Let´s return now to the neural tube
and discuss how the actual nervous system comes into existence.
The neuroepithelial 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 sclerotome
that portion 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 they´re gonna 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 cerebrospinal fluid.
Now, one of the hallmarks of the nervous system is it has regional specialization
and the regions of the spinal cord are gonna become motor or sensory.
And the real distinctive feature of the spinal cord in that regard is a sulcus limitans.
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 limitans
separates the upper part which is called the alar plate from the lower part
which is gonna be called the basal plate.
The alar plate is gonna be specifically sensory
whereas the basal plate is gonna be specifically motor.
So in the basal plate, we have neurons that are going to extend outward
pushing their axons to the myotome to innervate muscle.
In the alar plate, we´re going to have sensory neurons
but one interesting thing about the alar 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 gonna 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 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 myotome.
Sensory axons in the posterior root 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 pseudounipolar axons.
They actually go both directions
and within the area indicated in yellow, the alar 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, there are gonna be several interneurons that develop in the alar 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 sulcus limitans,
we have a alar plate sensory and below it, a basal plate motor.
The same organization is present in the brain stem
but takes on a 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 gracile
and cuneate nucleus in the alar or upper portion.
Then, below there, we have a variety of motor nuclei.
One thing to note that we do see in the brain stem
are the existence of very large tracts like the pyramidal or corticospinal tract
which is taking motor information from the brain down to the spinal cord.
We also have sensory tracts going up to the brain
but we´re not gonna highlight those.
But just be aware, all of your brainstem has large tracts
travelling through them up and down.
Now, we go into the cranial medulla and it´s gonna 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 limitans still separates the alar and basal plate
but instead of being in an anterior posterior organization,
they´re now located lateral alar plate and medial basal plate.
But they still have the same functions.
Sensory in the alar, 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 we will look at a more developed illustration of this area.
We can see that that´s become the olivary 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 gonna belabor the different types of sensory and motor activity
in the alar 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 afferent,
special visceral, and general visceral afferents
are all clustered in the area of the alar plate.
Likewise, the general visceral efferent, special visceral efferent,
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 pons.
Just as the superior olivary nucleus migrated
from the alar plate down, the pontine nuclei,
very large structures are going to be sensory in origin from the alar 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 pons.
The only thing we need to remember about the pons
is that it´s got the cerebellum growing directly posteriorly off of it.
There´s a massive ballooning of neuroepithelial cells.
Let´s take a look at that here and we can see in this sagittal cut,
we have the pons 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 neuroephithelium
and the various structures, the cerebellum,
its anterior, posterior lobe, the tonsils and the flocculonodular lobe
will all develop off of their on the posterior side of the fourth ventricle.