WEBVTT 00:00:00.000 --> 00:00:01.950 PATRICK: With cells named after stars and 00:00:01.950 --> 00:00:04.740 neurons that branch out like bare trees in the winter, 00:00:04.740 --> 00:00:07.440 the nervous system is beautiful under a microscope, 00:00:07.440 --> 00:00:10.275 but sometimes it looks like a cluttered mess. 00:00:10.275 --> 00:00:12.030 In this video, I'll teach you how to look at 00:00:12.030 --> 00:00:15.180 the histology of the different types of nervous system cells, 00:00:15.180 --> 00:00:18.120 so you can appreciate what you see under a microscope. 00:00:18.120 --> 00:00:19.815 If you're new to the channel, welcome. 00:00:19.815 --> 00:00:21.420 My name is Patrick and this channel is all 00:00:21.420 --> 00:00:23.490 about anatomy and how we learn about it. 00:00:23.490 --> 00:00:25.500 As always, I have the accompanying notes for 00:00:25.500 --> 00:00:28.005 this video linked in the description if you want to check those out. 00:00:28.005 --> 00:00:29.505 Otherwise, let's get started. 00:00:29.505 --> 00:00:32.370 Our biggest challenge in learning nervous system histology is 00:00:32.370 --> 00:00:35.835 figuring out big picture anatomy from microscopic anatomy. 00:00:35.835 --> 00:00:38.325 But we can use some clues to help us out. 00:00:38.325 --> 00:00:41.085 We can split the nervous system into the central nervous system, 00:00:41.085 --> 00:00:44.160 which includes the brain and spinal cord, and the peripheral 00:00:44.160 --> 00:00:47.040 nervous system, pretty much all the nerves that branch out 00:00:47.040 --> 00:00:49.640 from that. We will see some different structures and cell 00:00:49.640 --> 00:00:52.670 types depending on where we look. But the overall purpose 00:00:52.670 --> 00:00:56.520 of the nervous system is to send and receive electrical signals. 00:00:56.520 --> 00:00:58.835 That helps us deduce the anatomy of interest. 00:00:58.835 --> 00:01:01.735 Like the power lines that send electricity through a city, 00:01:01.735 --> 00:01:05.225 each nerve is made of clusters of smaller neuron cells. 00:01:05.225 --> 00:01:08.525 Each of these little circles are part of individual neurons. 00:01:08.525 --> 00:01:11.075 If we took a transverse cross-section of a nerve, 00:01:11.075 --> 00:01:14.300 we'd get a slide like this, just like this electrical cable. 00:01:14.300 --> 00:01:18.080 When we slice a nerve long ways for a longitudinal view, 00:01:18.080 --> 00:01:21.410 we see the long axons running the length of the nerve. 00:01:21.410 --> 00:01:24.860 While a picture like this nerve cross-section seems 00:01:24.860 --> 00:01:27.980 overly busy at first, see it for what it is: 00:01:27.980 --> 00:01:31.240 neurons and the tissue that wraps them into little bundles. 00:01:31.240 --> 00:01:34.700 If you're already familiar with the connective tissue around muscle bundles, 00:01:34.700 --> 00:01:37.260 then the naming conventions are going to come easily for nerves. 00:01:37.260 --> 00:01:39.800 Remember how from muscles you have the perimysium, 00:01:39.800 --> 00:01:44.360 epimysium, and endomysium? In nerves, you keep the same prefixes, 00:01:44.360 --> 00:01:48.665 but instead of mysium for muscle, you have neurium for nerves. 00:01:48.665 --> 00:01:51.200 The outermost layer is the epineurium, 00:01:51.200 --> 00:01:54.160 a layer of dense, irregular connective tissue. 00:01:54.160 --> 00:01:57.210 Then each bundle, or fascicle, is wrapped 00:01:57.210 --> 00:02:00.330 in a thinner connective tissue called the perineurium, 00:02:00.330 --> 00:02:03.240 while each neuron cell and all of its accessories 00:02:03.240 --> 00:02:06.720 are wrapped in endoneurium. In this slide, you can 00:02:06.720 --> 00:02:08.700 clearly see the dense tissue wrapping 00:02:08.700 --> 00:02:12.105 up the neuron bundle here—that's the perineurium— 00:02:12.105 --> 00:02:15.420 while each light-colored neuron has a dark ring around it, 00:02:15.420 --> 00:02:17.445 that's the endoneurium. 00:02:17.445 --> 00:02:21.000 The cell that lives inside that connective tissue is called a neuron. 00:02:21.000 --> 00:02:24.750 We're only looking at a small section of it on a cross-section view. 00:02:24.750 --> 00:02:27.525 I'm guessing you've seen a picture that looks like this before. 00:02:27.525 --> 00:02:31.770 The cliche illustrated neuron with all the tidy pieces in it. 00:02:31.770 --> 00:02:35.810 On this slide, we're looking at this small section of the neuron, 00:02:35.810 --> 00:02:38.570 but a bunch of them. That's because actual neuron cells can 00:02:38.570 --> 00:02:41.975 be really long and impossible to fit under a microscope slide. 