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Dialogue: 0,0:00:00.00,0:00:01.95,Default,,0000,0000,0000,,PATRICK: With cells named after stars and
Dialogue: 0,0:00:01.95,0:00:04.74,Default,,0000,0000,0000,,neurons that branch out like bare trees in the winter,
Dialogue: 0,0:00:04.74,0:00:07.44,Default,,0000,0000,0000,,the nervous system is beautiful under a microscope,
Dialogue: 0,0:00:07.44,0:00:10.28,Default,,0000,0000,0000,,but sometimes it looks like a cluttered mess.
Dialogue: 0,0:00:10.28,0:00:12.03,Default,,0000,0000,0000,,In this video, I'll teach you how to look at
Dialogue: 0,0:00:12.03,0:00:15.18,Default,,0000,0000,0000,,the histology of the different types of nervous system cells,
Dialogue: 0,0:00:15.18,0:00:18.12,Default,,0000,0000,0000,,so you can appreciate what you see under a microscope.
Dialogue: 0,0:00:18.12,0:00:19.82,Default,,0000,0000,0000,,If you're new to the channel, welcome.
Dialogue: 0,0:00:19.82,0:00:21.42,Default,,0000,0000,0000,,My name is Patrick and this channel is all
Dialogue: 0,0:00:21.42,0:00:23.49,Default,,0000,0000,0000,,about anatomy and how we learn about it.
Dialogue: 0,0:00:23.49,0:00:25.50,Default,,0000,0000,0000,,As always, I have the accompanying notes for
Dialogue: 0,0:00:25.50,0:00:28.00,Default,,0000,0000,0000,,this video linked in the description if you want to check those out.
Dialogue: 0,0:00:28.00,0:00:29.50,Default,,0000,0000,0000,,Otherwise, let's get started.
Dialogue: 0,0:00:29.50,0:00:32.37,Default,,0000,0000,0000,,Our biggest challenge in learning nervous system histology is
Dialogue: 0,0:00:32.37,0:00:35.84,Default,,0000,0000,0000,,figuring out big picture anatomy from microscopic anatomy.
Dialogue: 0,0:00:35.84,0:00:38.32,Default,,0000,0000,0000,,But we can use some clues to help us out.
Dialogue: 0,0:00:38.32,0:00:41.08,Default,,0000,0000,0000,,We can split the nervous system into the central nervous system,
Dialogue: 0,0:00:41.08,0:00:44.16,Default,,0000,0000,0000,,which includes the brain and spinal cord, and the peripheral
Dialogue: 0,0:00:44.16,0:00:47.04,Default,,0000,0000,0000,,nervous system, pretty much all the nerves that branch out
Dialogue: 0,0:00:47.04,0:00:49.64,Default,,0000,0000,0000,,from that. We will see some different structures and cell
Dialogue: 0,0:00:49.64,0:00:52.67,Default,,0000,0000,0000,,types depending on where we look. But the overall purpose
Dialogue: 0,0:00:52.67,0:00:56.52,Default,,0000,0000,0000,,of the nervous system is to send and receive electrical signals.
Dialogue: 0,0:00:56.52,0:00:58.84,Default,,0000,0000,0000,,That helps us deduce the anatomy of interest.
Dialogue: 0,0:00:58.84,0:01:01.74,Default,,0000,0000,0000,,Like the power lines that send electricity through a city,
Dialogue: 0,0:01:01.74,0:01:05.22,Default,,0000,0000,0000,,each nerve is made of clusters of smaller neuron cells.
Dialogue: 0,0:01:05.22,0:01:08.52,Default,,0000,0000,0000,,Each of these little circles are part of individual neurons.
Dialogue: 0,0:01:08.52,0:01:11.08,Default,,0000,0000,0000,,If we took a transverse cross-section of a nerve,
Dialogue: 0,0:01:11.08,0:01:14.30,Default,,0000,0000,0000,,we'd get a slide like this, just like this electrical cable.
