Anatomy of the larynx and trachea

Transcript

The cervical viscera are composed of three layers named after their primary function.

The most superficial layer is the endocrine layer which contains the thyroid and parathyroid glands, then there’s the respiratory layer which contains the larynx and trachea and finally, the deepest layer is the alimentary layer which contains the pharynx and esophagus.

Focusing on the respiratory layer of cervical viscera, this contains the larynx and trachea that routs air to the lungs, and gives everyone their own unique voice… and saying that just made me aware of my own voice on this video recording!

So, let’s start with the larynx which is responsible for voice production and maintaining a patent airway.

The larynx is located in the anterior neck at the level of the bodies of the C3 to C6 vertebrae and connects the inferior part of the pharynx with the trachea. It also contains the vocal cords, or vocal folds, so it’s responsible for voice production.

Now, the larynx has a skeleton… which is hardly a skeleton; rather, it’s formed of nine cartilages: thyroid, cricoid and epiglottic cartilages which are single and arytenoid, corniculate and cuneiform cartilages which come in pairs of two.

The thyroid cartilage is the largest cartilage and its superior border lies opposite the C4 vertebra.

It’s formed by two plate-like laminae which unite in the middle to form the laryngeal prominence also known as the Adam’s apple, which is well marked in males and less visible in females.

Superior to this prominence, the laminae diverge to form a V-shaped notch called the superior thyroid notch, and in the middle of the inferior border of the cartilage, a shallow indentation can be found called the inferior thyroid notch.

The posterior border of each lamina projects superiorly as the superior horn and inferiorly as the inferior horn. The superior border and the superior horns attach to the hyoid bone through a membrane called the thyrohyoid membrane.

This membrane has a thick median part which represents the median thyrohyoid ligament and two lateral parts which represent the lateral thyrohyoid ligaments.

The inferior horns articulate with the lateral surfaces of the cricoid cartilage and form the cricothyroid joints which allow rotation and gliding of the thyroid cartilage resulting in changes in the length of the vocal folds.

The cricoid cartilage is shaped like a ring with a thin anterior band and a broad posterior band, and it’s the only complete ring of cartilage to encircle any part of the airway.

Its posterior part is the lamina of the cricoid, and the anterior part is the arch, which attaches superiorly to the inferior margin of the thyroid cartilage through the median cricothyroid ligament, and inferiorly, to the first tracheal ring through the cricotracheal ligament.

The cricoid cartilage is also a key landmark in the neck, because it’s situated at the level of the C6 vertebra.

This is where the carotid artery can be compressed against the C6 vertebra and also, this is where the larynx and trachea join, where the pharynx joins the esophagus, and the point where the recurrent laryngeal nerve enters the larynx.

The arytenoid cartilages are paired, pyramidal-shaped cartilages that articulate with the lateral parts of the superior border of the cricoid cartilage lamina.

Each arytenoid cartilage has an apex superiorly, where the corniculate cartilage can be found. Connecting the arytenoid and epiglottic cartilages there is a thin, submucosal sheet of connective tissue called the quadrangular membrane.

This membrane has a free superior margin that forms the aryepiglottic ligament. This ligament is covered by mucosa to form the aryepiglottic fold.

Posterior to the ary-epiglottic folds, there are small nodules that represent the corniculate and cuneiform cartilages.

The quadrangular membrane also has a free inferior margin that forms the vestibular ligament, which is covered by mucosa, forming the vestibular fold.

Now, back to the arytenoid cartilage, it’s apex attaches to this ary-epiglottic fold or mucosal covered ary-epiglottic ligament, which is the superior aspect of the quadrangular membrane.

The arytenoid cartilage also has a vocal process anteriorly that provides the posterior attachment for the vocal ligament, as well as a large muscular process that projects laterally from its base and acts as a lever to which the posterior and lateral cricoarytenoid muscles are attached.