00:02:41.975 --> 00:02:43.640 But depending on where we're looking, 00:02:43.640 --> 00:02:46.175 we can identify different pieces of them. 00:02:46.175 --> 00:02:49.310 This big boy is called the cell body, or soma, 00:02:49.310 --> 00:02:50.855 which has a nucleus inside. 00:02:50.855 --> 00:02:54.605 We need to remember that, as cool and specialized as these cells are, 00:02:54.605 --> 00:02:58.070 neurons are still cells with DNA and organelles. 00:02:58.070 --> 00:03:01.055 Branching out from there are any number of dendrites, 00:03:01.055 --> 00:03:04.295 branches that collect electrical impulses from other cells. 00:03:04.295 --> 00:03:07.490 They sum up here at the axon hillock, where an impulse will 00:03:07.490 --> 00:03:11.860 travel down the axon, this long piece here. The axon can be 00:03:11.860 --> 00:03:16.915 over 95% of the volume of the neuron cell and they can be long, 00:03:16.915 --> 00:03:20.890 like over a meter long. These axons are what we just cut open on the 00:03:20.890 --> 00:03:24.400 cross-section and most of what we see on longitudinal sections. 00:03:24.400 --> 00:03:27.490 Finally, the neuron ends at the axon terminals, 00:03:27.490 --> 00:03:30.760 these tiny branches here. They send messages in the 00:03:30.760 --> 00:03:34.255 form of neurotransmitters to other cells through synapses. 00:03:34.255 --> 00:03:36.685 But that's the Platonic model of a neuron. 00:03:36.685 --> 00:03:40.465 In reality, neurons are one of the most diverse cell types in the body. 00:03:40.465 --> 00:03:43.870 Some axons are thin, bare cables, while some have a squishy 00:03:43.870 --> 00:03:46.540 layer around them that helps them transmit signals faster. 00:03:46.540 --> 00:03:48.260 It's called the myelin sheath. 00:03:48.260 --> 00:03:51.430 We say that those neurons are myelinated. 00:03:51.430 --> 00:03:54.940 The neurons we saw in longitudinal view are really myelin 00:03:54.940 --> 00:03:58.405 with axons inside. In this cross-section view, 00:03:58.405 --> 00:04:01.960 you can see the tiny axon with the marshmallowy myelin all 00:04:01.960 --> 00:04:04.600 around it and endoneurium around that. 00:04:04.600 --> 00:04:08.260 Length and diameters can change too. Like, the myelinated 00:04:08.260 --> 00:04:13.300 type 1a fibers are anywhere from 4 to 20 micrometers wide. 00:04:13.300 --> 00:04:17.800 Type B fibers are 1–4 micrometers wide, while the unmyelinated 00:04:17.800 --> 00:04:23.005 type C fibers are only 0.2–1.5 micrometers wide. 00:04:23.005 --> 00:04:25.610 The wider and more myelinated the neuron, 00:04:25.610 --> 00:04:28.275 the faster it transmits electrical impulses. 00:04:28.275 --> 00:04:33.620 Like those type 1A's send signals at 70–120 meters a second, 00:04:33.620 --> 00:04:38.135 while type C conducts at 0.5–2.5 meters per second. 00:04:38.135 --> 00:04:40.265 That's a pretty big difference in size and speed. 00:04:40.265 --> 00:04:42.050 Not only can axons vary, 00:04:42.050 --> 00:04:43.940 but the branching pattern can vary too. 00:04:43.940 --> 00:04:47.385 The most common type of neuron is a multipolar neuron. 00:04:47.385 --> 00:04:51.880 It has one axon and a cell body with a bunch of branching dendrites. 00:04:51.880 --> 00:04:55.030 You'll usually spot these on the brain and spinal cord. 00:04:55.030 --> 00:04:57.190 Meanwhile, bipolar neurons have 00:04:57.190 --> 00:05:01.260 a long axon and a single dendritic tree poking out the other end. 00:05:01.260 --> 00:05:04.960 You only see these in certain sensory systems, like the nose and 00:05:04.960 --> 00:05:09.100 retina, since they only send afferent, or sensory, information. 00:05:09.100 --> 00:05:12.025 Finally, unipolar neurons are what they sound like. 00:05:12.025 --> 00:05:15.520 They have a cell body and a single axon, no dendrites. 00:05:15.520 --> 00:05:18.425 But neurons aren't the only type of cell in the nervous system. 00:05:18.425 --> 00:05:21.870 We also have glial cells, essentially supportive cells. 00:05:21.870 --> 00:05:25.295 For instance, astrocytes or star-shaped cells, 00:05:25.295 --> 00:05:29.210 support and protect our neurons by regulating the blood-brain barrier, 00:05:29.210 --> 00:05:32.975 helping form synapses, and clearing excess neurotransmitters. 00:05:32.975 --> 00:05:35.510 They're hard to see with traditional light microscopes, 00:05:35.510 --> 00:05:37.595 so unless you have an electron microscope, 00:05:37.595 --> 00:05:39.095 you probably won't get quizzed on it. 00:05:39.095 --> 00:05:42.160 Oligodendrocytes are another fun one. They help make the 00:05:42.160 --> 00:05:45.280 myelin sheath around neurons in the brain and spinal cord, 00:05:45.280 --> 00:05:48.280 while Schwann cells make the myelin in the peripheral nerves. 