so large.
It's the major target of the retina in the tree shrew and
in the squirrel.
Squirrels are very similar.
If you looked at a squirrel brain, you would see a similar
optic lobe in the midbrain.
If you removed the whole brain, they would protrude
from the neocortex.
The neocortex in the human would completely cover the
midbrain here.
If you look at the human from the surface, you
see the huge neocortex.
You see the cerebellum behind it.
The midbrain has just been covered up by these other
structures.
But not in tree shrew, because of this huge midbrain.
Cerebral peduncles, relatively small.
You can see all the same structures in all three of
these brains.
But in relative terms, the sizes of the components are
very different.
So what do we call that in evolution, when we get these
big size changes that are related to the function,
functional specializations of animals?
We call it mosaic evolution.
Mosaic referring to the different functional parts.
And in this case, some of those pieces have become much
larger because of the important of the functions.
What's the other kind of evolution where the whole
brain expands?
We have a lot of that, especially in the large
mammals where everything grows proportionately.
And a lot of the forebrain, especially the
endbrain, grows like that.
You call it concerted evolution.
It all goes in concert.
So you have both kinds of things in evolution.
You have increases in size of the entire central nervous
system, the entire forebrain, and especially the endbrain.
But then you also see evidence for mosaic evolution.
And we get functional specializations and
enlargement of specific structures, as we've seen a
number of times now.
Now, so the long axons passing through we've mentioned are
the ascending visceral sensory.
And we sometimes refer to those that come through here,
the dorsal longitudinal fasciculi, a typical
anatomical name.
You don't have to learn it.
But I want you to see the principles.
They're very simple.
It's dorsally located.
It's longitudinal, meaning it doesn't go in this plane.
It goes at right angles to the screen there.
It goes rostral caudal direction
in the nervous system.
Dorsal longitudinal fasciculus, remember what that
word means?
It means ribbon.
It's a ribbon of fibers that carry
visceral information forward.
And then the ascending somatosensory.
We've talked about the spinothalamic tract, with the
spinotectal.
Now, yes, there's a few spinoreticular in there too.
There are not too many left here.
Most spinoreticular axons have terminated by the time you get
up to the, especially the rostral and the midbrain.
But then we have the medial lemniscus.
Those are the axons that became the dorsal column
axons, that terminated in the dorsal column nuclei.
And then axon from there went across the midline in the
SENs, in the very medial position in the hindbrain.
So we call it the medial lemniscus.
So those are the two somatosensory
bundles we've seen.
And of course the trigeminal lemniscus is added to it when
you go by the fifth nerve nucleus
in the rostral hindbrain.
So those are all the somatosensory axons.
And then I mentioned the ascending cerebellar output,
which in animals with a very large cerebellum, animals that
have highly developed coordinated movement of the
distal muscles in particular, the cerebellum enlarges a lot.
And that contributes to the input to the motor cortex
controlling movement.
The cerebellum also has descending pathways directly
to the spinal cord.
And then the corticopontine and corticospinal axons of the
cerebral peduncle, that huge bundle at the base of the
human brain.
So there's the peduncle in the rodent here.
Our embryonic human, it would be about the same size.
And then you see the other descending axons.
I've got all the descending axons there in blue.
The tectospinal near the midline, rubrospinal further
from the midline, corticospinal at the bottom.
Now, one more topic in this lecture.
We'll get on to the next one.
I can simplify the organization in the midbrain
in this way.
We can divide it into somatic and limbic.
And I'm broadening the term somatic here to mean than just
somatosensory.
All the somatic senses, that includes vision and audition,
and we can separate it from the limbic.
So you can do it this way.
Here's our midbrain.
And the dots there are over the limbic regions, central
gray and ventral tegmental area.
Remember if we stimulate there, animals act like
they're in pain, if you're stimulating
in the central gray.
But they will also show, depending on where you're
stimulating, animals can show sexual behavior, aggressive
behavior, defensive behavior.
Defensive tends to be the dominant one.
The most important for keeping the animal alive.
And then the ventral part here is the reward
area, you could say.
But it gets inputs associated with the functions that are
rewarding, so feeding and sexual behavior in particular.
