The first muscle I'm going to talk about is the cricothyroid muscle.
The cricoid has two muscles which are attached to it.
The first of which is the cricothyroid and we've also got the cricoarytenoid muscle,
which I'll come onto next.
We're looking anteriorly at the larynx.
You can see these muscles attaching to the anterior narrow arch of the cricoid cartilage.
You can see that this muscle has two distinct parts.
You've got a straight part and an oblique part.
The straight part or vertical part is attached to the inferior margin of the thyroid cartilage
and the oblique part is attached, you can see here, to the inferior horn of the thyroid
cartilage.
So let's take a look at the action of this muscle.
I'll just get rid of this muscle here which is the inferior pharyngeal constrictor and
remember, this attaches along the oblique line of the thyroid cartilage.
So important to remember is the cricothyroid joint is one of the articulations of the laryngeal
cartilages.
And this muscle produces an action at this joint.
So the inferior horn of the thyroid cartilage articulates with a facet on the lateral aspect
of the cricoid cartilage.
So when this muscle contracts, you can see that if it pulls this way, it's going to bring
the thyroid forward and this part can pull it downwards.
So you get forward and downward movement of the thyroid cartilage when the cricothyroid
muscle contracts.
So it's pulled downwards and forwards.
So what are the implications of this movement?
Well, just imagine if the thyroid cartilage is pulled forward and downwards, what's going
to happen to the vocal ligaments inside?
They're going to be stretched.
So if you stretch the vocal ligaments, you're going to get more tension in the vocal cords.
A greater amount of tension in the vocal cords is going to produce a higher pitched sound.
Remember the guitar string analogy.
If you've got a loose guitar string, it's going to make a very low sound.
If you've got a very tight guitar string, it'll produce a higher pitched sound.
So by bringing the thyroid cartilage forwards and downwards, tension is placed on these
vocal cords and it allows a higher pitched sound to be produced.
So the cricothyroid muscle is innervated by the external branch of the superior laryngeal
nerve, which comes from the vagus nerve.
So out of all the intrinsic muscles of the larynx (which I'm going to talk about), this
is the only one which is innervated by the external branch of the superior laryngeal
nerve.
All the rest of the muscles which I'm going to talk about are innervated by the recurrent
laryngeal branch of the vagus.
Remember that this one is an exception and it's innervated by the external branch of
the superior laryngeal nerve.
So next, we're going to look at some muscles which attach to the cricoid cartilage, to
the arytenoid cartilage.
So you can guess what these are called.
These are the cricoarytenoid muscles.
If I rotate the larynx around to the back and we'll just get rid of the thyroid cartilage
temporarily, we've got some muscles which sit in the posterior and the upper parts of
the cricoid arch, at the back of the cricoid cartilage.
They're not shown on this model, but they are called the posterior cricoarytenoid muscles.
You've got the lateral cricoarytenoid muscles.
These two parts have different functions.
So we'll first take a look at the posterior cricoarytenoid muscles.
So these, remember I mentioned that there's two depressions on either side of the posterior
lamina of the cricoid cartilage.
This is for the origin of the posterior cricoarytenoid muscles.
This is where the muscles originate.
So I've just switched over to a posterior view of the larynx.
It's showing these muscles clearly.
We're looking at the back.
This is the broad lamina at the back of the cricoid cartilage.
We've got the epiglottis up here shown with its mucosa, the aryepiglottic folds.
You can see the cuneiform and corniculate cartilages suspended in that.
And then you've got the posterior cricoarytenoid muscles attaching on the oval depressions
on either side of the midline of the posterior surface of the cricoid cartilage.
So these muscles insert onto the muscular process of the arytenoid cartilage.
If you remember, the arytenoid cartilage has two processes.
You've got the muscular process, which sticks out posteriorly and laterally and you've got
the vocal process which sticks out anteriorly and serves as a point of attachment for the
vocal ligament.
So just coming back to the 3D model, I'll just rotate the model so we can look superiorly.
You'll be able to see these two processes.
So you can see the process sticking anteriorly, the vocal process and the muscular process
sticking out posteriorly and laterally.
So the posterior cricoarytenoid muscle attaches onto this muscular process.
So what this does when it contracts is, if you imagine this muscular process being pulled
around this way, it's going to swivel the arytenoid cartilages around on their access,
so it will laterally rotate the arytenoid cartilages.
So remember that the distance between the two vocal folds is called the rima glottidis.
If you were to swivel the arytenoid cartilages around on their axis, laterally rotating them,
this is going to produce an opening of the rima glottidis.
So the posterior cricoarytenoid muscle opens the rima glottidis and it's the only muscle
that does this.
So it's important to get a grip of the sort of movements that occur at the cricoarytenoid
joint.
So the arytenoid cartilages are really interesting because they can be laterally rotated like
I showed you here.
They can also be abducted, so they can be moved side to side.
And they can also be rocked backwards and forwards.
So there's a few movements that the arytenoid cartilages can do.
So the next part of the cricoarytenoid muscle is the lateral cricoarytenoid.
This originates on the upper part of this cricoid arch.
So it sits laterally on the cricoid cartilage and it inserts again onto the muscular process.
But because of its different origin, it's going to produce a different action on the
arytenoid.
Because it sits in front of the muscular process, when it contracts, it's going to produce the
opposite action to the posterior cricoarytenoid muscle.
So when the lateral cricoarytenoid muscle contracts, it's going to pull the muscular
process this direction.
It's going to internally rotate the arytenoid cartilages and it's going to close the rima
glottidis.
So this results in adduction of the vocal cords and it brings the vocal cords together.
So in this diagram, you can see the lateral cricoarytenoid here sitting on the top of
the cricoid arch, on the upper part of the cricoid arch.
So this is a view from the medial aspect.
It's been kind of dissected in half.
You can see the thyroid cartilage has been cut here and you're looking at the medial
aspect of the lateral cricoarytenoid here.
It originates from the upper parts of the cricoid cartilage and it inserts into the
muscular process, but it produces the opposite action of the cricoarytenoid posterior muscle.
So remember I said that all the muscle, intrinsic muscles of the larynx are innervated by the
recurrent laryngeal nerve except for the cricothyroid muscle which is innervated by the external
branch of the superior laryngeal nerve.