PATRICK: With cells named after stars and
neurons that branch out like bare trees in the winter,
the nervous system is beautiful under a microscope,
but sometimes it looks like a cluttered mess.
In this video, I'll teach you how to look at
the histology of the different types of nervous system cells,
so you can appreciate what you see under a microscope.
If you're new to the channel, welcome.
My name is Patrick and this channel is all
about anatomy and how we learn about it.
As always, I have the accompanying notes for
this video linked in the description if you want to check those out.
Otherwise, let's get started.
Our biggest challenge in learning nervous system histology is
figuring out big picture anatomy from microscopic anatomy.
But we can use some clues to help us out.
We can split the nervous system into the central nervous system,
which includes the brain and spinal cord, and the peripheral
nervous system, pretty much all the nerves that branch out
from that. We will see some different structures and cell
types depending on where we look. But the overall purpose
of the nervous system is to send and receive electrical signals.
That helps us deduce the anatomy of interest.
Like the power lines that send electricity through a city,
each nerve is made of clusters of smaller neuron cells.
Each of these little circles are part of individual neurons.
If we took a transverse cross-section of a nerve,
we'd get a slide like this, just like this electrical cable.
When we slice a nerve long ways for a longitudinal view,
we see the long axons running the length of the nerve.
While a picture like this nerve cross-section seems
overly busy at first, see it for what it is:
neurons and the tissue that wraps them into little bundles.
If you're already familiar with the connective tissue around muscle bundles,
then the naming conventions are going to come easily for nerves.
Remember how from muscles you have the perimysium,
epimysium, and endomysium? In nerves, you keep the same prefixes,
but instead of mysium for muscle, you have neurium for nerves.
The outermost layer is the epineurium,
a layer of dense, irregular connective tissue.
Then each bundle, or fascicle, is wrapped
in a thinner connective tissue called the perineurium,
while each neuron cell and all of its accessories
are wrapped in endoneurium. In this slide, you can
clearly see the dense tissue wrapping
up the neuron bundle here—that's the perineurium—
while each light-colored neuron has a dark ring around it,
that's the endoneurium.
The cell that lives inside that connective tissue is called a neuron.
We're only looking at a small section of it on a cross-section view.
I'm guessing you've seen a picture that looks like this before.
The cliche illustrated neuron with all the tidy pieces in it.
On this slide, we're looking at this small section of the neuron,
but a bunch of them. That's because actual neuron cells can
be really long and impossible to fit under a microscope slide.
But depending on where we're looking,
we can identify different pieces of them.
This big boy is called the cell body, or soma,
which has a nucleus inside.
We need to remember that, as cool and specialized as these cells are,
neurons are still cells with DNA and organelles.
Branching out from there are any number of dendrites,
branches that collect electrical impulses from other cells.
They sum up here at the axon hillock, where an impulse will
travel down the axon, this long piece here. The axon can be
over 95% of the volume of the neuron cell and they can be long,
like over a meter long. These axons are what we just cut open on the
cross-section and most of what we see on longitudinal sections.
Finally, the neuron ends at the axon terminals,
these tiny branches here. They send messages in the
form of neurotransmitters to other cells through synapses.
But that's the Platonic model of a neuron.
In reality, neurons are one of the most diverse cell types in the body.
Some axons are thin, bare cables, while some have a squishy
layer around them that helps them transmit signals faster.
It's called the myelin sheath.
We say that those neurons are myelinated.
The neurons we saw in longitudinal view are really myelin
with axons inside. In this cross-section view,
you can see the tiny axon with the marshmallowy myelin all
around it and endoneurium around that.
Length and diameters can change too. Like, the myelinated
type 1a fibers are anywhere from 4 to 20 micrometers wide.
Type B fibers are 1–4 micrometers wide, while the unmyelinated
type C fibers are only 0.2–1.5 micrometers wide.
The wider and more myelinated the neuron,
the faster it transmits electrical impulses.
Like those type 1A's send signals at 70–120 meters a second,
while type C conducts at 0.5–2.5 meters per second.
That's a pretty big difference in size and speed.
Not only can axons vary,
but the branching pattern can vary too.
The most common type of neuron is a multipolar neuron.
It has one axon and a cell body with a bunch of branching dendrites.
You'll usually spot these on the brain and spinal cord.
Meanwhile, bipolar neurons have
a long axon and a single dendritic tree poking out the other end.
You only see these in certain sensory systems, like the nose and
retina, since they only send afferent, or sensory, information.
Finally, unipolar neurons are what they sound like.
They have a cell body and a single axon, no dendrites.
But neurons aren't the only type of cell in the nervous system.
We also have glial cells, essentially supportive cells.
For instance, astrocytes or star-shaped cells,
support and protect our neurons by regulating the blood-brain barrier,
helping form synapses, and clearing excess neurotransmitters.
They're hard to see with traditional light microscopes,
so unless you have an electron microscope,
you probably won't get quizzed on it.
Oligodendrocytes are another fun one. They help make the
myelin sheath around neurons in the brain and spinal cord,
while Schwann cells make the myelin in the peripheral nerves.
Quick summary: this all started with our bundles of neurons
organized into peripheral nerves like electrical wires in a cable.
But we still have some big deal nervous tissue to tackle,
the central nervous system, including brain and spinal cord.
Luckily for us, we can get our bearings with
the spinal cord similarly to how we did with the peripheral nerves.
The longitudinal section looks familiar but different,
but the transverse cross-section is super unique.
This cross section is this diagram, or what I call the butterfly pancake view.
At the tissue level, let's see what we're working with.
There are two different colors to work with,
which come from myelin status.
Since those myelin sheaths are so fatty and fluffy,
think of myelinated fibers like marshmallows that make up white matter,
while those dense, slow, unmyelinated fibers are the metallic
skewers that poke through them, making up the gray matter.
Look, I know that sounds backwards.
The darker color should be gray matter, right?
But I don't make the rules.
Take it up with management.
Since the gray matter is arranged into this shape,
we label those segments horns,
and we have anterior, lateral, and dorsal horns.
But there's another big component to the central nervous
system: the brain. Before we get to neurons,
we have a few layers of connective tissue called the meninges.
If you've heard of the disease
meningitis, it's inflammation of these layers.
The most superficial layer is the dura mater,
a layer of dense connective tissue that sticks to the skull.
Deeper than that is the arachnoid layer,
which is thin and looks like spider webs,
hence the name, and connects to the delicate thin pia mater underneath.
Aside from some connective tissue around blood vessels,
all the other structures of the brain can be classified as nervous tissue.
But like I said, layers. Let's look at these two different
colors, since their tissue level anatomy is different.
The outermost layer of the cerebrum is the cerebral cortex,
and deeper than that, the subcortical white matter.
The cerebral cortex only has six layers of its own,
and luckily, only a couple of cell types to differentiate between,
the most common of which are pyramidal neurons,
named because sure, they look like pyramids,
I guess. But they're also easy to spot because they
stay in a dark blue color and have a really big nuclei.
This is really just the tip of the iceberg with neuroanatomy.
But I didn't want to make this video overwhelming.
If you need more help with histology in general though,
I've made a bunch of videos that you can find in a playlist right here.
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Have fun, be good. Thanks for watching.