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Larynx

Cartilages, ligaments, membranes and muscles of the larynx.

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We’ve all been there. The lights are pointing at you, your mic in hand, the audience is waiting with bated breath, you open your mouth, and let go. It sounds like angels are playing on heavenly harps. It feels like you alone have been given the gift of music. Until you open your eyes and realized that even the running shower water couldn’t drown this. But even if you’re not going to win the next Grammy Award, it doesn’t mean that the sounds you make are any less fascinating. Did you ever stop and wonder where on earth this sound actually comes from? Well, we’re going to find out all about it in our tutorial on the structures of the larynx.

So, you’ve finally decided to learn about the larynx, and yet, a little voice in your head is telling you to click out of this tutorial and watch some more pleasant kitten videos instead. Don’t do it. I know. It seems daunting, but we’ve broken up this topic into three easily manageable parts. First, we’ll cover all the cartilages which are found in the larynx and the joints that they form, then we’ll tackle the ligaments and the membranes stretching between these cartilages, and finally, we’ll address all the muscles found in the larynx and the movements that they produce. So, although it might be quite a complex structure, it won’t seem so daunting when it’s broken down into smaller components.

First up, though, let’s talk a little bit about what the larynx is and where we find it. The larynx, also commonly known as the voice box, is located in the anterior neck anterior to the cervical part of the esophagus. It’s quite handy to think about it as the link between the pharynx and the trachea. It’s pretty easy to get confused between the pharynx and the larynx, but if you remember that both food and air pass through the pharynx, but only air passes through the larynx, that should make things a little easier.

Although the larynx is mobile during swallowing and the process of making sound, it’s said that at rest, the larynx sits at the level of the third to sixth cervical vertebrae. This is correct for your average adult male, but in females and children, it tends to sit slightly higher. The larynx also tends to be larger in males and one of its cartilages, the thyroid, continues to grow until the age of forty. It projects anteriorly in the midline forming what we know as the Adam’s apple.

The larynx serves many important and diverse functions. For instance, it protects our airways from large swallowed matter, which could choke us, it regulates the flow of air into the lungs, and finally, it is involved in the production of sound known as phonation.

So, let’s take a look at the general anatomy of the larynx as a whole, using this illustration of a posterolateral view of the larynx. Let’s start with the laryngeal inlet. The laryngeal inlet is the entrance from the pharynx into the larynx bordered by the free edge of the epiglottis anteriorly, laterally by the aryepiglottic folds, and posteriorly by the cuneiform and corniculate tubercles as well as the interarytenoid notch. Don’t’ worry about these structures too much. We’ll be going over them later in the tutorial.

At its most basic level, the larynx contains three main spaces. The first is the vestibule. It is the most superior space of the larynx extending from the laryngeal inlet to the vestibular folds. The middle smallest space of the larynx is known as the ventricle and refers to the small space between the vestibular folds and the vocal folds. The most inferior space within the larynx, continuous with that of the trachea, is known as the infraglottic cavity. It stretches between the inferior aspect of the vocal folds and the first tracheal ring. The last structure we should define here is the opening between the vocal folds and the arytenoid cartilages known as the rima glottidis.

So now that we’ve had a general look at the larynx, we can now turn our attention to the cartilages of the larynx.

So once again here, we have our main illustration, which we will be using throughout this tutorial, and it presents a beautiful view of all the cartilages and ligaments of the larynx from the posterolateral aspect. The hyoid bone here is always a good way to orientate yourself as it sits anterior to the larynx and is easy to identify, but it’s not considered part of the larynx.

Now regarding the cartilages, there may be some conflicting information out there. So, it’s universally agreed that we have three unpaired cartilages – the epiglottis, the thyroid, and the cricoid cartilages. The paired cartilages are the ones where you might struggle. The arytenoid, the corniculate, and the cuneiform cartilages will appear in all the sources that you’ll look at, but the triticeal cartilages may or not be mentioned. Want to know why? Well, stay with us and you’ll find out.

