The Nervous System, Part 1: Crash Course Anatomy & Physiology #8 — Transcript

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1. 0:00This morning was a typical morning for me. I woke up thinking about that dream that I
2. 0:03keep having about the guy in the sloth suit, and then I got dressed because I was cold,
3. 0:07and then I made some toast with butter ‘cause I was hungry, and then I let the dog out ‘cause
4. 0:11she was whining and staring and me, and then I made some tea but I let it cool off before
5. 0:14I drank it because I burned my mouth yesterday.
6. 0:16In addition to being just part of my morning ritual, all of these actions are examples
7. 0:21of what my nervous system does for me.
8. 0:23The weirdo dream, the sensation of cold air and hot tea, deciding what to put on the toast,
9. 0:27going to the door at the sound of the dog -- all that was processed and executed by
10. 0:31electrical and chemical signals to and from nerve cells.
11. 0:34You can’t oversell the importance of the nervous system.
12. 0:38It controls ALL THE THINGS!
13. 0:40All your organs, all your physiological and psychological reactions, even your body’s
14. 0:44other major controlling force, the endocrine system, bows down before the nervous system.
15. 0:49There is no “you” without it. There is no “me” without it. There’s no dogs
16. 0:52without it. There’s no animals. There’s no -- there’s no things -- there’s things.
17. 0:57It’s important. That’s why we’re dedicating the next several episodes to the fundamentals
18. 1:00of the nervous system -- its anatomy and organization, how it communicates, and what happens when
19. 1:05it gets damaged.
20. 1:06This is mission control, people!
21. 1:18Even though pretty much all animals -- except super simple ones like sponges -- have a nervous
22. 1:22system, ours is probably the most distinctive feature of our species.
23. 1:26From writing novels, to debating time travel, to juggling knives -- all of your thoughts,
24. 1:31and actions, and emotions can be boiled down into three principal functions -- sensory
25. 1:36input, integration, and motor output.
26. 1:39Imagine a spider walking onto your bare knee.
27. 1:41The sensory receptors on your skin detect those eight little legs -- that information
28. 1:46is your sensory input.
29. 1:47From there your nervous system processes that input, and decides what should be done about
30. 1:51it. That’s called integration -- like, should I be all zen about it and just let it walk
31. 1:55over me, or should I not be zen and freak out and run around screaming, “SPIDER!”?
32. 2:01Your hand lashing out to remove the spider, and maybe your accompanying banshee scream,
33. 2:05is the motor output -- the response that occurs when your nervous system activates certain
34. 2:09parts of your body.
35. 2:10As you can imagine, it takes a highly integrated system to detect, process, and act on data
36. 2:14like this, all the time.
37. 2:16And when we talk about the nervous system, we’re really talking about several levels
38. 2:19of organization, starting with two main parts: the central and peripheral nervous systems.
39. 2:24The central nervous system is your brain and spinal cord -- the main control center. It’s
40. 2:29what decided to remove the spider, and gave the order to your hand.
41. 2:32Your peripheral system is composed of all the nerves that branch off from the brain
42. 2:36and spine that allow your central nervous system to communicate with the rest of your body.
43. 2:40And since its job is communication, your peripheral system is set up to work in both directions:
44. 2:44The sensory, or afferent division is what picks up sensory stimuli -- like, “hey,
45. 2:49there’s an arachnid on you” -- and slings that information to the brain.
46. 2:51Your motor, or efferent division is the part that sends directions from your brain to the
47. 2:55muscles and glands -- like, “hey hand part, how ‘bout you do something about that spider.”
48. 3:00The motor division also includes the somatic, or voluntary nervous system, that rules your
49. 3:04skeletal muscle movement, and the autonomic, or involuntary nervous system, that keeps
50. 3:08your heart beating, and your lungs breathing, and your stomach churning.
51. 3:11And finally, that autonomic system, too, has its own complementary forces. Its sympathetic
52. 3:15division mobilizes the body into action and gets it all fired up, like “Gah! SPIDER!”
53. 3:20-- while the parasympathetic division relaxes the body and talks it down… Like, “it
54. 3:24wasn’t a black widow or anything; you’re fine, breathe!”
