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PROFESSOR: OK.
Today we want to continue talking about the hindbrain.
And we'll go through a little bit that gets a little hairy,
but I've tried to simplify it as much as I can.
And some of it the figures will be for reference not for
memorization.
I'll explain that as we go along.
We had talked about the basic organization of this hindbrain
as being similar to the spinal cord.
And I was using this figure to explain that.
There will be a little more of that today.
And I just reviewed major functions of the hindbrain.
And now I want to say a little bit about the neurons there,
especially the reticular formation of the hindbrain.
Nauta and his colleague Ramon-Moliner talked about the
isodendritic core of the brain stem, and I want to explain
what they meant by that and how it's been divided into
what we call a neuropil segment.
The neuropil simply refers to the substance of the brain,
especially the areas where all the connections are made,
where you can't see cell bodies.
And most of the stains we use don't show all the axons in
cells at all, but we just call it the neuropil.
And then we'll talk about some of these axons that have very
wide distributions and, no doubt, influencing the general
state of the nervous system.
When Nauta and Ramon-Moliner talked about the isodendritic
core of the brain stem they were contrasting what they
called the isodendritic neurons and the
idiodendritic neurons.
Isodendritic means neurons that all look pretty similar
to each other and to motor neurons.
So for example, you'll have a neuron with a few major
dendrites with their various branches and an axon.
Here's the nucleus.
That's pretty much like a motor neuron looks.
Nothing particularly specialized in the way it
looks because those neurons look the same whether they're
motor neurons in the spinal cord, or the hindbrain, or
midbrain, or whether they're reticular formation cells in
various parts of the brain.
They all look pretty similar.
When dendrites get more specialized we take cells
like, for example, in the retina where you have
dendrites that end in specific layers like this.
And some of them will end in more than one layer.
You have neurons in the olfactory bulbs that we call
the mitral cells.
They're large cells that have a dendrite--
I won't show the dendrites in this region but they go out to
near the surface and they form this very dense network of
processes sort of like a bush.
Obviously a highly specialized cell.
Those are all idiodendritics.
So the word ideo means specialized or particular.
And that's all we mean.
Let's look at some of these pictures.
This is a young rat.
The section was made with a parasagittal cut.
These are rather thick sections so you can see they
don't cut off many of the processes of a cell.
if they happen to get in the right plane they can see a lot
of the single cell.
And what you're looking at here is multiple reticular
formation cells each of which has a dendritic spread that
you could place in a little disk like that.
So the neuropil in the reticular formation seems to
be divided into these little modules, these little disks
all parallel to each other.
If you come to a cell group that's a group of motor
neurons, like this is the 12th nerve nucleus, the hypoglossal
nucleus controlling tongue movements, it's totally
different from these types of idsodendritic neurons in these
neuropil segments.
They don't have the same orientation, they look pretty
much like motor neurons in the spinal cord.
You could confuse these with those two except for the
dendritic orientation.
And then you also see some cells here with axons that
don't behave this way at all but they're very widespread.
And I think he shows that the Golgi method, you can see,
stain is very particular.
It will just stain a few cells.
If you're lucky it'll just get the ones you're interested in
but it doesn't always do that.
So you have to prepare a lot of different material to get
the interesting things.
The cell group here, it didn't stain anything in.
It did stain a few of the corticospinal spinal tract,
this means pyramidal tract, another name for the
corticospinal tract in the hindbrain.
Now look at this one.
Let me blow it up a little bit.
This is from the same anatomists, not from Cajal,
but the Scheibels, who were more recent neuroanatomists.
They were a couple that did a lot of Golgi work and
published many pictures like this.
They found this single cell here in the lower part of the
hindbrain we call the medulla oblongata that is the
elongation of the spinal cord, the medulla spinalis.
And here's it's axon.
It divides here and has a rostral branch with
terminations all along and going all the way up into the
midline part of the thalamus.
These are cell groups in the thalamus, this is the
subthalamus here, and there's even a branch going down into
the hypothalamus.
It projects into-- this is is the midbrain here.
