Okay, welcome back to this tutorial on the cranial nerve nuclei.

If you have Sylvius 4 software, go ahead and open it and follow along with me.

If you don't, that's fine, just enjoy the tour.

Maybe if you have Sylvius, you may just want to watch the remaining segment of

this tutorial, and then, go back and repeat what I do and do it on your own

time in your own way. So, however you like let's begin by

getting into Sylvius 4. And I'm going to open up the Brainstem

Cross Sectional Atlas and I'll just click on All structures.

So this opens up the Atlas within Sylvius 4 that's devoted to the brainstem and the

spinal cord. And the default here is to show us the,

the very top section, which as the navigator tool indicates is taken from the

upper part of the brainstem. So, this is really the junctional region

between the thalamus in the brainstem. We recognize the brainstem by these

prominent cerebral peduncles that are taking shape, but we know we're still in

the caudal part of the diencephalon because we have the mammillary bodies

present along the ventral midline. So let's just get down one section below

and now this is looking more like the midbrain, because we see these massive

cerebral peduncles on the ventral lateral surface.

We see a small cerebral aqueduct that tell us that we're cutting through the region

of the brain called the midbrain. Now, what's attached to the midbrain?

Well, the oculomotor nerve. So here's the oculomotor nerve and we can

follow its roots back towards a semantic motor nucleus along the dorsal midline and

a visceral motor nucleus that we should expect to find setting very near it.

And indeed, if we follow these axons back, we have the oculomotor nuclei which are

the somatic motor nucleus of the third nerve.

And then, just superior to the oculomotor nucleus is our Edinger-Westphal nucleus,

the source of the parasympathetic outflow through the third cranial nerve.

Okay, going just a little more inferior now.

We are rear, near a region where the midbrain is beginning to join the pons.

So we still see some cerebral peduncle, but now we're looking at some nuclei and

some fibers that are associated with the pons.

So we know we're right near that junctional region.

That's very near the region where the trochlear nerve inner, emerges from the

posterior part of the brainstem. And indeed here is our trochlear motor

nucleus. Again, what we expect to find?

We would expect to find a somatic motor nucleus near the dorsal midline of the

tegmentum of the brainstem. So there is our trochlear motor nucleus,

very small nucleus. And the trochlear nerve is a very small

nerve, so that's nerve IV. Nerve V is near the center of the pons, so

we need to section into the pons a little bit.

So we need to section through the pons just a little bit in order to get to the

trigeminal nerve. And at this level, we see there, broad,

massive nerve roots of the trigeminal nerve on either side.

And if we follow these nerve roots back, they connect to a set of nuclei.

So, more lateral is where we find our chief sensory nucleus of the trigeminal

complex, also called the principal sensory nucleus.

And just to the medial side of that nucleus we would find the branchial motor

nucleus, the trigeminal motor nucleus. Now, the mesencephalic nucleus actually

begins at about this level, it's found right about in this region.

Very small nucleus, but if I select it we can recognize this nucleus as it runs in

the superior direction, just along the lateral aspect of the gray matter that is

just on a lateral margin of the fourth ventricle as it begins to close off into

the cerebral peduncle. So this nucleus really just trails off as

a thin collection of cells. That's it.

Just medial to the tract of this nucleus, which are the axons feeding this nucleus.

So right in this position is where we find the cells that give rise to the

mesencephalic division of the trigeminal. Okay, well, let's get back to the

trigeminal nerve. So here again is the trigeminal nerve.

The chief sensory or the principal nucleus of the trigeminal complex and the

trigeminal motor nucleus. Now, at this level, we are getting very

close to those cranial nerves that we would find along the border between the

pons and the medulla. We're not quite there yet, but we're

getting close to it. So if we go down one section, now, we're

beginning to see some of those nerves and nuclei.

So what are those nerves again? It's nerves VI, VII, and VIII, the

abducens, the facial, and the vestibulocochlear.

So the abducens nerve is connected to the abducens nucleus, the somatic motor

nucleus here along the dorsal midline of the tegmentum of the pons.

And we can actually see some of the axons of the abducens nucleus that are growing

out in the ventral direction to exit right about here at the junction of the pons and

the medulla, pretty close to the midline. Now, also, in this level of the pons, we

have our facial motor nucleus, and this is another branchial motor nuclues.

So it's not going to be along the dorsal margin of the tegmentum.

But set back in the intermediate and lateral region of the tegmentum due the

embryological migration of these brancho meric motor neurons.