00:05:48.280 --> 00:05:51.580 Quick summary: this all started with our bundles of neurons 00:05:51.580 --> 00:05:55.180 organized into peripheral nerves like electrical wires in a cable. 00:05:55.180 --> 00:05:58.090 But we still have some big deal nervous tissue to tackle, 00:05:58.090 --> 00:06:01.430 the central nervous system, including brain and spinal cord. 00:06:01.430 --> 00:06:03.460 Luckily for us, we can get our bearings with 00:06:03.460 --> 00:06:06.895 the spinal cord similarly to how we did with the peripheral nerves. 00:06:06.895 --> 00:06:09.820 The longitudinal section looks familiar but different, 00:06:09.820 --> 00:06:13.000 but the transverse cross-section is super unique. 00:06:13.000 --> 00:06:18.670 This cross section is this diagram, or what I call the butterfly pancake view. 00:06:18.670 --> 00:06:21.055 At the tissue level, let's see what we're working with. 00:06:21.055 --> 00:06:22.795 There are two different colors to work with, 00:06:22.795 --> 00:06:24.430 which come from myelin status. 00:06:24.430 --> 00:06:27.295 Since those myelin sheaths are so fatty and fluffy, 00:06:27.295 --> 00:06:31.645 think of myelinated fibers like marshmallows that make up white matter, 00:06:31.645 --> 00:06:35.770 while those dense, slow, unmyelinated fibers are the metallic 00:06:35.770 --> 00:06:38.935 skewers that poke through them, making up the gray matter. 00:06:38.935 --> 00:06:40.690 Look, I know that sounds backwards. 00:06:40.690 --> 00:06:42.835 The darker color should be gray matter, right? 00:06:42.835 --> 00:06:44.500 But I don't make the rules. 00:06:44.500 --> 00:06:45.570 Take it up with management. 00:06:45.570 --> 00:06:48.055 Since the gray matter is arranged into this shape, 00:06:48.055 --> 00:06:50.500 we label those segments horns, 00:06:50.500 --> 00:06:54.220 and we have anterior, lateral, and dorsal horns. 00:06:54.220 --> 00:06:56.650 But there's another big component to the central nervous 00:06:56.650 --> 00:06:59.365 system: the brain. Before we get to neurons, 00:06:59.365 --> 00:07:02.635 we have a few layers of connective tissue called the meninges. 00:07:02.635 --> 00:07:04.225 If you've heard of the disease 00:07:04.225 --> 00:07:07.195 meningitis, it's inflammation of these layers. 00:07:07.195 --> 00:07:09.790 The most superficial layer is the dura mater, 00:07:09.790 --> 00:07:13.165 a layer of dense connective tissue that sticks to the skull. 00:07:13.165 --> 00:07:15.310 Deeper than that is the arachnoid layer, 00:07:15.310 --> 00:07:17.230 which is thin and looks like spider webs, 00:07:17.230 --> 00:07:21.820 hence the name, and connects to the delicate thin pia mater underneath. 00:07:21.820 --> 00:07:24.400 Aside from some connective tissue around blood vessels, 00:07:24.400 --> 00:07:28.105 all the other structures of the brain can be classified as nervous tissue. 00:07:28.105 --> 00:07:31.300 But like I said, layers. Let's look at these two different 00:07:31.300 --> 00:07:34.010 colors, since their tissue level anatomy is different. 00:07:34.010 --> 00:07:37.420 The outermost layer of the cerebrum is the cerebral cortex, 00:07:37.420 --> 00:07:40.240 and deeper than that, the subcortical white matter. 00:07:40.240 --> 00:07:43.195 The cerebral cortex only has six layers of its own, 00:07:43.195 --> 00:07:46.780 and luckily, only a couple of cell types to differentiate between, 00:07:46.780 --> 00:07:49.045 the most common of which are pyramidal neurons, 00:07:49.045 --> 00:07:51.610 named because sure, they look like pyramids, 00:07:51.610 --> 00:07:54.400 I guess. But they're also easy to spot because they 00:07:54.400 --> 00:07:57.805 stay in a dark blue color and have a really big nuclei. 00:07:57.805 --> 00:08:00.640 This is really just the tip of the iceberg with neuroanatomy. 00:08:00.640 --> 00:08:02.905 But I didn't want to make this video overwhelming. 00:08:02.905 --> 00:08:05.305 If you need more help with histology in general though, 00:08:05.305 --> 00:08:08.335 I've made a bunch of videos that you can find in a playlist right here. 00:08:08.335 --> 00:08:11.070 As always, thank you to my patrons on Patreon. 00:08:11.070 --> 00:08:12.930 If you haven't, I'd appreciate it if you 00:08:12.930 --> 00:08:16.050 subscribe and hit the bell so you get notified when I post a new video. 00:08:16.050 --> 00:08:19.300 Have fun, be good. Thanks for watching.