Name: 自主神經系統。速成班A&P #13 (Autonomic Nervous System: Crash Course A&P #13)
Uploaded: 2021-01-14T05:32:14.000Z
Duration: 8 min 49 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

情態副詞

No matter what you’re a fan of, you can probably think of two characters who fit the

description. Holmes and Watson. Brennan and Booth. Kirk and Spock. Ron and Hermione.

程度副詞

They’re close. They understand each other in ways that others don’t, and you really

can’t picture one without the other. But when when happens, they usually react in different ways.

Like, if you’re snooping around Hogwarts at night, and you come across some beastie,

the smart money’s on Hermione to come up with a spell that saves the day, while Ron

probably standin’ there with his mouth open.

And when Sherlock starts to go crazy because there hasn’t been a fiendishly clever jewel

heist or whatever for him to figure out, you can count on Watson to calm him down. Mostly.

My brother John, who is a writer of books, will tell you that these characters are foils

-- they’re not opposites, exactly. They’re contrasts, and the more you see them together,

the more they seem to complement or round one another out.

And that is basically your autonomic nervous system in a nutshell.

Your autonomic system is the branch of your peripheral nervous system that regulates the

functions of your internal organs, like your heart and stomach, and also controls your

smooth and cardiac muscles, and your glands.

All things that you you do not consciously control, so, yes, you could say it has a lot of power over you.

But the funny -- or maybe confusing -- thing about this system is that its effects on your

organs and muscles and glands are by no means consistent.

At any given moment, whether you happen to be totally relaxed or completely flipping

your wig, your autonomic system is constantly making involuntary, fine-tuned adjustments

to your body, based on what signals your central nervous system is picking up.

This could mean changing your body temperature, sending extra blood to a particular area,

slowing your heart beat, or tweaking your stomach secretions.

Its effects change, depending on the situation you’re in -- and also which part of your

autonomic system is in charge at that moment.

Because this weird little corner of your nervous system that keeps you alive is actually run

Two divisions that serve the same organs, but they create opposite effects in them,

battling it out back and forth, to either excite your body’s functions or subdue them.

One of them is dedicated to amping you up and preparing you for activity -- that’s

your sympathetic nervous system. And the other one talks you down and effectively undoes

what its foil did. And that is the parasympathetic nervous system.

Together, they’re what make your body experience stress, fear, relaxation, and defiance. Courage

If there’s an epic novel going on in your body right now it is probably being written by these two.

One of the two divisions of your autonomic nervous system is called the sympathetic system.

That sounds kinda nice, doesn’t it? It’s like understanding, and calming, and telling

WRONG! Contrary to its comforting name, the sympathetic system is what sounds your internal

alarm bells. It’s the hardware behind the famous “fight or flight” response. It

If the sympathetic is for “fight or flight,” the parasympathetic is for “resting and

digesting” -- it’s responsible for maintaining your body and conserving energy for later.

But, when you explore the anatomy of these two systems, like we’re gonna do today,

Because, even though their basic components are essentially the same, their physical structures

turn out to be different in a few really important ways. And those differences can help explain

why they act like the foils that they are, and why sometimes you feel more like Sherlock

First big difference: the nerves of these two divisions originate at different sites in your body.

Your sympathetic fibers are thoracolumbar -- meaning that they originate from between

your thoracic vertebrae where your ribs attach, and the lumbar vertebrae just inferior to your ribs.

Early anatomists saw how a network of nerves radiating from the middle of the spine like

this could quickly coordinate the functions of many major organs at once. So it was called

the sympathetic system, from the Greek words for “feeling together.”

But the nerve fibers of your parasympathetic system begin both above and below where

They’re craniosacral, meaning they sprout from the base of your brain and also from

your sacral spinal cord, just superior to the tailbone.

And because the roots of these nerve fibers basically frame the starting points of the

sympathetic nerves, they were called parasympathetic -- literally “beside the sympathetic.”

Another difference between these two foils? Their ganglia.

Unlike your sensory or motor neurons, where a single axon can reach all the way from your

spinal cord to whatever muscle or touch receptor it works with, both parts of your autonomic

system require two neurons in order to work.

And those two neurons meet in ganglia -- clusters of neuron cell bodies that house millions of synapses.

But where these ganglia appear relate to  their function, and which division of the

Sympathetic ganglia are found closer to the spinal cord, because in those fight-or-flight

moments of high excitement or activity, they need to be able to send a single message far

This way, excitatory signals traveling into a ganglion near the spine -- ganglion being

the singular of ganlia -- can trigger action potentials in a whole bunch of other neurons

that lead to many different effectors, like the heart, and lungs, and stomach, and adrenal glands.

By contrast, most parasympathetic ganglia are found way out from the spine -- near,

Because this system is responsible for taking care of particular functions only when you

have the time and energy to do it -- like digesting food or excreting waste -- it uses

It’s more like Commissioner Gordon calling Batman on the batphone, one on one just to

talk about how things are going and you know whether Alfred’s doing OK after his meningitis.

It’s a private conversation -- not everybody needs to be involved.

Anyway, because the ganglia of these two divisions appear in different places in your body, it

also makes sense that their neurons themselves have slightly different forms, namely the length of their axons.

Now, ganglia can be kind of complex -- it actually comes from the Greek word for “a

knot in a string” -- so when dealing with neurons around these structures, we look at

the fibers before they run into the ganglion, as well as after they come out of it.

Understandably enough, the axon lengths of the neurons before the ganglion are called

the preganglionic fibers, and the ones coming out are postganglionic.

The key here is that, in the sympathetic system, the preganglionic fibers are much shorter

Which makes sense, when you think about it, because sympathetic ganglia are really close

to the spinal cord, and the axons don’t have, or need, very far to go from the central

nervous system. But they do have a lot of distance to cover, on the other side of the

ganglion, in order to reach their effectors. So naturally the fibers leading out of the ganglia are a lot longer.

And, foils being what they are, the reverse is of course true for the parasympathetic system.