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let's return now to the neural tube and
discuss how the actual nervous system
comes into existence the neuro
epithelial cells that make up the neural
tube start to differentiate and
proliferate pretty crazily essentially
they're gonna migrate from a more
central region near the neural canal and
push outward as daughter cells come into
existence and need more space as they
push more laterally they're gonna form
an intermediate zone and finally a
marginal zone now the marginal zone
meets the sclera tone that portion of
the somite that's forming the meninges
like the dura mater and the arachnoid
mater and that limits the amount of
space available for the spinal cord to
inhabit so the intermediate zone and
marginal zone quickly fill the space
inside the neural canal and are going to
start differentiating into actual
neurons the central canal at the core of
the neural tube is lined by ependymal
cells just like the vesicles in the
brain but unlike the vesicles in the
brain the ependymal cells inside the
spinal cord will not create choroid
plexus or cerebral spinal fluid now one
of the hallmarks of the nervous system
is it has regional specialization and
the regions of the spinal cord are going
to become motor or sensory and the real
distinctive feature of the spinal cord
in that regard is a sulcus limit ends
and in this picture you can see that the
neural tube has a light blue and a dark
blue region and that's reflecting the
fact that the sulcus limit hands
separates the upper part which is called
the ALR plate from the lower part which
is going to be called the basal plate
the ALR plate is going to be
specifically sensory whereas the basal
plate is going to be specifically motor
so in the basal plate we have neurons
that are going to extend outward pushing
their axons to the Maya tome to
innervate muscle in the ALR plate we're
going to have sensory neurons but one
interesting thing about the ALR plate is
that it's going to have axons that go up
to the brain
rather than out to the skin to get to
the skin we're going to have cells
inside the dorsal root ganglia or
posterior root ganglia that extend to
the skin and extend back to the spinal
cord now the top and bottom of the
neural canal are going to be closed by
what's called a roof plate and a floor
plate
they don't really contain too many axons
but keep the spinal cord in more or less
a tube and it's in its orientation other
parts of the nervous system will
actually open up and we'll see that in
just a bit so let's review the functions
of each one of these areas motor neurons
shown here in green extend out to
muscles in the developing Maya tone
sensory axons in the posterior ganglia
remember those are coming from neural
crest cells are going to extend their
processes out to the skin but also
extend a process to the spinal cord so
these are pseudo unipolar axons they
actually go both directions and within
the area indicated in yellow the ALR
plate we have sensory axons receiving
information from the skin and heading up
to the brain it's going to extend axons
up to other parts of our central nervous
system
lastly they're going to be several
interneurons that develop in the ALR
plate these are neurons that connect the
sensory activity in the spinal cord to
the same levels motor activity and
that's what allows us to have
coordinated reflexes so remember that on
either side of the silica slim attends
we have an ALR plate sensory and below
it a basal plate motor this same
organization is present in the brainstem
but takes on the slightly different
appearance depending on which area of
the brainstem we're in in the very
caudal medulla close to the spinal cord
it looks pretty much like the spinal
cord we have sensory nuclei like the
grass aisle and Kuni eight nucleus in
the lr or upper portion then below there
we have a variety of motor nuclei one
thing to note that we do see in the
brainstem are the existence of very
large
rats like the pyramidal or corticospinal
tract which is taking sensory pardon me
motor information from the brain down to
the spinal cord we also have sensory
tracts going up to the brain but we're
not going to highlight those but just be
aware all of your brain stem has large
tracts traveling through them up and
down now we go into the cranial medulla
and it's going to look a little bit
different here the roof plate has opened
up a little bit and that's making the
cranial medulla look a little more like
a book that's been opened our sulcus
limit and still separates the ALR and
basal plate but instead of being in an
anterior posterior organization they're
now located lateral ALR plate and medial
basal plate but they still have the same
functions sensory in the LR motor in the
basal one thing that does happen here is
that we have some sensory neurons break
loose and migrate anteriorly you can see
that in the picture here and when we
look at a more developed illustration of
this area we can see that that's become
the Oliveri nucleus very distinctive
structure in the brainstem and acts as a
sensory relay area now this is not a
neuroscience review so I'm not going to
belabor the different types of sensory
and motor activity in the ALR and basal
plate but just be aware all of the
neuroscience you've ever learned is
represented in this process of
development so we do have the various
sensory types there so special somatic
afferent general somatic a third special
visceral and general visceral a Ferentz
are all clustered in the area of the ALR
plate likewise the general visceral
efferent special viscerally Ference and
general somatic efferent nuclei are all
located in or at least originating from
the basal plate so the developmental
basis for all those nuclei is present
early on let's move a little further up
and we're now in the ponds just as the
superior olivary nucleus migrated from
the ALR plate down the pontine nuclei
very
structures are going to be sensory in
origin from the hailer plate and migrate
to actually take up a spot anterior to
the basal plate but basal plate still
contains all the motor nuclei in the
ponds the only thing we need to remember
about the ponds is that it's got the
cerebellum growing directly posterior
off of it as a massive ballooning of
neuro epithelial cells let's take a look
at that here and we can see in this
sagittal cut we have the ponds sitting
ventrally or anteriorly we have the
cerebral aqueduct and fourth ventricle
posterior to that and then the
cerebellum develops off of it as an
extension of the neuro epithelium and
the various structures of the cerebellum
its anterior posterior lobe the tonsils
and the flaco nodule lobe will all
develop off of there on the posterior
side of the fourth ventricle
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you