Parasympathetic Nervous System: Crash Course A&P #15

Parasympathetic Nervous System: Crash Course A&P #15


Consider your heart for a moment. For the average person — at rest, like you
probably are, sitting there watching me — the heart beats at around 60 beats per minute. Once a second. Nice and easy. But if you were to somehow disconnect your
heart from your autonomic nervous system, things, as you might imagine, would change. But, your heart would not stop. Actually,
it would be the opposite. It would speed up. It would start beating at around 100 beats
per minute — and that’s just at rest. With your heart beating two-thirds faster
than normal, before you even broke a sweat, your cardiac muscle would experience a lot
of extra wear and tear. The surrounding blood vessels would be under enormous pressure.
And your body would suddenly require — and waste — a lot of energy. Basically, you’d be out of balance. Part of what keeps your heart under control
is your parasympathetic nervous system. It’s often described as the calming side
of your autonomic system — a kind of antidote to the effects of stress created by the sympathetic
system. But it’s really much more than that. Unlike your sympathetic division, which lets
you deal with the crisis of the now, the parasympathetic system allows your body to handle … everything
else. It not only calms you down after being stressed
out, it’s what allows you to digest food, to reproduce, to excrete waste, to fight off
infections. Basically, it lets you do the business of
living. But our bodies can only do that when they
are in balance, somewhere between excitement and inhibition, both aroused enough and calm
enough to keep things working. So the parasympathetic system is why our hearts
don’t pump so hard that they explode, sure. But it also explains a lot of other stuff
about our bodies. Oh, but just one thing? Learning about the parasympathetic system
is going to involve a lot of memorizing. Hope that doesn’t stress you out. You will recall that our sympathetic & parasympathetic
systems not only have different functions — more or less engaging the same organs to opposite
effects — they also have different structures. Their ganglia, for example, are located in
different places: The sympathetic ganglia are located near the spinal cord, while on the
parasympathetic side, they’re close to the effectors. And likewise, the use of neurotransmitters in the
two systems is similar, but not quite the same. In both systems, neurons release acetylcholine,
or ACh, in their preganglionic synapses. But in your parasympathetic system, the postganglionic
neurons release ACh at their synapses with the effector organs, too… …as opposed to in the sympathetic system,
where effectors get a dose of norepinephrine instead. But the biggest anatomical difference between
these two systems has to do with the physical networks that they form as they reach throughout
your body. While the sympathetic nerves all spring from
the thoracolumbar area of your spinal cord, right around your midsection, the nerves of
the parasympathetic division are craniosacral. And with the exception of a couple of sacral
nerves near the tailbone that run to the bladder and genitals and rectum, most of these nerves
never go through the spinal cord. Instead, they run right from the brain almost
all the way to their effectors. There are 12 of these cranial nerves, and they vary
in terms of what kinds of neurons they contain. I mean, we’re talking about the autonomic system
here, but they are not all autonomic motor fibers. Some of your cranial nerves also carry motor
fibers that control voluntary functions, like moving your eyeballs around. And others carry only sensory fibers, which
relay data to and from your sensory organs. And, you know, just to keep things interesting,
some of your cranial nerves carry both motor and sensory neurons. So, which ones are where? And what exactly
does each one of these 12 nerves do? As anatomists, we have to keep track of the
human wiring-diagram that are cranial nerves, because you don’t want to end up like some
sidekick in a ‘90s action movie who has to defuse a bomb all by himself. SHOULD I CUT THE RED WIRE OR THE BLACK WIRE? Honestly, though, if you find yourself inside
of somebody’s brain stem you probably shouldn’t be cutting anything. Since all 12 of these cranial nerves are important,
you’re gonna have to come up with some kind of mnemonics to help you keep track of both
their names and their functions. You’ll need to know what each one is called, whether
it’s a sensory nerve, a motor nerve, or both. And the map that we follow of the cranial
nerves is based on a ventral view of the brain — looking at its underside, with the anterior
portion at the top, and posterior on the bottom. First, let’s tackle the names. Starting at the top, the first cranial nerve
you encounter is the olfactory nerve, which takes scent information gathered by the nose
and sends it to the brain. Followed by the optic nerve, which does the
same, but with visual data. Then there’s oculomotor, which controls four of the six
muscles that control the movements of your eyes. The next nerve, near the center of the brain’s
ventral side, is the trochlear nerve, which controls a single muscle in the eye,
and it lets you do this. Just below that is the trigeminal nerve, the
largest of the cranial nerves, which branches into three main strands — hence the ‘tri’
— and innervates the face and jaw muscles. After that there’s the abducens, which stimulates
the muscles that let your eyes do this — from side to side, followed by the facial nerve,
which operates the muscles that make most facial expressions possible. Then there’s the auditory nerve. You can
probably guess what that’s for. You might notice that, up until the auditory
nerve, the cranial nerves mostly control organs in the front of the cranium — mainly the
eyes and facial muscles. But as you work your way down, the nerves
