Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video

Lecture 12: Larynx and Trachea

Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the cartilages and membranes of the larynx
2. Identify the intrinsic and extrinsic muscles of the larynx
3. Describe the innervation of the larynx and its clinical significance
4. Explain the anatomy of the vocal cords and their function
5. Describe the anatomy of the trachea and its relations
6. Correlate anatomical features with clinical conditions

Larynx Overview

The larynx is a complex organ situated in the anterior neck at the level of the C3 to C6 vertebrae in adults. It is continuous superiorly with the laryngopharynx, into which it opens, and inferiorly with the trachea. The larynx serves three essential functions: protection of the lower airway (by closing during swallowing to prevent aspiration), respiration (by regulating airflow to the lungs), and phonation (by vibration of the vocal folds to produce voice).

The larynx is composed of a cartilaginous skeleton (nine cartilages in total), connecting membranes and ligaments, intrinsic muscles that move the cartilages relative to each other, and a mucosal lining. The position of the larynx changes with age: it is relatively higher in infants (allowing simultaneous breathing and swallowing) and descends during childhood.

<image>Panel A: Midsagittal section of the head and neck showing the larynx at the C3-C6 vertebral level. Panel B: Larynx continuous with the laryngopharynx above and trachea below. Panel C: Hyoid bone, thyroid cartilage, cricoid cartilage, and epiglottis shown in relation to the pharynx posteriorly and thyroid gland anterolaterally. Panel D: Vertebral levels C3-C6 labeled alongside the laryngeal structures.</image>

Laryngeal Cartilages

The nine cartilages of the larynx comprise three unpaired median cartilages and three pairs of smaller cartilages.

The thyroid cartilage is the largest laryngeal cartilage and is formed by two quadrilateral laminae that meet anteriorly at an angle. In males, this angle is approximately ninety degrees, creating the prominent laryngeal prominence (Adam's apple); in females, the angle is approximately one hundred twenty degrees, resulting in less prominence. The superior horns project upward and are connected to the hyoid bone by the thyrohyoid membrane. The inferior horns project inferiorly and articulate with the cricoid cartilage at the cricothyroid joints, which permit rocking and gliding movements that tense the vocal folds. The oblique line on the external surface of each lamina provides attachment for the sternothyroid, thyrohyoid, and inferior constrictor muscles.

The cricoid cartilage is the only complete cartilaginous ring in the airway, making it essential for maintaining airway patency. It is shaped like a signet ring, with a narrow anterior arch and a tall posterior lamina. Located at the level of C6, it marks several important anatomical boundaries: the junction between the larynx and trachea, the junction between the pharynx and esophagus, and the level at which the inferior thyroid artery enters the larynx and the recurrent laryngeal nerve enters the larynx. The superior surface of the posterior lamina bears facets for articulation with the arytenoid cartilages.

The epiglottis is a leaf-shaped elastic cartilage that projects upward behind the tongue and hyoid bone. Its lower end (petiolus) is attached to the inner surface of the thyroid cartilage just below the thyroid notch. The hyoepiglottic ligament connects it to the hyoid bone, and the thyroepiglottic ligament attaches it to the thyroid cartilage. During swallowing, the larynx elevates and the epiglottis folds posteroinferiorly to cover the laryngeal inlet, directing the food bolus laterally into the piriform fossae and away from the airway.

The arytenoid cartilages are paired pyramid-shaped cartilages that sit on the superior border of the cricoid lamina. Each has an apex (pointing upward), a base (articulating with the cricoid), a vocal process (projecting anteriorly, giving attachment to the vocal ligament), and a muscular process (projecting laterally, giving attachment to the intrinsic muscles that move the vocal folds). The cricoarytenoid joints permit rotation and gliding movements of the arytenoids, which are essential for abducting and adducting the vocal folds.

