Autonomic Nervous System: Crash Course A&P #13


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. 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. Not at all. 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 by two competing interests. 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 and cowardice. Panic and peace. If there’s an epic novel going on in your body right
now it is probably being written by these two. Let’s talk about names for a minute. 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 you that it’s not so bad after all. 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 is synonymous with stress. 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. I recognize that this is confusing. But, when you explore the anatomy of these
two systems, like we’re gonna do today, they start to make a little more sense. 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 than Watson, or the other way around. 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 the sympathetic ones do. 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 autonomic system they’re serving. 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 and wide, like the Bat Signal. 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, or even inside of their effector organs. 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 more specific, strategic signals. 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 than the postganglionic ones. 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. Since parasympathetic ganglia are so close
to, or even inside of, their effector organs, the preganglionic fibers are a lot longer. They extend from the cranium and sacrum where
they start, out to the lungs or liver or bladder — wherever their effector is — where they
reach their ganglion. From there, the postganglionic fibers are super short —
just long enough to communicate with their effector. So, once again — it’s anatomy and
physiology — the structure of each of these systems is related to its function. The sympathetic nervous system is set up in
such a way that even a small stress signal sent down one path could trigger a response
in many effectors at once. Which is one reason why your reaction to a
sudden, stressful event can feel so all-encompassing. By the same token, the resting and digesting
that’s overseen by the parasympathetic system doesn’t require urgent, all-hands-on deck
communication. If you need to process a burrito or take a nap or maybe a trip to the bathroom, it can
communicate with the organs involved, one on one. But still none of this tells us how these
systems do what they do — how these nerves communicate with your organs, and muscles,
and glands in times of either stress, or relaxation. We’ll start that next week, with a white-knuckle
ride through your sympathetic nervous system. So, between now and then, rest up. For now, you learned the basic two-part system
of your autonomic nervous system. Mainly that it consists of two primary, complementary
divisions, the sympathetic nervous system, which arouses your body, and the parasympathetic
nervous system, which is charge of resting, digesting, and repair. We also talked through the three main anatomical
differences between the two systems. Their nerve fibers originate in different parts
of the body, the ganglia of the sympathetic system are located close to the spinal cord,
while the ganglia of the parasympathetic system are close to their effectors. And finally, the nerve
fibers themselves have different structures. Announcing Thomas Frank, our new “Headmaster
of Learning,” whose generous contribution on Patreon helps keep Crash Course alive and
well for everyone. Thank you, Thomas. If you want to help us keep making great videos like
this one, you can check out Patreon.com/CrashCourse This episode of Crash Course was cosponsored by
Link, Kelly Naylor, Tim Webster, and Steven Meekel. This episode was written by Kathleen Yale,
edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. Our director is Nicholas
Jenkins, the script supervisor and editor is Nicole Sweeney, the sound designer is Michael
Aranda, and our graphics team is Thought Café.

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