If I were to ask you, what are some of the
fastest muscles in the human body? You might
think of muscle groups like the quads, the
hamstrings, or maybe even the calf muscles.
Muscles that get involved in things like
sprinting or jumping. If I were to ask you,
what are some of the most coordinated muscles
in the human body? You might think of muscles
that move the hands and the fingers but none
of those muscle groups that I just mentioned
encompass both of those characteristics.
That title belongs to a group of muscles
called the extra ocular muscles or the muscles
that move the eye. So, in today, video, we're
going to show you this awesome dissection that
we made through the skull so that you can take
a look at these amazing eye muscles and we can
figure out why they're some of the most fast and
coordinated muscles in the human body. Anatomical
awesomeness is awaiting us. So, let's do this.
[Intro]
So, let's take a look at a real human
eye and these extra ocular muscles. Now,
we're going to zoom in a bit here. So, let me
actually orient you to what we're looking at
by using this skull. We're going to have
to remove or we did remove this portion
of the skull right here so we could see
more clearly into the orbit which is just
the eye socket. We're also going to show
you a view where we remove this plate of
bone here so you can see a superior
view or the upper portion of the eye.
Let's start with that lateral view and
you can see how amazing this eye is. If
I put a little pressure on it, you see
the cornea bulge out and the cornea is
just that clear portion of the eye that
you'll see on someone when you look at
them from the side and then obviously
you can see the white part of the eye here.
Now, the white part of the eye is called the
Sclera, and the sclera is important because
it's made of a tough connective tissue and
this is where the extraocular muscles will
attach so they can pull and tug on the eye
and move it in different directions. Now,
Extraocular just means outside the eye. Extra
means outside, ocular referring to the eye and
you can see the first extraocular muscle
right here. Look how awesome this muscle
is and you can definitely see it attaching to
the sclara or the white portion of the eye.
Now, this muscles actually called the Lateral
Rectus and there are six total extraocular
muscles and they're actually pretty easy
to learn to memorize because four of the
muscles have the word "rectus" in the name.
We've seen the first one called the lateral
rectus and rectus just means straight
because the muscle fibers are going in
this straight fiber orientation as you can see
from this muscle but here's the lateral rectus.
We also will have an inferior rectus
on the under portion or inferior aspect of
the eye, a superior rectus above the eye and
I can even show you that from the top view.
If we show you from the top view, I actually
do have to mention another muscle that's on
top of the superior rectus. This muscle here
is called the Levator Palpebrae superioris.
It's a mouthful but the name actually
may make sense. Levator means it lifts or
elevates. Palpebrae refers to the eyelid.
So, in other words, it opens your eye.
If I move that out of the way, we can get to the
superior rectus here and on the medial aspect of
the eye, look how cool this one is, this is the
medial rectus but you can see with the four rectus
muscles how they attach to different portions
of the eye so they could move it in different
directions. It could go upward or elevation,
downward which should be depression, could move
the eye outward which should be abduction or even
move the eye inward which should be adduction.
And what about the last two extra ocular
muscles? These two are my favorites because
they have a cool unique that I'm going
to mention in just a second but they
also are pretty easy to remember because
they have the word "oblique" as part of
their name. And oblique just because they
attach to the eye at a slant or an angle.
The first one is called the Superior Oblique
and you can see this cool muscle coming in and
attaching at a slant or an angle to the top
portion of the eye or the superior aspect of
the eye and then we can see the Inferior
Oblique here, again coming in at a slant
or an angle to attach to the inferior
aspect or that underside of the eye.
Now, because of the angle at which they attached
eye, this gives them the ability to pull the eye
in a direction that you might think like towards
the corners, like outward and upward or downward
and inward but the reality is to get to all the
corners or those specific directions of gaze.
You're going to need a combination of those
obliques and as well as some of the rectus
muscles but that unique function that I mentioned
or at least alluded to with these oblique muscles,
these muscles can do something super cool.
They can actually rotate the eye inward or
internally and rotate the eye externally
almost like just twisting the eyeball
which can be pretty helpful for your vision when
you say like tilt your head from side to side.
Now, for those of you who love every little anatomical detail,
I'm going to put the specific functions of those
muscles in the description below but let's talk
about how and why these muscles are some of
the most coordinated muscles in the human
body and let's say you're reading something
on a page or on a screen and I want you to
think about how subtle of a movement these
muscles have to create for the eye so that
your eye can scan letter by letter by letter as
you're reading across that page or that screen.