The corniculate cartilages are small conical cartilages that sit on the apices of the arytenoid cartilages and are contained within the aryepiglottic folds. The cuneiform cartilages are small rod-shaped cartilages also within the aryepiglottic folds, anterior to the corniculates; they help stiffen these folds.

<image>Panel A: Anterior, lateral, and posterior views of the thyroid cartilage (blue) showing laminae, laryngeal prominence, superior and inferior horns, and oblique line. Panel B: Cricoid cartilage (green) showing signet ring shape with narrow arch and tall lamina, and epiglottis (yellow) showing leaf shape and petiolus. Panel C: Arytenoid cartilages (red) showing pyramid shape with vocal and muscular processes. Panel D: Corniculate (orange) and cuneiform (purple) cartilages in the aryepiglottic folds with scale bar included.</image>

Laryngeal Membranes and Ligaments

The membranes and ligaments of the larynx are divided into extrinsic connections (linking the larynx to adjacent structures) and intrinsic connections (linking the laryngeal cartilages to each other).

The extrinsic membranes include the thyrohyoid membrane and the cricotracheal membrane. The thyrohyoid membrane spans from the superior border of the thyroid cartilage to the hyoid bone, suspending the larynx from the hyoid. It is thickened in the midline as the median thyrohyoid ligament and at its lateral edges as the lateral thyrohyoid ligaments. The membrane is pierced on each side by the internal laryngeal nerve and the superior laryngeal artery (from the superior thyroid artery), which enter the larynx to provide sensory innervation and blood supply to the supraglottic region. The cricotracheal membrane connects the inferior border of the cricoid cartilage to the first tracheal ring.

The intrinsic membranes include the quadrangular membrane and the cricothyroid membrane (conus elasticus). The quadrangular membrane extends from the lateral edge of the epiglottis to the arytenoid cartilages. Its free upper edge forms the aryepiglottic fold, which constitutes the lateral boundary of the laryngeal inlet. Its free lower edge forms the vestibular ligament, which underlies the vestibular fold (false vocal cord). The cricothyroid membrane (conus elasticus) is a fibroelastic membrane that extends from the superior border of the cricoid cartilage upward to attach to the thyroid cartilage anteriorly and to the vocal processes of the arytenoid cartilages posteriorly. The free upper edge of this membrane is thickened to form the vocal ligament, which is the core of the vocal fold (true vocal cord). Anteriorly, the membrane thickens to form the median cricothyroid ligament, which spans the gap between the cricoid and thyroid cartilages.

The cricothyroid membrane has great clinical importance as the site of emergency airway access. The median cricothyroid ligament is approximately nine millimeters in height, is relatively avascular, and lies in a subcutaneous position, making it accessible for cricothyrotomy when upper airway obstruction prevents endotracheal intubation.

<image>Panel A: Midsagittal section showing the thyrohyoid membrane connecting thyroid cartilage to hyoid with internal laryngeal nerve and superior laryngeal vessels piercing it. Panel B: Quadrangular membrane extending from epiglottis to arytenoid with free edges forming aryepiglottic fold above and vestibular ligament below. Panel C: Cricothyroid membrane (conus elasticus) with its free upper edge as vocal ligament and anterior thickening as median cricothyroid ligament. Panel D: Cricothyroid membrane highlighted as the site for emergency airway access.</image>

Interior of the Larynx

The interior of the larynx is divided into three regions by two pairs of folds that project into the lumen.

The vestibule (supraglottis) is the uppermost region, extending from the laryngeal inlet (aditus) to the vestibular folds. The laryngeal inlet is bounded anteriorly by the epiglottis, laterally by the aryepiglottic folds, and posteriorly by the interarytenoid notch between the two arytenoid cartilages. The vestibule is lined by respiratory mucosa.

The ventricle (also called the laryngeal sinus or ventricle of Morgagni) is the narrow space between the vestibular folds above and the vocal folds below. A diverticulum called the laryngeal saccule extends upward from the ventricle between the vestibular fold and the thyroid cartilage; it contains mucous glands that lubricate the vocal folds.

