The Pharynx and Larynx
Anatomy
Pharynx
The pharynx is the superior-most portion of the aerodigestive tract. It is divided into three regions: the nasopharynx, oropharynx, and hypopharynx. The nasopharynx begins at the skull base and extends inferiorly to the level of the soft palate. The nasal cavities communicate posteriorly with the nasopharynx via the choanae. Additionally, the Eustachian tube opens into the nasopharynx, providing a drainage route for the middle ear. Posterosuperior to the Eustachian tube opening is the cartilaginous base of the Eustachian tube, which forms an elevation of the nasopharyngeal wall called the torus tubarius. The salpingopharyngeus and levator veli palatini muscles extend from the torus. Posterior to the torus is the fossa of Rosenmüller (also known as the pharyngeal recess), which is the most common site of origin of nasopharyngeal carcinoma (see Carcinoma of the Pharynx). The adenoid is a collection of lymphoid tissue located at the roof of the nasopharynx, opposite the choanae.
The oropharynx extends from the level of the soft palate to the vallecula (the space between the base of tongue and the epiglottis), which is at the level of the hyoid bone. It is posterior to, and contiguous with, the oral cavity. The oropharynx is divided into several subsites: the base of tongue, soft palate and uvula, tonsillar region (including tonsillar fossae, anterior tonsillar pillar, and posterior tonsillar pillar), and the posterior pharyngeal wall. The base of tongue consists of the posterior one-third of the tongue and contains the lingual tonsils and the glossoepiglottic folds.
The hypopharynx is the inferior-most portion of the pharynx. It extends from the vallecula to the level of the cricoid cartilage, where it terminates at the esophageal introitus. The hypopharyngeal subsites include the piriform sinuses, the postcricoid region, and the posterior hypopharyngeal wall. The piriform sinuses are paired recesses that extend anteriorly on either side of the larynx to form a space between the outer surface of the cricoid cartilage and the thyroid cartilage. They are bounded superiorly by the pharyngoepiglottic folds and inferiorly by the cricoid cartilage. The postcricoid region is where the hypopharynx meets the posterior laryngeal wall.
The outer muscular layer of the pharynx comprises the stacked superior, middle, and inferior constrictor muscles. The fibers of the constrictor muscles are oriented circumferentially around the pharynx. The inner muscular layer of the pharynx includes the longitudinally oriented stylopharyngeus, palatopharyngeus, and salpingopharyngeus muscles.
→Blood Supply, Lymphatic Drainage, and Innervation
The blood supply to the pharynx is from branches of the external carotid artery, including the ascending pharyngeal, facial, maxillary, lingual, and superior thyroid arteries. Venous drainage is through the pharyngeal venous plexus, which drains into the internal jugular vein. Lymphatic drainage of the nasopharynx is to the retropharyngeal, high level V, and level II lymph nodes. The oropharynx drains to the jugulodigastric, retropharyngeal, and parapharyngeal lymph nodes in levels II through IV. The hypopharynx drains to the jugulodigastric, retropharyngeal, paratracheal, and paraesophageal lymph nodes.
The pharynx is innervated by the pharyngeal plexus, which is composed of contributions from the vagus nerve, (cranial nerve X), glossopharyngeal nerve (cranial nerve IX), maxillary division of the trigeminal nerve (cranial nerve V2), and sympathetic trunk. Sensory innervation to the superior nasopharynx is via cranial nerve V2, while the remainder of the pharynx derives its sensation from the glossopharyngeal nerve . Motor innervation of all pharyngeal muscles is by the vagus nerve, with the exception of the stylopharyngeus muscle, which is innervated by the glossopharyngeal nerve.
Larynx
The larynx protects the lower respiratory tract and contains the anatomic components necessary for phonation. The laryngeal skeleton is composed of multiple cartilages. The thyroid cartilage is a shield-shaped structure that forms the anterior aspect of the larynx. At its superior edge, it contains a midline prominence with a central notch known as the thyroid notch; this prominence is colloquially referred to as the “Adam’s apple,” as it is more prominent in males. Posterosuperiorly, the thyroid cartilage has two horns, or superior cornua, and is suspended from the hyoid bone (located superiorly and attached to the thyroid cartilage via the thyrohyoid membrane and ligaments). Inferior to the thyroid cartilage (and connected to it via the cricothyroid membrane) is the cricoid cartilage. The inferior cornua of the thyroid cartilage articulate with the cricoid cartilage, forming the cricothyroid joint. The cricoid cartilage is unique in that it forms a complete circumferential ring around the larynx (unlike the thyroid cartilage or tracheal rings, which are open and devoid of cartilage posteriorly). Overlying the thyroid and cricoid cartilages are the strap muscles, which include the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles.
