What up everyone, my name is Patrick and thank you for joining me on our little chat about
the nervous system today.
I’m so glad you could make it.
Now you’re probably coming in thinking that the nervous system is the body’s electrical
system, which is like, kiiiiiinda true but also way more complicated than that.
So in this video, we’re gonna set you up to learn the gross anatomy of the nervous
system and set you up so you can learn how nerves transfer information from point to
point b.
[intro]
Okay, big picture here, we have a nervous system for the purpose of sending information
quickly around the body.
We’ve got other systems for communication, like the endocrine and immune systems, but
the nervous system is responsible for making certain it happen /fast/.
So to do all that high speed communication, we have a couple anatomical systems within
one: the central nervous system which includes the brain, brain stem, and spinal cord, and
the peripheral nervous system which is everything else, including all those nerve roots that
branch off the spine.
So let’s break those down.
Ok, obviously the brain is involved in the nervous system, but where students get mixed
up is thinking it’s this like squishy ball of electricity.
When in reality, the brain is way more complicated (understatement of the decade) — it plays
with hormones and communication from your microbiome, so while it’s the chief organ
of the central nervous system, I don’t want you to think of it as isolated to the nervous
system.
But, the brain /does/ still need to be able to get information and commands out to the
rest of the body as well as receive information from it
That’s where our brain stem comes in handy.
Now, full disclosure, I used to think that the brain stem was like a plug for the spinal
cord into the brain, but it’s not /just/ that— it’s an organ with three parts,
the midbrain, pons, and medulla.
That brain stem leads into the spinal cord which is a long bundle of nervous tissue,
and since it’s ya know, kind of important to protect, it’s wrapped up in a bunch of
different protective soft tissues and gets protected by those bony vertebrae.
We’ll revisit this when we get to the musculoskeletal system, but when you’re looking at a skeleton,
the spinal cord does not go through this bone.
This is where the chunky vertebral body and discs stack on top of each other.
/THIS/ little canal is where the spinal cord goes.
And those little gaps between the vertebrae let branches pass out of the spinal cord.
Those little branches have two hookups on each side called ganglia or ganglion for singular,
there’s the anterior or ventral root that sends down motor signals from the brain, and
a posterior or dorsal root that sends up sensory info.
If you want a little acronym for that, SPAM.
Sensory is posterior, anterior is motor.
And all of those ganglia single out into spinal roots which is where we finally make the transition
to the peripheral nervous system.
As always, anatomists are the worst at naming things, so most of those peripheral nerves
are going to follow the typical anatomical regions naming pattern.
The main leg nerve is going to be femoral nerve, and so on.
The naming is straightforward, but I’ve got a video that you can find right there
if you need a refresher.
But sometimes those nerve branches bundle into things called plexuses, networks of nerves
that head to the same general target.
For example, the brachial plexus stems out of multiple levels in the neck and go under
the clavicle and branch off down each arm, which is still the boring anatomical name
we’d expect — brachial for arm.The plexus is still made of individual nerves, but bundle
together and hang out for a bit before splitting off.
Now, it makes sense that these are all in the peripheral nervous system since they branch
off the spinal cord.
But, there are twelve nerves that branch directly off the brain and brain stem called cranial
nerves that give us a bit of a hard time.
One controls the muscles of the eye, one transmits sensations from your nose, another does facial
expressions, there’s a bunch
Does it really matter which category they’re in, no.
But if you’re on a first date with an anatomist and need something to talk about, remember,
cranial nerves.
Now, within the nervous system you also going to hear things like sympathetic and parasympathetic
nervous systems.
Those are harder to visualize since they’re multisystemic, so check out my nervous system
playlist right there.
Hopefully something helpful will be there including cranial nerves and motor units .
Okay, that’s big picture anatomy.
*sips water-deep exhale* Okay, cellular anatomy time.
In order to understand /how/ a nerve actually sends a signal, we need to zoom into the cellular
level.
This is the nerve cell you’ll see the most, what we call a neuron, and like I said before
/it’s not just an electrical wire/.
This thing isn’t the only cell in the nervous system either.
You’ve also got glial cells, like Schwann cells, and microglia and astrocytes.
But we care about the neuron so much since this is the main way a nervous impulse gets
around your body, whether it’s going to another neuron, a gland, or a muscle.
Here’s what I want you to remember — no matter what, this thing is still a living
cell.
It has a nucleus with DNA, it has mitochondria and other organelles that you’re used to
hearing about from bio.
A neuron isn’t just an electrical wire, it’s a cell.
And this big chunker right here is the cell body, or soma.
It ends up being really important in receiving information from these dudes, the dendrites
and sending it down this long axon.
How I remember the terminology by the way, is that the /dendrites/ kind of look and sound
like tentacles, and when you look at them they totally look like spindly tentacles,
while the long axon reminds me of an axle in a car.
And those axons are /all/ different sizes.
They can be as short as a millimeter, or they can be longer than a meter like your sciatic
nerve which goes from your lower back to your big toes.
They can also be different diameters, which affects their transmission speed— a thicker
nerve can send a faster signal than a thinner nerve.
So if you think about this cell like it’s a full scale human body, these dendrites are
kind of like your ears or nose.
They /take in/ molecular information and send it down the line.
And these inputs can be /so many things/.
They can excite the neuron to do something or inhibit it from doing something.
There are so many combinations out there, but for now, know that these dendrites is
where the neuron receives information from.
And it all gets summed up at the axon hillock, this root looking piece right here.
To me, the axon hillock is like when you're the designated driver taking your drunk friends
through the taqueria drive-through at 1 am.
Look, the analogy isn’t perfect.
So those signals get transmitted down the axon and end up at these little feet looking
things, the axon terminals.
Makes sense, the axon /terminates/ here.
If the purpose of this neuron cell is to communicate information, it has to be able to take information
from somewhere, like from the dendrites, and then communicate that information elsewhere.
And within those axon terminals are chemicals called neurotransmitters that get passed onto
the next cell in what’s called a synapse.
I’ll go into that in more depth in the next video in the playlist.
The last things are these little water wing looking things, Schwann cells, what makes
up the myelin sheath around the axon.
They’re these fatty, insulating cells that increase transmission speed along the axon.
The comparison that gets made all the time is how the myelin sheath is like the plastic
coating around your phone charger.
Not because it keeps the exterior from getting shocked, but because it actually helps conduct
electricity more efficiently.
Obviously, there’s more precise anatomy we could get into anytime we talk about the
nervous system, but this video sets you up to understand the next video which is all
about nervous system physiology including action potentials and synapses, so check that
one out here.
A big thank you goes to my supporters on Patreon, Diana and Jessica.
If this video helped you out, consider joining them on Patreon, or subscribing and liking
the video.
Have fun, be good.
Thanks for watching.