We began at the top of this atlas or the most supeior section.
Which is very near the junction of the thalamus and the midbrain.
We recognizde elements of the thalamus here, all this gray matter alnog the
dorsal and medial aspect. Just to the lateral wall of the third
ventricle. Down below we have the mammillary bodies,
which is part of the hypothalamus. So we are definitely in a portion of the
posterior diencephalon, but we are also starting to get into the mid-brain.
And we know that, because of these massive bundles of white matter that are beginning
to take shape in the ventral lateral surface of the brain.
That's where we expect to find the Cerebral peduncles.
In fact, what we have here is the confluence of the internal capsule into
the Cerebral peduncles. So, if I select the Cerebral peduncles,
and then just go down one section We are clearly in the mid brain, and the cerebral
peduncles are one of those key features that will allow you to be able to
recognize this part of the brain. Another feature would be what's going on
with the ventricular system, so we just saw the third ventricle opening up.
In the region of the diencephalon. N the mid-brain we don't expect to see the
third ventricle. Rather, we expect to see the narrow
cerebral aqueduct, and indeed that's what we see here.
So he presence of the peduncles, the presence of the cerebral aqueduct is
confirmation that we are looking at the mid-brain.
Well, let's think about again that outside to inside perspective.
What else do we see from the outside when we think about the mid brain?
Well, you should be thinking about a cranial nerve.
Cranial nerve three, and indeed we have cranial nerve three here at the Oculomotor
nerve. And so again another characteristic
feature of the mid-brain, cranial nerve three is derived from a sematic motor
nucleus, so long the dorsal midline of the take momentum of the brain stem.
This is the Oculumotor nucleus, it also has a pair of sympathetic motor component
from the Edinger Westphal nucleus. So those are the cranial nerve nuclei
associated with this level of the mid brain.
But there's lots of other features here that's worth noting in the mid brain.
There's a very large spherical nucleus in the mid brain that looks a little bit
orange-ish. In a fresh specimen of the human brain
that has been sectioned open. This is called the red nucleus, very
prominent nucleus of the tegmentum in the mid-brain.
Between the red nucleus and the cerebral peduncle is the substantia nigra, which is
really two nuclei in one. There is a compact layer of cells on the
dorsal margin of the nigra. And these are the famous dopamine neurons
of the mid brain that project broadly to the fore brain including the basal
ganglia, parts of the prefrontal cortex of the frontal lobe as well as the amygdala
in the hippocampus Then there is a inferior part to the substantianagra,
which is very much like the globas palettus in the basal ganglia.
We'll say much more about all of this when we talk about the basal ganglia, but for
now I would highlight that the substantiangra is found in the mid-brain.
Okay, let's look at the dorsal part of the mid-brain, and again if we think about
what we see from the outside when we look at the mid-brain you may recall that there
are these four bumps that we see from the outside.
They're called little hills, or colliculi, which means little hill.
There are a pair of superior colliculi and a pair of inferior colliculi.
We are at the level of the superior colliculus.
The superior, of these little hills. One on either side of the mid-line.
And just below the superior colliculi, around the cerebral aquaduct is a bit of
gray matter, that we call the periaqueductal gray.
All right, well, let's go down a section to a lower level of the mid-brain.
And at this level, we are below the superior colliculus, and we are at the
level where we Are beginning to develop the inferior colliculus.
We're actually a little bit below that little hill, strategically, Because we
want to see the nucleus of the trochlear nerve, which is found here along the
dorsal midline of the tegumentum. Right in line, but inferior to where we
saw the ocular motor nuclei. We still have a small cerebral aqueduct
surrounded by a large periaqueductal gray, nucleus so that is consistent with what we
saw in a more superior level. But now you'll note that the inferior part
of the section looks quite a bit different.
We still see something that resembles the cerebral penuncle, but now we're beginning
to see some[UNKNOWN] of these axons in the ventral part.
Of this section. In fact, what we're getting into is a set
of nuclei that is defining the features of the pons.
So this is very near the juction of the pons and the midbrain.
We have nuclei in the base of the pons calls the pontine nuclei that give rise to
axons to called the pontocerebellar fibers because they're going from the pons to the
cerebellum. Well let's go ahead and section down into
the pons. We'll actually skip this section.
And go to the widest part of the pons, where the ponce is sending axons out into
the cerebellum. And then we'll work our way, back up to
those sections we just passed by. Well, we're in the pons.
How do we know that? Well, because we see, these pontene nuclei
defining much of this basal or ventral aspect of the pons giving rise to these,
to this white matter, called the ponto cerebellal fibers.
This accounts for that transverse orientation.
Of axons we see in the pons that is bridging the long axis of the brain stem.