Why don’t we start with the epiglottis, possibly, the best-known cartilage of the larynx. It’s a leaf-shaped piece of elastic cartilage which sits posterior to the hyoid bone and anterior to the laryngeal inlet allowing the passage of air. The main function of the epiglottis is to close off this inlet during swallowing and prevent food from entering the airways. As you can see, the epiglottis has a free edge and an attached part. The free superior part is broad, rounded, and occasionally, has a notch in the midline.

The stalk of the epiglottis is the attached inferior stem-like part of this cartilage. It is connected to the posterior surface of the laryngeal prominence of the thyroid cartilage by the thyroepiglottic ligament. It’s quite interesting that the anterior or the lingual surface of the epiglottis is covered by mucosa very similar to the mucosa of the tongue and even contains some taste buds. The posterior or laryngeal surface is covered in ciliated mucosa typical of the respiratory tract.

Let’s move on now to the largest cartilage of the larynx – the thyroid cartilage. It’s actually formed of two hyaline cartilage laminae which fuse in the midline. The inferior two-thirds form the laryngeal prominence or the Adam’s apple. You’ll also often hear it refer to as the thyroid angle. Remember the epiglottis cartilage attaches to the posterior aspect of this eminence. Above the prominence, we have a V-shaped superior thyroid notch.

As the thyroid cartilage extends posterolaterally, it expands into a superior horn and inferior horn, which are also known as the superior or inferior cornua. Here, it is easy to identify a raised ridge just anterior to the cornu known as the oblique line. It is limited superiorly by a rounded raised area known as the superior thyroid tubercle, which is located just anterior to the root of the superior cornu and inferiorly by the inferior thyroid tubercle on the inferior border of the lamina.

We’re now moving on to the last unpaired cartilage of the larynx – the cricoid cartilage. The cricoid cartilage is a complete ring of hyaline cartilage between the thyroid cartilage and the trachea. It can be divided into two parts – the anterior arch of the cricoid cartilage which is curved, while the posterior lamina of the cricoid cartilage is flattened.

In the midline of the lamina, we find a raised ridge known as the vertical ridge of the cricoid cartilage which creates two concavities to either side of it. Although smaller, the cricoid cartilage is thicker and stronger than the thyroid cartilage. You’ll notice over here a round depression, and this is known as the articular surface of the cricoid cartilage which articulates with the inferior cornu of the thyroid cartilage. Along the superior border of the cricoid lamina is the arytenoid articular surface which articulates with the smaller arytenoid cartilage.

We’re now moving on to the paired cartilages of the larynx, and first stop, we have the arytenoid cartilages, which we just mentioned. These little hyaline cartilages are pyramidal in shape with three surfaces and a base which sits on the cricoid cartilage. It tapers into an apex which articulates with the corniculate cartilage above.

There are two projections on the arytenoid cartilage. The vocal process is an elongated sharp projection of the anterior surface of the arytenoid cartilage while the muscular process is rounded and projects posterolaterally. These are important attachment sites and we’ll cover the structures which anchor them later on in the tutorial.

That brings us to the corniculate cartilages, which sit on top of the arytenoid cartilages. These little elastic cartilages are considered minor cartilages and it’s not hard to see why. They’re conical in shape and can sometimes be fused to the arytenoid cartilages.

Now, we’re looking at the cuneiform cartilages. They are tiny, elongated nodules of elastic cartilage located within the aryepiglottic folds which we’ll see shortly.

Okay, that brings us to our controversial pair – the triticeal cartilages. Okay, so these cartilages are not necessarily controversial, but there are definitely plenty of sources which choose to omit them when talking about the laryngeal cartilages. That’s because they’re extremely variable. A study found that the elastic triticeal cartilages were only present in about a third of their sample members and could be found either bilaterally or unilaterally. When present, these cartilages are suspended in the free edge of the thyrohyoid membrane.