It's got many branches in the center part of the central
grey of the midbrain, many branches in the reticular
formation of the hindbrain.
Then it's got a caudal branch that terminates in reticular
formation here.
It terminates in one of the dorsal column
nuclei, nucleus gracilis.
And its branch even extends down into the rostral end of
the spinal cord.
Many reticular formation cells of the brain stem have this
kind of widespread projection.
It does not mean that the function of the reticular
formation is just changing the state of the brain because
connections there also control many very specific functions,
inherited movement patterns or fixed action
patterns for example.
Now if we go back to this picture of these cell groups I
want to talk a little bit about what these columns of
cells are like.
We're just looking at a single cross section here through the
caudal hindbrain.
I've labeled the major areas.
We draw lines around the different groups of cells that
we can make out.
And in Nissl stain we see specific groups of motor
neurons, three of them here, visceral motor, somatic motor,
like the ones controlling tongue movements we saw in the
previous slide.
But there's also what we call a branchial motor column.
So I want to explain what the branchial motor column is.
Visceral motor, they could be cells controlling heart rate,
for example, or breathing.
We've talked a little bit about the ones controlling
heart rate.
Many of those contribute their axons to the vagus nerve.
And then here, the secondary sensory cells, which we'll
talk about later.
What is the branchial motor column?
The body and the brain have an interesting segmentation.
For example, in the spinal cord we know that each spinal
nerve, separate from the others, and they enter what we
call one segment.
The question is does that segment--
anything-- does it correspond to anything in reality?
After all, it forms that way because the vertebrae form and
the nerves come out in between the vertebrae.
But we do talk about various segments of the
cord, as you know.
Well, prior to that formation the mesoderm is segmented.
These are called the somites, the segmented mesoderm, and
they continue through the entire level of the spinal
cord up into the caudal hindbrain.
The somites, OK?
There is some segmentation in the mesoderm when you get up
into the brain here.
They're called somitomeres, and they're only incompletely
segmented not as completely separated as the somites.
But in addition, there's some more specialized ones we call
the branchial arches.
They are collections of mesodermal the tissues that
form the gill arches in fish.
And in humans they form the jaws, the auditory ossicles,
so the bones of the middle ear, for example, the hyoid in
the neck, the pharyngeal skeleton
including the thyroid cartilage.
This all derives from the branchial arches.
Its history is a bit different in the
mammals and in the fishes.
And they're innervated by cranial nerves
for the most part.
And that's shown here.
You see the cranial nerves have somewhat different types
of organization than the spinal nerves.
The branchial arches are innervated by these cranial
nerves, 5, 7, 9 and 10.
And we'll learn about what all these cranial nerves are.
And by the midterm you'll have memorized all the cranial
nerves and their major functions.
It's not hard after you encounter them multiple times
and discuss them.
I'm not asking you to memorize them all at once
right at the beginning.
I just want you to try to understand what I'm saying.
This will grow on you.
Here's a picture of a two-day chick embryo that was taken so
it reveals the somites that are forming in just two days
of incubation in the chick.
And you see they extend up into the head region but only
the lower part.
Mostly, it's below the head region.
I want to say a little bit about, now, what's given more
reality to the segmentation not just in spinal cord but in
the brain too.
And that is their gene expression.
Now the somites and some of the segmentation in the
hindbrain had been-- and, of course, the branchial arches--
were all known before we had gene expression data at all.
But the gene expression data has added a lot to the meaning
of these terms.
The origins are very ancient.
We refer to many of these genes as homeobox genes
because of a particular sequence we call the homeobox
that we can find pretty closely related from one
species to the next.
We can trace them all the way back to primitive
invertebrates.
And we see them-- they were discovered in fruit flies,
where mutations in these genes caused major changes in the
body segmentation.
And I want to give some examples of the expression
patterns of these genes.
Here at the top I show a fruit fly.
Look at the DNA in drosophila.
In a single chromosome we can find these different genes
laid out in this sequence.
And they are expressed in the same sequence in the body.
