Let's do mouth and take a superficial view of the head and
neck so we can look at how the dermatomes work in this region.
Superiorly coming down over the eyes to the superior aspect of the nose,
the dermatome is supplied by the ophthalmic nerve.
Below that coming over the cheeks down to the upper
lip, the dermatome is supplied by the maxillary nerve.
Then finally, the area coming down over the
jaw is supplied by the mandibular nerve.
And you'll see in parentheses, they have a roman numeral and an Arabic numeral
and those all stand for divisions of the trigeminal nerve.
That's how the trigeminal nerve or cranial nerve V gets its name. It has these 3 divisions, 5I, 5II, and 5III.
You'll also notice we're really only covering the anterior half of the face here.
And the posterior aspect is going to be supplied by cervical spinal nerves.
So let's swing around to a posterior view to see those.
Here, we see C2, C3, C4, and C5 and you might be wondering well where's C1.
Well, it turns out that the C1 spinal nerve is only a motor nerve
and it doesn't actually have any sensory innervation in this dermatome distribution.
Now let's talk about some of the
muscles of the head and neck.
And one of the ways we group the muscles is by their
actions and the first group we're going to look at
are called the muscles of facial expression
and they tell you exactly what they're doing.
Their muscles that cause
expressive movements of the face.
The first one we're going to look at
is actually a 2-part muscle joined by a wide flat tendon called the epicranial aponeurosis.
Aponeurosis is just another word
for a really wide flat tendon.
It has a frontal belly and an occipital belly and therefore
together we call this muscle the occipital frontalis.
And you can kind of imagine when these muscle bellies contract,
it's going to cause the eyebrows to be raised and wrinkle the forehead.
Let's swing around to an anterior view to see
some of the other muscles of facial expression.
We have these circular muscles that go around the eyes called the orbicularis oculi
and those are going to be good for forceful closure of the eyelids.
We have some muscles on the bridge of the nose called
the nasalis that will cause flaring of the nostrils.
We have more circular muscles around the mouth called orbicularis oris
and that's going to cause, as you might imagine, squinching down around the mouth.
We have a wide flat muscle called the platisma, very superficial, very thin,
it really just attaches into the skin and it causes a tightening of the skin around the neck area.
And we have something
called the buccinator.
When you see the word buccal (b u c c a l),
it usually means something in the cheek area and so this is a small muscle in the cheek
that acts to keep food pressed up against
the area between the upper and lower teeth.
When we talk about innervation of these muscles of
facial expression, we talk about the facial nerve.
In order to see the facial nerve though, we have to remove
the largest of our salivary glands which is called the parotid gland.
And the name is actually descriptive.
Parotid means par otid or next to the ear.
And as we can see, it's a very large salivary
gland sitting just anterior to the ear.
In order to see the facial nerve that's running
through this, we're going to have to fade that out.
And that way we can actually see that it's coming out of this region and branching
into various branches to supply all of these muscles of facial expression.
Now let's talk about the mandible. The mandible
is a pretty special bone within the skull
because it's essentially the only one that moves and
that movement is going to be very important for chewing.
Anteriorly, we have the body of the mandible and then we
have this vertically oriented portion called the ramus.
And the area where the 2 meet is something called the angle.
If we look at the ramus, we see these 2 projections, one posterior and one anterior.
The posterior one is more rounded and that's the head
and the more anterior one is more angulated and this is called the coronoid process.
If we swing around to a posterior view to
see some internal features of the mandible,
the first thing we see are these openings and
these are the mandibular foramens or canal.
And this is the passageway for vessels but also
something called the inferior alveolar nerve which will supply the teeth.
If that name is a little confusing, I can understand why
because we said alveolar in the context of the lungs.
And alveolus was the tiny air sac where gas exchange took place.
Well, alveolus just means like a tiny hollow thing.
So in this context, alveola refers to the little hollow part of a tooth socket.
So those are the 2 times you're probably going to hear the word alveolar.
Speaking of the teeth, we have teeth that are basically symmetric on either side of the midline
and on the upper and lower portions of our mouth.
We have 3 molars most posteriorly then anterior we
have 2 premolars and then canines and then 2 incisors.
We use these teeth for chewing and of course with
chewing we're going to have some sort of movement.
And unlike all of the other parts of the skull
that have sutures where there is no movement,
at the mandible we do have an actual movable joint
called the temporomandibular joint or TMJ for short.
Again, we have this process coming off of the mandible called the coronoid process
and then this one more posteriorly called the head.
And it's the head that's going to actually
participate in this temporomandibular joint.
And it's going to do so at that area of the zygomatic arch,
which we said was a combination of zygomatic bone and temporal bone.
If we zoom in a little bit more, we see that we're going to have some support structures here
to keep this attached and so we'll have a temporomandibular ligament.
And because there's a lot of movement here, this
is actually going to be an actual synovial joint,
a joint like the knee or the elbow or one of the joints you
typically think of as a joint, something that's highly movable.
And it's highly movable because it has all the features of what we call synovial joint
including a joint capsule with a joint space.
One thing that's pretty unique about the temporomandibular joint though
is that that joint cavity is separated into 2 separate ones by an articular disk.
So that articular disk actually separates the 2 and
the fact that you have 2 separate synovial cavities
on either side of this articular disk
allows for a great degree of movement and gliding at the temporomandibular joint.
The muscles that move the jaw during chewing are called the
muscles of mastication. Mastication just means chewing.
And there are 4 of them. In the lateral view, we see a wide flat one called
the temporalis and then a thicker more narrow one called the masseter.
And in order to see the other two, we have to do a posterior view because they are deeper
on the surface of the mandible and those are the medial and lateral pterygoids.
And you can see that they're attaching at all different angles and that's a pretty good
thing when it comes to chewing because it allows for a lot of different types of movements.
So again, we see the lateral pterygoid here attaching to the head.
For example, we see the medial pterygoid attaching at a different point down by the angle.
And so when they contract, they're going to cause
different types of movements on the mandible.
Some of those movements include pulling the jaw forward called protraction or protrusion,
but also the opposite direction which will be retraction or retrusion.
Depressing the jaw is actually something
that's usually a passive thing.
If you just relax your muscles, gravity will
take over but the muscles of mastication can,
under circumstances, forcefully open the jaw but
generally most of the muscle is dedicated to elevation
because it's working against gravity but also
compressing food between the 2 sets of teeth.
So, the muscles that elevate have
to be the thickest, biggest ones.