Introduction to Muscle Actions of the Larynx
This is a tutorial on the muscles of the larynx. When you're thinking of the muscles of the larynx, it's useful to think in terms of the functions of the larynx and also to think of the different compartments of the larynx.
In terms of the compartments of the larynx, which I went over in the last tutorial, think about the laryngeal inlet and then think about the three chambers – you’ve got the vestibule, the middle chamber and the infraglottic compartment. Think about how these can close and open in relation to the function of the larynx, to aid respiration and to produce voice and phonation.
For instance, if you want to protect the respiratory tract, you're going to want to stop stuff coming into the airways, into the trachea below. You’re going to want to narrow the vestibule. You want to constrict and oppose these two mucosal surfaces and you want to bring the rima vestibuli together.
Remember, the rima vestibuli is the space between the vestibular folds on either side and the rima glottidis is the distance between the two vocal folds. You’ll want to narrow the rima glottidis and the rima vestibuli and you'll want to close off the laryngeal inlet by bringing the epiglottis forward and closing the laryngeal inlet.
All these actions to close the various compartments and folds will prevent things coming into the trachea.
And in terms of phonation, if you imagine the lungs blowing out air. If you imagine air coming out towards you, towards the screen, it would come out through your trachea and rub, blow past the vocal folds and it would produce noise. The air rushing past the vocal folds produces a frequency of vibration in these vocal folds. This frequency of vibration produces noise.
This is just similar to a musical instrument. If you pluck a guitar string, it oscillates at a certain frequency. That produces noise. If you think about adjusting the tension of the guitar string, that's going to change the pitch of the sound produced. If you have a loose guitar string, it'll produce a very low sound and if you stretch the guitar string it’s tight, it will produce a very high sound.
When we talk about the muscles of the larynx, think how these can affect the tension on the vocal ligament and vocal folds which would thereby produce a change in the pitch of the sound produced if air blows past it.
think about how the laryngeal inlet can be opened or closed, how the vestibule can be narrowed, how the middle chamber can be narrowed and how the vestibular folds and vocal folds can be brought together, narrowing or expanding the distance between the rima glottidis and the rima vestibuli.
Muscles of the Larynx - Part 1
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. 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.
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. The inferior horn of the thyroid cartilage articulates with a facet on the lateral aspect of the cricoid cartilage.
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. You get forward and downward movement of the thyroid cartilage when the cricothyroid muscle contracts. It’s pulled downwards and forwards.
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. 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. 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.
The cricothyroid muscle is innervated by the external branch of the superior laryngeal nerve, which comes from the vagus nerve. 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.
Next, we're going to look at some muscles which attach to the cricoid cartilage, to the arytenoid cartilage. 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.
We’ll first take a look at the posterior cricoarytenoid muscles. 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.
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.
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.
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. You can see the process sticking anteriorly, the vocal process and the muscular process sticking out posteriorly and laterally.
The posterior cricoarytenoid muscle attaches onto this muscular process. 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.
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. The posterior cricoarytenoid muscle opens the rima glottidis and it's the only muscle that does this.
It’s important to get a grip of the sort of movements that occur at the cricoarytenoid joint. 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 are a few movements that the arytenoid cartilages can do.
The next part of the cricoarytenoid muscle is the lateral cricoarytenoid. This originates on the upper part of this cricoid arch. 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.
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. This results in adduction of the vocal cords and it brings the vocal cords together.
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. 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.
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.
Muscles of the Larynx - Part 2
We’ve looked at muscles which attach to the cricoid to the thyroid and produced downward and forward movement which helps to tighten the vocal cords and produce higher pitched sounds. And then we looked at the posterior and lateral cricoarytenoid muscles which attach to the cricoid to the arytenoid cartilages. And now we're going to look at the muscles which connect the arytenoid cartilages together.
We’ve got interarytenoid muscles because they connect; they run between both cartilages, the arytenoid cartilages. You’ve got two muscles which run between the arytenoid cartilages. We've got a transverse arytenoid muscle and we've got an oblique arytenoid muscle.
We’re looking at a posterior view of the cricoid with the arytenoid cartilage sitting on top. We've got the transverse arytenoid muscle running from the posterior lateral surface of one arytenoid cartilage to the posterior and lateral surface of the other arytenoid cartilage. You can see that if this muscle contracts, it's going to bring the arytenoid cartilages closer together. It adducts the arytenoid cartilages. And again, it's innervated by the recurrent laryngeal nerve.
And then we've got the oblique arytenoid muscles. These originate posteriorly on the surface of the muscular process. Now we've seen two muscles attaching to this muscular process already, the posterior and lateral cricoarytenoid. And now we're seeing a third. This is the oblique arytenoid muscle which originates from the posterior surface of the muscular process. And then it runs obliquely (which gives its name) and attaches on the opposite cartilage. It attaches at the back of the apex of the other arytenoid cartilage.
You’ve got two of these. The other one obviously has the same origin and insertion. It runs from the posterior surface of the muscular process of the arytenoid cartilage and inserts on the apex of the other arytenoid cartilage.
If we just rotate this model around, the oblique arytenoid muscle is quite interesting because some sources say that the muscle continues past. When it reaches the apex of the arytenoid cartilage, it continues around and forms this muscle which attaches to the epiglottis. This is referred to as the aryepiglottic muscle and it's sometimes considered part of the oblique arytenoid muscle. It’s considered a continuation of the oblique arytenoid muscle.