So if you lay the chromosome out in a particular way, you
see where they're expressed, laid out in the same way from
anterior to posterior in the fly.
Now in the mouse you have the same kind of thing, the genes
are still in that same order.
And these are paralogous groups, that is, they're
related to these genes in the fruit fly.
And we see them not just in one chromosome, but here in
four different chromosomes of the mouse we see
these similar genes.
And they, likewise, this is in the fruit fly, are expressed,
at least here in the nervous system they're showing them
expressed, in rostral de coral order, just like they are in
the body of the fruit fly.
And if you take the proteins they express and
get markers for them--
so now we're looking at how those genes are expressed.
We can stain for the protein markers and we see here
they've taken three different genes that have a different
expression pattern in the embryonic day nine and one
half mouse embryos.
Notice that this one is expressed up here in the brain
and then it's not expressed just caudal to that but then
it's expressed again further caudally.
Here's one that's not expressed in the head region,
then it's expressed more caudally, and finally in the
tail region.
Here's one that's even more caudal in it's expression.
And these are where the three genes are found in the
chromosomes.
Wolpert, in 2002, published this figure where he
summarizes the expression patterns of a bunch of
different hox genes, which we call them hox genes
usually in the mouse.
And here he's given their abbreviated names and showing
where they're expressed in different levels of the
nervous system.
This is from cervical to the caudal-most spinal cord, and
he doesn't include the ones that are expressed so much
further rostrally.
But you can see now that there are different expression
patterns at different levels.
Often they have a very strict anterior limit.
Anterior to that they're not expressed at all, or I should
say rostral limit.
And then they're expressed in various distances after that
as we go caudally.
Now if we go back a minute to the development of the
hindbrain, we see at a certain stage the appearance of a
segmentation that was known just from structural studies.
This is a more modern structural study.
It shows a scanning electron micrograph published by Allman
in his 2000 book, the one that I've given some readings from.
And you can see these bumps that appear.
And those segments of their hindbrain are called
rhombomeres, rhombo meaning rhombencephalon for hindbrain.
And the question is what is their reality?
First of all, do they have different
gene expression patterns?
And they do.
Usually it's eight rhombomeres, and I called them
eight here.
In this particular figure they show seven.
You see a little bit of variability in the way they're
talked about because of some ambiguity at the ends there.
Here are different hox genes and other
transcription factors.
And look at the way they're expressed.
Here's one that's expressed in the third rhombomere and the
fifth rhombomere, but not in the one in between and not
below or above it.
And when you put all these together, you get sort of what
looks like a bar code.
And each rhombomere then has a unique pattern, not a single
gene marking it but a pattern of genes that distinguish it
from the other rhombomeres.
It doesn't mean we understand all that much about them but
if you look at the different rhombomeres here and their
expression boundaries of the genes here they've laid out
the expression patterns for just five of them.
These five listed at the bottom here.
Different hox genes, OK?
And then they did a simple experiment.
They injected embryonic precursor cells that were
labeled, and they injected them at different stages.
They injected them before the rhombomeres formed and shortly
after the rhombomeres formed.
And they found out that if they injected them before the
rhombomeres formed the cells would spread across the
boundaries of the rhombomeres.
Remember, the rhombomeres weren't explicit yet.
If they injected afterwards, so now they presumably were
beginning to express these different genes, you can see
what happened.
The cells became restricted to a single rhombomere, a single
segment of the hindbrain.
And that was not done all that long ago.
It was cited in the Striedter book.
And there are more recent studies on the function of
these genes too.
All right.
So I want to continue talking about the hindbrain.
And we're going to go back to these secondary sensory and
motor neuron groupings in the alar and basal plate regions
of the hindbrain.
And as we go over them again and again I think some of it
will begin to be remembered.
I hope so.
I know it's not easy because you're getting words.
I mean, neuroanatomy is learning a language, right?
In fact Andrew has told me that in the book sometimes I
will mention something once and I'll define the term.
And then another page later the term's used again and he's
scratching his head because he knows he saw that some place
before but can't remember where.
