[MUSIC PLAYING]
Welcome to this video mini-lecture, where we will be covering histology topics of the
nervous system.
My goal is for you to have a better conceptual understanding of the components of the central
nervous system and the peripheral nervous system, as well as the location of these important structures.
My name is Christopher Demas and I'll be walking you through these topics.
We are going to be focusing on the central nervous system primarily during this lecture,
which includes the brain and the spinal cord.
I will start by drawing a midsagittal or medial slice of the brain and the spinal cord to
reveal its inner structures.
Notice the folded nature of the cerebral cortex -- both the sulci and the gyri, the inner folds
and the our protrusions of the brain respectively that serve to increase its surface area.
This structure is repeated in the cerebellum, which is drawn at a posterior of the brain.
You might be thinking, why does the brain have such a convoluted structure?
And it's a really good question.
This folded structure is vital for the brain's ability to support the 100 billion neurons
within its structure, which is roughly the number of stars within the Milky Way galaxy.
The only way for the brain to fit all these neurons is for it to fold and fill every nook
and cranny of the bony cranium.
Now that we are getting a sense of the structure of the brain let's take a closer look at the
inside of the cortex by taking a coronal slice through this brain.
Even though this slice only includes the right hemisphere of the brain I'm going to reconstruct
the left hemisphere for completeness.
You once again see the repeated folded structure of sulci and gyri within the cortex.
Another important aspect of the structure of the brain I would like to point out is
the location of the white and the gray matter within the brain.
You notice that there is gray matter on the outside and white matter towards the inside.
There's actually a very important reason for this.
And we will discuss it more later in this lecture.
The last part of the anatomy that I want to depict before moving on is the spinal cord.
We're going to follow the full spinal cord and look at its structure and cross-section.
Notice that the dorsal or posterior side of the spinal cord, which is to the right becomes
the top of the drawing.
This is the way that most diagrams in your textbooks and lectures display the spinal
cord in cross-section.
So I decided to depict it in a similar manner.
Now I'm going to go ahead and label the dorsal horn and the ventral horn, which represents
the upper and lower portion of this butterfly structure within the spinal cord.
Attached to the spinal cord on the right is a peripheral nerve root, which includes the
ventral root and dorsal root.
Once again, we find that the spinal cord consists of gray and white matter, which I'll draw
in now.
Now that we are oriented and have a better sense of the anatomy of the brain and the
spinal cord let's look at the histology of each section.
We will start by focusing on the cerebral cortex of the brain.
The neurons of the brain do not live by themself and actually require the support of specialized
cells known as glial cells.
In fact these supporting cells vastly outnumber the neurons as there are about 10 glial cells
for every neuron.
Let's take a look at the neurons suspended in part of the cortex.
Neurons are only able to survive when there is adequate nutrition and oxygen supply.
So let's place a capillary in this picture.
Notice that the capillary is not touching the neuron.
Within the brain, the neurons are separated from the circulation by the blood-brain barrier.
The brain is a very sensitive and relatively immune protected organ.
And without this barrier we would be highly susceptible to infection and toxins.
The barrier's provided by astrocytes who job it is to facilitate diffusion of nutrients
and waste products between capillaries and the neurons.
Astrocytes are actually the most abundant glial cell and as mentioned previously is
responsible for creating that blood-brain barrier.
Next we're going to cover two more glial cell types -- the oligodendrocyte and microglia.
Oligodendrocytes are responsible for creating the insulating myelin sheath around the axon
of neurons while microglia are important for phagocytosis and active immune defense within
the brain.
You might recall an autoimmune disease of oligodendrocytes known as multiple sclerosis.
In this disease the immune system targets oligodendrocytes leading to the degradation
of the myelin sheath.
This ultimately leads to the neurological symptoms of multiple sclerosis including numbness,
muscle spasms, balance problems, lack of coordination, and even blindness.
And as mentioned the microglia are specialized for phagocytosis and important for the active
immune defense within the brain as well as maintenance of neurons such as removal of
plaques and unnecessary connections.
Finally, in the upper right-hand corner, we have simple columnar cells known as ependymal cells.
They line the cerebral spinal fluid filled ventricles that bathes the brain.
Some of these specialized cells produce cerebral spinal fluid in organized structures known
as the choroid plexus.
I'm going to go ahead and draw in some of the cerebral spinal fluid filled ventricles
now.
If you look in the sagittal cut brain, it's located in the center of the brain where I'm
drawing it in now.
And there's also a thin canal within the spinal cord where it's located.
In the coronal section there's also a fluid filled ventricle, which are drawn in.
The choroid plexus is drawn in purple.
Now that we have focused on the central nervous system and its components let's take a look
at the peripheral nervous system and the structure of the neurons.
The motor neurons start in the gray matter of the ventral horn and travel through the
ventral root out to the target muscle.
I'll draw the motor neuron in red.