I’m sure you’ve noticed me talking about the hyaline and elastic cartilage, so just to recap, the thyroid, cricoid, and most of the arytenoid cartilages consist of hyaline cartilage. Hyaline cartilage is mostly formed of type two collagen which makes it quite firm and strong. That makes sense as the listed cartilages connect the pharynx to the trachea and our attachments sites for various muscles and ligaments.

The remaining cartilages – that is, the corniculate, the cuneiform, the triticeal cartilages, the epiglottis, and the vocal processes of the arytenoid cartilages – consist of elastic cartilage. Elastic cartilage as well as collagen contains elastic fibers. Its defining quality is, of course, flexibility versus the strength of hyaline cartilage. This is what allows them to be flexible and able to assist in performing the various functions of the larynx.

So, we’ve briefly touched on the articular surfaces which some of the laryngeal cartilages possess, but it’s a good roundup of all the joints of the larynx in one place.

We have three pairs of joints in the larynx, and the first is the synovial cricothyroid joint, which is formed by the articular surfaces of the inferior cornu of the thyroid cartilage and the lateral aspects of the cricoid cartilage. You can now see the capsule which covers the joint highlighted now.

Next stop are the synovial cricoarytenoid joints located between the lateral aspects of the superior edge of the cricoid cartilage and the bases of the arytenoid cartilages. Just like the cricothyroid joints, they are enclosed in fibrous capsules.

Our final pair of joints are the aryteno-corniculate joints. These little cartilages can either be synovial or cartilaginous.

So that brings us to the end of our exploration of the laryngeal cartilages. But let’s keep our momentum going and continue on to the ligaments and membranes of the larynx.

The soft tissues of the larynx can be divided into an extrinsic group, which connects parts of the larynx to other structures, and an intrinsic group, which connects parts of the larynx to each other. First up, let’s talk about the extrinsic structures.

So, right now, you can see the thyrohyoid membrane, highlighted in green, and it is a broad fibroelastic membrane attached inferiorly to the superior margin of the thyroid cartilage and its superior horns and superiorly to the superior margin of the body of the hyoid and its greater horns. The medial part of the thyrohyoid membrane is thickened and forms the median thyrohyoid ligament. The lateral free edges of the membrane between the tips of the greater horns of the hyoid and superior horns of the thyroid cartilage are also thickened. These structures are known as the lateral thyrohyoid ligaments. Suspended in them, you’ll find our little triticeal cartilages which we discussed in the first section of this tutorial.

In the thinner membrane between the anterior and lateral thyrohyoid ligaments, you’ll find a little aperture which allows the passage of the superior laryngeal artery and the internal laryngeal nerve. Stretching between the inferior edge of the cricoid cartilage and the first tracheal ring, we find the cricotracheal ligament, and it blends with the fibroelastic tissue found between the tracheal rings.

The last little extrinsic ligament we’re talking about is the hyoepiglottic ligament, and just like most structures of the larynx, the name of this ligament is a big giveaway to its location. This elastic band attaches to the upper border of the hyoid bone and the anterior surface of the free part of the epiglottis.

So, we’re now moving on to the intrinsic soft tissues or structures which link parts of the larynx to each other. We’re starting this section with the quadrangular membrane, which is a layer of submucosa and these broad thin sheets of connective tissue extend from the lateral edges of the epiglottis to the arytenoid cartilages. The quadrangular membrane is covered in mucosa and its superior free border is known as the aryepiglottic fold, which forms the lateral border of the laryngeal inlet.

At the inferior end of each aryepiglottic fold is the cuneiform tubercle, which contains the cuneiform cartilages we looked at earlier. The inferior free edge of the quadrangular membrane forms the vestibular fold, also commonly known as the false vocal cord. These folds do not play a part in sound production, but rather are protective structures. Also attached to the epiglottis is the thyroepiglottic ligament. As you can see, it connects the stalk of the epiglottis to the posterior surface of the thyroid cartilage. We’ve actually already encountered this ligament when we were talking about the epiglottis.

