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’ll 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.
And 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 is part of individual neuron cells.
If we took a transverse cross section of a nerve, we’d get a slide like this.
Just like this electrical cable.
And when we slice a nerve long ways for a longitudinal view and see the long axons running
the length of the nerve.
So 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.
And if you’re already familiar with the connective tissue around /muscle/ bundles,
then the naming conventions are gonna come easy for nerves.
Remember how for muscles you have perimysium, epimysium and endomysium?
Well in nerves, you keep the same prefixes, but instead of -mysium for muscle, you have
-nerium for nerves.
So the outermost layer is 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 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 and we’re only
looking at a small section of it on a cross section.
I’m guessing you’ve seen a picture like this before — the cliche illustrated neuron
with all the tidy pieces, right?
We’re only looking at this small section.
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 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 a myelin sheath, so we say that those neurons are myelinated.
The neurons we saw on longitudinal view are really myelin with axons inside, and 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 4-20 micrometers wide, Type B fibers
are 1 to 4 micrometers wide, while the unmyelinated Type C fibers are only 0.2 to 1.5 micrometers
wide.
And the wider and more myelinated the neuron, the faster it transmits electrical impulses.
So those type 1As send signals at 70 to 120 meters a second while type C conducts at half
to two and half meters per second.
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 ion the brain and spinal cord.
Meanwhile, bipolar neurons have a long axon and a single dendritic tree poking out the
other end and 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 kind of 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 the brain and spinal cord.
Luckily for us, we can get our bearings with the spinal cord similarly to how we did the
peripheral nerves.
The longitudinal section looks familiar, but different and the transverse cross section
is super unique.
This cross section is this diagram, or what I call the butterfly pancake.
So 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, but I don’t
make the rules.
Take it up with management.
Since the gray matter falls into this shape, we label these segments horns.
And we have anterior, lateral, and dorsal horns.
But there’s another big component to the central nervous system, the brain.
Let’s look at these two different colors, since their tissue level anatomy is different.
Before we get to neurons, we have a few layers of connective tissue called the meninges.
If you’ve heard of the diseases /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.
And 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.
The outermost layer of the cerebrum is the cerebral cortex, and deeper than that, the
/sub/cortical white matter.
The cerebral cortex has 6 layers of its own and 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 stain a dark blue color and have real big
nuclei.
75% of cells in the cerebral cortex will be pyramidal cells.
These other dots are called stellate cells, that look kind of like stars too, but they’re
really tiny so they also get called granular cells.
The deepest layer of the cerebral cortex features fusiform cells, which gives away their shape.
/Fusiform/ is the name for a shape — just like fusiform muscles, these cells are spindle
shaped.
And remember that this is gray matter, they’re unmyelinated neurons.
When we get to the subcortical white matter, we’ll see more oligodendroglial cells, some
of the cells that actually /make/ the myelin.
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 them and you
can find a playlist of them here.
As always, thanks to my patrons over on Patreon.
And if you haven’t I’d appreciate it if you subscribed and hit the bell so you get
notified when I post a new video.
Have fun, be good.