Larynx

Cartilaginous framework and ligaments

The larynx is composed of three large unpaired cartilages (cricoid, thyroid, and epiglottis) and three paired smaller cartilages (arytenoid, corniculate, and cuneiform), making a total of nine individual cartilages. The thyroid cartilage is the largest of the laryngeal cartilages and is composed of hyaline cartilage. It forms the anterior and lateral portions of the larynx and has no posterior component. The broad flat right and left halves (laminae) of the cartilage fuse anteriorly in the midline to form a V-shaped anterior projection called the laryngeal prominence (commonly called the “Adam’s apple”).

The Adam’s apple is typically more prominent in males after puberty. This is due to the influence of the hormone testosterone, which stimulates the overall growth of the larynx resulting in a deeper voice with time. Superior to this prominence is the superior thyroid notch. A less prominent inferior thyroid notch is present along the base of the thyroid cartilage. The posterior margin of each lamina extends upward into a superior horn and downward into an inferior horn. The longer superior horn, along with the entire superior border of the thyroid cartilage, attaches to the hyoid bone by the thyrohyoid membrane. This board fibroelastic sheet has a thick median region called the median thyrohyoid ligament as well as lateral parts called the lateral thyrohyoid ligaments, which directly attach to the superior horn. The inferior horn bears a facet on its medial surface with which its articulates with the posterolateral surface of the cricoid cartilage to form the cricothyroid joint.

The cricoid cartilage is a much smaller signet ring-shaped hyaline cartilage located directly below the thyroid cartilage. It forms the inferior aspect of the larynx and is connected to the trachea inferiorly. It is the only complete ring of cartilage that encircles the airway. The cricoid cartilage has a narrow anterior arch (band portion) and a wider posterior lamina (signet portion) with a midline ridge that serves as a surface of attachment for the esophagus. A dense band of connective tissue, the cricothyroid ligament, attaches the cricoid cartilage to the inferior border of the thyroid cartilage. The cricotracheal ligament attaches the cricoid cartilage to the first tracheal ring.

Located on the superolateral aspects of the wider posterior cricoid cartilage lamina are the paired pyramidal-shaped arytenoid cartilages. Each cartilage has a superior apex, an anterior vocal process and a large lateral muscular process. Attached to the apices of the arytenoid cartilage are the small, paired and conical- shaped corniculate cartilages.

The last unpaired cartilage, the epiglottis, is a large leaf-shaped elastic cartilage that is covered by mucous membrane. It is attached by its stalk to the inner aspect of the angle formed by the laminae of the thyroid cartilage via the thyroepiglottic ligament in the midline. The epiglottis is also attached to the hyoid bone by the hyoepiglottic ligament which extends from the anterosuperior surface of the epiglottis to the body of the hyoid bone. The epiglottis projects superiorly into the pharynx, with its upper margin just behind the root of the tongue. As its name suggests (epi = above, glottis = mouth of windpipe), it sits above the laryngeal opening (inlet). During swallowing, as the larynx moves up and forwards, the epiglottis swings downward to close off the laryngeal inlet, and thus prevents materials from entering the airway.  

A thin layer of connective tissue, the quadrangular membrane extends between the lateral borders of the epiglottis and the anterolateral margins of the arytenoid cartilage. Its free lower edge is thickened and forms the vestibular ligament. This ligament is enclosed by a fold of mucous membrane to form the vestibular fold (false vocal cord) which extends from the thyroid cartilage to the arytenoid cartilage.

The final cartilaginous components of the larynx are the two small club-shaped cuneiform cartilages that lie superior and anterior to the corniculate cartilages. They do not directly attach to any other laryngeal cartilage but are suspended within and strengthen a fibro-elastic membrane called the aryepiglottic membrane. This membrane forms the free superior edge of the quadrangular membrane, which as described earlier, connects the arytenoid cartilages to the lateral borders of the epiglottis. It is covered by mucosa to form the aryepiglottic fold. On the posterior aspect of the aryepiglottic folds both the corniculate and cuneiform cartilages are seen as small nodules surrounding the laryngeal inlet.