Now the next structure can be a bit confusing due to its inconsistencies in literature, so we’re going to be breaking it down for you. Overall, the structure that we’re talking about is the cricothyroid membrane, or ligament, and it refers to the entirety of the connective tissue between the cricoid and thyroid cartilages. Now, the most medial part of the cricothyroid membrane is quite a bit thicker and is known as the median cricothyroid ligament, sometimes known as the anterior cricothyroid ligament. Its inferior attachment is found at the superior border of the cricoid arch, while superiorly, it attaches to the inferior border of the thyroid cartilage,

Now the thicker and more lateral portions are known as the conus elasticus. They attach inferiorly to the superior border of the cricoid arch and lamina. Superiorly, however, instead of attaching to the inferior margin of the thyroid cartilage, the conus elasticus attaches to the posterior aspect of the thyroid angle. Posteriorly, it also attaches to the lower part of the arytenoid cartilage up to the lip of the vocal processes.

The superior free edges of the conus elasticus are much thicker and are given their own name, and they are known as the vocal ligaments. They attach to the vocal processes of the arytenoid cartilages and the inner surface of the thyroid cartilage angle. In reality, vocal ligaments are covered in a mucous membrane. The two structures combined are known as the vocal folds. The vocal folds are, of course, what we commonly know as the vocal cords and are essential for producing sound.

And that finally wraps up the membranes and the ligaments of the larynx. Let’s move on now to look at the final piece of the puzzle and learn all about the muscles and the movements of the larynx.

Similarly to the other soft tissues of the larynx, the muscles can be divided into intrinsic and extrinsic groups. The extrinsic muscles generally tend to be attached to the hyoid bone, which as we saw previously is connected to the thyroid cartilage by the thyrohyoid membrane. This arrangement means that the muscles acting on the hyoid bone also mobilize the larynx.

The extrinsic muscles of the larynx generally do one of two things – they are to elevate or depress the larynx during swallowing and speech. The depressors include most of the infrahyoid muscles, notably, the sternohyoid muscle, the sternothyroid muscle, and the omohyoid muscles. The list of elevators is a little bit more extensive. So, the last of the infrahyoid muscles is the thyrohyoid muscle, which actually elevates the larynx when the hyoid is fixed. The rest of the elevators are the suprahyoid muscles – the digastric, the stylohyoid, the geniohyoid, and the mylohyoid, as well as the stylopharyngeus. Some sources also include the palatopharyngeus and the salpingopharyngeus in this list.

So rather than moving the larynx as a whole, the intrinsic muscles rather move as its separate components. All but one muscle of the larynx are innervated by the inferior laryngeal nerve which is a branch of the recurrent laryngeal nerve of the vagus nerve. It’s also helpful to appreciate the actions of these intrinsic muscles when looking down from the superior view, so we’ll use this illustration a lot in this part of the tutorial, so it’s good to make sure that you’re familiar with all the visible cartilages – for instance, the thyroid cartilage, the cricoid cartilage, and, of course, the arytenoid and corniculate cartilages.

We also can see a number of ligaments here such as the vocal ligaments and the conus elasticus and, of course, we’ll get to know all the muscles as we progress through this part of the tutorial. Now let’s have a look at each of them in a little bit more detail and talk about what role they play in sound production.

So, let’s start with the exception to our innervation rule – the cricothyroid muscle. Unlike the rest of the intrinsic muscles, it is innervated by the external laryngeal nerve, which is a branch of the superior laryngeal nerve of the vagus nerve. So, this muscle actually has two parts with a single origin at the anterolateral superior edge of the cricoid cartilage. The straight part inserts into the inferior margin of the thyroid cartilage, while the oblique part inserts into the inferior horn of the thyroid cartilage. These muscles draw the thyroid cartilage anteroinferiorly which in turn lengthens and stretches the vocal ligament, and that results in a high-pitched sound.

