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PATRICK: With cells named after stars and

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neurons that branch out like bare trees in the winter,

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the nervous system is beautiful under a microscope,

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but sometimes it looks like a cluttered mess.

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In this video, I'll teach you how to look at

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the histology of the different types of nervous system cells,

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so you can appreciate what you see under a microscope.

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If you're new to the channel, welcome.

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My name is Patrick and this channel is all

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about anatomy and how we learn about it.

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As always, I have the accompanying notes for

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this video linked in the description if you want to check those out.

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Otherwise, let's get started.

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Our biggest challenge in learning nervous system histology is

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figuring out big picture anatomy from microscopic anatomy.

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But we can use some clues to help us out.

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We can split the nervous system into the central nervous system,

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which includes the brain and spinal cord, and the peripheral

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nervous system, pretty much all the nerves that branch out

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from that. We will see some different structures and cell

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types depending on where we look. But the overall purpose

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of the nervous system is to send and receive electrical signals.

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That helps us deduce the anatomy of interest.

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Like the power lines that send electricity through a city,

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each nerve is made of clusters of smaller neuron cells.

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Each of these little circles are part of individual neurons.

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If we took a transverse cross-section of a nerve,

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we'd get a slide like this, just like this electrical cable.

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When we slice a nerve long ways for a longitudinal view,

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we see the long axons running the length of the nerve.

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While a picture like this nerve cross-section seems

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overly busy at first, see it for what it is:

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neurons and the tissue that wraps them into little bundles.

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If you're already familiar with the connective tissue around muscle bundles,

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then the naming conventions are going to come easily for nerves.

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Remember how from muscles you have the perimysium,

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epimysium, and endomysium? In nerves, you keep the same prefixes,

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but instead of mysium for muscle, you have neurium for nerves.

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The outermost layer is the epineurium,

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a layer of dense, irregular connective tissue.

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Then each bundle, or fascicle, is wrapped

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in a thinner connective tissue called the perineurium,

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while each neuron cell and all of its accessories

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are wrapped in endoneurium. In this slide, you can

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clearly see the dense tissue wrapping

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up the neuron bundle here—that's the perineurium—

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while each light-colored neuron has a dark ring around it,

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that's the endoneurium.

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The cell that lives inside that connective tissue is called a neuron.

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We're only looking at a small section of it on a cross-section view.

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I'm guessing you've seen a picture that looks like this before.

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The cliche illustrated neuron with all the tidy pieces in it.

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On this slide, we're looking at this small section of the neuron,

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but a bunch of them. That's because actual neuron cells can

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be really long and impossible to fit under a microscope slide.

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But depending on where we're looking,

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we can identify different pieces of them.

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This big boy is called the cell body, or soma,

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which has a nucleus inside.

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We need to remember that, as cool and specialized as these cells are,

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neurons are still cells with DNA and organelles.

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Branching out from there are any number of dendrites,

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branches that collect electrical impulses from other cells.

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They sum up here at the axon hillock, where an impulse will

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travel down the axon, this long piece here. The axon can be

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over 95% of the volume of the neuron cell and they can be long,

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like over a meter long. These axons are what we just cut open on the

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cross-section and most of what we see on longitudinal sections.

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Finally, the neuron ends at the axon terminals,

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these tiny branches here. They send messages in the

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form of neurotransmitters to other cells through synapses.

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But that's the Platonic model of a neuron.

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In reality, neurons are one of the most diverse cell types in the body.

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Some axons are thin, bare cables, while some have a squishy

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layer around them that helps them transmit signals faster.

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It's called the myelin sheath.

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We say that those neurons are myelinated.

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The neurons we saw in longitudinal view are really myelin

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with axons inside. In this cross-section view,

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you can see the tiny axon with the marshmallowy myelin all

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around it and endoneurium around that.

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Length and diameters can change too. Like, the myelinated

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type 1a fibers are anywhere from 4 to 20 micrometers wide.