Dialogue: 0,0:01:14.30,0:01:18.08,Default,,0000,0000,0000,,When we slice a nerve long ways for a longitudinal view,
Dialogue: 0,0:01:18.08,0:01:21.41,Default,,0000,0000,0000,,we see the long axons running the length of the nerve.
Dialogue: 0,0:01:21.41,0:01:24.86,Default,,0000,0000,0000,,While a picture like this nerve cross-section seems
Dialogue: 0,0:01:24.86,0:01:27.98,Default,,0000,0000,0000,,overly busy at first, see it for what it is:
Dialogue: 0,0:01:27.98,0:01:31.24,Default,,0000,0000,0000,,neurons and the tissue that wraps them into little bundles.
Dialogue: 0,0:01:31.24,0:01:34.70,Default,,0000,0000,0000,,If you're already familiar with the connective tissue around muscle bundles,
Dialogue: 0,0:01:34.70,0:01:37.26,Default,,0000,0000,0000,,then the naming conventions are going to come easily for nerves.
Dialogue: 0,0:01:37.26,0:01:39.80,Default,,0000,0000,0000,,Remember how from muscles you have the perimysium,
Dialogue: 0,0:01:39.80,0:01:44.36,Default,,0000,0000,0000,,epimysium, and endomysium? In nerves, you keep the same prefixes,
Dialogue: 0,0:01:44.36,0:01:48.66,Default,,0000,0000,0000,,but instead of mysium for muscle, you have neurium for nerves.
Dialogue: 0,0:01:48.66,0:01:51.20,Default,,0000,0000,0000,,The outermost layer is the epineurium,
Dialogue: 0,0:01:51.20,0:01:54.16,Default,,0000,0000,0000,,a layer of dense, irregular connective tissue.
Dialogue: 0,0:01:54.16,0:01:57.21,Default,,0000,0000,0000,,Then each bundle, or fascicle, is wrapped
Dialogue: 0,0:01:57.21,0:02:00.33,Default,,0000,0000,0000,,in a thinner connective tissue called the perineurium,
Dialogue: 0,0:02:00.33,0:02:03.24,Default,,0000,0000,0000,,while each neuron cell and all of its accessories
Dialogue: 0,0:02:03.24,0:02:06.72,Default,,0000,0000,0000,,are wrapped in endoneurium. In this slide, you can
Dialogue: 0,0:02:06.72,0:02:08.70,Default,,0000,0000,0000,,clearly see the dense tissue wrapping
Dialogue: 0,0:02:08.70,0:02:12.10,Default,,0000,0000,0000,,up the neuron bundle here—that's the perineurium—
Dialogue: 0,0:02:12.10,0:02:15.42,Default,,0000,0000,0000,,while each light-colored neuron has a dark ring around it,
Dialogue: 0,0:02:15.42,0:02:17.44,Default,,0000,0000,0000,,that's the endoneurium.
Dialogue: 0,0:02:17.44,0:02:21.00,Default,,0000,0000,0000,,The cell that lives inside that connective tissue is called a neuron.
Dialogue: 0,0:02:21.00,0:02:24.75,Default,,0000,0000,0000,,We're only looking at a small section of it on a cross-section view.
Dialogue: 0,0:02:24.75,0:02:27.52,Default,,0000,0000,0000,,I'm guessing you've seen a picture that looks like this before.
Dialogue: 0,0:02:27.52,0:02:31.77,Default,,0000,0000,0000,,The cliche illustrated neuron with all the tidy pieces in it.
Dialogue: 0,0:02:31.77,0:02:35.81,Default,,0000,0000,0000,,On this slide, we're looking at this small section of the neuron,
Dialogue: 0,0:02:35.81,0:02:38.57,Default,,0000,0000,0000,,but a bunch of them. That's because actual neuron cells can
Dialogue: 0,0:02:38.57,0:02:41.98,Default,,0000,0000,0000,,be really long and impossible to fit under a microscope slide.
