1 00:00:00,000 --> 00:00:01,950 PATRICK: With cells named after stars and 2 00:00:01,950 --> 00:00:04,740 neurons that branch out like bare trees in the winter, 3 00:00:04,740 --> 00:00:07,440 the nervous system is beautiful under a microscope, 4 00:00:07,440 --> 00:00:10,275 but sometimes it looks like a cluttered mess. 5 00:00:10,275 --> 00:00:12,030 In this video, I'll teach you how to look at 6 00:00:12,030 --> 00:00:15,180 the histology of the different types of nervous system cells, 7 00:00:15,180 --> 00:00:18,120 so you can appreciate what you see under a microscope. 8 00:00:18,120 --> 00:00:19,815 If you're new to the channel, welcome. 9 00:00:19,815 --> 00:00:21,420 My name is Patrick and this channel is all 10 00:00:21,420 --> 00:00:23,490 about anatomy and how we learn about it. 11 00:00:23,490 --> 00:00:25,500 As always, I have the accompanying notes for 12 00:00:25,500 --> 00:00:28,005 this video linked in the description if you want to check those out. 13 00:00:28,005 --> 00:00:29,505 Otherwise, let's get started. 14 00:00:29,505 --> 00:00:32,370 Our biggest challenge in learning nervous system histology is 15 00:00:32,370 --> 00:00:35,835 figuring out big picture anatomy from microscopic anatomy. 16 00:00:35,835 --> 00:00:38,325 But we can use some clues to help us out. 17 00:00:38,325 --> 00:00:41,085 We can split the nervous system into the central nervous system, 18 00:00:41,085 --> 00:00:44,160 which includes the brain and spinal cord, and the peripheral 19 00:00:44,160 --> 00:00:47,040 nervous system, pretty much all the nerves that branch out 20 00:00:47,040 --> 00:00:49,640 from that. We will see some different structures and cell 21 00:00:49,640 --> 00:00:52,670 types depending on where we look. But the overall purpose 22 00:00:52,670 --> 00:00:56,520 of the nervous system is to send and receive electrical signals. 23 00:00:56,520 --> 00:00:58,835 That helps us deduce the anatomy of interest. 24 00:00:58,835 --> 00:01:01,735 Like the power lines that send electricity through a city, 25 00:01:01,735 --> 00:01:05,225 each nerve is made of clusters of smaller neuron cells. 26 00:01:05,225 --> 00:01:08,525 Each of these little circles are part of individual neurons. 27 00:01:08,525 --> 00:01:11,075 If we took a transverse cross-section of a nerve, 28 00:01:11,075 --> 00:01:14,300 we'd get a slide like this, just like this electrical cable. 29 00:01:14,300 --> 00:01:18,080 When we slice a nerve long ways for a longitudinal view, 30 00:01:18,080 --> 00:01:21,410 we see the long axons running the length of the nerve. 31 00:01:21,410 --> 00:01:24,860 While a picture like this nerve cross-section seems 32 00:01:24,860 --> 00:01:27,980 overly busy at first, see it for what it is: 33 00:01:27,980 --> 00:01:31,240 neurons and the tissue that wraps them into little bundles. 34 00:01:31,240 --> 00:01:34,700 If you're already familiar with the connective tissue around muscle bundles, 35 00:01:34,700 --> 00:01:37,260 then the naming conventions are going to come easily for nerves. 36 00:01:37,260 --> 00:01:39,800 Remember how from muscles you have the perimysium, 37 00:01:39,800 --> 00:01:44,360 epimysium, and endomysium? In nerves, you keep the same prefixes, 38 00:01:44,360 --> 00:01:48,665 but instead of mysium for muscle, you have neurium for nerves. 39 00:01:48,665 --> 00:01:51,200 The outermost layer is the epineurium, 40 00:01:51,200 --> 00:01:54,160 a layer of dense, irregular connective tissue. 