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Type B fibers are 1–4 micrometers wide, while the unmyelinated

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type C fibers are only 0.2–1.5 micrometers wide.

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The wider and more myelinated the neuron,

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the faster it transmits electrical impulses.

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Like those type 1A's send signals at 70–120 meters a second,

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while type C conducts at 0.5–2.5 meters per second.

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That's a pretty big difference in size and speed.

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Not only can axons vary,

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but the branching pattern can vary too.

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The most common type of neuron is a multipolar neuron.

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It has one axon and a cell body with a bunch of branching dendrites.

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You'll usually spot these on the brain and spinal cord.

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Meanwhile, bipolar neurons have

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a long axon and a single dendritic tree poking out the other end.

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You only see these in certain sensory systems, like the nose and

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retina, since they only send afferent, or sensory, information.

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Finally, unipolar neurons are what they sound like.

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They have a cell body and a single axon, no dendrites.

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But neurons aren't the only type of cell in the nervous system.

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We also have glial cells, essentially supportive cells.

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For instance, astrocytes or star-shaped cells,

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support and protect our neurons by regulating the blood-brain barrier,

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helping form synapses, and clearing excess neurotransmitters.

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They're hard to see with traditional light microscopes,

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so unless you have an electron microscope,

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you probably won't get quizzed on it.

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Oligodendrocytes are another fun one. They help make the

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myelin sheath around neurons in the brain and spinal cord,

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while Schwann cells make the myelin in the peripheral nerves.

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Quick summary: this all started with our bundles of neurons

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organized into peripheral nerves like electrical wires in a cable.

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But we still have some big deal nervous tissue to tackle,

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the central nervous system, including brain and spinal cord.

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Luckily for us, we can get our bearings with

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the spinal cord similarly to how we did with the peripheral nerves.

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The longitudinal section looks familiar but different,

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but the transverse cross-section is super unique.

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This cross section is this diagram, or what I call the butterfly pancake view.

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At the tissue level, let's see what we're working with.

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There are two different colors to work with,

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which come from myelin status.

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Since those myelin sheaths are so fatty and fluffy,

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think of myelinated fibers like marshmallows that make up white matter,

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while those dense, slow, unmyelinated fibers are the metallic

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skewers that poke through them, making up the gray matter.

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Look, I know that sounds backwards.

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The darker color should be gray matter, right?

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But I don't make the rules.

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Take it up with management.

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Since the gray matter is arranged into this shape,

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we label those segments horns,

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and we have anterior, lateral, and dorsal horns.

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But there's another big component to the central nervous

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system: the brain. Before we get to neurons,

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we have a few layers of connective tissue called the meninges.

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If you've heard of the disease

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meningitis, it's inflammation of these layers.

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The most superficial layer is the dura mater,

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a layer of dense connective tissue that sticks to the skull.

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Deeper than that is the arachnoid layer,

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which is thin and looks like spider webs,

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hence the name, and connects to the delicate thin pia mater underneath.

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Aside from some connective tissue around blood vessels,

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all the other structures of the brain can be classified as nervous tissue.

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But like I said, layers. Let's look at these two different

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colors, since their tissue level anatomy is different.

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The outermost layer of the cerebrum is the cerebral cortex,

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and deeper than that, the subcortical white matter.

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The cerebral cortex only has six layers of its own,

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and luckily, only a couple of cell types to differentiate between,

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the most common of which are pyramidal neurons,

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named because sure, they look like pyramids,

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I guess. But they're also easy to spot because they

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stay in a dark blue color and have a really big nuclei.

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This is really just the tip of the iceberg with neuroanatomy.

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But I didn't want to make this video overwhelming.

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If you need more help with histology in general though,

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I've made a bunch of videos that you can find in a playlist right here.

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As always, thank you to my patrons on Patreon.

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If you haven't, I'd appreciate it if you

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subscribe and hit the bell so you get notified when I post a new video.

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Have fun, be good. Thanks for watching.