The larynx can be divided into three regions: the supraglottis, glottis, and subglottis. The supraglottis consists of several subsites, including the epiglottis, aryepiglottic folds, arytenoid cartilages, and false vocal folds. The epiglottis is a fibroelastic cartilaginous flap located posterior to the base of the tongue at the laryngeal inlet. At rest, the epiglottis is oriented in an upright position, allowing the passage of air from the pharynx into the trachea. However, during deglutition, or the act of swallowing, the epiglottis covers the laryngeal inlet, diverting material away from the airway. Laterally, the mucosa of the epiglottis is contiguous with that overlying the arytenoid cartilages, forming the aryepiglottic folds. The arytenoid cartilages are paired, pyramidal structures to which the true vocal folds are attached. At their base, the arytenoid cartilages articulate with the superolateral aspects of the cricoid cartilage, forming the cricoarytenoid joints. Movement of the arytenoid cartilages at these joints allows for movement of the vocal folds, producing speech. The false vocal folds are paired projections of laryngeal mucosa, located just superior to the true vocal folds. They assist in glottic closure during actions such as swallowing and valsalva but are usually not involved in phonation.
The space between the false vocal folds and true vocal folds is called the ventricle. The apex of the ventricle is the level at which the supraglottis transitions to the glottis. The glottis contains the true vocal folds and extends inferiorly to 0.5 cm below their inferior edge. Posteriorly, the true vocal folds are attached to the vocal processes of the arytenoid cartilages. Anteriorly, the two vocal folds meet at the anterior commissure, where they attach to the thyroid cartilage. The medial (free) edge of the vocal fold is its vibratory surface and is highly specialized for phonation. Details regarding how the vocal folds function in speech production are given in a subsequent section of this chapter. Histologically, the vocal fold is composed of five layers. The outermost layer consists of epithelial cells, followed by three layers of elastic connective tissue: the superficial lamina propria, intermediate lamina propria, and deep lamina propria. The intermediate and deep lamina propria are collectively referred to as the vocal ligament. The innermost layer is the vocalis muscle.
The subglottis is the inferior-most portion of the larynx. Its superior border is a horizontal plane 1 cm inferior to the anterior true vocal folds . Inferiorly, the subglottis extends to the inferior border of the cricoid cartilage.
The fibrous framework of the larynx is formed by a membrane of fibroelastic tissue. Above the ventricle is the quadrangular membrane, which extends from the lateral epiglottis to the false cord. Below the ventricle is the conus elasticus, which has attachments to the thyroid cartilage superoanteriorly, the cricoid cartilage inferiorly, and the arytenoid cartilage superoposteriorly. These ligamentous structures connect the cartilages and are also thought to provide a barrier to spread of glottic pathology out of the larynx.
The intrinsic muscles of the larynx function to modulate the position of the vocal folds. A summary of the actions of the most important of these muscles follows:
- Thyroarytenoid (TA): parallel and lateral to the vocal folds; shortens and thickens the vocal folds, reduces tension, as well as rotates the arytenoids medially, adducting the vocal folds
- Lateral cricoarytenoid (LCA): rotates the arytenoids medially, adducting the vocal folds
- Posterior cricoarytenoid (PCA): rotates the arytenoids laterally, abducting the vocal folds
- Interarytenoid: slides the arytenoids medially, adducting the vocal folds
- Cricothyroid: increases the length and tension of the vocal folds
→Blood Supply, Lymphatic Drainage, and Innervation
The vascular supply of the larynx is via the superior and inferior laryngeal arteries, which are branches of the superior and inferior thyroid arteries, respectively. Venous drainage of the larynx is via the superior and inferior laryngeal veins, which drain into the internal jugular vein and the brachiocephalic (innominate) vein, respectively. In general, lymphatic drainage of the larynx is to cervical lymph nodes in levels II through IV. The supraglottis primarily drains to the upper jugular lymph nodes, while the subglottis drains to the pretracheal and lower jugular nodes. The glottis has very limited lymphatic drainage, and, unlike the supraglottis and subglottis, drainage does not occur to the contralateral side.
Motor innervation of the intrinsic muscles of the larynx is via the recurrent laryngeal nerve, with the exception of the cricothyroid muscle, which is innervated by the external branch of the superior laryngeal nerve. Sensory innervation of the supraglottis and glottis is via the internal branch of the superior laryngeal nerve, while the subglottis derives its sensation from the recurrent laryngeal nerve.
