. Welcome to today's tutorial on the cranial
nerve nuclei. This topic pertains to one of our
foundational core concepts in the field of neuroscience.
And that is, the brain is the body's most complex organ.
And we turn our attention in this lesson to one of the most complex regions of that
organ, the brain stem. So my learning objectives for you in this
tutorial are for you to be able to identiry the major subdivisions of the
brain stem. And the spinal cord.
Although our focus will be on the brain stem, primarily.
And I want you to be able to do so in representative transverse cross sections.
That, is, not just from looking at the outside, but from looking at the inside.
And I want you to be able to have a discussion.
Maybe with a family member or a friend or a classmate.
And I want you to discuss the relationship between the cranial nerves, that I trust
you have some familiarity with now, and the nuclei that we find within the brain
stem. And so, what I'm referring to are the
clusters of cells that receive sensory information from the cranial nerves or the
clusters of motor neurons that grow out the axons that run through cranial nerves
on their way to muscle. Okay.
So let me just remind you of the basic organization of our sensory and motor
systems. Beginning with our sensory system.
So obviously we have information coming from the outside world and information
coming from within our own body. That is feed into the central nervous
system via our special sensory cells that we find in our special sensory organs like
the eye, and the ear, and the nose, and the tongue.
But we also have this vast sensory surface, our skin, as well as sensory
cells that are growing out receptor endings in our muscles, in our joints, in
our tendons. And all of this information informs the
brain about the internal state of the body and the movements of the body through this
environment. Well, all of this information is conveyed
via axons that run in nerves. And those nerves are spinal nerves and
cranial nerves. So these nerves are grown from cells.
And those cells typically reside in ganglia, which are small clusters of cells
just outside of the central nervous system.
Now, the special sensory organs are a bit different, and we'll talk about them in
due course. So when we focus on the cranial nerves now
I want you to just have in mind a nerve which is very much like one of our spinal
nerves. Except these nerves are attached to the
brain stem. Alright, so these are sensory components
that are fed up into higher levels of processing through pathways that we'll
come to study one at a time over the next couple of weeks.
So, that's the sensory side. There's also a motor component to most of
nerves and most of our cranial nerves. And that motor component is concerned with
activating skeletal muscles, and activating muscles associated with the
viscera. That is, smooth muscles, the muscles of
the heart, cardiac muscle, as well as the secretory cells in our glands.
These effector elements are engaged by axons that grow out of hte central nervous
system. And for skeletal muscle, for example,
those axons are derived from motor neurons, that reside in the ventral horn
of the spinal cord. And in a collection of nuclei in the brain
stem. Now, the activation of smooth muscle,
cardiac muscle, glandular tissue is just a bit more complicated.
That gets us into a division of the motor system that we call the visceral motor
system, or sometimes it's called the autonomic nervous system.
And here, we have. A chain of neurons that connect the
central nervous system to the effector cells that, do the contraction, be it in
the heart or in the blood vessels or in the, viscera.
So we'll talk about that topic in, a few weeks from now.
And hopefully, the organization of that autonomic nervous system will be clear.
For now, I just want you to get the big picture that the motor output is derived
from, from two principle kinds of motor systems, a visceral motor system and a
somatic motor system. And as we'll see, the somatic motor system
can be further derived in terms of the embryological origins of the tissue that.
Is innervated. Now what we're leaving out from this
diagram of course, is the wonderful integration of sensation, and the
production of commands from motor output that takes place at higher levels of
processing. Namely in the cerebral cortex and in
circuits that run through sub cortical structures like the basal ganglia, the
thalamus and even the cerebullum. Well, all of the sensory motor integration
will be the topic of forthcoming tutorials so, so hang tight on that.
Alright so, with that as the background lets now focus in on the cranial nerves
themselves. And the brain stem.
So, I will remind you of what the surface features of the brain stem look like.
First beginning with the basic subdivisions of the brain stem.
The brain stem is conventionally divided into a mid brain, pons, and the medulla
oblangata. All of which sits between the forebrain,
which would include the Thalamus and parts of the basal ganglia.
Which is basically what we have from about this region on in the anterior direction
and the superior direction, and then below the madela we have the spinal cord.