Dialogue: 0,0:02:41.98,0:02:43.64,Default,,0000,0000,0000,,But depending on where we're looking,
Dialogue: 0,0:02:43.64,0:02:46.18,Default,,0000,0000,0000,,we can identify different pieces of them.
Dialogue: 0,0:02:46.18,0:02:49.31,Default,,0000,0000,0000,,This big boy is called the cell body, or soma,
Dialogue: 0,0:02:49.31,0:02:50.86,Default,,0000,0000,0000,,which has a nucleus inside.
Dialogue: 0,0:02:50.86,0:02:54.60,Default,,0000,0000,0000,,We need to remember that, as cool and specialized as these cells are,
Dialogue: 0,0:02:54.60,0:02:58.07,Default,,0000,0000,0000,,neurons are still cells with DNA and organelles.
Dialogue: 0,0:02:58.07,0:03:01.06,Default,,0000,0000,0000,,Branching out from there are any number of dendrites,
Dialogue: 0,0:03:01.06,0:03:04.30,Default,,0000,0000,0000,,branches that collect electrical impulses from other cells.
Dialogue: 0,0:03:04.30,0:03:07.49,Default,,0000,0000,0000,,They sum up here at the axon hillock, where an impulse will
Dialogue: 0,0:03:07.49,0:03:11.86,Default,,0000,0000,0000,,travel down the axon, this long piece here. The axon can be
Dialogue: 0,0:03:11.86,0:03:16.92,Default,,0000,0000,0000,,over 95% of the volume of the neuron cell and they can be long,
Dialogue: 0,0:03:16.92,0:03:20.89,Default,,0000,0000,0000,,like over a meter long. These axons are what we just cut open on the
Dialogue: 0,0:03:20.89,0:03:24.40,Default,,0000,0000,0000,,cross-section and most of what we see on longitudinal sections.
Dialogue: 0,0:03:24.40,0:03:27.49,Default,,0000,0000,0000,,Finally, the neuron ends at the axon terminals,
Dialogue: 0,0:03:27.49,0:03:30.76,Default,,0000,0000,0000,,these tiny branches here. They send messages in the
Dialogue: 0,0:03:30.76,0:03:34.26,Default,,0000,0000,0000,,form of neurotransmitters to other cells through synapses.
Dialogue: 0,0:03:34.26,0:03:36.68,Default,,0000,0000,0000,,But that's the Platonic model of a neuron.
Dialogue: 0,0:03:36.68,0:03:40.46,Default,,0000,0000,0000,,In reality, neurons are one of the most diverse cell types in the body.
Dialogue: 0,0:03:40.46,0:03:43.87,Default,,0000,0000,0000,,Some axons are thin, bare cables, while some have a squishy
Dialogue: 0,0:03:43.87,0:03:46.54,Default,,0000,0000,0000,,layer around them that helps them transmit signals faster.
Dialogue: 0,0:03:46.54,0:03:48.26,Default,,0000,0000,0000,,It's called the myelin sheath.
Dialogue: 0,0:03:48.26,0:03:51.43,Default,,0000,0000,0000,,We say that those neurons are myelinated.
Dialogue: 0,0:03:51.43,0:03:54.94,Default,,0000,0000,0000,,The neurons we saw in longitudinal view are really myelin
Dialogue: 0,0:03:54.94,0:03:58.40,Default,,0000,0000,0000,,with axons inside. In this cross-section view,
Dialogue: 0,0:03:58.40,0:04:01.96,Default,,0000,0000,0000,,you can see the tiny axon with the marshmallowy myelin all
Dialogue: 0,0:04:01.96,0:04:04.60,Default,,0000,0000,0000,,around it and endoneurium around that.
Dialogue: 0,0:04:04.60,0:04:08.26,Default,,0000,0000,0000,,Length and diameters can change too. Like, the myelinated
Dialogue: 0,0:04:08.26,0:04:13.30,Default,,0000,0000,0000,,type 1a fibers are anywhere from 4 to 20 micrometers wide.