Cavity of the larynx

Now that we have discussed the external cartilaginous skeleton, let us take a look inside the lumen of the larynx. The mucosa lined cavity of the larynx extends from its superior opening (laryngeal inlet) to the inferior border of the cricoid cartilage which is continuous with the lumen of the trachea. The laryngeal cavity is divided into three regions:

• Vestibule:  between the laryngeal inlet and the vestibular folds
• Middle part: between the vestibular folds above and vocal folds below
• Infraglottic cavity: between the vocal fold and the trachea

The lateral walls of the middle part of the laryngeal cavity bulge outward to form lateral recesses (laryngeal ventricle) between the vestibular fold and the vocal fold. Each ventricle has an elongated blind tubular extension, the laryngeal saccule, that projects anterosuperiorly between the vestibular fold and the thyroid cartilage. Each saccule is lined with mucus glands that produce mucus to lubricate the vocal folds.

The vocal apparatus of the larynx is called the glottis and consists of two vocal folds (true vocal cords) and the rima glottidis. Each mucous membrane-covered vocal fold contains a vocal ligament that extends from the inner surface of the thyroid cartilage to the vocal process of the corresponding arytenoid cartilage. Running parallel to each vocal ligament is the vocalis muscle which is responsible for adjusting the tension of the vocal folds. The vocal folds are the “true” structures that produce sound as air passes over them, whereas the vestibular folds (false vocal cords) have no role in sound production but protect the vocal cords.

The opening between the vocal cords is referred to as the rima glottidis. The shape of this opening depends on the position of the vocal folds.  Rotational movements of the arytenoid cartilages at the cricoarytenoid joints can separate (abduct) the vocal folds, widening the rima glottidis or appose (adduct) the vocal folds and narrow the rima glottidis. Typically, during breathing, the vocal cords are abducted however, during swallowing, they are adducted to close the rima glottidis. The qualities of the sound produced (e.g. pitch) depends on the length, tension, and position of the vocal folds. Normally, during phonation, the vocal cords are closely apposed resulting in a slit-like rima glottidis. As expired air passes over the cords, it causes them to vibrate, producing sound. The sound is then modified by tongue, jaw and lip movements into speech.

Test yourself on the larynx with this quiz.

Muscles

There are two groups of muscles that are associated with the larynx, the extrinsic and intrinsic muscles. The extrinsic laryngeal muscles move the larynx as a whole. They consist of the suprahyoid muscles that elevate the hyoid bone and the larynx during swallowing and vocalization, and the infrahyoid muscles that depress the hyoid bone and the larynx.

The small intrinsic laryngeal muscles are responsible for moving various components of the larynx. They modify the length and tension of the vocal cords as well as the shape of the rima glottidis during breathing, swallowing and vocalization. 

Blood supply and lymphatic drainage

The arterial supply of the larynx is by the superior and inferior laryngeal arteries which are branches of the thyroid arteries. The larynx is drained by corresponding veins, namely the superior and inferior laryngeal veins. The lymphatic vessels above the vocal folds drain into the superior deep cervical lymph nodes whereas those below the vocal folds drain first to nodes around the trachea (pretracheal and paratracheal nodes) and subsequently into the inferior deep cervical lymph nodes.

Innervation

The right and left superior and inferior laryngeal nerves which are branches of the vagus nerve, the tenth cranial nerve (CN X), provide motor and sensory innervation to the larynx. Each superior laryngeal nerve divides into the internal and external laryngeal nerves. The internal laryngeal nerve accompanies the superior laryngeal artery through the thyrohyoid membrane and provides the sensory and autonomic innervation of the laryngeal cavity to the level of the vocal cords. The smaller external laryngeal nerve provides motor innervation to the cricothyroid muscle.

The recurrent laryngeal nerves which are ascending branches of the vagus nerves continue toward the larynx as the right and left inferior laryngeal nerves. They provide motor innervation to all the intrinsic muscles of the larynx except the cricothyroid muscles and sensory innervation to the laryngeal cavity below the vocal cords.