So the brain stem is an intermediate part of the central nervous system between
forebrain and spinal cord. It's a very complicated place.
And it includes most of what we call the hind brain.
From embryological development of the nervous system.
So the fore brain sits the anterior and superior.
The hind brain is the brain stem, and the spinal cord is inferior or cottal to the
brain stem. Okay, now let's look at the ventral
surface of the brain stem and just quickly review the cranial nerves.
And they're laid out for you in this figure from the textbook that we refer to.
I would also refer you to my tutorial where I actually showed you the surface of
the human brain stem in the lab. And I also have with me today the same
model of the brain stem that we used. And, I think, rather than going through
this figure, I'll just show you quickly this model.
And if you feel like you're good on the brain stem and the superficial aspects of
it, feel free to skip over this part. But otherwise let's just take a few
minutes and review the surface features of the brain stem.
Well, I trust that you all can see this, and this brain stem is also produced for
you and fully annotated and movable within Sylvius software.
So if you have access to Sylvius, you might want to get that out and follow
along, and click along with me. Alright, so what we're looking at is the
ventral surface of the brain stem. This resembles the drawing that we had in
the previous slide in the upper right. So, from about this level on up is
actually forebrain, so what we're looking at here, these large egg, egg-shaped
structures, that's actually part of the basal ganglia.
Okay, and the hypothalamus and the thalamus on the dorsal aspect.
Which I'll show you more in just a moment. So the brain stem would be this region
right in here. So we have the mid brain, pons, the
medulla oblongata. And just below we have spinal cord.
And I want you to notice the cranial nerves that are visible here in this
model. Well, we've been talking mainly about the
brain stem, so we're concerned with cranial nerves 3 through 12.
Cranial nerve one, the olfactory nerve, is not present in this model, and it doesn't
attach to the brain stem. Neither does cranial nerve two, the optic
nerve. However for completeness, we do have the
optic nerve here in this brain stem model. And the nice thing about this model is
that you can follow the optic nerve to the chiasm and then we can turn the model
around and note its progression from the optic tract right to the posterior.
Pole of the thalamus, a region called the lateral geniculate nucleus that we'll talk
about in a later session. Well, that's cranial nerve two.
As I said, it does not attach to the brain stem so we won't be talking about it
further in this tutorial, but let's find the rest of the cranial nerves in this
model. So, cranial nerve three This right here.
That's the oculomotor nerve, it's the nerve that emerges from the space right
between these massive stalks that define the ventral surface of the midbrain.
These stalks are the cerebral peduncles. The space between them is the
interpeduncular fossa, and Cranial nerve three, the ocular motor nerve, emerges
right in that fossa. Okay, cranial nerve 4 is a bit unusual.
As you may recall, it's the trochlear nerve.
It's unusual because it emerges from the dorsal aspect of the brain stem.
It's the only motor ax, the only motor nerve to do so.
All the other motor nerves exit the ventral.
And lateral regions of the central nervous system.
But here's the trochlear nerve. You can see it's a fine little thread that
wraps around the posterior lateral aspect of the quarter mid-brain.
And then joins up to make its way to the orbit, with the other ocular motor nerves.
So trochlear nerve, nerve 4, nerve 5, is this very large protruding nerve, very
obvious nerve. It plunges right through the fibers of the
pons, the transverse fibers of the pons, so it's a very easy nerve to recognize in
a whole brain specimen or in our illustrations.
So that's the trigeminal nerve, nerve 5. Now next we come to the junction of the
pons and the medulla, and along that junction are three pairs of cranial
nerves. From medial to lateral, they are the
abducens nerve, nerve 6, the facial nerve, which is the seventh cranial nerve, and
then the vestibulocochlear nerve. The eighth cranial nerve.
So 6, 7, and 8. If you notice where cranial nerve 8 is,
just below it are the nerve roots, nerve rootlets of 9 and 10.
So we have the nerve rootlets of the glossopharyngeal nerve, nerve 9.
The rootlets of the vagus nerve just below the glossopharyngeal nerve.
So the vagus nerve is cranial nerve 10. Cranial nerve 11, is a series of rootlets.
Which, in the model, it looks like these wings that come off on either side of the,
