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.

(Intro)

You will recall that our sympathetic and 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 just 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, 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 traveling 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, 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