tend to innervate the lower and more posterior portions of the head. Like the glossopharyngeal nerve, which leads
to your tongue and your pharynx. That’s followed by your vagus nerve — you
should definitely remember that one — and then the spinal accessory nerve — which has
to do with moving your head and shoulders, and not whether your belt matches your shoes. Lastly there’s the hypoglossal, the nerve
that allows you to swallow and talk, among other things that you do with your mouth and
tongue. That was a lot of information and probably new
words, so how are you gonna remember it all? Well, by finding a way to remember the first
letter of each name, in order. Which is: O-O-O .. T-T … A-F-A … G-V-S-H. That doesn’t spell anything useful at all.
There is a mnemonic that you’ll probably hear in school that goes like this: On old Olympus’ towering top, a Fin and
German viewed some hops. That’s pretty weird sounding — not terribly
easy to remember. I mean, Olympus? Fin? Hops? There’s gotta be something more relevant
to us 21st century science lovers. Like, the Lord of the Rings fans out there might prefer
something along the lines of: Onward old orcs! Toward the Argonath for a
Great Villain! Slay Hobbits! I’m just trying to help. Whatever device you use to remember the names
of the cranial nerves, you also have to keep track of their functions — that is, whether
they’re sensory, motor, or both. So, again from top to bottom, a lot of teachers use
this sequence of S’s, M’s and B’s to remember: Some say marry money, but my brother says
big brains matter more. That one’s not so bad. But I don’t know, maybe
you’ll have better luck with something like this: Sorry, Sherlock — Mean Moriarty Beat Me,
But Some Bobbies Busted Moriarty Masterfully! You are, of course, invited to think up your
own. And feel free to share them in the comments, hopefully there will be some good ones down there
— anything would be better than Fins and Hops But if you’re going to commit one cranial nerve
type to memory, it should be 10, the vagus nerve. This long and extensive nerve stretches from
near the brainstem down to most of your visceral organs, including your heart, lungs, and stomach. The vagus nerves work as a two-way street,
ferrying incoming sensory information from the peripheral system to the brain, and transmitting
outgoing motor instructions from the brain to the rest of the body. So it’s a “B” nerve, because it has
“both” sensory and motor functions. And usually you don’t notice this nerve at work,
because its functions are mostly automatic. Say you’ve had a really stressful day, so
your sympathetic system is charged up. You come home, crash on the couch, mow down a
half a pizza. Your stomach sends signals to your brain through
the sensory nerve axons in your vagus nerve, telling you that your belly is full of starch
and protein and fat. Your brain sees that your stomach is churning
away, which is a usual parasympathetic activity, so it sends signals back down through the
vagus nerve, triggering other parasympathetic responses — like slowing down your heart rate,
putting some glucose back into storage, and reducing all that norepinephrine that your
sympathetic system was pumping out all day. Soon, you start feeling more relaxed. Which
is just one reason why, for some people, eating is a way of reducing stress and anxiety. In
fact, it can feel so good that even though your stomach is full, you might continue eating. So, like I mentioned before, it can be easy
to think of the two divisions of your autonomic system as opposites or even rivals, but that’s
a little off the mark. Looking at your body as a whole, you should
picture them as two sides of a scale — sometimes it’s balanced in the middle, and sometimes it leans
to the left or right, depending on what’s happening. That balance is the essence of homeostasis, and
as you’ll recall, homeostasis is the key to life. Here’s something else that’s important
for life: sex. It mostly falls within the parasympathetic
domain of “necessary but not an emergency.” But in order to effectively do it, you need
help from both systems. First, the parasympathetic system has to make
sure you’re calm enough to even think about sex, and then funnel extra blood away from
your muscles and down to your genitals — which is why too much stress and anxiety can lead
to sexual dysfunction. But you also need a burst of that sympathetic
system to excite you, and keep you excited. So like two sides of the scale, the balance
depends on having the right amount of both. The rate of action potentials travelling through
each division is known as your “sympathetic tone” and your “parasympathetic tone.” And, most of the time, our parasympathetic
tone is actually dominant, keeping down the caged animal that is your sympathetic response. That’s why you need your parasympathetic
system to keep your heart from racing like a rabbit’s. And why, most of the time, our
bodies can do the eating, and sex-having, and all of the other fun tasks that make up
the business of living. Today on Crash Course: Anatomy & Physiology
we looked at the parasympathetic nervous system, its structural differences from the sympathetic
system, and the 12 cranial nerves. We also looked at the vagus nerve as a way of understanding
how parasympathetic responses work, and how they make our lives possible. Big shout out and thank you to our Headmaster
of Learning, Thomas Frank whose generous contribution on Patreon keeps 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 co-sponsored
by Jennifer K. Koons, Tim Wisard, and Mich Acosta. This episode of Crash Course was filmed in
the Doctor Cheryl C. Kinney Crash Course Studio. We got a plaque. I wanted to show you the
plaque before we put it up on the wall. It 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, our sound designer is Michael
Aranda, and the graphics team is Thought Café

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