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  • This morning was a typical morning for me. I woke up thinking about that dream that I

  • keep having about the guy in the sloth suit, and then I got dressed because I was cold,

  • and then I made some toast with buttercause I was hungry, and then I let the dog outcause

  • she was whining and staring and me, and then I made some tea but I let it cool off before

  • I drank it because I burned my mouth yesterday.

  • In addition to being just part of my morning ritual, all of these actions are examples

  • of what my nervous system does for me.

  • The weirdo dream, the sensation of cold air and hot tea, deciding what to put on the toast,

  • going to the door at the sound of the dog -- all that was processed and executed by

  • electrical and chemical signals to and from nerve cells.

  • You can’t oversell the importance of the nervous system.

  • It controls ALL THE THINGS!

  • All your organs, all your physiological and psychological reactions, even your body’s

  • other major controlling force, the endocrine system, bows down before the nervous system.

  • There is noyouwithout it. There is nomewithout it. There’s no dogs

  • without it. There’s no animals. There’s no -- there’s no things -- there’s things.

  • It’s important. That’s why were dedicating the next several episodes to the fundamentals

  • of the nervous system -- its anatomy and organization, how it communicates, and what happens when

  • it gets damaged.

  • This is mission control, people!

  • Even though pretty much all animals -- except super simple ones like sponges -- have a nervous

  • system, ours is probably the most distinctive feature of our species.

  • From writing novels, to debating time travel, to juggling knives -- all of your thoughts,

  • and actions, and emotions can be boiled down into three principal functions -- sensory

  • input, integration, and motor output.

  • Imagine a spider walking onto your bare knee.

  • The sensory receptors on your skin detect those eight little legs -- that information

  • is your sensory input.

  • From there your nervous system processes that input, and decides what should be done about

  • it. That’s called integration -- like, should I be all zen about it and just let it walk

  • over me, or should I not be zen and freak out and run around screaming, “SPIDER!”?

  • Your hand lashing out to remove the spider, and maybe your accompanying banshee scream,

  • is the motor output -- the response that occurs when your nervous system activates certain

  • parts of your body.

  • As you can imagine, it takes a highly integrated system to detect, process, and act on data

  • like this, all the time.

  • And when we talk about the nervous system, were really talking about several levels

  • of organization, starting with two main parts: the central and peripheral nervous systems.

  • The central nervous system is your brain and spinal cord -- the main control center. It’s

  • what decided to remove the spider, and gave the order to your hand.

  • Your peripheral system is composed of all the nerves that branch off from the brain

  • and spine that allow your central nervous system to communicate with the rest of your body.

  • And since its job is communication, your peripheral system is set up to work in both directions:

  • The sensory, or afferent division is what picks up sensory stimuli -- like, “hey,

  • there’s an arachnid on you” -- and slings that information to the brain.

  • Your motor, or efferent division is the part that sends directions from your brain to the

  • muscles and glands -- like, “hey hand part, howbout you do something about that spider.”

  • The motor division also includes the somatic, or voluntary nervous system, that rules your

  • skeletal muscle movement, and the autonomic, or involuntary nervous system, that keeps

  • your heart beating, and your lungs breathing, and your stomach churning.

  • And finally, that autonomic system, too, has its own complementary forces. Its sympathetic

  • division mobilizes the body into action and gets it all fired up, likeGah! SPIDER!”

  • -- while the parasympathetic division relaxes the body and talks it downLike, “it

  • wasn’t a black widow or anything; youre fine, breathe!”

  • So that’s the organization of your nervous system in a nutshell. But no matter what part

  • youre talking about, theyre all made up of mainly nervous tissue, which youll

  • remember is densely packed with cells.

  • Maybe less than 20 percent of that tissue consists of extracellular space. Everything else? Cells.

  • The type of cells youve most likely heard of are the neurons, or nerve cells, which

  • respond to stimuli and transmit signals.

  • These cells get all the publicity -- theyre the ones that were always thanking every

  • time we ace an exam or think up a snappy comeback to an argument.

  • But these wise guys really account for just a small part of your nervous tissue because

  • they are surrounded and protected by gaggles of neuroglia, or glial cells.

  • Once considered just the scaffolding or glue that held neurons together, we now know that

  • our different glial cell types serve many other important functions, and they make up

  • about half of the mass of your brain, outnumbering their neuron colleagues by about 10 to 1.

  • Star-shaped astrocytes are found in your central nervous system and are your most abundant

  • and versatile glial cells. They anchor neurons to their blood supply, and govern the exchange

  • of materials between neurons and capillaries.

  • Also in your central nervous system are your protective microglial cells -- theyre smaller

  • and kinda thorny-looking, and act as the main source of immune defense against invading

  • microorganisms in the brain and spinal cord.

  • Your ependymal cells line cavities in your brain and spinal cord and create, secrete,

  • and circulate cerebrospinal fluid that fills those cavities and cushions those organs.

  • And finally your central nervous system’s oligodendrocytes wrap around neurons, producing

  • an insulating barrier called the myelin sheath.

  • Now, over in your peripheral nervous system, there are just two kinds of glial cells. Satellite

  • cells do mainly in the peripheral system what astrocyte cells do in the central system -- they

  • surround and support neuron cell bodies. While Schwann cells are similar to your oligodendrocytes,

  • in that they wrap around axons and make that insulating myelin sheath.