So he would like me, when this book is published to put boxes
or something in the margin so people can more
quickly find it.
I tell them, well, I have a plan to put a glossary, but
that might be more cumbersome.
So I like this idea.
And any suggestions like that that you have just
give them to me.
Send me by email or post them on a Stellar forum.
Now what I've done here is taken a figure from Nauta and
Feirtag, this figure where he compares the columns in the
spinal cord and the columns in the hindbrain.
But they also show the trajectories of the nerves
carrying input to these cell group and output from them to
the periphery.
So here in the spinal cord we have three
major columns of cells.
You could call them the somatic sensory column.
It's where the secondary sensory cells receive any
input from body surface.
They're all there in the alar plate, the dorsal-most part of
the spinal grey matter.
And then we have this intermediate
column, it's small.
That's where the lateral horn forms, thoracic and upper
lumbar region.
That's the visceral motor column, the preganglionic
motor neurons in the sympathetic nervous system.
And then finally, in the ventral-most part of the grey
matter here is where we have, he calls it the
somatic motor column.
This is where the motor neurons are that innervate the
striated muscles of the body.
So three major columns there, one sensory and two for the
output cells, the motor neuron.
Now he tries to do the same thing here for the hindbrain.
So here's the alar plate, three columns of secondary
sensory neurons.
He calls them general sensory here and
special somatic sensory.
Now those terms, I would say, they're becoming a little bit
less common now.
So you'll see in the figure I've used in the book that
I'll show you now I don't call them
special and general anymore.
General somatic sensory just refers to sensory input like
in the cord, but now it's coming in the trigeminal
nucleus of the hindbrain.
So this is the fifth nerve here.
I lost my color.
The fifth cranial nerve.
And here, caudally, of course, we have the dorsal column
nuclei if we're going to say that's in the hindbrain.
But then, more rostrally we have the auditory nuclei and
the vestibular nuclei.
He just calls them special somatic sensory here.
And then finally, the visceral sensory.
Notice that the axons carrying the input don't come in
through dorsal routes in the same way they do in the cord.
These do come in mostly dorsally, but the most dorsal
are the auditory and vestibular.
The trigeminal comes in more at the middle level whereas
the visceral sensory comes in through a mixed nerve,
contains sensory and both input and output fibers like
the seventh cranial nerve or the ninth or the
10th cranial nerve.
And then if you look here in the motor groups you notice
that the branchial motor column, like the facial nerve,
is the seventh facial nerve.
The axons leave the nucleus, and they
have a peculiar route.
They tend to go dorsally first and then they loop
around and come out.
Very typical of the axons of the facial motor nucleus.
And then we have the visceral motor column, which sends
axons into the ninth and 10th cranial nerves, some in the
seventh as well.
That's a useful slide to be familiar with.
Just study it so you understand what
he's trying to say.
And you've seen that figure now multiple times.
You should know the relationship to the spinal
cord, alar and basal plates for the secondary sensory and
the motor neurons.
It's useful to know that at least for recognition.
I'm not likely to ask you to draw it.
Students find that pretty difficult, and usually I can't
understand their drawings anyways.
I might ask you something about it though.
But I want to give you a little more about cranial
nerves, names of some cell groups, and then fibers
passing through.
And I'm going to show you a figure that--
Well first of all, let's talk something
about the cranial nerves.
Traditionally we talk about 12 of them.
And in any medical school textbooks you learn that there
are 12 cranial nerves.
Olfactory, optic, occulomotor, trochlear, trigeminal,
abducens, facial, auditory and vestibular, and then the
glossopharyngeal, and--
where are we, eighth?
No, yeah ninth.
And then the 10th the vagus, 12th the hypoglossal.
So not too difficult to learn.
But I just want you to know that if we look at all of the
animals, all of the vertebrates, you can make out
25 cranial nerves.
Some are found only in certain groups of vertebrates.
And in some cases, nerves that in the mammals are single,
like the facial nerve, also the glossopharyngeal, number
nine, and the vagus, number 10, they contain they contain
two distinct parts.