That requires an incredible amount
of coordination and fine motor control
and if we didn't have this level of fine motor
control with our eye muscles, your eye movements
would be jerky and you'd likely miss things
as you're reading across the screen or just
you'd miss things as you - as you were scanning
through looking at something just in daily life.
So, this gives us the why we need this level of
coordination but how does our body accomplish
this level of coordination with these muscles? The
extraocular muscles are made up of the smallest
motor units found throughout the but what does
this mean? Well, if you've watched some of our
previous videos, you may have heard me mention
motor units and motor unit recruitment before
but if you haven't heard of this, a motor unit
is the motor neuron and the muscle fibers it
controls and an important concept to understand
how motor units function is that when that motor
neuron sends the signal, all of the muscle fibers
that it controls will contract with full force.
So, that means you don't want to have
one motor unit and therefore, one motor
neuron controlling every single muscle fiber
within a whole muscle. That would be bad. So,
for example, if the lateral rectus extraocular
muscle only had one motor unit and therefore,
one motor neuron controlling every single muscle
fiber within the muscle, that means every time
it sent the signal, every single muscle fiber
contracts and that muscle would just whip the
eye out to the outside and we wouldn't have any
of that fine motor control but instead, we break
up the muscles into multiple motor units and these
extraocular muscles have very small motor units.
We're talking one motor neuron controlling
three to 10 muscle fibers. Compare that to
the biceps - the biceps has motor units
the size of one motor neuron controlling
over 1,000 muscle fibers. So, you could see if I
can engage only three to 10 muscle fibers within
these extraocular muscles, I can get that fine
motor control and get those subtle movements
with eye position but what if I did want to whip
or move my eye quickly in a certain direction?
I think we could all agree that there are
plenty of situations where this is necessary
or beneficial to move our eye quickly so
that we could maybe view something coming
our way that maybe dangerous and we could react
to it and one of the ways that we can move these
muscles or engage these muscles so the eye moves
quickly is just simply recruit more motor units.
So, instead of engaging maybe just those three to
10 muscle fibers that we would do in those subtle
fine movements, we could recruit more motor units
and nearly engage almost all of the muscle fibers
and move that eye as quickly as possible. Now,
that is not unique just to these extraocular
muscles, we could do that any of our skeletal
muscles like with the biceps, I'm going to recruit
less motor units doing something like this versus
engaging a lot more doing something like this.
So, there's something else or some other
things we need to consider when it comes
to understanding why these extraocular muscles
are some of the fastest in the human body. One
of the things that contributes to the speed
of these muscles and how fast they can move
the eye is that they are built up with a
great amount of fast twitch muscle fibers.
Now, you may have heard of fast twitch fibers
versus slow twitch fibers and as the names imply,
the fast twitch fibers contract with greater
velocity but they fatigue rather quickly
whereas the slow twitch fibers contract with
less velocity but they are fatigue resistant.
So you might think, "Well, these
extraocular muscles must be mostly
made up of the fast twitch fibers", and
yes, they have a great amount of them and
that does contribute to their speed but
we still need to have adequate amounts of
slow twitch fibers mixed into these muscles
because think of it from this perspective;
we need to have some level of fatigue resistance
built into these muscles because we need to be
able to our gaze for an extended period of
time without these muscles crapping out or
fatiguing on us and if you think about it from
the idea of that we're constantly contracting
these muscles and moving our eye throughout
the day, again a need for fatigue resistance.
So again, yes, the fast twitch fibers are
important for the speed of these muscles
but there's still also one other thing we need to
consider that contributes to the speed of these
extraocular muscles. The extraocular muscles are
overbuilt and you might be thinking "How are they
overbuilt? These are pretty small muscles." well,
what I mean by "overbuilt" is that these muscles
have a nice ratio of their size compared to how
much weight they have to move. The eyeball doesn't
weigh very much and you could definitely get away
with having smaller extraocular muscles and still
maintain that fine motor control and be able to
make those subtle adjustments in eye position
because remember, we didn't need to recruit that
many muscle fibers and motor units to do those
subtle adjustments but if the muscle is smaller
overall, it wouldn't be able to generate as much
force and wouldn't be able to move the eye as
quickly but instead, we have is nice overbuilt
muscles with this nice ratio of their size
compared to the weight they have to move
so that you can move the eyeball really quickly
when you say have to look over here or over here.
And speaking of moving your eyeballs, let's move
those eyes in the direction of the sponsor of
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