So here's our facial motor nucleus and the facial motor nucleus gives rise to axons

that do something very strange. They actually sweep across the top of the

abducens nucleus, so there is our nerve root of the facial nerve and the facial

motor nucleus itself. So, the axons didn't exactly grow up and

around, actually, the neurons themselves migrated in the anterior lateral position

and they simply elongated the axon as they migrated down.

So these cells used to up here somewhere, but ended up migrating down.

And with it, drew, they drew out their axons.

So the seventh nerve ends up exiting the brainstem in this lateral position out

here, and just to the lateral side of the facial nerve, we expect to find the

vestibulocochlear nerve, which is right in this domain here.

So the vestibulocochlear nerve is related to a set of nuclei called vestibular

nuclei. We see several of them here in this

cluster, near lateral aspect of the tegmentum of the pons.

We don't yet have the cochlear nucleus, that's found in slightly more inferior

position in the upper part of the medulla. So we'll get to that in just a moment.

Now also in the lateral aspect of this tegmentum, we would expect the sensory

nucleui associated with the trigeminal complex.

And now that we're below the trigeminal nerve, we're going to be below that chief

sensory nucleus. What we find is our spinal nucleus of the

trigeminal complex. And just to the lateral side of that

nucleus are the axons from the trigeminal nerve that supply the nucleus.

We call this the spinal trigeminal tract. And then just to the medial side of the

tract is the spinal trigeminal nucleus. Okay, now, let's go down one section, and

now, we're in the upper part of the medulla.

The section looks a lot bigger, but really, it's a magnification of the

section for clarity purposes not to scale. So, the brainstem is actually getting a

bit smaller now that we go from pons to medulla but our magnification is actually

increased. Now, unfortunately, we didn't have an

optimal section to put into Sylvius to demonstrate the cochlear nuclei, so

they're, they're not labeled. But they're present here nevertheless.

So notice that there is this big dark bundle of white matter here called the

inferior cerebellar peduncle. And I say dark, because this is an image

of sections through the brainstem where the white matter is made to look dark and

the gray matter is made to look light. I should have said that right from the

start. I'm sorry about that.

But just to clarify, the white matter is dark, the gray matter is light.

So, this is a big bundle of white matter in the dorsolateral aspect of the medulla

that we'll talk about in a later session. This provides signals from the spinal cord

to reach the cerebellum. Just to the lateral margin of that bundle

here in the upper medulla, we see some gray matter which is unstained in this

view of the brain. And so, this gray matter, that's part of

the cochlear nuclei complex. In fact, this little tag of tissue here,

that's a bit of the eight nerve. So the cochlear nuclei are in the lateral

margin just outside that inferior cerebellar peduncle, the vestibular

division is just on the medial edge of this inferior cerebellar peduncle.

So here, we have a couple of divisions of the vestibular nuclear complex.

They would connect up with axons that are coming right here from this vestibular

cochlear nerve. Now, we are also beginning to encounter

the rostral part of this very interesting visceral sensory nucleus, called the

nucleus of the solitary tract. So here is the nucleus.

There is something called the solitary tract.

It's actually the axons from nerves VII, IX, and X that supply signals into this

nucleus. So there is the nucleus, and if we go down

one section, we see this a bit more distinctively.

There's something like a bullseye shape where the nucleus is surrounding the

tract. Kind of an unusual configuration if you

can compare that to the situation we have with the trigeminal system.

In the trigeminal system, we have the tract that sits out here lateral to the

nucleus, but in the solitary system, the tract is right in the middle of the

nucleus, so the gray matter actually surrounds the nucleus with respect to the

solitary tract. So, it's an interesting shape.

It's a visual distinctive as we look into a section through the upper part of the

medulla that is stained to, to resemble a myelin stain where white matter is dark

and gray matter is light. Okay.

So, also here, in the upper medulla, you should be thinking about a somatic motor

nucleus, the hypoglossal nucleus. We would expect to find it along the

dorsal margin of the tegmentum and there is our hypoglossal nucleus.

Now, if you'll remember where the hypoglossal nerve roots exit the

brainstem, they do so right along this cleft between the medullary pyramid and

the olive. So, we can imagine if we took a section

through here, what would we see? We might expect to see fibers emerging

between the pyramid and the olive and that's exactly what we see here in this

section. So there's the hypoglossal nerve and we

can follow its trajectory, right between the medullary pyramid, which now we see in

cross section, a dense bundle of white matter and the olive.