"Pons" means bridge. And these pontocerebellar fibers give rise
to these massive bundles of axons that enter the cerebellum, called the middle
cerebellar peduncle. Now in the region of the tegmentum of the
pons, we have some cranial nerve nuclei of note.
We have the chief sensory nucleus of the trigeminal complex.
Also called the principle sensory nucleus. And the trigeminal motor nucleus.
That are connected up with the trigeminal nerve roots.
Which we see here. Now, above the tegmentum of course, that
cerebral aqueduct has now opened up into a large fourth ventricle.
And so, lateral and dorsal to that foruth ventricle is the cerebellum.
Now, you may be wondering about these dark bundles of white matter that we see here.
In the lateral aspect, of the ventricles here.
This is the superior cerebellar peduncle. Yet another major bundle of axons
associated with the cerebellum. So we have the middle peduncle.
By far, the largest of the three cerebellar peduncles.
And the superior cerebellar peduncle. I'm going to select this structure.
And then retrace our steps, and go up a few sections, and see what happens to this
peduncle. This peduncle Is the means by which the
cerebellum sends signals out of the cerebellum to reach our motor circuits in
the thalamus and so these peduncles are leaving the cerebellum and as they do so
they work their way into the tegmentum of the ponds.
Where they cross the mid-line, so we see a crossing of this peduncle called a
decussation in the upper part of the pons, and that's really the primary feature I
wanted to highlight in the sections between.
The level of the trochlear nucleus and the trigeminal nerve.
So let's go back to the section that we were just viewing and highlight a few
other features. We notice in this basal region.
While many of the axons running through have defesiculated, we're beginning to see
a re-condensation of axons into a tight bundle, this will emerge on the other side
of the Pons as an important pathway called the medulary pyramid, which contains among
other axons, the cortical spinal tract. So we'll say much about that pathway in
this course. All right, so let's go down one section,
which is the level of the cottle part. Of the pons and, I'll deselect the
superior peduncle here for a moment, and show you some other features that are
present in the tegmentum of the pons. Again, starting with cranial nerve nuclei.
So now we have a different division of our trigeminal nuclear complex.
This is called the spinal nucleus of the trigeminal complex.
This is part of the trigeminal nucleus that extends from the level of the
principle or chief sensory nucleus all the way down to the merging of the caudal
medulla with the upper cervial spinal cord.
So there's a nucleus, and then just lateral to the nucleus is a tract, called
the spinal trigeminal tract. We have other important cranial nerve
nuclei here. We have the facial motor nucleus in the
colu pons, which gives rise to axons that sweep across The tegmentum, and loop
around an important grey matter structure, just below the pornter/g here.
And these axons then arc back out and exit the ventral lateral aspect of the pons.
What they loop around is the Abducens nucleus.
So here's the Abducens nucleus, a somatic motor nucleus along the dorsal mid line of
the tegmentum, It gives rise to the axons of the abducens nerve that exit near the
junction of the pons in the medulla. We've accounted for the abducens nerve as
it exits near the mid line of the pons. We've accounted for the facial nerve that
exits a bit more laterally. The third of these pairs of nerves at this
junctional region Is the vestibular cochlear nerve.
And at this level, we're beginning to get into some of the nuclei that are
associated with that nerve. This is one of the vestibular nuclei that
we have in this region of the. Pons here is another one.
Because there are multiple nuclei here, receiving incoming vestibular signals.
We call this the vestibular nuclear complex.
All right. Lt's move now out of the pons and into the
upper part of the medella. Here again we expect to find vestibular
nuclei. We have yet an additional vestibular
nucleus here, part of that nuclear complex, but we also have the auditory
component of the eighth nerve which becuse we didn't achieve an optimal section
through it in sylvias we elected not to label it.
But we see a little bit of it here. It's this grey matter that forms this rind
around the outside of the dorsal-lateral aspect of the medulla.
And what it's surrounding is a, is a large mass of white matter, that I'll get to in
a second. So this grey matter that we see out here ,
this is part of the Cochlear nuclear complex.
So there are multiple division of this complex.
We can call them collectively the cochlear nuclei.
So we've got our vestibular nuclei and then our cochlear nuclei associated with
the eighth nerve. Well this big massive bundle of white
matter. This is the inferior cerebellar peduncle,
one of the three major stalks that attaches the cerebellum to the brain stem.
This is inferior, and it attaches from below, mainly conveying signals from the
spinal cord but also signals from an important nucleus of the medulla that I'll
mention in just a moment. Well, we also have elements of the
trigeminal system here, here is our spinal trigeminal tract and our spinal trigeminal
nucleus. We'll see this all the way through the
medulla. Now, in the more ventral apsects of the
medulla, those fibers that were penetrating through the basal region of
the pons and coalescing back into a nice compact bundle of white matter, have now
formed the medullary pyramids. And at this level, these axons are almost
entirely the corticospinal tract. That is the pathway by which the cortex
governs the motor systems of the spinal cord.
