I want you to picture this: you’re kicking back in your room, you’re on your 3rd hour
of a YouTube binge-watch, your butt’s happy in that spot and doesn’t plan on moving.
And all of a sudden, your roommate busts in and is like “THERE’S A FIRE IN THE KITCHEN!”
so you jump up and /that’s/ when it hits you.
Your heart is beating /way/ faster than it should, you feel exceptionally lightheaded,
and even though your house is on fire, you need to sit down like right now.
Folks who experience that sensation too severely may have POTS - or postural orthostatic tachycardia
syndrome.
And it’s not their fault.
POTS is a type of dysautonomia, a dysfunction of the autonomic nervous system, that portion
of the nervous system that controls some of those bodily functions that run in the background
like heart rate, breathing, even pupil dilation.
Diseases to this system range from deadly to mildly annoying.
So — what do we need to know about our body’s autopilot mode?
[INTRO]
What’s up and welcome to another anatomy and physiology lesson, my name is Patrick
and today we’re talking about the autonomic nervous system.
This is one of those parts of physiology that you’re gonna use regardless of whether you
go into medicine, how drugs work, even fitness — everyone needs to know how the autonomic
nervous system works.
So we start where we always do, the BIG PICTURE: the autonomic nervous system takes care of
those bodily functions that happen without you even thinking of them. Obviously, examples
are coming.
If you’ve been keeping up with this series, you know that the nervous system has two main
functions: sense things and move things. We call these sensory nerves and motor nerves.
And we have two ways that we can tell our body to move, either voluntarily or involuntarily.
The deliberate, voluntary movement falls within the somatic nervous system. When you lift
that tasty burrito up to your mouth, you’re doing that voluntarily, that falls under somatic
control. “this is what your Patreon money goes to by the way”
But /some/ things happened automatically during burrito time: you started salivating, a couple
of hormones were released, and your stomach got ready to welcome the incoming burrito.
Notice that /none/ of those functions had to do with receiving information.
All the functions of the autonomic nervous system are motor. And those automatic functions
can be a bunch of different things, but in general, they fall into the excited
“fight or flight” category, what’s called the sympathetic nervous system or the more
chill “rest and digest” category, the parasympathetic nervous system.
Technically, there’s a third enteric nervous system that plays a role in digestion. Not
surprising considering enteric is Greek for intestine. I’m not gonna cover that in this
video, but just so you know, it’s a thing.
So broadly speaking, you have the speed up and the slow down, the punk rock and the R&B,
the salsa picante and the mint chip ice cream, the high strung Type A and libertarian who
just needs them to chill out for like a second.
To get a bit more scientific, the Sympathetic nervous system is way more active during high
stress situations and gets you ready to /move/. This means speeding up your heart rate, stimulating
sweat production, dilating your pupils to let more light in, and stimulating glucose
production and release from the liver.
I mean a fter all, something stressful is happening.
You might need to run away from a bear or from the crushing anxiety about your student
debt.
But while it’s stoking certain organs for exercise, it’s also inhibiting other functions
that you /don’t/ need to be active during stress.
When you’re running away from that bear, you don’t need to worry about digesting
food, and you definitely don’t want to stop to pee.
So your sympathetic nervous system communicates with your stomach, intestines, and bladder
to inhibit those bodily functions from happening.
At least while the stressor is still there.
On the other hand, your Parasympathetic nervous system does the opposite. It /slows down/
heart rate, and /stimulates/ the stomach to digest food, and for your bladder to drain.
So while these two things seem like they’re evenly balanced, like all things should be,
the sympathetic nervous system actually encompasses more anatomy.
It attaches to blood vessels and sweat glands while the parasympathetic doesn’t hook into
the skin or musculoskeletal system at all. So overall it’s a much bigger system than
the parasympathetic.
But hold up, we’ve been talking about these systems as very abstract ideas up until now.
So what exact pieces of anatomy are we talking about when we learn about these systems?
This is the point where if you haven’t watched the other videos in my nervous systems, you
might want to pause this video and watch those. We’re about to go pretty high brow.
Okay, so we have a central nervous system with a brain and spinal cord in it, right.
And that spinal cord has a bunch of different branches off of it that make up the peripheral
nervous system.
Both branches of the autonomic nervous system have roughly the same structure, a kind of
a two piece set up that we’ll zoom in on.
We have the spinal cord, a neuron that plugs into something called a ganglion and another
neuron that connects to the target organ, anything from the pupils to the colon.
We call the connection from spinal cord to ganglion the preganglionic neuron, while the
postganglionic neuron connects ganglion to organ.
You have these bilaterally, so on both your left and right sides.
And in both the sympathetic and parasympathetic systems once an action potential travels down
the myelinated preganglionic neuron, it releases a specific neurotransmitter, acetylcholine,
or ACh to the ganglion. Depending on what organ it’s going to, the postganglionic
neuron might release acetylcholine again or norepinephrine to its target.
And we’ll just mention this right now because i’m sure it’s coming in the comments.
Norepinephrine can be /both/ a neurotransmitter between neurons OR a hormone in the bloodstream.
The context is everything.
So that’s our general context for autonomic neurons, but there are some exceptions.
Like the sympathetic nervous system can take one of those super-fast myelinated preganglionic
neurons, bypass the ganglion entirely, and hook it up directly to chromaffin cells in
the adrenal gland which release excitatory hormones into the bloodstream. —-
Unfortunately for your poor, sleep-deprived student brains there /are/ some differences
between the sympathetic and parasympathetic divisions that you have to know.