And the olive is made from this fantastically looking gray matter nucleus,

called the inferior olivary nucleus, and some white matter that's on the outside of

that nucleus. So we'll talk about the inferior olivary

nucleus when we talk about cerebellar function.

Really, a fascinating story about how this inferior olivary nucleus induces

plasticity in the cerebellum. But for now, we'll just use it as a

visible landmark that allows us to know where to expect to find the emergence of

the twelfth cranial nerve between the olive and the pyramid.

Alright. Let's get back up to the dorsal tegmentum.

We talked about the somatic motor nucleus on the midline.

We talked about the visceral sensory nucleus that is a bit more lateral.

And our special sensory nuclei, the vestibular nuclei.

And then, our general sensory nucleu in the far lateral region.

But between the motor and the sensory, we might expect to find some visceral motor.

And indeed, we have a major visceral motor structure here, called the dorsal motor

nucleus of the vagus nerve. So this is where many of our

parasympathetic visceral motor cells reside that grow out axons through the

vagus nerve. Also contributing to that vagus nerve are

parasympathetic neurons that are involved with inhibiting the rate of contraction of

the heart and the contractility of the ventricle.

Those parasympathetic visceral motor neurons come from this obscure, hard to

see, ambiguus nucleus in the medulla called the nucleus ambiguus.

Sometimes it's nice when the names tell us something about what we expect to see and

this is one such case. So here is the nucleus ambiguus.

As I mentioned, it sort of has two parts to it, there is the cardioinhibitory

parasympathetic outflow. But then there's also the branchial motor

outflow, which goes to the muscles of the pharynx and the larynx, also via cranial

nerves IX and X. Okay.

Well let's just follow down a few more sections and we'll get below the level of

the hypoglossal nucleus and nerve here. What we will encounter, however, is the

spinal trigeminal nucleus. And we'll see that we still have a tract

to the lateral side of the nucleus. And as we go through the brainstem, at

least in Sylvius, we use anatomical convention to recognize different

subdivisions of this spinal trigeminal nucleus.

There are really multiple subdivisions that have different functions and the

sensations of pain and temperature derived from the face and the oral cavity.

We won't get into all of that now, but I'll just point out to you that that's why

we see these various divisions in Sylvius here.

Now, these divisions eventually seem to run right into the dorsal horn of the

spinal cord. So there's this bright, compact part

called the substantia gelatinosa and I want you to notice as we get into the

brainstem, there is also a bright compact part of the spinal trigeminal nucleus that

we call the gelatinosa part. So again, very much parallel,

organizational system to what we see in the dorsal horn of the spinal cord, which

as we'll come to see in a forthcoming tutorial, also is concerned with pain and

temperature sensation from the body. Well, I hope you enjoyed that brief tour

of the brainstem, and just a little bit about the spinal cord, we'll have more

about the spinal chord in the later session.

I would encourage you to go through the tutorial notes, find those cranial nerve

nuclei that are highlighted in Bold font, and then, get into Sylvius and find them

here. And if you have any trouble, I'll just

remind you that we have a glossary feature built into Sylvius, which will give you a

chance to look up a term that might be of interest to you.

So for example, let's just type in trigeminal.

See what happens, we find a number of targets come up.

If we want to see the trigeminal nerve, we have an opportunity to view it in

different views, including the view of our brainstem model here.

If we want to find a particular nucleus, such as the spinal trigeminal nucleus,

again, we have different views, some of which are included in the tutorial, others

are unique to Sylvius. This one in particular you might enjoy.

This is an animation that illustrates for you how pain and temperature signals get

from the trigeminal nerve into the brainstem.

And the sounds that you heard are an indication of the synaptic junctions

between one axon, and the next neuron in the pathway.

I bet you didn't know synapses made sound. Of course, I'm joking.

Synapses don't make sound. But we put them in this animation just to

highlight the fact that there is a junction from one neuron to the next

neuron. And it will be important for you to be able to speak about the pathways

with the precision of language that conveys your understanding of the

locations of cell bodies, of axons, and synaptic connections within the pathways.

So we'll be getting to that in the next few tutorials, which will turn our

attention from the gray matter structures to the pathways that allow these gray

matter structures to be interconnected into a sensory system.

So I look forward to talking to you now in more depth about our sensory systems

beginning with our capacity for mechanosensation.

So I'll see you next time.

Cranial Nerve Nuclei, part 4

Medical Neuroscience

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