The Nervous System, Part 1: Crash Course A&P #8


Transcript Provided by YouTube:

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


This post was previously published on YouTube.

Photo credit: Screenshot from video.