So that's limbic.
That named the limbic because of the positions of the
structures closely connected with these
regions in the endbrain.
They're at the limbus or the fringes of the hemisphere.
And we'll see that.
We'll have five lectures on the limbic system
later on in the class.
And here we are in the midbrain.
It's got its own special region.
But the rest here is somatic.
That includes the entire reticular formation, including
the red nucleus there, that we don't include in
the reticular formation.
But also we can lump together with this reticular formation,
the auditory and visual areas of the midbrain tectum and the
auditory tectum.
Those are all somatic.
Now, if you follow those two regions forward into the
tweenbrain, which we're going to talk about next, you can do
it very easily.
The limbic areas you follow into the hypothalamus and this
structure right at the top, the epithalamus.
Those are the limbic regions of the tweenbrain.
The somatic regions are the subthalamus and the older
parts of the thalamus are related to this area I've got
in striations there.
And the white areas are related to the more official
parts there of the tectum.
That's all somatic.
So we basically have the hypothalamus and epithalamus
and then the rest.
We're in limbic and somatic.
That's the organization that we'll be talking a lot about
in the class.
So once we're above the midbrain now,
everything is forebrain.
And the major subdivisions we've been talking about are
the ones we just mentioned.
The hypothalamus and epithalamus on the one hand,
related to the limbic system.
And I will try to be consistent.
If I'm using colors, I'll put that in red.
And then the somatic areas, if I'm using
colors, I'll put in blue.
Thalamus and subthalamus, relating more to somatic
sensory and motor system.
Now, that subdivision is validated in the anatomical
organization of the major outputs of the endbrain.
Because they're organized into two major bundles.
We call them the medial and the lateral forebrain bundles.
One related to the limbic system.
One related to the other parts.
Somatic in nature.
And so we'll review that now.
And then I don't know, we probably
won't get to this today.
But I'll talk about a little evidence
on forebrain evolution.
A topic very interesting to me in that I've written about it
in several places in the book.
You need a few more descriptive terms.
It's not very complicated here.
Here's our embryonic brain on the right.
And I've taken a section through the endbrain there.
You can see the ventricle in the middle.
Here's the walls and the neural tube.
They're thickening more here in the ventral and the ventral
lateral region.
The matrix layer where all the cells are dividing, becomes
very, very thick early in development.
And we can divide it.
It forms these three major bumps there, when seen from
the ventricle side.
We call the upper one here, the dorsal ventricular ridge.
And then we have the lateral and
medial ganglionic eminences.
They didn't stick to the same terminology.
But that's the way it's called in the literature.
The one we'll hear the most about is the one I put in red
there, the dorsal ventricular ridge, because of what that
becomes in the adult animals.
Many of these cells are becoming corpus
striatum, of course.
That's where the striatum develops.
Right there.
But not all of them.
And it's the ones that don't become corpus striatum that
are especially interesting from the standpoint of
evolutionary studies.
Let me just give you a little preview on endbrain evolution,
so we don't leave it all to the end.
Because there's recent data from studies of expression
patterns of regulatory genes.
Remember, we talked about the Hox genes or homeobox genes.
And by comparing gene expression in different
species, we can come to some conclusions about what's
related to what in these different brains.
And it's not always simple from the structure to make
those comparisons.
Because if we have only the morphological data, the
cytoarchitecture and connection patterns, we can
come to some of the same conclusions.
But having the gene expression data gives us, especially the
controversial areas, we can come to firmer conclusions.
So this is the expression patterns of several different
genes or groups of genes that they've labeled here in this
figure green, blue, and that, what is it, pink.
And they've done that in mice.
This is mice here.
There it's the word "mouse." And here's a bird.
This is the chick.
Here's a turtle.
I don't why they picked the turtle, but
it represents reptiles.
And the frog here, representing amphibians.
And you see these genes for example that you find
expressed in the neocortex of mammals, you do find expressed
in these other animals.
Now, that contradicted some of the earlier structural studies
which said there was nothing like
neocortex in other animals.
But in fact, there is a dorsal cortex in the amphibian and in
reptiles, including the turtle here, a dorsal cortex.
