This is a tutorial on the muscles of mastication.
So the muscles of mastication are innervated by the trigeminal nerve.
They're innervated by the mandibular branch of the trigeminal nerve.
So that's V3.
When you're testing the trigeminal nerve, you're testing muscles of mastication and
facial sensation.
So the muscles of mastication are innervated by the mandibular branch of the trigeminal
nerve.
Most of the muscles of facial expression are innervated by the facial nerve, cranial nerve
seven.
So the trigeminal nerve is cranial nerve no.
5 and the muscles of mastication are innervated by V3, so the mandibular branch.
There are four muscles that you need to know which are involved in mastication -- the temporalis,
the masseter, the medial pterygoid and the lateral pterygoid.
I'll begin by showing you the temporalis muscle, which is this muscle here, this big muscle
which sits in the temple fossa of the skull.
So if I just show you the temple fossa, you can see this indentation on the lateral aspect
of the skull.
This is where the temporalis sits.
This muscle inserts -- all the muscles of mastication inserts onto the mandible because
the mandible is the bone that moves to cause mastication.
It's the bone which is involved in chewing.
So the temporalis inserts onto the coronoid process of the mandible.
That's this anterior process here.
If we just have a look at that, it just inserts here.
If I just show you the other side, you can see that it inserts just there, the coronoid
process of the mandible.
Just looking at the points of insertion and its origin, you can see that if the muscles
were to contract, it would cause the mandible to elevate and it can pull it back in a posterior
direction.
If you just look at the direction of the fibers so it also causes the mandible to retract.
So there's four important movements to know with regard to the mandible.
You've got retraction (that's movement posteriorly), you've got protrusion (that's movement in
this direction anteriorly) and you've got elevation (so bringing the mandible upwards
in a superior direction) and you've got depression (which brings the mandible inferior).
So four movements of the mandible.
So looking at the origin and the insertion of the temporalis muscle, you can see that
it elevates it and it can also retract it looking at the direction of the fibers.
So if you put your fingers in the temple region on your face and you clench your muscles as
if you're chewing, you can actually feel this muscle working.
When you're testing the trigeminal nerves, this is what you do to your patients.
You put your fingers in the temple region and ask them to chew, grind their teeth, so
you can feel this muscle working.
So if I just bring back the muscles that I got rid of, this muscle here is the masseter.
This is another muscle involved in mastication.
This muscle actually has two parts.
It's got a superficial part and a deep part, which isn't actually very clear on this model,
but the deep part inserts a bit more posteriorly in the zygomatic arch and the superficial
part inserts more anteriorly on the zygomatic arch.
So if you remember my tutorial on the skull, you've got this arch here, which is referred
to as the zygomatic arch and it's comprised of the zygomatic process of the maxilla and
the zygomatic bone.
So the masseter has two parts -- the deep and superficial part which originates on the
zygomatic arch.
It inserts onto the lateral aspect of the lateral surface of the ramus of the mandible.
So again, this is a muscle you can feel on yourself.
If you put your fingers over the angle of your mandible and clench your teeth together,
you can feel this muscle working.
So what this muscle does is it elevates and retracts the mandible.
It just elevates the mandible, it doesn't retract the mandible.
So the next two muscles of mastication are the pterygoid muscles.
You've got the lateral and the medial pterygoid muscles.
These muscles are called the pterygoid muscles because they insert onto the pterygoid process.
So if you've watched the tutorials on the skull, bones of the skulls, you'll remember
that the pterygoid process is a process which extends downwards from the sphenoid bone.
The lateral and medial pterygoid muscles onto the lateral plate of the pterygoid process.
So this is what I found a bit confusing.
The lateral pterygoid muscle is called the lateral pterygoid because it inserts onto
the lateral surface of the lateral pterygoid plate.
The medial pterygoid muscle is called the medial pterygoid because it inserts onto the
medial surface of the lateral pterygoid plate.
I'll just show you this quickly on a different model.
We've got the temporalis here, the masseter.
And then if I remove the masseter, you can see there are some muscles that lie deep to
the mandible.
If I just rotate it around, you can see on the inside surface of the mandible that there
are two muscles.
Unfortunately, this model doesn't actually have the medial pterygoid muscles, so what
we're looking at here is the lateral pterygoid muscles.
If I just show you from the outside, you can see how they sit underneath the mandible on
the interior surface.
I'll just swing it around again.
What you've got here are the lateral pterygoid muscles.
There's two parts.
We've got the superior lateral pterygoid and the inferior lateral pterygoid.
So this superior lateral pterygoid originates on the infratemporal surface of the greater
wing of the sphenoid and it inserts superiorly on the condylar process of the mandible.
This is the posterior process on the mandible.
The inferior lateral pterygoid inserts a little bit lower on the neck of the condyle of the
mandible.
It originates on the lateral plate of the pterygoid process.
So just to show you again.
The pterygoid process is this thing that's pointing down.
If I rotate it around, if you keep your eyes on this process here, this downward pointing
process, this is the pterygoid process.
So if I just remove the masseter and remove the mandible temporarily, you can see the
pterygoid process sticking down.
So it's this bone here, this thing sticking down.
And you've got medial and lateral plate.
So you've got this medial plate here and you've got the lateral plate.
So the pterygoid muscles originate on the lateral plate, but the medial pterygoid (which
isn't shown here) inserts on the medial surface and the lateral pterygoid inserts on the lateral
process.
This model isn't entirely accurate because this muscle isn't showing to be inserting
on the lateral plate, but in reality, it inserts on this lateral plate of the pterygoid process.
If I just get the mandible back, it inserts onto the condyle of the mandible.
So just to repeat that, you've got the pterygoid process here, which is part of the sphenoid
bone.
You've got the lateral plate and the medial plate.
But the confusing thing is that the medial pterygoid doesn't insert on the medial plate
(which is this thing here), it actually originates on the medial surface of the lateral pterygoid
plate and it inserts on the medial surface of the mandible.
So that's the medial pterygoid muscle, which isn't shown here unfortunately, but it originates
on the medial surface of the lateral pterygoid and it inserts onto the medial surface of
the angle of the mandible.
So the lateral pterygoid has two heads.
It's got two muscles.
It's got the superior and inferior lateral pterygoid.
The superior lateral pterygoid joins with the capsule of the temporomandibular joint
higher up on the condyle of the mandible and the inferior lateral pterygoid inserts on
the neck of the condyle of the mandible.
So those are the four muscles of mastication.
You've got the temporalis, the masseter (which is this muscle here) and you've got the medial
and lateral pterygoid muscles.
So the medial pterygoid muscles are involved in elevation and side to side movements of
the mandible.
The lateral pterygoid is involved in protrusion and side to side movements of the mandible.