Aerodigestive Anatomy and Embryology with Radiologic Correlates

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What are the three primary subunits of the larynx?

The larynx serves three important functions: airway protection, phonation, and swallowing. The larynx is also the portal to the airway; thus anatomical and functional patency is critical to normal breathing. It is sometimes easier to compartmentalize the larynx to better focus on discussion and study. The larynx is commonly divided into three subsections to aid in the description of pathologic processes: the supraglottis, glottis, and subglottis ( Fig. 71.1 ). These anatomic subunits have different histologic characteristics and harbor different benign laryngeal disease processes.

The supraglottis extends from the rostral edge of the epiglottis to the middle of the laryngeal ventricle. This subsection includes the laryngeal surface of the epiglottis, false vocal folds (also known as the vestibular folds), aryepiglottic folds, and superior portions of the arytenoid cartilages. The glottis extends superiorly from the mid-ventricle to 1 centimeter below the true vocal folds. This subsection contains the true vocal folds and arytenoid cartilages. The subglottis extends from 1 centimeter below the true vocal folds to the most caudal edge of the cricoid cartilage.

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What cartilages compose the larynx?

The larynx is composed of three unpaired cartilages and three paired cartilages ( Fig. 71.2 ). Two of the unpaired cartilages, the thyroid and cricoid cartilages, are palpable externally and serve as surgical landmarks. The epiglottis and the paired cartilages—the arytenoid, cuneiform, and corniculate cartilages—are internal to the larynx. The cuneiform and corniculate cartilages are sometimes jointly referred to as sesamoid cartilages and serve as structural support to the aryepiglottic folds. The hyoid bone is the most superior aspect of the larynx and serves as an important insertion site for several extrinsic (suprahyoid and infrahyoid) laryngeal muscles. These cartilages can be seen in modified barium swallow studies used to assess dysphagia ( Fig. 71.3 ).

The thyroid cartilage is formed by two alae that fuse at the midline. Each ala contains both superior and inferior cornu, the former tethering to the hyoid via the lateral thyrohyoid ligament, and the latter serves as an articulating joint with the cricoid cartilage. In the midline there is a small notch that helps define the “Adam’s apple,” which serves as a second connection to the hyoid via the median thyrohyoid ligament.

The cricoid cartilage is the only complete cartilaginous ring of the airway. It has a characteristic three-dimensional shape that is often compared to a signet ring. The ring is oriented such that the narrow portion is anterior in the neck and the larger plate resides posteriorly, offering additional height upon which the arytenoid cartilages sit on its superior surface. The vocal process of the arytenoid cartilage forms the posterior attachment of the vocal folds, which extend to their anterior attachment at the internal midline of the thyroid cartilage.

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What muscles are found within the larynx?

The larynx utilizes two separate groups of muscles, the extrinsic and intrinsic laryngeal muscles, to perform tasks related to phonation, airway protection, and swallowing. The extrinsic musculature connects one structural element of the larynx to another structural element outside of the larynx; conversely, the intrinsic musculature connects one structural element inside the larynx to another.

The extrinsic musculature includes muscles that elevate and depress the laryngeal framework within the neck; while they play a small role in phonation and as accessory muscles of breathing, they are primarily utilized for laryngeal elevation during swallowing. A combination of the activation of the geniohyoid, digastric, mylohyoid, thyrohyoid, or stylohyoid muscles results in laryngeal elevation, while laryngeal depression can be achieved with activation of the strap muscles (sternohyoid, sternothyroid, and omohyoid). The extrinsic musculature has various innervations, including cervical rootlets and cranial nerves V and VII. The final extrinsic muscle group comprises pharyngeal constrictors. The constrictors, all innervated by the pharyngeal plexus, help advance a food bolus into the esophagus. The superior constrictor does not insert on the larynx, but the middle and inferior constrictors do, resulting in elevation and posterior translation with swallows.

Intrinsic musculature is primarily associated with airway protection and phonation. They are all paired muscles except for the interarytenoid muscle ( Fig. 71.2 ). These muscles are classified as adductors or abductors based on the movement of the vocal folds with activation. Adduction results in medial approximation of the vocal folds for phonation, cough, or airway protection. The adductors include the thyroarytenoid, lateral cricoarytenoid, and interarytenoid muscles. Abduction results in lateral excursion of the vocal folds during breathing. The lone abductor is the posterior cricoarytenoid muscle. The cricothyroid muscle is considered an intrinsic laryngeal muscle but it does not directly cause adduction or abduction of the vocal folds. The cricothyroid muscle works to pivot the thyroid cartilage anteriorly along the axis created by the cricothyroid joint, lengthening and tensing the vocal folds to increase pitch.

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What type of mucosa is found in the larynx?

The majority of the mucosa is columnar respiratory epithelium. However, stratified squamous epithelium overlies the vibratory portions of the true vocal fold. The transition points from the respiratory epithelium to the squamous epithelium occur at the superior and inferior arcuate lines; the former is within the laryngeal ventricle, while the latter rests just below the true vocal fold.

This stratified squamous epithelium is thought to be protective against trauma caused by high-frequency collisions of the vocal folds during phonation. Despite this protective quality, the existence of different epithelia and transitions between epithelial types is important for understanding both malignant and benign pathologies. Understanding epithelial types in the larynx can help understand the likely locations for malignancy (squamous cell carcinoma of the glottis) and benign pathology (recurrent respiratory papillomatosis commonly occurs at the transition points between respiratory and squamous epithelia).

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