And it has that same regulatory gene.
It doesn't mean this is neocortex in these animals.
But it means they came, very likely came, they
have similar origins.
We know neocortex, structurally speaking, is more
elaborate in animals than this dorsal cortex
in these other animals.
But look at it in the chick here.
Where's the cortex?
It's supposed to be above the ventricles.
Here it is above the ventricle here.
The dorsal cortex is mostly about the
ventricle, even in the frog.
But the ventricle is over here now.
There's no ventricle there in the bird.
So this has all been called striatum in the bird.
All striatum.
The "H" means hyperstriatum.
But look at the dorsal ventricular ridge area.
It grew and stayed in the same place.
It was very prominent here in the turtle.
In the frog, it's not so separate.
We couldn't be sure what happened to the dorsal
ventricular ridge if these studies hadn't been done.
But it's there.
It's just part of a neural tube there.
And the same gene is expressed in one area medially here.
That's true in the turtle too.
And there it is in the chick.
It just forms a more ventral part of what
has been called striatum.
In fact, anatomists have called that the ectostriatum.
But it expresses those genes related to
dorsal ventricular ridge.
And then everything ventrally, the part that becomes the
corpus striatum in mammals, does so also in the chick, the
turtle, the frog.
But look at the dorsal ventricular ridge structures.
That's what was most interesting to the people
doing these studies.
The only structures that express those genes related to
the dorsal ventricular ridge of these other animals are in
the amygdala.
I call it an outpost of the corpus
striatum, a caudal outpost.
And in fact, if you look structurally at the amygdala,
you see that some of it seems like cortex.
Some of it seems like striatum.
It's actually related to both.
But actually, it's a dorsal ventricular ridge derivative.
And there's a little bit of the septal area also, just
like in these other animals.
So we'll come back to that later.
First, I want to talk about major features of the
forebrain structure.
And then I'll just do a little review of what
we've covered so far.
You can check how much is sinking in.
So here's the forebrain in our picture of
the embryonic brain.
And in our sections, we have the endbrain and the
tweenbrain.
That's what makes up the forebrain.
In this level, the endbrain is folded back.
It's grown a lot.
It's folded back over the tweenbrain.
And as we know, when it gets bigger and bigger, it folds
back also at the midbrain.
So the major subdivisions are the tweenbrain and endbrain.
If we just start with the tweenbrain or diencephalon, we
think first of all the major structures are the
hypothalamus and thalamus, the hypothalamus
below, thalamus above.
But we can lump together with hypothalamus, that epithalamus
at the top.
We can follow the limbic structures of the midbrain
right into both the epithalamus and hypothalamus.
And the thalamus is more related to the subthalamus.
That's the fourth major component of the tweenbrain.
So hypothalamus and epithalamus, characterized by
visceral inputs from below.
We're tracing sensory pathways.
We're going to see also visceral
inputs coming in there.
Its connections with the endbrain and the membrane, we
call limbic system connections.
That means in the midbrain, they're connected with those
limbic structures, limbic system structures, that I
showed you in the pictures.
And its functions include, especially the hypothalamus,
the gating of pathways
ascending through the thalamus.
This is a neglected issue somewhat.
The hypothalamus has very specific projections to parts
of thalamus, mostly we call the older thalamus.
But it actually reaches all of it, because of its heavy
projections to reticular formation of the midbrain,
which then projects forward into the thalamus.
And then the other major division, thalamus and
subthalamus, somatic parts of the tweenbrain.
And I'm pointing out synonymous terms here.
Thalamus is really
embryologically the dorsal thalamus.
And subthalamus is
embryologically the ventral thalamus.
So we talk about dorsal and ventral thalamus.
And anatomists will use these terms synonymously.
But if you're just talking about the adult, these terms,
thalamus and subthalamus, are the common ones.
Sorry.
I have to apologize sometimes for all the names we use.
But actually I'm cutting way down on them.
So the ones I commonly use and use a lot, you
should try to learn.
But you will learn them I think if you're
paying close attention.
If you're understanding what I'm saying, you should learn
them fairly easily.