And you can separate them in embryology but then the fibers
run together in the cranial nerve.
But in some animals they are not separate like that.
So if you looked at every stage you'd have to say, well,
now we're up to 15.
And then we look at the lateral line nerves, remember
we talked about electroreception as the
sensory specialization in many animals.
Probably, it may be that the very earliest mammals had some
electroreception.
But anyway, animals with the lateral line organs have
multiple cranial nerves innervating them.
There's at least six that we don't have at all.
So we're up to 21.
And then there's a few more.
Now humans actually have a terminal nerve in addition to
an olfactory nerve way up in front, but it's so
insignificant and we can see it mostly early in the embryo
and then it's not obvious in adults.
Some adults don't seem to have it at all.
So we just leave it out.
That would be 22.
And there's still a few more like the epiphyseal nerve, the
pineal eye.
Now we still have a pineal gland but we don't have the
nerve, at least we don't call it a cranial nerve because it
never leaves the brain.
So there's another one.
So it's become traditional to just list 12 cranial nerves,
and it's certainly adequeate for the human.
So, we'll review now the location of the cell groups.
Secondary sensory neurons in alar plate, motor neurons in
the basal plate, and then the axons passing through.
Now this is a figure for your reference.
I'm going to show it to you, and don't try to memorize it
just try to understand it.
All I've done is added some detail to give it a more
adequate picture of the caudal hindbrain.
And notice I've used some color coding there.
I've shown sensory and ascending fibers in green.
You don't see the green too well here but those are green
here and here.
Spinothalamic tract, medial lemniscus.
And then here, trigeminal nucleus, these.
Vestibular and auditory.
So ascending fibers and sensory cell groups I've named
or indicated in the green.
And then in the blue I've shown the outputs or the
descending tracts that we tend to associate
them with motor functions.
So here's the branchial motor column coming from these
nuclei, the nucleus ambiguus for swallowing and
vocalization, the facial nerve, the facial motor for
emotional expressions and other expressions in the face.
Masculatory nucleus is part of the fifth nerve for chewing
movements, jaw movements.
And then this is very much like a ventral root, call it
the somatic motor column, innervating the eye muscles
and tongue muscles.
And then descending fibers, corticospinal in this group at
the bottom, and rubrospinal coming from the midbrain.
We'll be talking about that a little later when we review
the midbrain.
So I just want you to understand how that's
organized, do you know the meaning of such a figure?
So now, think of all those groups of cells
and ask this question.
How big are those groups?
We're seeing them in one little cross section of the
brain stem.
What if we saw the whole brain, and we want to know the
full extent.
And that's what Nauta does in this picture.
Again, I just want you to understand what it's about.
Imagine the brain is transparent and you could see
all those groups.
This is what he comes up with.
And it's pretty interesting because what it shows, for one
thing, it's a human brain stem, which has some
distortions that you don't find in rodents or in the
human embryo.
And usually in a neuroanatomy text they use the human brain.
Mine is a little unusual in that I don't focus on the
human brain because I'm interested in how this
originated, why it is like it is, and what it's for.
Notice this big one here.
That's the trigeminal.
That's the sensory input coming in from the face.
It terminates in secondary sensory cells in a nucleus
that's largest up here in the rostral hindbrain right near
where the trigeminal nerve enters the brain in the
rostral hindbrain, or the pontine region.
But then it has a descending component that goes all the
way down into the upper part of the spinal cord.
Others are much more restricted.
Like here's the cochlear nucleus, it's
just in this region.
Same for the vestibular nuclei but the vestibular nuclei have
a somewhat greater rostral caudal extent.
The visceral motor--
the visceral sensory, rather, nucleus of the solitary tract,
it's also elongated there.
And here he shows in the top view, he's showing the motor
nuclei on the left, secondary sensory cells on the right.
And you see this extent.
The motor nuclei tend to be more restricted but some of
them, especially nucleus ambiguus there, are elongated
groups of cells that in a cross section of the human
brain stem you have a little bit of trouble even finding
them because there aren't that many cells in a cross section.