Between the bases of the arytenoid cartilages and the superolateral surfaces of the lamina of the cricoid cartilage there are the cricoarytenoid joints which allow some important movements in approximating, tensing and relaxing the vocal cords by allowing the arytenoid cartilages to slide toward or away from one to another, to tilt anteriorly and posteriorly and to rotate.

Now, the elastic vocal ligaments form the submucosal skeleton of the vocal folds and extend from the junction of the laminae of the thyroid cartilage anteriorly to the vocal process of the arytenoid cartilage posteriorly.

The vocal ligaments are the thickened, free superior border of the conus elasticus. The other component of the conus elasticus is the lateral cricothyroid ligament, which extends laterally between the vocal folds and the superior border of the cricoid.

The conus elasticus blends anteriorly with the median cricothyroid ligament, and along with the overlying mucosa it closes the tracheal inlet except for the central zone called rima glottidis which is the opening between the vocal folds.

The epiglottic cartilage lies posterior to the root of the tongue and hyoid bone and anterior to the laryngeal inlet and it forms the superior part of the anterior wall and the superior margin of the inlet.

It consists of elastic cartilage and gives flexibility to the epiglottis, which is a heart-shaped cartilage covered with mucous membrane. It also functions as a valve, closing off the superior opening of the larynx during swallowing.

Now, the inferior end of the epiglottic cartilage, called the stalk of the epiglottis, is attached by the thyroepiglottic ligament to the angle formed by the thyroid laminae. Time for a break! Can you identify the cartilages of the larynx?

Now, the laryngeal cavity extends from the laryngeal inlet through which it communicates with the laryngopharynx, to the level of the inferior border of the cricoid cartilage where it continues with the tracheal cavity.

The laryngeal cavity is divided in three regions: the laryngeal vestibule which is located between the laryngeal inlet and the vestibular folds, the middle part which is the central cavity between the vestibular and vocal folds, and the infraglottic cavity which is located between the vocal folds and the inferior border of the cricoid cartilage, where it continues with the tracheal lumen.

In the middle part of the laryngeal cavity there are two recesses called the laryngeal ventricles that extend laterally between the vestibular and vocal folds, and inside each ventricle there’s an outpouching called the laryngeal saccule which is lined with mucosal glands.

Now, the vocal folds are the sharp folds of mucous membrane overlying and incorporating the thyro-arytenoid muscles and the vocal ligaments, which represent the medial free edge of the conus elasticus.

Each of the vocal folds also contains the vocalis muscle which are slender muscle slips medial to the thyro-arytenodid muscles, lateral to the vocal ligament and contain exceptionally fine fibers.

These folds produce audible vibrations when their free margins are closely opposed during phonation, and air pushed out of our lungs.

When these folds are completely closed, this prevents entry of air into the lungs. Finally, the vocal folds, together with the vocal processes and the rima glottidis form the glottis. Now, the shape of the rima glottidis varies according to the position of the vocal folds.

So, during normal breathing the rima is narrow and wedge-shaped, but during forced respiration it’s wide and trapezoidal.

Also, during phonation, the rima is slit-like. The pitch of the voice is changed by variations in the tension and length of the vocal folds, in the width of the rima glottidis, and in the intensity of the expiratory effort.

This is the reason why the male voice typically becomes lower pitch after puberty, because the vocal folds increase in length.

Now, the vestibular folds extend between the thyroid and arytenoid cartilages, which mostly play a protective role, and don’t really participate in sound production so they are termed the false vocal cords.

They consist of two thick folds of mucous membrane enclosing the vestibular ligaments. Let’s take another break and see if you can identify the main structures in the laryngeal cavity.

Moving on to the laryngeal muscles which are divided into two groups: extrinsic muscles and intrinsic muscles.

The extrinsic muscles move the larynx as a whole and are represented by the infrahyoid muscles, which depress the hyoid and larynx, and the suprahyoid muscles, along with the stylopharyngeus which elevate the hyoid and larynx.

On the other hand, the intrinsic muscles move different laryngeal components to alter the length and shape of the vocal folds and rima glottidis.