So these are the structures that get somatic inputs.
Somatic inputs from the lemniscal pathways.
They also get connections from the midbrain, tectum and
tegmentum, the somatic parts of the midbrain.
And here's our division, just to remind you
what happened to them.
In the midbrain, you maintain these components of the
tweenbrain.
So if we now go to the endbrain, we can learn the
pallial structures and the striatal structures.
By pallium, it means the outer bark.
That is, the surface.
And the pallium has somatic and limbic parts, just like
the midbrain does.
The limbic cortex is the older, that is larger in the
very primitive animals.
And the most primitive of chordates that had a
hemisphere, like the hagfish and the lamprey, that's pretty
much all you're going to find if it hasn't a cortex.
And then you have the nonlimbic cortex, which is
parts of the dorsal cortex.
Very small in amphibians and reptiles, but huge in mammals.
That's the neocortex.
And then, the subpallial structures, sometimes we just
call them subcortical, the corpus
striatum and the pallidum.
Sometimes we use the term "striatum" to
mean all these things.
If you hear are the term "striatum," they might mean
just the dorsal striatum, which is the biggest part.
They might mean the entire striatum, dorsal and ventral.
And they might even lump the pallidum with it.
But the pallidum, or globus pallidus for the dorsal part
and the ventral pallidum for the ventral part, these are
the output structures.
There are some outputs going down to the midbrain, directly
from the more dorsal parts of these striatal structures.
But the main outputs come from the pallidal structures.
So what is the neocortex?
We talk about primary, sensory, and
motor cortical areas.
Those are unimodal neocortical areas.
They're dominated by one input.
Does that mean that's all they get?
Well, no.
There's a little bit of input from other senses too.
But studies of them, physiological studies, have
been totally focused on one input, because that's mainly
what you can find.
You have to try pretty hard to find the other influences from
other modalities.
These are the primary sensory and motor areas.
The motor area is probably a specialty of the
somatosensory cortex.
And then we have unimodal association cortex and
heteromodal association cortex.
And when we talk about the cortex, we'll define these
things in a lot more detail.
But the long connections across the cortex come from
these association areas, not from the primary sensory and
motor areas.
They do have long axons going out.
But they go subcortically.
The long axons within the cortex just go to adjoining
areas, with its primary sensory and motor.
These are the areas, the association areas, that send
the long axons to other parts of the brain, to other parts
of the cortex rather.
And then we have the limbic cortex.
That includes the olfactory cortex, that you see mainly
from underneath.
But it also includes what we sometimes call paleocortical,
older cortical, because we can trace them more easily in the
more primitive animals.
And then the corpus striatum, dorsal and ventral.
The ventral striatum includes the olfactory tubercle, which
gets direct olfactory input.
The dorsal striatum is sometimes called neostriatum.
Many anatomists are trying to get away from the words "neo"
and "paleo," because they say we don't really know enough
about what's new and what's old.
But I think we actually know quite a bit.
It's not that difficult.
But we'll usually use dorsal striatum
here and ventral striatum.
And then the two related structures related to dorsal
striatum is the globus pallidus, the pale globe, just
because of the way it looks in human dissections.
And then the ventral pallidum gets heavy input from ventral
striatal structures.
Let's look at those two major pathways that give some proof
about the validity of this kind of
division of the endbrain.
And I've shown here a tweenbrain section and a
endbrain section.
And I've outlined two major pathways, one related to the
limbic system, one related to the somatic system.
They're a little small here.
Here's the tweenbrain section.
And the medial forebrain bundle are these axons that
are coming through the hypothalamus.
They're concentrated in the lateral hypothalamus.
That's the medial forebrain bundle.
They include a large bundle of axons located right here,
coming from the hippocampus.
And then this is where the cerebral peduncle is forming,
axons coming out of the hemisphere, coming through the
corpus striatum there from the neocortex.
That we call the lateral forebrain bundle.
And many of those axons coming from the thalamus going this
way, but many others are coming this way.
Both these bundles are two-way streets.
It's axons going in both directions.
So if we look at this one, here is where the medial
forebrain bundle is, here in the developing endbrain.