Just above the medullary pyramids is really a, quite a lovely nucleus.
It's called the inferior olivary nucleus. And this is the nucleus that gives rise to
a bulge In the ventral lateral aspect of the medulla that we call the olive.
So you may recall from our surface views of the medulla, seeing the columns or the
medullary pyramid. And just lateral to it is the inferior
olive. Or the olive for short.
That's what we call it from the outside when we're looking at a cross section as
we are here. We recognize this as the inferior olivary
nucleus. Well, let's move on.
Now you may remember between the olive and the medulary pyramid exits the axions of
the 12th nerve, the hypoglosal nerve. We can see the hypoglossal nerve roots
here as they pass between the pyramid and the olive and their source is the
hypoglossal nucleus. Another somatic motorn nucleus that we
find on the dorsal mid line of the tegmentum in the medulla.
Now, just lateral to that nucleus we have the dorsal motor nucleus of vagus.
And right next the that nucleus is a sensory nucleus, the nucleus of the
solitary tract. And that's grey matter that surrounds this
solitary compact bundle of white matter. Called the solitary tract.
The nucleus of the solitary tract is a visceral sensory nucleus.
The dorsal motor nucleus, a vagus is a parasympathetic visceral motor outflow
nucleus. So, notice again the organization that we
have here. Somatic.
Motor, visceral motor, visceral sensory and then we have special sensory and
general sensory. Our special sensroy at this level are
still vestibula nuclei and then our general sensory is still the spinal
trigeminal nucleus in the spinal trigeminal tract.
Now, I also should point out one important white matter pathway that we'll spend a
fair amount of time on when we talk about our mechanosensory system.
And it is a medial ribbon of white matter, that sits right on top of the medullary
pyramids, its called the medial lemniscus. So I'm going to select the medial
lemniscus, so I can show you how this structure, this medial ribbon of white
matter, runs all the way through the brain stem.
I haven't highlighted it until this section, because we probably see it best
in the middle to lower part of the medulla.
But indeed we can track it all the way up from the caudal medulla to the thalamus.
And we will do so when we study our mechanosensory pathways.
For this structure, conveys Our sense of touch from the body below the face, from
the brain stem to the thalamus. Okay.
Let's go now to the caudal part of the medulla and highlight a few structures of
interest. Now, what we find is still a spinal
trigeminal nucleus in tract. But now, in the dorsal part of this
section, we've lost the fourth ventricle entirely, and what we have are a set of
gray matter nuclei that are bounded by white matter pathways.
And these nuclei are called the dorsal column nuclei because they sit right on
top of the dorsal columns of white matter that rise up from the spinal cord.
And there are two of them. There's a medial tract called the gracile
tract, which ends in a gracile nucleus. And then there's a more lateral tract
called the cuneate tract that ends in the cuneate nucleus.
Now, these nuclei, these dorsal column nuclei, they grow the axons that form the
medial meniscus. So they grow axons that sweep around
through the tegmentum of the brain stem, called the internal arcurate fibers.
And they form the medial limniscus. The medial limniscus axons derive from the
contralateral, or the opposite side dorsal column nuclei.
So there's a crossing. There's a decussation here.
There's one other important decussation to note.
At this level of the medulla. And it's a decusation of the medullary
pyramids. Notice how one pyramid is starting to
stream up and over the mid line on top of the other pyramid.
If we go down one final section, we'll see this crossing more obviously.
This is where the cortical spinal tract crosses the mid-line.
Actually as we'll seen in a few weeks about 90% to 95% of the axons cross some
remain on the same side, but the vast majority of them cross and here's where
the do so, in the caudal medulla. Well, that completes our brief tour of the
internal anatomy of the brain stem. We'll have the remainder of the course to
reinforce this anatomy and come back to it.
We'll do so when we talk about our long pathways.
For somatic sensation and motor control. And then we'll apply this knowlege to
consider the localization of injury in clinical cases whrer there has been some
focal damage. And what will be importatn to emphasize at
that point in your learning in the regional anatomy.
That is, where do various systems come together in small places in the nervous
system? And the brain stem is one such important
place, where even very small injuries of nerve tissue can give rise to widespread
and disparate neurological signs and systems.
So, I look forward to having that level of discussion with you in a few weeks.
But for now, take your time. And go through the brain stem sections and
identify the principal grey matter and white matter structures that are
highlighted for you in the tutorial notes, including those that I've highlighted here
in this video. And, use this knowledge, we'll apply this,
we'll discuss it as we go along. So, thanks for your attention, and I'll
see you next time.
