I need to show you something real quick. This is my desk. This is where I film all my videos,
and underneath it is a bunch of cables.
I put a little tray under here,
I added some velcro and zip ties and it looks way better and more manageable.
Your body does the same thing but with nerves.
Your nerves branch out from the spinal cord, and sometimes they bundle up with other nerves
to form something called a plexus. Think of it like cable management, but for nerves.
If you’re new here, my name is Patrick and this channel is all about anatomy and physiology. This
is the first video in a multi-video series where I teach you the 4 big nerve plexuses of the body.
You’ll find the rest of the videos in a playlist linked in the description and up here in the card.
But before we go deep on the anatomy, we have to ask, what is a plexus and why do we care?
[This video was sponsored by KenHub]
A quick refresher on nervous system basics.
You’ve got a central nervous system which includes the brain and spinal cord,
and a peripheral nervous system which is all the nerves that branch off that
spinal cord. When we study plexuses, we’re focused on the peripheral nervous system,
but we end up referencing the spinal column when identifying where the nerves poke out.
Nerves can send signals from the brain to the rest of the body and get it to
move through motor nerves, but you’ve also got nerves that send information
from the body to the brain — these are called sensory nerves. And when we’re
talking about nerves themselves, really we’re talking about bundles of axons,
the long spindly part of the neuron, which is the main cell type of the nervous system.
I know, I know — glial cells have their loyal fan base, but we’ve gotta stay focused.
Each of those peripheral nerves branch out from the spinal cord
through gaps between each vertebrae called a spinal foramen or foramina in the plural.
This is one of the many anatomy words that just means a hole. So a spinal foramen is
a gap in the spine that allows nerves to exit the spinal cord.
Those little nerves that poke off from the spinal cord are called the spinal nerves.
They’re mixed nerves, which means they send both motor signals down and send sensory signals up.
That’s because each spinal nerve is the bundling together of an
anterior and posterior root, each of which handles different signals.
The anterior, or ventral root, handles the motor signals while the posterior, or dorsal root
handles the sensory information. When those two combine, the resulting spinal nerve pokes through
the vertebral foramina, again, that gap between vertebrae, and it handles both kinds of signals.
See, all this is just cable management applied to anatomy!
From there, the spinal nerve splits again into a ventral and dorsal ramus,
or rami for plural. Unlike the spinal roots the
rami are still mixed — dealing with both sensory and motor information.
From there, those peripheral nerves take a bunch of different paths to get to their final
destination depending on which nerve or which plexus we’re interested in.
When it finally gets to its destination, we say that that nerve innervates that piece of
anatomy. for instance, we can say that the sciatic nerve innervates the hamstring
muscles since a nerve signal will get to the hamstring muscle by traveling from the brain,
down the spinal cord, through the sciatic nerve,
and get the semimembranosus muscle, one of the hamstrings, to contract.
Now we can finally get to the crux of the video — a plexus is when different spinal
nerves nerves twist, intertwine, and combine with other nerves and eventually split into
their individual nerves that innervate their more distal targets. Plexuses can also refer to
bundles of blood vessels or lymphatic vessels more broadly, but I’m gonna focus on nerves.
The word Plexus comes from the Greek word for “braid” and when you look at a real life plexus,
you can see the similarities. Nerves twist and recombine and split to form structures
that look less like electrical cords and more like networks or webs of string.
Now here’s the key takeaway of this video, each plexus uses the structure of the body
to best send nervous signals to the destination. Plexuses don’t exist to
infuriate anatomy students, they exist because they’re ultimately good design. The same
can’t be said for other nerves in the body — looking at you recurrent laryngeal nerve.
Alright so what does that actually look like in practice? Well, take a look at the brachial plexus which branches out from spinal roots in the neck to innervate
the arm. Or the sacral plexus which branches out from the lower back and sacrum to innervate most
of the leg. From there, innervation will be pretty intuitive. Like a lot of nerves take
the shortest path from its plexus to the muscle it innervates, which has the added
benefit of making naming easier. There are always exceptions, but for the most part,
if you have a good understanding of gross anatomy, you’ll be able to piece together nerve names.
And actually, speaking of which, let’s talk about naming conventions.
Those bony vertebrae end up being really useful reference points,
since it’s easy to find where you are on the spine by counting vertebrae. But
we have two different naming conventions rules— one for the spinal nerves branching
out of the cervical vertebrae, and another for everything inferior.
Starting below Cervical vertebra 1 (the atlas), spinal nerves are named for the
vertebrae under them. So the spinal nerve pokes out between the 1st and 2nd cervical
vertebrae is named cervical spinal root 2, or just C2. Spinal nerve C5 pokes out between the
fourth and fifth cervical vertebrae, etc. Here’s my little memory trick:
I imagine each vertebra like a flower pot with a number on it, and each spinal nerve is the flower that matches it.
The exception is spinal nerve C8, which is a weird little guy because there isn’t an
eighth cervical vertebra. It pokes out between cervical vertebra 7 and thoracic vertebra 1.
Inferior to that, the spinal nerves get their name from the vertebrae above
where they poke out from the spinal cord. So spinal root T1 pokes out
below the first thoracic vertebrae, T2 below the second thoracic vertebrae, and so on.
That pattern continues down to the sacrum, but since the sacral vertebrae are all fused together,
there aren’t intervertebral foramina. Instead, there are holes in the sacrum
called sacral foramina for the nerves to poke through. Those are just named S1, S2,
etc. And there’s one little pair of coccygeal nerves to round us out.
So as we go through the rest of the videos in this series,
we’re going to focus on what the plexus is and how to name it, but also on what it innervates.
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You can find the plexus playlist in this link here, and you can get a refresher
on neuron anatomy in this video here. Have fun, be good. Thanks for watching.