The motor neuron starts at the cell body and then proceeds down the axon to the axon terminal.
Also I'd like to point out that motor nerves in the peripheral nervous system additionally
have a protective coat of myelin like the neurons in the central nervous system.
This myelin, however, is different since it's generated and maintained by Schwann cells.
Now what I'm going to go ahead and do is label the important structures that you should be
familiar with with the motor neuron including the dendrites, the cell body, the axon, and
the myelin sheath.
Now I'd like to draw in another neuron cell type called a sensory nerve.
Sensory neurons pass through the dorsal root and synapse in the dorsal horn as shown here
in blue.
Sensory nerves were actually confusing to me when I was learning this the first time.
So I wanted to take a moment to point out that within neurons the cell body does not
need to be found at the terminus of the neuron.
The cell body in the sensory neuron is in fact found in between the dendrites and the
terminal axon.
You can see the bolds that the cell bodies make in the dorsal root, which might help
you remember which side of the spinal cord the sensory neurons innervate.
Lastly, I wanted to point out the short interruptions of myelin between the myelin sheaths called
nodes of Ranvier.
You may have already learned about this.
But myelin helps to increase the conduction speed of a nerve impulse through a process
called saltatory conduction.
At the beginning of this lecture, I mentioned that I would answer the question of why there
is white and gray matter in the brain and the spinal cord.
And I think now is a good time to revisit that question.
Looking at neurons at a higher magnification, you can see how the composition of the neuron
is different depending if you are at the dendrites in the middle of the axon or if you are at
the end of the axon.
The myelin sheath found around the axon is made mostly of lipids, which gives it a white appearance.
The terminal end actually has a different composition and retains fewer lipids leading
to it having a darker tinge.
For this reason, most of the neurons begin with their dendrites in the gray matter.
And the nerve tracks, which contain axons is what makes the white matter.
This is why the inner layer of the spinal cord-- the horn-- is gray in color and the
surrounding spinal cord is white.
Lastly, I want to take a moment to point out the layers of neurons within the cortex of
the cerebrum and the cerebellum.
The cerebral cortex consists of six layers, which I will draw in now.
The first most superficial layer is called the molecular layer, which contains a few
scattered neurons and consists mainly of extensions of apical dendrites, horizontally oriented
axons, and glial cells.
The second layer of the cerebral cortex is the external granular layer, which contains
small pure middle neurons.
The following layer is the external pure middle layer, which contains predominantly small
and medium sized pyramidal neurons.
The fourth layer is the internal granular layer, which contains different types of stellateneurons.
The fifth layer is the internal pure middle layer, which contains large pure middle neurons.
The final layer is the multiform layer or the polymorphic layer, which also contains
large pure middle neurons and many small spindle-like pure middle and multiform neurons.
The major takeaway for this particular diagram is the order of the layers.
I wouldn't worry about the type of neurons or the shape and structure.
Next we're going to take a look at the cerebellar cortex, which consists of three layers of
cells.
We're going to go ahead and zoom in on that portion of the cerebellar cortex.
Now I'm going to go ahead and insert an H&E stain slide of the cerebellar cortex.
As you can see the outermost layer is called the molecular layer, which is mainly filled
with fine axons and dendrites from the cells below.
There is a single layer of Purkinje cells that lie at the boundary of the molecular
and the granular layers.
These giant cells send their dendrites into the molecular layer and their axons into the
white matter.
The third and final layer is the granular layer that contains mostly granular cells.
Keep in mind that the layers of the cerebellar cortex are easy to visualize on the standard
H&E stain.
However, the six cerebral cortical layers are much harder to distinguish.
And it's why I drew them rather than use and H&E stain slide.
And with that we are finished with the topics this mini-lecture.
Let's take a moment to summarize what we've learned.
First, there are two main divisions of the nervous system, which we have discussed --
the central nervous system -- the brain and the spinal cord -- and the peripheral nervous
system, which includes everything else.
There are different cells as part of each of these divisions.
The central nervous system contains specific glial cells, such as the myelin producing
oligodendrocytes, the astrocytes involved the blood-brain barrier, microglia, inactive
immunity, and ependymal cells that produce cerebral spinal fluid.
Remember that the autoimmune attack of oligodendrocytes
is what leads to the neurologic disease known as multiple sclerosis.
Within the peripheral nervous system we looked at the composition of motor and sensory neurons
and their relationship to the spinal cord.
These neurons also contain myelin,
but it is produced by the peripheral nervous system specific cell known as Schwann cells.
We then looked at the six layers of the cerebral cortex and the three layers of the cerebellar cortex.
These layers make up the gray matter of the brain and cerebellum from the vast number
of cell bodies in these regions.
They send their axon nerve tracks into the white matter of the brain and the spinal cord,
some of which synapse in the gray horn of the spinal cord.
[MUSIC PLAYING]
I hope you found this to be helpful.
And feel free to review this lecture as many times as you would like.
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