41 00:01:54,160 --> 00:01:57,210 Then each bundle, or fascicle, is wrapped 42 00:01:57,210 --> 00:02:00,330 in a thinner connective tissue called the perineurium, 43 00:02:00,330 --> 00:02:03,240 while each neuron cell and all of its accessories 44 00:02:03,240 --> 00:02:06,720 are wrapped in endoneurium. In this slide, you can 45 00:02:06,720 --> 00:02:08,700 clearly see the dense tissue wrapping 46 00:02:08,700 --> 00:02:12,105 up the neuron bundle here—that's the perineurium— 47 00:02:12,105 --> 00:02:15,420 while each light-colored neuron has a dark ring around it, 48 00:02:15,420 --> 00:02:17,445 that's the endoneurium. 49 00:02:17,445 --> 00:02:21,000 The cell that lives inside that connective tissue is called a neuron. 50 00:02:21,000 --> 00:02:24,750 We're only looking at a small section of it on a cross-section view. 51 00:02:24,750 --> 00:02:27,525 I'm guessing you've seen a picture that looks like this before. 52 00:02:27,525 --> 00:02:31,770 The cliche illustrated neuron with all the tidy pieces in it. 53 00:02:31,770 --> 00:02:35,810 On this slide, we're looking at this small section of the neuron, 54 00:02:35,810 --> 00:02:38,570 but a bunch of them. That's because actual neuron cells can 55 00:02:38,570 --> 00:02:41,975 be really long and impossible to fit under a microscope slide. 56 00:02:41,975 --> 00:02:43,640 But depending on where we're looking, 57 00:02:43,640 --> 00:02:46,175 we can identify different pieces of them. 58 00:02:46,175 --> 00:02:49,310 This big boy is called the cell body, or soma, 59 00:02:49,310 --> 00:02:50,855 which has a nucleus inside. 60 00:02:50,855 --> 00:02:54,605 We need to remember that, as cool and specialized as these cells are, 61 00:02:54,605 --> 00:02:58,070 neurons are still cells with DNA and organelles. 62 00:02:58,070 --> 00:03:01,055 Branching out from there are any number of dendrites, 63 00:03:01,055 --> 00:03:04,295 branches that collect electrical impulses from other cells. 64 00:03:04,295 --> 00:03:07,490 They sum up here at the axon hillock, where an impulse will 65 00:03:07,490 --> 00:03:11,860 travel down the axon, this long piece here. The axon can be 66 00:03:11,860 --> 00:03:16,915 over 95% of the volume of the neuron cell and they can be long, 67 00:03:16,915 --> 00:03:20,890 like over a meter long. These axons are what we just cut open on the 68 00:03:20,890 --> 00:03:24,400 cross-section and most of what we see on longitudinal sections. 69 00:03:24,400 --> 00:03:27,490 Finally, the neuron ends at the axon terminals, 70 00:03:27,490 --> 00:03:30,760 these tiny branches here. They send messages in the 71 00:03:30,760 --> 00:03:34,255 form of neurotransmitters to other cells through synapses. 72 00:03:34,255 --> 00:03:36,685 But that's the Platonic model of a neuron. 73 00:03:36,685 --> 00:03:40,465 In reality, neurons are one of the most diverse cell types in the body. 74 00:03:40,465 --> 00:03:43,870 Some axons are thin, bare cables, while some have a squishy 75 00:03:43,870 --> 00:03:46,540 layer around them that helps them transmit signals faster. 76 00:03:46,540 --> 00:03:48,260 It's called the myelin sheath. 77 00:03:48,260 --> 00:03:51,430 We say that those neurons are myelinated. 78 00:03:51,430 --> 00:03:54,940 The neurons we saw in longitudinal view are really myelin 79 00:03:54,940 --> 00:03:58,405 with axons inside. In this cross-section view, 80 00:03:58,405 --> 00:04:01,960 you can see the tiny axon with the marshmallowy myelin all 81 00:04:01,960 --> 00:04:04,600 around it and endoneurium around that. 82 00:04:04,600 --> 00:04:08,260 Length and diameters can change too. Like, the myelinated 83 00:04:08,260 --> 00:04:13,300 type 1a fibers are anywhere from 4 to 20 micrometers wide. 84 00:04:13,300 --> 00:04:17,800 Type B fibers are 1–4 micrometers wide, while the unmyelinated 85 00:04:17,800 --> 00:04:23,005 type C fibers are only 0.2–1.5 micrometers wide. 86 00:04:23,005 --> 00:04:25,610 The wider and more myelinated the neuron, 87 00:04:25,610 --> 00:04:28,275 the faster it transmits electrical impulses. 