If you imagine this oblique arytenoid muscle attaching from the muscular process to the apex of the arytenoid cartilage opposite and then you've got this aryepiglottic muscle which might be a continuation of this muscle, but it attaches from the arytenoid cartilage and it runs up in the aryepiglottic fold to attach to the epiglottis, so you've got this aryepiglottic muscle.
You can see if this aryepiglottic muscle were to contract, it would close the laryngeal inlet – bring the epiglottis down and close the laryngeal inlet. Whether or not the aryepiglottic muscle is considered a part of the oblique arytenoid or not, just remember that there is a muscle that there is a muscle attaching from the arytenoid cartilage to the epiglottis and it can close the laryngeal inlet.
The interarytenoid muscle, the transverse arytenoid and the oblique arytenoid and the aryepiglottic parts or the aryepiglottic muscle are innervated by the recurrent laryngeal nerve.
The last muscle we're going to talk about is the thyroarytenoid muscle. The best way to show you this is through a cross section. I'm going to rotate the thyroid cartilage around laterally and I'm going to slice it through the middle. We’re now looking at a slice of the larynx. I've cut it through the midline, so you can see it like that. I'll just rotate it around laterally and we'll be able to look at the inside of this cartilage now.
Remember between the cricoid cartilage and the thyroid cartilage, you've got this cricothyroid membrane. The thyroarytenoid muscle attaches from the thyroid cartilage to the arytenoid cartilage. It originates around this area here. It attaches to the thyroid angle and a little bit to the cricothyroid membrane below. And then it attaches to the anterolateral surface of the arytenoid cartilage. It’s quite a broad muscle.
You've also got some fibers given off from the thyroarytenoid muscle onto the epiglottis. You’ve got fibers coming up on the lateral surface of the epiglottis. This is referred to as the thyroepiglottic muscle or the thyroepiglottic part of the thyroarytenoid muscle. Remember, you've also got the aryepiglottic muscle which is a part of the oblique arytenoid muscle.
The thyroarytenoid muscle is often thought of as two parts. You’ve got the vocalis muscle and you've got the thyroarytenoid part. It depends on the source you look at, but different sources tell you different thing. In this tutorial, we're going to look at the vocalis muscle as a separate muscle and the thyroarytenoid as a separate muscle.
The thyroarytenoid muscle originates on the thyroid angle and below, it has attachments to the cricothyroid ligament and then it inserts onto the anterolateral surface of the arytenoid cartilage. And then it gives off these fibers which attach to the lateral surface of the epiglottis. This is the thyroepiglottic muscle. And I'll just draw on this muscle here which connects the arytenoid to the epiglottis. And this is the aryepiglottic muscle.
I'm just showing you this diagram here which shows a nice illustration of what I've shown you. We’re looking at the same kind of section with the thyroid cut through the midline. You've got these fibers going up to the epiglottis, the thyroepiglottic muscle. You've got the broad thyroarytenoid muscle and you've got the aryepiglottic muscle here.
The thyroarytenoid muscle has two parts. We’re going to look at the vocalis muscle, which is considered a part of the thyroarytenoid muscle.
Just coming back to the 3D model, what we're going to do is we're going to rotate it around anteriorly and then we're going to look superiorly down at the larynx. From this view, you can see the attachment of the vocalis muscle. The vocalis muscle runs lateral to the vocal ligament. You can see the vocal ligament here. The vocalis muscle, it originates on the lateral surface of the vocal process and it inserts along the vocal ligament. It attaches along the vocal ligament and it also attaches at the thyroid angle.
The function of the thyroarytenoid muscle and the vocalis muscle is to adjust the tension in the vocal cords. Remember we looked at the cricothyroid muscle which could stretch the vocal ligament and thereby produce higher pitched sounds, the thyroarytenoid muscle, when it contracts, it brings the arytenoid cartilage closer to the thyroid cartilage. This results in a slackening or a loosening of the vocal ligaments. This produces lower pitched sounds.
the vocalis muscle which we're looking at separately has the function of making small adjustments which result in changing the pitch and the quality of the voice and phonation and the sounds produced, whereas the broad sheets of muscles, the rest of the thyroarytenoid muscle is responsible for larger adjustments of the tension in the vocal cords.
I just thought I'd show you this diagram as well just to show you the relationship of the thyroarytenoid muscle to the other structures in the larynx. We’re looking at a coronal section here. You can see the epiglottis here, the aryepiglottic folds dropping downwards from the lateral aspect of the epiglottis. And then you've got the vestibular folds folding around the vestibular ligament. And then you've got the laryngeal ventricle and you've got the vocal fold folding around the vocal ligament.
I'll just draw on the vocal fold here. This is the most medial structure I'm going to show you. And then we're just inside that. We've got the cricothyroid ligament. That connects from the – this is the cricoid cartilage down here. So attaching from the cricoid cartilage, you've got the cricoid ligament. And remember the upper border of the cricoid ligament forms the vocal ligament where the vocal folds fold around. And then just lateral to that, you've got the thyroarytenoid muscle, you can see that here.
That should give you some idea of the kind of relationship of the muscles to the folds and the membranes and ligaments and that kind of thing.
That’s the thyroarytenoid muscle. We’ve covered all the muscle of the larynx now. It’s useful just to think in terms of how they affect the opening and closing of the rima glottidis, how they affect the tension of the vocal cords and these functions affect phonation and protection of the respiratory tract.