55. 3:26So that’s the organization of your nervous system in a nutshell. But no matter what part
56. 3:30you’re talking about, they’re all made up of mainly nervous tissue, which you’ll
57. 3:34remember is densely packed with cells.
58. 3:36Maybe less than 20 percent of that tissue consists of extracellular space. Everything else? Cells.
59. 3:41The type of cells you’ve most likely heard of are the neurons, or nerve cells, which
60. 3:45respond to stimuli and transmit signals.
61. 3:47These cells get all the publicity -- they’re the ones that we’re always thanking every
62. 3:51time we ace an exam or think up a snappy comeback to an argument.
63. 3:54But these wise guys really account for just a small part of your nervous tissue because
64. 3:58they are surrounded and protected by gaggles of neuroglia, or glial cells.
65. 4:03Once considered just the scaffolding or glue that held neurons together, we now know that
66. 4:07our different glial cell types serve many other important functions, and they make up
67. 4:12about half of the mass of your brain, outnumbering their neuron colleagues by about 10 to 1.
68. 4:18Star-shaped astrocytes are found in your central nervous system and are your most abundant
69. 4:21and versatile glial cells. They anchor neurons to their blood supply, and govern the exchange
70. 4:27of materials between neurons and capillaries.
71. 4:29Also in your central nervous system are your protective microglial cells -- they’re smaller
72. 4:34and kinda thorny-looking, and act as the main source of immune defense against invading
73. 4:37microorganisms in the brain and spinal cord.
74. 4:40Your ependymal cells line cavities in your brain and spinal cord and create, secrete,
75. 4:44and circulate cerebrospinal fluid that fills those cavities and cushions those organs.
76. 4:48And finally your central nervous system’s oligodendrocytes wrap around neurons, producing
77. 4:53an insulating barrier called the myelin sheath.
78. 4:56Now, over in your peripheral nervous system, there are just two kinds of glial cells. Satellite
79. 5:00cells do mainly in the peripheral system what astrocyte cells do in the central system -- they
80. 5:05surround and support neuron cell bodies. While Schwann cells are similar to your oligodendrocytes,
81. 5:10in that they wrap around axons and make that insulating myelin sheath.
82. 5:14So don’t sell your glial cells short -- they’re in the majority, cell-wise. But of course
83. 5:18when it comes to passing tests and winning arguments, most of the heavy lifting is done
84. 5:22by the neurons. And they’re not all the same -- they’re actually highly specialized,
85. 5:26coming in all shapes and sizes -- from tiny ones in your brain to the ones that run the
86. 5:30entire length of your leg.
87. 5:32But they do all share three super-cool things in common.
88. 5:35Number 1. They’re some of the longest-lived cells in your body. There’s a lot of debate
89. 5:39right now about whether you’re actually born with all of the neurons you’ll ever
90. 5:43have, but some research suggests that, at least in your brain’s cerebral cortex, your
91. 5:47neurons will live as long as you do.
92. 5:49Cool fact number 2. They are irreplaceable. It’s a good thing that they have such longevity,
93. 5:54because your neurons aren’t like your constantly- renewing skin cells. Most neurons are amitotic, so
94. 5:59once they take on their given roles in the nervous system, they lose their ability to
95. 6:02divide. So take care of ‘em!
96. 6:04And number 3. They have huge appetites. Like a soccer-playing teenager, neurons have a
97. 6:10crazy-high metabolic rate. They need a steady and abundant supply of glucose and oxygen,
98. 6:15and about 25 percent of the calories that you take in every day are consumed by your
99. 6:20brain’s activity.
100. 6:21Along with all these wonderful qualities, your neurons also share the same basic structure.
101. 6:25The soma, or cell body, is the neuron’s life support. It’s got all the normal cell
102. 6:29goodies like a nucleus, and DNA, mitochondria, ribosomes, cytoplasm.
103. 6:33The bushy, branch-like things projecting out from the soma are dendrites. They’re the
104. 6:37listeners -- they pick up messages, news, gossip from other cells and convey that information to the cell body.
105. 6:43The neuron’s axon, meanwhile, is like the talker. This long extension, or fiber, can
106. 6:47be super short, or run a full meter from your spine down to your ankle. We’ve got a few
107. 6:52different axon layouts in our body, but in the most abundant type of neuron, the axons
108. 6:56transmit electrical impulses away from the cell body to other cells.
