Muscles of the Larynx – Part 1 – 3D Anatomy Tutorial

Muscles of the Larynx – Part 1 – 3D Anatomy Tutorial


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.