Physiology
Laryngeal Reflexes
The larynx reflexively closes in response to tactile and chemical stimuli in order to protect the airway from aspiration. Depending on the individual and the degree of stimulation, laryngospasm may be triggered, wherein the vocal folds undergo extended adduction (closure). This response is of particular concern when attempting difficult intubations, as trauma to the larynx can induce laryngospasm, precluding passage of the endotracheal tube through the glottis. In some cases, overstimulation of the larynx can result in arrhythmia or bradycardia. The etiology of this response may be related to the modulation of heart rate in response to respiration, which occurs under normal conditions.
Deglutition
The act of swallowing requires extensive coordination between the muscles of the oral cavity, pharynx, and larynx to ensure that aspiration does not occur. Deglutition begins with the oral phase, in which the tongue assists in propelling the bolus (of food, liquid, or saliva) posteriorly into the oropharynx. This initiates the pharyngeal phase of deglutition, during which the soft palate elevates to separate the oropharynx from the nasopharynx. Failure of this step can result in reflux of the bolus material into the nose. Laryngeal closure is achieved through tight adduction of the vocal folds. In addition, the false vocal cords also adduct to provide additional protection. The longitudinal pharyngeal muscles contract, elevating the pharynx and larynx. The hyoid bone is also elevated by the digastric and stylohyoid muscles. This results in retroversion of the epiglottis, so that it covers the laryngeal inlet and directs the bolus toward the piriform sinuses. Simultaneously, the true and false vocal folds close to prevent aspiration. Sequential contraction of the pharyngeal constrictor muscles propels the bolus through the hypopharynx. At the inferior boundary of the hypopharynx, the cricopharyngeus muscle (upper esophageal sphincter) relaxes, allowing the bolus to enter the esophagus and beginning the esophageal phase of deglutition. In addition to the anatomic changes that occur to prevent aspiration during swallowing, the act of deglutition also directly inhibits respiration (at the level of the brainstem) while the larynx is closed. Aside from initiation, the remainder of the deglutition process is reflexive and involuntary.
Phonation
The process of phonation, or voicing, begins with the expiration of air through the trachea and the glottis. Adequate expiratory force is necessary to generate vocal fold motion to produce sound; the minimal amount of force necessary is termed the phonation threshold pressure. As air is expelled through the vocal folds, it causes them to oscillate, forming a mucosal wave and generating sound. In order for this to occur, the vocal folds must be approximated; expulsion of air through insufficiently adducted vocal folds results in a whispered or breathy voice quality (or complete aphonia if the glottic gap is too great). On the other hand, hyperadduction of the vocal folds can prevent any air from passing through the glottis, resulting in a strained voice quality or aphonia . There are multiple theories as to how oscillation of the vocal folds is initiated and propagated. The prevailing concept involves a combination of the myoelastic properties of the vocal folds as well as the aerodynamic properties governing the flow of air through the glottis. As air is expired, subglottic pressure increases until it is sufficient to force the vocal folds apart. Subsequent decrease in air pressure gradient, Bernoulli’s effect and the elastic recoil of the vocal folds serve to reapproximate the vocal folds. This allows subglottic pressure to build again, and the cycle repeats. Many different factors impact voice quality. Modulation of vocal fold length and tension changes the pitch of the voice, while changes in length and caliber of the vocal tract can affect resonance. Finally, formation of the vibratory sound waves into phonemes is referred to as articulation and is assisted by the action of the tongue, palate, pharynx, and lips.
Mucociliary Clearance and Paranasal Sinus Drainage
The mucosal lining of the posterior nasal cavity and sinus cavities is composed of pseudostratified columnar epithelium that contains both ciliated and nonciliated cells. The cilia beat at a rate of 10-20 times per second in a coordinated fashion to direct mucus toward natural drainage pathways. In the sinuses, mucus is propelled at a rate of 3-25 mm per minute in the direction of the natural sinus ostia. From there, the mucus is transported to the nasopharynx and eventually swallowed. The ability of the respiratory epithelium to direct mucus drainage is referred to as mucociliary clearance. This phenomenon explains why opening a second sinus ostium is an ineffective method of treating sinus blockage—mucus will continue to be directed toward the obstructed natural ostium rather than the newly created drainage pathway. A more effective approach, therefore, is to enlarge the existing natural ostium. Functional endoscopic sinus surgery (FESS), which can be used to treat cases of sinus obstruction, is discussed further in Functional Endoscopic Sinus Surgery.