Dialogue: 0,0:04:13.30,0:04:17.80,Default,,0000,0000,0000,,Type B fibers are 1–4 micrometers wide, while the unmyelinated
Dialogue: 0,0:04:17.80,0:04:23.00,Default,,0000,0000,0000,,type C fibers are only 0.2–1.5 micrometers wide.
Dialogue: 0,0:04:23.00,0:04:25.61,Default,,0000,0000,0000,,The wider and more myelinated the neuron,
Dialogue: 0,0:04:25.61,0:04:28.28,Default,,0000,0000,0000,,the faster it transmits electrical impulses.
Dialogue: 0,0:04:28.28,0:04:33.62,Default,,0000,0000,0000,,Like those type 1A's send signals at 70–120 meters a second,
Dialogue: 0,0:04:33.62,0:04:38.14,Default,,0000,0000,0000,,while type C conducts at 0.5–2.5 meters per second.
Dialogue: 0,0:04:38.14,0:04:40.26,Default,,0000,0000,0000,,That's a pretty big difference in size and speed.
Dialogue: 0,0:04:40.26,0:04:42.05,Default,,0000,0000,0000,,Not only can axons vary,
Dialogue: 0,0:04:42.05,0:04:43.94,Default,,0000,0000,0000,,but the branching pattern can vary too.
Dialogue: 0,0:04:43.94,0:04:47.38,Default,,0000,0000,0000,,The most common type of neuron is a multipolar neuron.
Dialogue: 0,0:04:47.38,0:04:51.88,Default,,0000,0000,0000,,It has one axon and a cell body with a bunch of branching dendrites.
Dialogue: 0,0:04:51.88,0:04:55.03,Default,,0000,0000,0000,,You'll usually spot these on the brain and spinal cord.
Dialogue: 0,0:04:55.03,0:04:57.19,Default,,0000,0000,0000,,Meanwhile, bipolar neurons have
Dialogue: 0,0:04:57.19,0:05:01.26,Default,,0000,0000,0000,,a long axon and a single dendritic tree poking out the other end.
Dialogue: 0,0:05:01.26,0:05:04.96,Default,,0000,0000,0000,,You only see these in certain sensory systems, like the nose and
Dialogue: 0,0:05:04.96,0:05:09.10,Default,,0000,0000,0000,,retina, since they only send afferent, or sensory, information.
Dialogue: 0,0:05:09.10,0:05:12.02,Default,,0000,0000,0000,,Finally, unipolar neurons are what they sound like.
Dialogue: 0,0:05:12.02,0:05:15.52,Default,,0000,0000,0000,,They have a cell body and a single axon, no dendrites.
Dialogue: 0,0:05:15.52,0:05:18.42,Default,,0000,0000,0000,,But neurons aren't the only type of cell in the nervous system.
Dialogue: 0,0:05:18.42,0:05:21.87,Default,,0000,0000,0000,,We also have glial cells, essentially supportive cells.
Dialogue: 0,0:05:21.87,0:05:25.30,Default,,0000,0000,0000,,For instance, astrocytes or star-shaped cells,
Dialogue: 0,0:05:25.30,0:05:29.21,Default,,0000,0000,0000,,support and protect our neurons by regulating the blood-brain barrier,
Dialogue: 0,0:05:29.21,0:05:32.98,Default,,0000,0000,0000,,helping form synapses, and clearing excess neurotransmitters.
Dialogue: 0,0:05:32.98,0:05:35.51,Default,,0000,0000,0000,,They're hard to see with traditional light microscopes,
Dialogue: 0,0:05:35.51,0:05:37.60,Default,,0000,0000,0000,,so unless you have an electron microscope,
Dialogue: 0,0:05:37.60,0:05:39.10,Default,,0000,0000,0000,,you probably won't get quizzed on it.