109. 7:00For us students of biology, it’s a good thing that nerve cells aren’t all identical.
110. 7:04Because their differences in structure are one of the ways that we tell them apart, and classify them.
111. 7:08The main feature we look at is how many processes extend out from the cell body.
112. 7:12A “process” in this case being a projecting part of an organic structure.
113. 7:1799 percent of all your neurons are multipolar neurons, with three or more processes sticking
114. 7:22out from the soma -- including one axon, and a bunch of dendrites.
115. 7:25Bipolar neurons have two processes -- an axon and a single dendrite -- extending from opposite
116. 7:31sides of the cell body. They’re pretty rare, found only in a few special sensory places,
117. 7:35like the retina of your eye.
118. 7:36Unipolar neurons, on the other hand, have just one process, and are found mostly in
119. 7:40your sensory receptors.
120. 7:41So, if you ever find yourself probing around someone’s nervous tissue, remember these
121. 7:44three terms to help you figure out what you’re looking at.
122. 7:47But because we’re talking physiology here as well as anatomy, we have to classify these
123. 7:51cells in terms of their function, and that basically comes down to which way an impulse
124. 7:55travels through a neuron in relation to the brain and spine.
125. 7:58Our sensory, or afferent, neurons pick up messages and transmit impulses from sensory
126. 8:03receptors in say, the skin or internal organs, and send them toward the central nervous system.
127. 8:08Most sensory neurons are unipolar.
128. 8:10Motor, or efferent, neurons do the opposite -- they’re mostly multipolar, and transmit
129. 8:14impulses away from the central nervous system and out to your body’s muscles and glands.
130. 8:19And then there are interneurons, or association neurons, which live in the central nervous
131. 8:24system and transmit impulses between those sensory and motor neurons. Interneurons are
132. 8:28the most abundant of your body’s neurons and are mostly multipolar.
133. 8:31OK! It’s applied knowledge time! Let’s review everything we’ve learned so far in
134. 8:35terms of that spider on your knee.
135. 8:37Those eight creeping legs first activate your unipolar sensory neurons in the skin on your
136. 8:41knee, when they sense something crawling on you. The signal travels up an axon wrapped
137. 8:46in Schwann cells and into your spinal cord, where it gets passed on to several multipolar interneurons.
138. 8:52Now, some of those interneurons might send a signal straight down a bunch of multipolar
139. 8:55neurons to your quadriceps muscle on your thigh, triggering you to kick your leg out
140. 9:00before you even know what’s going on.
141. 9:02Other interneurons will pass that signal to neurons that carry it up your spinal cord to your brain.
142. 9:06That’s where your body first recognizes that thing as a spider, and the connections
143. 9:10between neurons interpret and split the signal so that you can either scream, and start swinging
144. 9:15your arms wildly about...or...remain calm, and with dignity remove the spider from your person.
145. 9:21It’s all based on the connections between neurons.
146. 9:23Which brings me to a whole new question: How?
147. 9:26How in the name of Jean-Martin Charcot do nerve cells use chemistry and electricity
148. 9:32to communicate with each other?
149. 9:33It’s one of the most stupifyingly awesome and complicated aspects of your nervous system,
150. 9:37and basically of all life and it is what we will cover in our next lesson.
151. 9:41Today you learned how sensory input, integration, and motor output of your nervous system basically
152. 9:46rules your world. We talked about how the central and peripheral systems are organized,
153. 9:50and what they do, and looked at the role of different glial cells in nervous tissue function.
154. 9:55We also looked at the role, anatomy, and function of neuron types in the body, both structurally
155. 10:00and functionally, and how everything plays out when you find a spider crawling on your skin.
156. 10:05Thank you for watching, especially to all of our Subbable subscribers, who make Crash
157. 10:08Course possible for themselves and for the whole rest of the world. To find out how you
158. 10:12can become a supporter, just go to subbable.com.
159. 10:14This episode was written by Kathleen Yale, the script was edited by Blake de Pastino,
160. 10:17and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael
161. 10:22Aranda, and our graphics team is Thought Café.