88 00:04:28,275 --> 00:04:33,620 Like those type 1A's send signals at 70–120 meters a second, 89 00:04:33,620 --> 00:04:38,135 while type C conducts at 0.5–2.5 meters per second. 90 00:04:38,135 --> 00:04:40,265 That's a pretty big difference in size and speed. 91 00:04:40,265 --> 00:04:42,050 Not only can axons vary, 92 00:04:42,050 --> 00:04:43,940 but the branching pattern can vary too. 93 00:04:43,940 --> 00:04:47,385 The most common type of neuron is a multipolar neuron. 94 00:04:47,385 --> 00:04:51,880 It has one axon and a cell body with a bunch of branching dendrites. 95 00:04:51,880 --> 00:04:55,030 You'll usually spot these on the brain and spinal cord. 96 00:04:55,030 --> 00:04:57,190 Meanwhile, bipolar neurons have 97 00:04:57,190 --> 00:05:01,260 a long axon and a single dendritic tree poking out the other end. 98 00:05:01,260 --> 00:05:04,960 You only see these in certain sensory systems, like the nose and 99 00:05:04,960 --> 00:05:09,100 retina, since they only send afferent, or sensory, information. 100 00:05:09,100 --> 00:05:12,025 Finally, unipolar neurons are what they sound like. 101 00:05:12,025 --> 00:05:15,520 They have a cell body and a single axon, no dendrites. 102 00:05:15,520 --> 00:05:18,425 But neurons aren't the only type of cell in the nervous system. 103 00:05:18,425 --> 00:05:21,870 We also have glial cells, essentially supportive cells. 104 00:05:21,870 --> 00:05:25,295 For instance, astrocytes or star-shaped cells, 105 00:05:25,295 --> 00:05:29,210 support and protect our neurons by regulating the blood-brain barrier, 106 00:05:29,210 --> 00:05:32,975 helping form synapses, and clearing excess neurotransmitters. 107 00:05:32,975 --> 00:05:35,510 They're hard to see with traditional light microscopes, 108 00:05:35,510 --> 00:05:37,595 so unless you have an electron microscope, 109 00:05:37,595 --> 00:05:39,095 you probably won't get quizzed on it. 110 00:05:39,095 --> 00:05:42,160 Oligodendrocytes are another fun one. They help make the 111 00:05:42,160 --> 00:05:45,280 myelin sheath around neurons in the brain and spinal cord, 112 00:05:45,280 --> 00:05:48,280 while Schwann cells make the myelin in the peripheral nerves. 113 00:05:48,280 --> 00:05:51,580 Quick summary: this all started with our bundles of neurons 114 00:05:51,580 --> 00:05:55,180 organized into peripheral nerves like electrical wires in a cable. 115 00:05:55,180 --> 00:05:58,090 But we still have some big deal nervous tissue to tackle, 116 00:05:58,090 --> 00:06:01,430 the central nervous system, including brain and spinal cord. 117 00:06:01,430 --> 00:06:03,460 Luckily for us, we can get our bearings with 118 00:06:03,460 --> 00:06:06,895 the spinal cord similarly to how we did with the peripheral nerves. 119 00:06:06,895 --> 00:06:09,820 The longitudinal section looks familiar but different, 120 00:06:09,820 --> 00:06:13,000 but the transverse cross-section is super unique. 121 00:06:13,000 --> 00:06:18,670 This cross section is this diagram, or what I call the butterfly pancake view. 122 00:06:18,670 --> 00:06:21,055 At the tissue level, let's see what we're working with. 123 00:06:21,055 --> 00:06:22,795 There are two different colors to work with, 124 00:06:22,795 --> 00:06:24,430 which come from myelin status. 