Look, I know, I can hear you groan from here. But, because I love you all so much, I’m
going to limit it to two big differences:
The first is their general location off of the spinal cord.
Sympathetic neurons have a cell body in the lateral gray column of the spinal cord, this
spot here, and branch out from thoracic vertebrae 1 to lumbar vertebrae 2, so kind of a middle
chunk of the spinal cord.
And from there, those preganglionic neurons can go superiorly or inferiorly to different
ganglia.
Or they could go to ganglia that have fused together. Fot example, the last cervical ganglion
and the first thoracic ganglion fuse into something called the stellate ganglion.
Side note: i found out that some surgeons have severe the stellate ganglion in cases
where people’s hands are extremely sweaty. I guess it pays to sweat the small stuff.
But there are a bunch of plexuses and other bundles of nerves too. You’ve probably heard
of the solar plexus before, this cluster of sympathetic nerves and ganglia in the chest.
The parasympathetic nervous system on the other hand branches out of the head and pelvis
— we’re talking cranial nerves 3, 7, 9, and 10 and sacral roots 2, 3, and 4. So it
makes a vertebral sandwich with the sympathetic nerve roots.
It makes sense when you know what they do.
Like cranial nerve 3, the oculomotor nerve, has motor control of the eye, so it can constrict
the pupil.
Or cranial nerve ten, the Vagus nerve, which is the big deal parasympathetic nerve that
can slow heart rate or stimulate digestion.
Then the sacral nerve roots are for bladder and genital stuff.
Big difference number 2 is what kind of receptors the postganglionic neurons use.
Remember, all the /pre/ganglionic neurons for both the sympathetic and parasympathetic
systems just spit out acetylcholine. So we call those cholinergic neurons.
And because neurotransmitters are just locks and keys, an acetylcholine neurotransmitter
fits into a cholinergic receptor. Two types of those, either nicotinic or muscarinic receptors.
But it’s a little different for postganglionic neurons.
In the parasympathetic nervous system, their postganglionic neurons usually spit out more
acetylcholine, so it’s another cholinergic neuron.
Sympathetic postganglionic neurons are another story.
[ad·​ren·​er·​gic] They mostly release norepinephrine to something
called an adrenergic receptor. Sometimes these neurons pump out more acetylcholine, but *in
general* they pump out norepinephrine.
And within those adrenergic receptors, you can have alpha 1 and beta 1 receptors that
kick off excitation of a neuron, and alpha 2 and beta 2 receptors that inhibit the target
tissue.
By the way I made these graphics into a little flow chart PDF for yall that I’ll link to
in the dooblydoo if you need a little help.
The easiest way to remember that is that adrenergic has word overlap with adrenaline, which is
where it gets its name from.
Obviously, your can tie adrenaline and excitement, which is perfect, because that’s what the
sympathetic nervous system does.
So we’ve got cholinergic receptors for acetylcholine, and adrenergic receptors for norepinephrine
and epinephrine.
And this is important when you remember that neurons have a bunch of different types of
channels that can let different ions in. There are a bunch of different types of channels
including something called ligand gated ion channels — receptors on the neuron that
are opened by chemicals
This lets sodium and calcium into the channel which excites the neuron and triggers an action
potential kind of like ion gated channels did.
Let’s take a look at a concrete example — probably the most important subject of
autonomic innervation: the heart.
Your little cardiac muscle has sympathetic and parasympathetic innervation. And remember
our two piece setup, preganglionic and postganglionic neurons.
For the sympathetic innervation of the heart, the preganglionic neurons come out from thoracic
roots one through four. This lines up with that same model we saw earlier, plus, T1-4
is right about where your heart is anyway.
Their ganglia are your typical paravertebral ganglia, running alongside the spine, then
branching out to postganglionic neurons to the heart.
And because we’re looking at sympathetic postganglionic neurons, we’re working with
adrenergic receptors.
We’ve got beta 1s in the sinoatrial node and atrioventricular nodes, those pacemaker
cells in the heart. And Beta 1s are excitatory, which means we can get that heart rate up
there, ready for exercise.
BUT, and I think this is dope, it’s also stimulating some beta 2 receptors, which means
that /some/thing’s gonna relax.
Exactly what we want blood vessels to do during exercise. This is the process of vasodilation,
the tiny muscles around blood vessels relax, which increases blood delivery.
So after sympathetic stimulation, heart rate increases, force of contraction increases
which means we can squeeze out blood more forcefully, even the electrical signal itself
is conducted faster.
But it will slow down eventually, and our big player there in the parasympathetic nervous
system is the vagus nerve, good ol’ cranial nerve 10.
See, the vagus nerve /also/ plugs into the SA node and AV node, and when it stimulates
either of those nodes, heart rate slows down.
There’s a /ton/ I’m not covering about the heart’s electrical activity there, but
that’d be like a 90 minute video.
And if you want to learn about how those voluntary, somatic motor functions work, I recommend
this video I did about motor units. And if you need more help with nervous system stuff,
check out this playlist on the topic.
Otherwise if this video helped you out, consider subscribing or liking the video. Big thank
you to my Patrons Diana and Jessica. Appreciate ya! Have fun, be good, see you next time!