Dialogue: 0,0:05:39.10,0:05:42.16,Default,,0000,0000,0000,,Oligodendrocytes are another fun one. They help make the
Dialogue: 0,0:05:42.16,0:05:45.28,Default,,0000,0000,0000,,myelin sheath around neurons in the brain and spinal cord,
Dialogue: 0,0:05:45.28,0:05:48.28,Default,,0000,0000,0000,,while Schwann cells make the myelin in the peripheral nerves.
Dialogue: 0,0:05:48.28,0:05:51.58,Default,,0000,0000,0000,,Quick summary: this all started with our bundles of neurons
Dialogue: 0,0:05:51.58,0:05:55.18,Default,,0000,0000,0000,,organized into peripheral nerves like electrical wires in a cable.
Dialogue: 0,0:05:55.18,0:05:58.09,Default,,0000,0000,0000,,But we still have some big deal nervous tissue to tackle,
Dialogue: 0,0:05:58.09,0:06:01.43,Default,,0000,0000,0000,,the central nervous system, including brain and spinal cord.
Dialogue: 0,0:06:01.43,0:06:03.46,Default,,0000,0000,0000,,Luckily for us, we can get our bearings with
Dialogue: 0,0:06:03.46,0:06:06.90,Default,,0000,0000,0000,,the spinal cord similarly to how we did with the peripheral nerves.
Dialogue: 0,0:06:06.90,0:06:09.82,Default,,0000,0000,0000,,The longitudinal section looks familiar but different,
Dialogue: 0,0:06:09.82,0:06:13.00,Default,,0000,0000,0000,,but the transverse cross-section is super unique.
Dialogue: 0,0:06:13.00,0:06:18.67,Default,,0000,0000,0000,,This cross section is this diagram, or what I call the butterfly pancake view.
Dialogue: 0,0:06:18.67,0:06:21.06,Default,,0000,0000,0000,,At the tissue level, let's see what we're working with.
Dialogue: 0,0:06:21.06,0:06:22.80,Default,,0000,0000,0000,,There are two different colors to work with,
Dialogue: 0,0:06:22.80,0:06:24.43,Default,,0000,0000,0000,,which come from myelin status.
Dialogue: 0,0:06:24.43,0:06:27.30,Default,,0000,0000,0000,,Since those myelin sheaths are so fatty and fluffy,
Dialogue: 0,0:06:27.30,0:06:31.64,Default,,0000,0000,0000,,think of myelinated fibers like marshmallows that make up white matter,
Dialogue: 0,0:06:31.64,0:06:35.77,Default,,0000,0000,0000,,while those dense, slow, unmyelinated fibers are the metallic
Dialogue: 0,0:06:35.77,0:06:38.94,Default,,0000,0000,0000,,skewers that poke through them, making up the gray matter.
Dialogue: 0,0:06:38.94,0:06:40.69,Default,,0000,0000,0000,,Look, I know that sounds backwards.
Dialogue: 0,0:06:40.69,0:06:42.84,Default,,0000,0000,0000,,The darker color should be gray matter, right?
Dialogue: 0,0:06:42.84,0:06:44.50,Default,,0000,0000,0000,,But I don't make the rules.
Dialogue: 0,0:06:44.50,0:06:45.57,Default,,0000,0000,0000,,Take it up with management.
Dialogue: 0,0:06:45.57,0:06:48.06,Default,,0000,0000,0000,,Since the gray matter is arranged into this shape,
Dialogue: 0,0:06:48.06,0:06:50.50,Default,,0000,0000,0000,,we label those segments horns,
Dialogue: 0,0:06:50.50,0:06:54.22,Default,,0000,0000,0000,,and we have anterior, lateral, and dorsal horns.
Dialogue: 0,0:06:54.22,0:06:56.65,Default,,0000,0000,0000,,But there's another big component to the central nervous
Dialogue: 0,0:06:56.65,0:06:59.36,Default,,0000,0000,0000,,system: the brain. Before we get to neurons,
Dialogue: 0,0:06:59.36,0:07:02.64,Default,,0000,0000,0000,,we have a few layers of connective tissue called the meninges.