125 00:06:24,430 --> 00:06:27,295 Since those myelin sheaths are so fatty and fluffy, 126 00:06:27,295 --> 00:06:31,645 think of myelinated fibers like marshmallows that make up white matter, 127 00:06:31,645 --> 00:06:35,770 while those dense, slow, unmyelinated fibers are the metallic 128 00:06:35,770 --> 00:06:38,935 skewers that poke through them, making up the gray matter. 129 00:06:38,935 --> 00:06:40,690 Look, I know that sounds backwards. 130 00:06:40,690 --> 00:06:42,835 The darker color should be gray matter, right? 131 00:06:42,835 --> 00:06:44,500 But I don't make the rules. 132 00:06:44,500 --> 00:06:45,570 Take it up with management. 133 00:06:45,570 --> 00:06:48,055 Since the gray matter is arranged into this shape, 134 00:06:48,055 --> 00:06:50,500 we label those segments horns, 135 00:06:50,500 --> 00:06:54,220 and we have anterior, lateral, and dorsal horns. 136 00:06:54,220 --> 00:06:56,650 But there's another big component to the central nervous 137 00:06:56,650 --> 00:06:59,365 system: the brain. Before we get to neurons, 138 00:06:59,365 --> 00:07:02,635 we have a few layers of connective tissue called the meninges. 139 00:07:02,635 --> 00:07:04,225 If you've heard of the disease 140 00:07:04,225 --> 00:07:07,195 meningitis, it's inflammation of these layers. 141 00:07:07,195 --> 00:07:09,790 The most superficial layer is the dura mater, 142 00:07:09,790 --> 00:07:13,165 a layer of dense connective tissue that sticks to the skull. 143 00:07:13,165 --> 00:07:15,310 Deeper than that is the arachnoid layer, 144 00:07:15,310 --> 00:07:17,230 which is thin and looks like spider webs, 145 00:07:17,230 --> 00:07:21,820 hence the name, and connects to the delicate thin pia mater underneath. 146 00:07:21,820 --> 00:07:24,400 Aside from some connective tissue around blood vessels, 147 00:07:24,400 --> 00:07:28,105 all the other structures of the brain can be classified as nervous tissue. 148 00:07:28,105 --> 00:07:31,300 But like I said, layers. Let's look at these two different 149 00:07:31,300 --> 00:07:34,010 colors, since their tissue level anatomy is different. 150 00:07:34,010 --> 00:07:37,420 The outermost layer of the cerebrum is the cerebral cortex, 151 00:07:37,420 --> 00:07:40,240 and deeper than that, the subcortical white matter. 152 00:07:40,240 --> 00:07:43,195 The cerebral cortex only has six layers of its own, 153 00:07:43,195 --> 00:07:46,780 and luckily, only a couple of cell types to differentiate between, 154 00:07:46,780 --> 00:07:49,045 the most common of which are pyramidal neurons, 155 00:07:49,045 --> 00:07:51,610 named because sure, they look like pyramids, 156 00:07:51,610 --> 00:07:54,400 I guess. But they're also easy to spot because they 157 00:07:54,400 --> 00:07:57,805 stay in a dark blue color and have a really big nuclei. 158 00:07:57,805 --> 00:08:00,640 This is really just the tip of the iceberg with neuroanatomy. 159 00:08:00,640 --> 00:08:02,905 But I didn't want to make this video overwhelming. 160 00:08:02,905 --> 00:08:05,305 If you need more help with histology in general though, 161 00:08:05,305 --> 00:08:08,335 I've made a bunch of videos that you can find in a playlist right here. 162 00:08:08,335 --> 00:08:11,070 As always, thank you to my patrons on Patreon. 163 00:08:11,070 --> 00:08:12,930 If you haven't, I'd appreciate it if you 164 00:08:12,930 --> 00:08:16,050 subscribe and hit the bell so you get notified when I post a new video. 165 00:08:16,050 --> 00:08:19,300 Have fun, be good. Thanks for watching.