It tends to be a small nucleus unless you can see it in a
view like this.
You can see that it actually contains quite a few cells
controlling our swallowing and vocalization.
And it gives rise to axons that go out to
several cranial nerves.
If I don't change the magnification it doesn't
advance this easily.
That's why I'm always doing that.
So I want to talk a little more about that fifth cranial
nerve, the trigeminal system and their terminations in the
hindbrain, and the what happens to the axons of those
cells because they are very much like in the spinal cord.
And we can talk about the channels of conduction just
like we did in the spinal cord.
I think it was important in evolution, and I'll explain
that here in a minute.
So the information comes in from the face.
and some of them enter a local reflex channel reaching motor
neurons pretty quickly.
And we'll show a pathway for the eye blink reflex.
We call eye blink a reflex even though eye blink is more
than a reflex.
It's actually a fixed action pattern because our motivation
to blink the eyes builds up somewhat
automatically over time.
And whether there's any input that changes to trigger the
eye blink we still blink the eyes.
You all know that.
You've probably had staring contests with your friends and
stare at each other and see who can wait the longest
without blinking his eye.
But in fact, the eyes eventually blink even though
you're trying very hard to prevent it.
And that fits the definition of fixed action patterns that
have a motivational component, which reflexes do not
necessarily have.
So it is a reflex in the way it's triggered but it's also a
fixed action pattern.
And then we can define lemniscal pathways.
Textbooks just talk about the trigeminal lemniscus carrying
information from the face forward,
rostrally into the thalamus.
But in fact, there's a trigeminal reticular pathway
just like the spinal reticular pathway.
And we're going to talk about that first because it's
important in evolution.
And then I want to spell out this hypothesis in a little
more detail.
That originally the axons coming in from one side of the
body do not decussate.
But then they evolve so that at least in midbrain and
forebrain the body surface on the right became represented
on the left and the left became
represented on the right.
So I want to explain why I think that happened.
So let's do that now starting with spinal reticular
[UNINTELLIGIBLE]
Here I'm showing the nature of the spinal reticular pathway
in the spinal cord.
Here are primary sensory axons in red.
The secondary sensory cells' axons ascend, mostly
ipsilaterally, but there are some that reach
the other side too.
So it's really bilateral, and I show that
especially here in the cord.
And a few of them might make it up into the forebrain.
We know that in mammals that have been studied, a few of
them make it right up into the caudal forebrain, the
diencephalon.
Well, trigeminal reticular I've simply added here.
I show primary sensory axons coming in through the fifth
cranial nerve contacting secondary sensory cells here
that I've shown in black, and it's very much like the spinal
reticular pathway.
Some crossed but mostly uncrossed.
And they go in both directions, mostly ascend
towards the more rostral parts of the brain.
All right.
So now let's talk about how that led to a predominantly
crossed pathway.
Remember I said that some of the fifth nerve input-- here's
the trigeminal nerve--
did reach the other side, though most of them were on
the same side.
When the optic inputs developed they grew out,
remember, from the tween brain.
It's a tween brain nerve, a diencephalic nerve.
In fact, the retina's part of the CNS
and the way it develops.
And here I have the axons coming in terminating
bilaterally.
They terminated right near the midline at the base of the
brain with a special role to synchronize the rhythm of
activity so it would match the rhythm of the sun.
And it still has that function in us and all other mammals.
So the question is then why did it become, not in its
hypothalamic termination there, but the further
extensions of it became mostly crossed?
I think it has a very simple reason that can apply just as
well to the trigeminal pathway.
Because when you have lateral eyes something happening over
here is in a different location than
something over here.
And if that could indicate something dangerous you want
to turn away from it and escape from it.
That's a very critical function.
You don't survive and pass on your genes unless you get away
from predators.
And to turn away like this, here's something approaching,
a primitive animal with lateral eyes, it triggers
input mostly in the right eye here and we
want him to turn away.
To do that we want to get to these neurons
that project caudally.
They already existed for the trigeminal system causing
flexion of the body on that left side.