And here is where the lateral forebrain bundle is forming,
here in the endbrain.
That's coming right out of the cortex.
So if I just show represented neurons, here are all cells
giving rise to inputs to the medial forebrain bundle,
including the hippocampus up there.
This is the medial pallium.
And it becomes hippocampus in more advanced animals.
And here are cells in the striatum, cells in the cortex,
sending axons in to the lateral forebrain bundle.
So this subdivision of limbic and somatic holds up very well
when we talk about connections.
And that of course is part of the basis for separating them
in the first place, in the thinking of Nauta and other
anatomists.
So I've just summarized for you here in this little table,
axons of the medial forebundle coming to and from the
olfactory cortex, other cortex we call limbic, but also some
subcortical structures like the amygdala and what we call
the basal forebrain.
It's not cortex.
It's just in front of they hypothalamus.
And then of course, the hypothalamus itself,
especially the lateral area and the limbic midbrain areas.
And then the lateral forebrain bundle related to striatum and
the neocortex.
Let's follow that output of the neocortex now in our
embryonic brain.
I've put it in orange here, the whole
lateral forebrain bundle.
So here it is, coming out of the neocortex.
And it's being joined by axons from the striatum.
And here it comes down.
As corticospinal final tract, it goes all the way down into
the spinal cord.
And now I'm showing you here the sections
where you find it.
Here it comes out of the cortex through the striatum.
Here it's the cerebral peduncle, coming along the
side of the tweenbrain.
There it is at the base of the midbrain.
Here it is at the base of the hindbrain.
Now, it's called pyramidal tract.
And there it's crossed over.
Descending in the spinal cord in the lateral position, is
the corticospinal tract.
And I've named them.
To follow them in the order I did here, you have to start
down here, cortical white matter to internal capsule,
then cerebral peduncles.
It includes the cortical pontine axons.
Then the primal tract and the corticospinal tract.
All the same axons.
Just getting different names at different levels.
They're always descriptive names, named by its shape or
by its origin and termination.
Like corticospinal tract, origin and termination.
Not that they all go to the spinal cord.
But the longest ones do.
Pyramidal tract, its got a pyramid shape at the base of
the hindbrain.
Cerebral peduncles, because it sort of forms a little foot at
the base of the midbrain.
Internal capsule, it's encapsulated by the corpus
striatum cells.
So it gets that term "internal capsule", right here, when it
goes through the striatum.
And then you see it in the cross-section.
So that's where all those names come from.
You notice it's the same bunch of axons that we can follow
from cortex to spinal cord.
I want you to remember the subdivision in the midbrain
that we can follow also; the tweenbrain, just showing it
again here.
And remember these two major tracts here.
They run, within red, the limbic structures.
And the neocortex, as its enlarged, has become related
to both systems, not just the somatic.
It might have originated as mainly somatic.
But any system, when it gets very large, ends up increasing
its projections.
And I just want to point out here, we can diagram it by
eliminating a lot of the smaller projections, just
taking the big ones.
Look at the main neocortical outputs, striatum, brain stem,
and spinal cord.
Here the whole brainstem is included, from the
diencephalon all the way back to the caudal hindbrand, and
of course the whole spinal cord.
The cortex projects to all of them.
It didn't start out like that.
And you won't find the dorsal cortex of a reptile with all
these projections.
But the neocortex of mammals has these projections.
And it projects very heavily to the striatum, as well.
The striatum has two major outputs.
One of them simply goes back to cortex.
The other goes to the brainstem.
But the cortex does have connections, smaller ones--
I've shown them in dash lines here--
to the limbic structures.
The limbic structures are defined really by this main
projection to the hypothalamus.
But it does project into the brainstem, like those limbic
midbrain areas also.
The hypothalamus then projects forward to
limbic structures primarily.
But there is a projection we can follow through the
thalamus, which would be in here, through the thalamus,
open to the neocortex.
And we'll be talking about that when we talk about the
system later.
And I think we better stop there.
I'll make your life a little more difficult by showing you
the complications.
But in fact this one is the one important one
now to keep in mind.
And I think as I remember when I wrote the book chapter, I
only used this one.