  • So don’t sell your glial cells short -- theyre in the majority, cell-wise. But of course

  • when it comes to passing tests and winning arguments, most of the heavy lifting is done

  • by the neurons. And theyre not all the same -- theyre actually highly specialized,

  • coming in all shapes and sizes -- from tiny ones in your brain to the ones that run the

  • entire length of your leg.

  • But they do all share three super-cool things in common.

  • Number 1. Theyre some of the longest-lived cells in your body. There’s a lot of debate

  • right now about whether youre actually born with all of the neurons youll ever

  • have, but some research suggests that, at least in your brain’s cerebral cortex, your

  • neurons will live as long as you do.

  • Cool fact number 2. They are irreplaceable. It’s a good thing that they have such longevity,

  • because your neurons aren’t like your constantly- renewing skin cells. Most neurons are amitotic, so

  • once they take on their given roles in the nervous system, they lose their ability to

  • divide. So take care ofem!

  • And number 3. They have huge appetites. Like a soccer-playing teenager, neurons have a

  • crazy-high metabolic rate. They need a steady and abundant supply of glucose and oxygen,

  • and about 25 percent of the calories that you take in every day are consumed by your

  • brain’s activity.

  • Along with all these wonderful qualities, your neurons also share the same basic structure.

  • The soma, or cell body, is the neuron’s life support. It’s got all the normal cell

  • goodies like a nucleus, and DNA, mitochondria, ribosomes, cytoplasm.

  • The bushy, branch-like things projecting out from the soma are dendrites. Theyre the

  • listeners -- they pick up messages, news, gossip from other cells and convey that information to the cell body.

  • The neuron’s axon, meanwhile, is like the talker. This long extension, or fiber, can

  • be super short, or run a full meter from your spine down to your ankle. Weve got a few

  • different axon layouts in our body, but in the most abundant type of neuron, the axons

  • transmit electrical impulses away from the cell body to other cells.

  • For us students of biology, it’s a good thing that nerve cells aren’t all identical.

  • Because their differences in structure are one of the ways that we tell them apart, and classify them.

  • The main feature we look at is how many processes extend out from the cell body.

  • A “processin this case being a projecting part of an organic structure.

  • 99 percent of all your neurons are multipolar neurons, with three or more processes sticking

  • out from the soma -- including one axon, and a bunch of dendrites.

  • Bipolar neurons have two processes -- an axon and a single dendrite -- extending from opposite

  • sides of the cell body. Theyre pretty rare, found only in a few special sensory places,

  • like the retina of your eye.

  • Unipolar neurons, on the other hand, have just one process, and are found mostly in

  • your sensory receptors.

  • So, if you ever find yourself probing around someone’s nervous tissue, remember these

  • three terms to help you figure out what youre looking at.

  • But because were talking physiology here as well as anatomy, we have to classify these

  • cells in terms of their function, and that basically comes down to which way an impulse

  • travels through a neuron in relation to the brain and spine.

  • Our sensory, or afferent, neurons pick up messages and transmit impulses from sensory

  • receptors in say, the skin or internal organs, and send them toward the central nervous system.

  • Most sensory neurons are unipolar.

  • Motor, or efferent, neurons do the opposite -- theyre mostly multipolar, and transmit

  • impulses away from the central nervous system and out to your body’s muscles and glands.

  • And then there are interneurons, or association neurons, which live in the central nervous

  • system and transmit impulses between those sensory and motor neurons. Interneurons are

  • the most abundant of your body’s neurons and are mostly multipolar.

  • OK! It’s applied knowledge time! Let’s review everything weve learned so far in

  • terms of that spider on your knee.

  • Those eight creeping legs first activate your unipolar sensory neurons in the skin on your

  • knee, when they sense something crawling on you. The signal travels up an axon wrapped

  • in Schwann cells and into your spinal cord, where it gets passed on to several multipolar interneurons.

  • Now, some of those interneurons might send a signal straight down a bunch of multipolar

  • neurons to your quadriceps muscle on your thigh, triggering you to kick your leg out

  • before you even know what’s going on.

  • Other interneurons will pass that signal to neurons that carry it up your spinal cord to your brain.

  • That’s where your body first recognizes that thing as a spider, and the connections

  • between neurons interpret and split the signal so that you can either scream, and start swinging

  • your arms wildly about...or...remain calm, and with dignity remove the spider from your person.

  • It’s all based on the connections between neurons.

  • Which brings me to a whole new question: How?

  • How in the name of Jean-Martin Charcot do nerve cells use chemistry and electricity

  • to communicate with each other?

  • It’s one of the most stupifyingly awesome and complicated aspects of your nervous system,

  • and basically of all life and it is what we will cover in our next lesson.

  • Today you learned how sensory input, integration, and motor output of your nervous system basically

  • rules your world. We talked about how the central and peripheral systems are organized,

  • and what they do, and looked at the role of different glial cells in nervous tissue function.

  • We also looked at the role, anatomy, and function of neuron types in the body, both structurally

  • and functionally, and how everything plays out when you find a spider crawling on your skin.

  • Thank you for watching, especially to all of our Subbable subscribers, who make Crash

  • Course possible for themselves and for the whole rest of the world. To find out how you

  • can become a supporter, just go to subbable.com.

  • This episode was written by Kathleen Yale, the script was edited by Blake de Pastino,

  • and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael

  • Aranda, and our graphics team is Thought Café.

This morning was a typical morning for me. I woke up thinking about that dream that I

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

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