Next, we have a group of muscles which can be collectively described as the adductors of the arytenoid cartilages. So, first stop, we’re looking at the only unpaired muscle of the larynx, and this is the transverse arytenoid muscle. It lies deep to the oblique arytenoid muscles which we’ll look at in a moment and attaches to the muscular processes of the arytenoid cartilages and the adjacent lateral border. With very similar attachments, we find the oblique arytenoid muscles, and these muscles are superficial to the transverse arytenoid muscle and stretch from the muscular process of the arytenoid cartilage to the apex of the contralateral arytenoid cartilage.

So, both the transverse and oblique arytenoid muscles do the same job of adducting the arytenoid cartilages and, therefore, closing the rima glottidis. Air forced between the adducted vocal cords produces tone.

Next up, we have a pair of cricoarytenoid muscles which true to their name, have attachments to the cricoid and arytenoid cartilages. Although similar in name, they are opposite in function. The lateral cricoarytenoid muscle, which is now highlighted in green, arises in the arch of the cricoid cartilage and inserts into the muscular process of the arytenoid cartilage. When the lateral cricoarytenoid muscle contracts, it acts to swivel the muscular process laterally which in turn adducts and shortens the vocal folds.

When the lateral cricothyroid muscle acts in isolation, it leaves a small opening in the posterior intercartilaginous space of the rima glottidis, and this facilitates a whisper-type of phonation.

The posterior cricoarytenoid muscle is antagonist, meaning that it does the opposite job. It arises in the posterior surface of the cricoid lamina and also inserts into the muscular process of the arytenoid cartilage, just like its lateral counterpart. These attachment points mean that when it contracts, it pulls the muscular process medially and in turn abducts and lengthens the vocal folds, opening the rima glottidis allowing air to flow down into the lungs. When the vocal folds are full opened, phonation is not possible.

Up next, we have the aryepiglottic muscle, and it is often considered to be an extension of the oblique arytenoid muscle as some of its fibers continue laterally past the apex of the arytenoid cartilage and continuing into the aryepiglottic fold. To be more specific, its attachments are the muscular process of the arytenoid cartilage and the lateral border of the epiglottis. It shares the same function as the transverse and oblique arytenoid muscles which is adduction of the arytenoid cartilages and closure of the rima glottidis.

Okay, let’s move on now and talk about the thyroarytenoid muscle. The thyroarytenoid muscle stretches between the angle of the thyroid cartilage and the adjacent cricothyroid ligament to the anterolateral surface of the arytenoid cartilage parallel to the vocal ligament. The thyroarytenoid ligament acts to draw the arytenoid cartilages anteriorly, in turn, shortening and relaxing the vocal ligament. And this results in a deeper pitch.

The vocalis muscle is similar to the thyroarytenoid muscle by its attachments and it also runs parallel and lateral to the vocal ligament and is positioned medial to the thyroarytenoid muscle. It originates on the lateral surface of the vocal process of the arytenoid cartilage and inserts into the vocal ligament on the same side. Due to such proximity to the thyroarytenoid muscle, the vocalis muscle is considered by some to be its deep part and by some to be a separate muscle.

The vocalis muscle has a variable function in terms of tensing and relaxing the vocal folds as it provides minute adjustments to the pitch by either tensing the anterior part of the vocal fold or relaxing the posterior part of the vocal fold. It’s especially used in singing as small nuances in pitch are very important.

So that brings us to our final intrinsic muscle of the larynx, and this is the thyroepiglottic muscle, which is also associated with the thyroarytenoid muscle. Both muscles share a single origin at the angle of the thyroid cartilage, yet the thyroepiglottic muscle has a very distinct separate insertion into the lateral aspect of the epiglottis. For this reason, some sources considered these two to be a single muscle, and some to be two independent muscles. It acts to widen the laryngeal inlet by pulling the vocal folds laterally.

So, while it’s useful to know each muscle individually, it’s also helpful to see them grouped by action to better understand the sound production in the larynx. Let’s recap.