And we want to do that when the input is from the right
for vision.
And that's the idea of why that would evolve that way.
Well that gives us an escape response.
But that's still a long ways from image-forming eyes
involved in localization, and oriented towards
things, and so forth.
Well, we know that the eye evolved and developed in
better some imaging capacity.
We know formation of visual images evolved independently
in different animals.
It evolved in the octopus, for example, separate from the
vertebrates.
But with formation of images and development of some
organization in those projections, the projections
at least that went beyond the hypothalamus, then it became
very useful for orienting towards things.
The main structure that became involved in that-- and it
might not have been the initial one but it certainly
became one pretty early in evolution, reached the roof of
the midbrain--
the optic tectum.
In order for the optic tectum, then, to steer the body
accurately towards something, the most efficient--
if it's not an escape movement but orienting towards
something-- is a cross projection.
That's easily solved.
We simply have outputs that elicit an escape stay
ipsilateral, as we already described.
But outputs that elicit orienting towards then simply
cross the midline.
And that's what happened.
So for orienting towards something, that the input's
coming from the right here and we want to turn towards it, it
crosses once at the optica and it reaches the opposite side
of the midbrain.
And then to orient towards it here it crosses again in order
to control turning towards it rather than away from it.
Flight movements.
Well, that visual input was so important that the other
inputs that came into the tectum that also gave spatial
information, including somatosensory, which initially
didn't go into the midbrain.
It came in through the hindbrain.
It now had to match the visual to be functional.
Auditory input evolved.
That had to match visual.
It also gives some spatial localization ability.
And experiments have been done where you can alter the
organization, distort the visual input
coming into the tectum--
this has been done mostly in chicks--
and the auditory and somatosensory inputs will
shift in order to match that distortion.
The visual is dominant in the tectum.
So I think the crossed visual input probably developed first
in the midbrain.
The other inputs came to match it.
It's the only way that made sense for function.
And then, of course, the further extensions, which
developed a little later into the forebrain, maintain that
crossed projection.
If you look for references, if you search the literature, you
won't find anybody else that actually presents this
hypothesis.
But I think now that we can think clearly about evolution,
we know what the demands were in evolution and we know a
little bit about what's primitive and what's less
primitive in the brain, I think that it's a reasonable
hypothesis.
Let's now talk a little bit about that trigeminal nerve,
and get a little bit further here before we have to
continue next time.
There's the position of the nucleus in the embryo, the
embryonic hindbrain.
That's where the secondary sensory cells are.
Here they are in the adult.
Fifth cranial nerve, or trigeminal
nerve, projects there.
We can follow the pathway the for eye blink.
It requires axons that go from that nucleus to the seventh,
or facial motor nucleus, to cause the eye blink.
For them to be short connections they come from the
trigeminal nucleus at that level, and that's not up at
the rostral end of the trigeminal nucleus, it's a
little bit more caudal.
Well, many of those trigeminal axons, primary sensory axons,
descend in the lateral edge of the hindbrain there.
And they terminate all along in that descending nucleus.
So that's the way the short connections would form causing
the eye blink.
And that is happening right at this level.
I'm showing the arrow there, that's where the eye blink
connection would form, like this connection right here.
Whereas if we want to ascend we send secondary sensory
axons across the midline.
Again, this is the greater number of fibers.
There is a trigeminal reticular, which we already
talked about, that ascend on the same side mostly.
Now we're going to talk about the more numerous ones that
cross not only from the rostral end here but also more
caudally, and they ascend towards the thalamus.
They do terminate in the tectum and other
places on their way.
But they reach the caudal thalamus, the ventral nucleus,
and that's where they reach cells that project the cortex.
So it's very much like the medial lemniscus or
spinothalamic tracts.
We usually don't separate them, though if you had to
separate them you'd say, well, it's more
like the dorsal column.
Medial lemniscus system would be the axons coming here from
this principle nucleus, the ones from the descending
nucleus resemble more in their functions the
spinothalamic tract.
OK.
We'll just review that next time when we start out.