And what these terms refer to are the embryological origins of the tissues that
are connected up with these nuclei in the brain stem.
So there are muscles that are derived from the embryological somites so we call the
axons that connect to those. Muscles, somatic motor axons.
And the nuclei that grew those axons, somatic motor nuclei.
And in the head and the neck region of the embryo, we have something called the
pharyngeal arches. And from the pharyngeal arches are derived
a variety of tissues. Including some of the muscles of the,
cranial region. And, deep within the neck.
And the motor neurons that innervate these derivatives of the pharyngeal arches that
become muscles are called branchial motor neurons, giving rise to branchial motor
axons. And so we can recognize the distinct set
of nuclei in the brain stem that are connected to these muscles derived from
the pharyngeal arches rather than the somites.
So it's useful to distinguish the somatic motor from the branchial motor, and we'll
do that as we look inside the brain stem. And finally there are smooth muscle cells,
of course cardiac muscle tissue and glandular tissue that is innervated by
axons that are controlled by the central nervous system.
And that out flow from the CNS comes from our visceral motor axons.
And those visceral motor neurons that grow those axons are found in the brain stem
and also found in the spinal cord. And we'll get to the spinal cord in a
later session, but for now I want to show you the brain stem nuclei that.
Throughout those visceral motor axons. Now on the sensory side we can also
recognize different distinctions. There are, is a general sensory system
derived from our skin surfaces, our muscles, our joints, structures in the
peripheral parts of the body, so to speak. And then there are special sensory
systems, or special sense organs. Such as our, our eye, our ear, our tongue,
our nose. So those are systems that are especially
dedicated to processing just a, a fine modality of information derived from our
environment. And then finally there are visceral
sensory axons that feed the central nervous system with information derived
from our visceral organs. Well as this color code makes explicit
these different cell modalities for motor control and for sensation connect with
different nuclei within the brain stem. And, in the tutorial notes, and here in
front of you now is a really beautiful drawing made by, my friend and colleague
Dr. Nell Cant here in the department of
neurobiology at Duke University. And it's a simplified way of explaining
how one can relate the devleopment of the neural tube, to the more complicated
mature forms that we see in the spinal cord and in the brainstem.
Well, we'll talk some about brain development in a few weeks.
But for now, I would just emphasize that the nervous system is derived from the
walls of a tube. And that's what we have here drawn
simplistically in the upper left. So imagine this as a cross section through
the caudal part of the developing embryo where the nervous system is going to form
the spinal cord. And I think as you may know already, the
dorsal part of that spinal cord is related to sensation.
Sensory signals derive from the peripheral parts of the body, or the superficial
parts of the body. And then sensory signals derive from the
visceral, the more interior organs within the body.
That's all found in the dorsal aspect of the developing neural tube that will form
the spinal cord. The ventral part on the other hand is
motor. And again there's a systematic
organization where the motor neurons that. Innervate the tissues derived from
somites, that is our stride and skeletal muscles.
They are innervated by cells that sit in the more inferior part, sorry the more
ventral part of this developing neural tube, whereas the motor neurons that are
going to supply motor signals. To those ganglia that innervate the
visceral organs. They come from this more intermediate
region right in here. Now that's the plan for the developing
neural tube in that region that forms the spinal cord, and then when we actually
look at what we see in the adult Central nervous system.
We see a spinal cord with this characteristic butterfly shape of gray
matter in the middle, and we have a dorsal horn where we find our sensory neurons,
and we find a ventral horn. Which is filled up with neurons that are
affiliated with motor output. With this large anterior part of the
ventral horn, where we find our somatic motor neurons.
Well, here's another way to conceptualize this.
And I really ant you, to do all of this me.
You might even want to stand up. If you're comfortable doing that, in a
position to do this. So I want you to take your two hands and I
want you to put them together in such a way as they resemble the illustration that
we have here in the upper left. Okay.
Let me clear our annotations so you can see that clearly.
So what I want you to do is I want you to make with your hands this figure.
Okay, so here we go. We're, we have just made a cross-section
of the developing neural tube. Now, I want us to note how our hands
resemble the distribution of these neurons related to sensation and motor control.
We have our fingers up on the dorsal aspect of this transverse section.
The fingers would represent our somatic sensory neurons whereas the palms of our
hands are sitting in a more anterior or ventral position.
The palms of our hands would represent our somatic motor neurons.
But we also have. These knuckles right.
Near the junction of our palms and our digits.
Sot he knuckles would represent the visceral elements.
With the visceral sensory elements right on the top of our knuckles.
And the visceral motor elements just on the anterior or ventral aspect of the
knuckles relative to this cross-section. Okay.
So we've got somatic sensation, dorsal, somatic motor control, ventral, and then
along the ridges formed by our knuckles we have visceral sensory and visceral motor.
All right. Now let's take this conceptualization up
into the brainstem. So what happens in the brainstem is that
the ventricular system expands. In the spinal cord we have a central
canal, a central space for the, flow through the spinal fluid, but in the brain
stem we have a large fourth ventricle So essentially, this face expands in the
following way. The sensory elements open up to form the
floor of the fourth ventricle. With the digits now still representing
somatic sensation, but now they are lateral, underneath the floor of the
fourth ventricle. And where would we find our somatic motor
cells? Well, we'd find them in the position of my
palms. Near the midline, in the dorsal part of
the tegmentum. Just underneath the fourth ventricle.
Now, notice where my knuckles are. My knuckles are somewhere between the
somatic motor. Region and the somatic sensory region.
And that's where we find our visceral sensory and our visceral motor nodes.
So again, a very simple neuro tube opens up to form a fourth ventricle with the
sensory systems moving far lateral. All right, now that simple picture
hopefully. Will stay with you.
Now, if you did that with me, hopefully the significance will stay with you, and
you'll have a picture of the developing brainstem.
Where our somatic sensory elements, and we'll also add now our special sensory
elements as we get into the cranial region, are to be found in a far lateral
position. And just medial to those, we would find
our visceral sensory. And special sensory systems relater to the
visceral, specifically our sense of taste. So these sensory elements are lateral.
Now more medial, we find the motor elements right along the.
Midline of the tegmentum underneath the ventricle, we would find our somatic motor
cells. And just between the somatic motor and the
sensory cells, we'll find our visceral motor cells.
Now, when it comes to the cranial region, there are different kinds of muscles.
Right? So in addition to the smooth muscles and
the striated muscles. There are striated muscles that are
derived from somites and those that are derived from the branchiolmeric arches or
the pharyngeal arches of the developing embryo.
These include many of the structures around the cranial region and into the
neck region and there are different motor neuorns that are growing out axons to
innervate either the striated, somatic somite derived muscle.
Or the striated muscles derived from the branchiomeres and those that are derived
from somites are receiving input from our somatic motor cells.
Which are found again right along this dorsal midline underneath the floor of the
fourth ventricle. The motor neurons that supply innervation
to the derivatives of the branchiomeres have migrated into an intermediate
position. So these motor neurons initially sat very
near our somatic motor neurons. But they migrate into a more intermediate
somewhat more anterior and lateral position and as they migrate their growing
axon is dragged down with it. So consequently we have motor neurons,
such as those that supply the muscles of facial expression.
That sit in this intermediate position and grow axons that loop around in this
fashion. And in this case what I'm describing for
you is the facial motor nucleus in the facial nerve.