The adductor and abductor muscles of the larynx open and close the rima glottidis by moving the vocal folds. This controls phonation or the production of sound. The main adductors are the lateral cricoarytenoid muscle, in addition to the transverse and oblique arytenoid muscles. The only abductor is the posterior cricoarytenoid muscles.

So, while adductors and abductors control phonation, or the production of sound, the tensors and relaxers control the pitch of the sound. The main tensors are the cricothyroid muscles, while the primary relaxers are the thyroarytenoid muscles. Fine adjustments to pitch made during speaking or singing are produced by the vocalis muscle.

Many of the intrinsic muscles also combine their actions together to act as a sphincter. This function has nothing to do with speech, but is rather protective. The lateral cricoarytenoids, transverse and oblique arytenoids, and the aryepiglottic muscles bring the laryngeal cartilages closer to the epiglottis thus closing off the laryngeal inlet. This action is a reflex in response to liquid or food particles in close proximity to the larynx.

Okay, so that wraps up the anatomy part of this tutorial, but before we finish for the day, let’s take a look at how this anatomy manifest in a clinical setting.

So, today, we’re going to be talking about muscle tension dysphonia. It refers to a difficulty or pain in sound production. It can manifest as change in the vocal quality, more effort required to make sound, voice fatigue, pain, or general discomfort. In extreme cases, aphonia, or inability to speak, may be another symptom. MTD can be primary or secondary. The exact cause of primary MTD is often unclear, ranging from allergies, an illness to emotional distress. Secondary MTD has a slightly different mechanism in that its primary cause is normally an underlying physical problem such as nodules, polyps, or vocal fold paresis which refers to paralysis due to nerve damage. In such cases, MTD develops due to the body’s attempt to compensate for whatever laryngeal muscle function is lacking.

So how would a physician diagnose this condition? During a routine examination, the tenderness of the neck muscles surrounding the larynx and the narrowing of the space between the hyoid and the thyroid would indicate tension in the laryngeal muscles. During a laryngoscopy, it often becomes apparent that the vocal folds are not used for speaking and the surrounding muscles tense to compensate for it.

The treatment usually includes voice therapy. For primary MTD, potential triggers to its cause are addressed as well. In secondary MTD, the underlying cause is treated as well as voice therapy for MTD.

Okay, so that’s everything that I wanted to cover and teach you today, but since it’s such a busy tutorial, it would make some sense to summarize everything before we go. So, let’s have a quick overview of what we talked about today.

So, our tutorial today on the larynx was based on three main areas of interest. The first of these was the cartilages of the larynx. Here we identified and spoke about the anatomy and functions of the epiglottis, the thyroid cartilage, the cricoid cartilage, the arytenoid cartilages, the corniculate cartilages, the cuneiform cartilages, and finally, the variably present triticeal cartilages.

We then went on to look at the main ligaments of the membranes of the larynx and we started with the large thyrohyoid membrane found between the thyroid cartilage and the hyoid bone. We saw it contained two defined ligaments – the median and lateral thyrohyoid ligaments. We then looked at the cricotracheal ligament which extended from the cricoid cartilage to the first cartilaginous ring of the trachea.

We then moved on to the quadrangular membrane which formed much of the vestibular of the larynx. Its superior free border formed the aryepiglottic fold while its inferior free border formed the false vocal cords known as the vestibular folds. After that, we looked at four final ligaments which were the thyroepiglottic ligament, the median cricothyroid ligament, the conus elasticus, and the all-important vocal ligaments.

Finally, we explored the many muscles of the larynx focusing mainly on its intrinsic muscles. These included the cricothyroid muscle, the posterior and lateral cricoarytenoid muscles, the oblique and transverse arytenoid muscles, the aryepiglottic muscle, the thyroarytenoid muscle, the vocalis muscle, and finally, the thyroepiglottic muscle.

And that’s it! Talk about a jampacked video tutorial! But trust me, you’ll be singing your way through exams SES after this. Until next time, happy studying!

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