Dialogue: 0,0:07:02.64,0:07:04.22,Default,,0000,0000,0000,,If you've heard of the disease
Dialogue: 0,0:07:04.22,0:07:07.20,Default,,0000,0000,0000,,meningitis, it's inflammation of these layers.
Dialogue: 0,0:07:07.20,0:07:09.79,Default,,0000,0000,0000,,The most superficial layer is the dura mater,
Dialogue: 0,0:07:09.79,0:07:13.16,Default,,0000,0000,0000,,a layer of dense connective tissue that sticks to the skull.
Dialogue: 0,0:07:13.16,0:07:15.31,Default,,0000,0000,0000,,Deeper than that is the arachnoid layer,
Dialogue: 0,0:07:15.31,0:07:17.23,Default,,0000,0000,0000,,which is thin and looks like spider webs,
Dialogue: 0,0:07:17.23,0:07:21.82,Default,,0000,0000,0000,,hence the name, and connects to the delicate thin pia mater underneath.
Dialogue: 0,0:07:21.82,0:07:24.40,Default,,0000,0000,0000,,Aside from some connective tissue around blood vessels,
Dialogue: 0,0:07:24.40,0:07:28.10,Default,,0000,0000,0000,,all the other structures of the brain can be classified as nervous tissue.
Dialogue: 0,0:07:28.10,0:07:31.30,Default,,0000,0000,0000,,But like I said, layers. Let's look at these two different
Dialogue: 0,0:07:31.30,0:07:34.01,Default,,0000,0000,0000,,colors, since their tissue level anatomy is different.
Dialogue: 0,0:07:34.01,0:07:37.42,Default,,0000,0000,0000,,The outermost layer of the cerebrum is the cerebral cortex,
Dialogue: 0,0:07:37.42,0:07:40.24,Default,,0000,0000,0000,,and deeper than that, the subcortical white matter.
Dialogue: 0,0:07:40.24,0:07:43.20,Default,,0000,0000,0000,,The cerebral cortex only has six layers of its own,
Dialogue: 0,0:07:43.20,0:07:46.78,Default,,0000,0000,0000,,and luckily, only a couple of cell types to differentiate between,
Dialogue: 0,0:07:46.78,0:07:49.04,Default,,0000,0000,0000,,the most common of which are pyramidal neurons,
Dialogue: 0,0:07:49.04,0:07:51.61,Default,,0000,0000,0000,,named because sure, they look like pyramids,
Dialogue: 0,0:07:51.61,0:07:54.40,Default,,0000,0000,0000,,I guess. But they're also easy to spot because they
Dialogue: 0,0:07:54.40,0:07:57.80,Default,,0000,0000,0000,,stay in a dark blue color and have a really big nuclei.
Dialogue: 0,0:07:57.80,0:08:00.64,Default,,0000,0000,0000,,This is really just the tip of the iceberg with neuroanatomy.
Dialogue: 0,0:08:00.64,0:08:02.90,Default,,0000,0000,0000,,But I didn't want to make this video overwhelming.
Dialogue: 0,0:08:02.90,0:08:05.30,Default,,0000,0000,0000,,If you need more help with histology in general though,
Dialogue: 0,0:08:05.30,0:08:08.34,Default,,0000,0000,0000,,I've made a bunch of videos that you can find in a playlist right here.
Dialogue: 0,0:08:08.34,0:08:11.07,Default,,0000,0000,0000,,As always, thank you to my patrons on Patreon.
Dialogue: 0,0:08:11.07,0:08:12.93,Default,,0000,0000,0000,,If you haven't, I'd appreciate it if you
Dialogue: 0,0:08:12.93,0:08:16.05,Default,,0000,0000,0000,,subscribe and hit the bell so you get notified when I post a new video.
Dialogue: 0,0:08:16.05,0:08:19.30,Default,,0000,0000,0000,,Have fun, be good. Thanks for watching.