Today we’ll look at the anatomy and physiology behind the motor unit, the pieces of anatomy
that take a signal from your brain and tell your muscles to lift that coffee cup to your
mouth, letting that sweet nectar of the gods fill your face with the life giving caffeine
that you’ve become so dependent on.
Was that too aggressive?
[intro]
What’s up everyone, my name is Patrick and I’m ohh so glad you’re joining me today
to talk about my very favorite part of the nervous system, the part where it becomes
the musculoskeletal system.
Okay, so we’re going to follow along with that example I said a second ago: my brain
has decided it’s going to use muscles to lift up my coffee cup.
This is totally independent of smelling the coffee, seeing the coffee, or daydreaming
about coffee.
This is literally just the one-way brain to muscle pathway we’re talking about here.
And something like picking up coffee involves multiple muscles, so we can’t go through
and pick apart each nerve and each muscle fiber.
Instead, we pare it down to the tiniest level and subject of the video: the motor unit,
motor just means that it’s responsible for movement.
I like to think of this as where nervous system meets our muscular system, so the actual unit
has one piece from each system: a bunch of muscle fibers and the motor neuron that innervates
them, or plugs into them.
The gross anatomy is pretty simple: we have sensory neurons that relay sights and sounds
and smells back to the brain, and motor neurons that go from your brain to muscles and tell
you to move.
And among all our motor neurons, we have some under somatic or voluntary control and some
that run on autopilot, the autonomic nervous system.
Now this is important, I’m making sure to specify what type of neuron plugs into the
muscle fibers because there are also two types of motor neurons:
Your upper motor neurons are responsible for taking the signal from the brain and passing
it down the brain stem and spinal cord to the lower motor neurons in the peripheral
nervous system.
This is important since upper doesn’t necessarily mean “above the neck” right like there
are technically lower motor neurons that connect the cranial nerves under your brain to muscles
that move your eye or tongue.
So when we’re talking about motor units, we’re laser focused on those lower motor
neurons, specifically where they interface with skeletal muscle to get them to contract.
It’s this whole thing but also not really.
It’s the whole lower motor neuron, but only /particular muscle/ cells.
I /do not/ want you to think of it like /this/ nerve plugs into the biceps muscle and innervates
the whole muscle.
You see this muscle?
Do you see a USB port on this thing?
It’s not a plug.
To see where it /does/ attach, we have to zoom in a little bit.
So at the end of every neuron are those axon terminals, and while they often connect with
other neurons, they can also connect to muscle /cells/.
That part where they actually meet up is called the neuromuscular junction.
Obviously neuro- for neuron and -muscular for muscle.
This itself is /not/ the motor unit, you have to zoom out for that.
I want to make this extra clear.
The neuromuscular junction isn’t the entire motor unit — it’s important, yeah, but
the proper motor unit is the motor neuron and the muscle fibers it innervates.
So now we’ve got our anatomy, but we still need to pick up the coffee.
So we’ve decided what we’re gonna do, we send an action potential down from the
brain, it ends up at the neuromuscular junctions, and this is so cool, it stimulates all the
terminals together, not just one or two, but every muscle fiber that attaches to that motor
neuron.
And that actually makes a lot of sense.
Like if you had a power strip with three phones plugged in, and then plugged the power strip
into the wall, all three of the phones get a charge.
In this case, the main cable is like the motor neuron and the individual phone chargers are
like the axon terminals attaching to the muscle fibers which are the phones.
That also means that one neuron couldn’t fire and stimulate other muscle fibers, even
if they’re in the same muscle belly.
But your body doesn’t fire all the motor neurons in a muscle at the same time anyway.
Your body recruits smaller units first, /but/ if that’s not enough to lift the thing or
jump the box or whatever, your body will recruit bigger units.
As a general rule, tiny muscles that need super fine, precise control like finger or
eye muscles will have only a few muscle fibers (like on the order of tens) per motor neuron
while those chunkier muscles in the core and limbs can have lots of muscle fibers per neuron,
like over a thousand.
To put this in perspective, let’s take your main calf muscle, the gastrocnemius.
This /definitely/ counts as one of those chunky muscles — it has over a million muscle fibers
in its muscle belly and about 580 motor axons, which means that each of those neurons has
almost 2,000 neuromuscular junctions each.
Again, that’s 2,000 axon terminals of each axon.
All the better for strong, forceful contraction.
But you don’t always want a strong contraction.
Sometimes you want a slower, more delicate contraction.
To me, the best example of this would be some kind of static hold where the weight increases.
Like if I’m holding one book in my hands, my bicep is for sure activated, but not all
the motor units are activated.
Now if the load increases, I have to activate more motor units to provide more force.
The same bicep is contracting, but more motor units were recruited to adjust to the greater
workload.
Now, here’s what’s extra cool.
We can also see motor units come in during adaptations to weight lifting.
Assuming you’ve never lifted or you’re just getting back into it, when you first
start, your muscles don’t actually grow right away.
Yes, your body adapts, but it does so by recruiting more motor neurons, which stimulates more
existing muscle fibers.
And with more muscle fibers working for you, you lift more weight.
In strength and conditioning you might hear those called neural adaptations, literally
adaptations of the nervous system to help you lift more weight.
So during the first few weeks of consistent lifting, you shouldn’t see noticeably bigger
muscles, but they will get stronger.
Everything in this video is very specific to the anatomical unit where the nervous system
meets the muscular system, which is great, but now you need to know how the muscle actually
contracts — I didn’t want to overwhelm you in this video, so I made a separate video
on the sliding filament theory that you can find here.
It explains everything that happens after an action potential gets to the neuromuscular
junction.
If you have other questions about the nervous system or muscular system, you can probably
find them in either of these playlists here.
Otherwise thank you to my Patrons Diana and Jessica, appreciate ya!
Have fun, be good.
Thanks for watching.