Name: 神経系、その1。クラッシュコース A&P #8 (The Nervous System, Part 1: Crash Course A&P #8)
Uploaded: 2021-01-14T05:32:13.000Z
Duration: 10 min 36 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

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 butter ‘cause I was hungry, and then I let the dog out ‘cause

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

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.

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 no “you” without it. There is no “me” without 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 we’re dedicating the next several episodes to the fundamentals

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

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

Imagine a spider walking onto your bare knee.

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

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

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

And when we talk about the nervous system, we’re 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, how ‘bout 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, like “Gah! SPIDER!”

-- while the parasympathetic division relaxes the body and talks it down… Like, “it

wasn’t a black widow or anything; you’re fine, breathe!”

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

you’re talking about, they’re all made up of mainly nervous tissue, which you’ll

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

The type of cells you’ve most likely heard of are the neurons, or nerve cells, which

These cells get all the publicity -- they’re the ones that we’re 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 -- they’re 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 -- they’re 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 they’re not all the same -- they’re actually highly specialized,

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

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

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

right now about whether you’re actually born with all of the neurons you’ll ever

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

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

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

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