The infraglottic cavity (subglottis) extends from the vocal folds to the lower border of the cricoid cartilage, where it becomes continuous with the trachea.

The vestibular folds (false vocal cords) are the upper pair of folds, formed by the vestibular ligaments covered by respiratory mucosa. They do not participate in phonation but serve a protective function by closing during the Valsalva maneuver (holding the breath against a closed glottis, as when straining).

The vocal folds (true vocal cords) are the lower pair of folds and are the structures responsible for voice production. Each vocal fold consists of the vocal ligament (the free upper edge of the conus elasticus), the vocalis muscle (the medial portion of the thyroarytenoid muscle), and a covering of stratified squamous epithelium. Unlike the rest of the larynx, the vocal folds lack a submucosa, so the epithelium is tightly bound to the underlying ligament; this arrangement allows the folds to vibrate with air passage to produce sound.

The rima glottidis is the opening between the two vocal folds and the arytenoid cartilages. It is the narrowest part of the laryngeal airway in adults. The shape of the rima changes depending on the activity: during quiet respiration, it is narrow and triangular; during forced inspiration, it is wide open; during phonation, the membranous portions of the folds (the intermembranous part) adduct while the posterior cartilaginous portions (the intercartilaginous part) may remain slightly open.

<image>Panel A: Coronal section through the larynx showing the vestibule above vestibular folds. Panel B: Ventricle between vestibular and vocal folds with saccule projecting superiorly, and infraglottic cavity below vocal folds continuing to trachea. Panel C: Superior laryngoscopic view of vocal folds with rima glottidis in triangular configuration during respiration and slit-like during phonation. Panel D: Rima glottidis wide open during forced inspiration with arytenoid cartilages labeled.</image>

Intrinsic Muscles of the Larynx

The intrinsic muscles of the larynx act exclusively on the laryngeal cartilages to control the position and tension of the vocal folds. They can be organized by function.

The posterior cricoarytenoid is the only muscle that abducts (opens) the vocal folds. It arises from the posterior surface of the cricoid lamina and inserts onto the muscular process of the arytenoid cartilage. When it contracts, it rotates the arytenoid cartilage laterally, swinging the vocal process (and attached vocal fold) away from the midline. This muscle is essential for breathing; bilateral paralysis causes the vocal folds to remain adducted, resulting in life-threatening airway obstruction.

The lateral cricoarytenoid is the primary adductor of the vocal folds. It arises from the lateral part of the cricoid arch and inserts onto the muscular process of the arytenoid. Contraction rotates the arytenoid medially, bringing the vocal folds together.

The transverse arytenoid (interarytenoid) is an unpaired muscle spanning between the two arytenoid cartilages. It adducts the arytenoids themselves, closing the posterior (intercartilaginous) part of the rima glottidis.

The oblique arytenoid muscles cross superficial to the transverse arytenoid in an X pattern from the muscular process of one arytenoid to the apex of the other. Some fibers continue as the aryepiglottic muscle into the aryepiglottic folds. These muscles help close the laryngeal inlet during swallowing.

The cricothyroid muscle is unique among the intrinsic muscles in several respects. It is the only intrinsic muscle located externally (on the anterior larynx), and it is the only one innervated by the external branch of the superior laryngeal nerve rather than by the recurrent laryngeal nerve. It arises from the anterior arch of the cricoid cartilage and inserts onto the inferior border and inferior horn of the thyroid cartilage. Contraction tilts the thyroid cartilage forward on the cricoid (or, equivalently, tilts the cricoid backward), increasing the distance between the thyroid and arytenoid cartilages and thereby stretching and tensing the vocal folds. Increased vocal fold tension produces a higher-pitched sound.

The thyroarytenoid muscle forms the body of the vocal fold. It arises from the inner surface of the thyroid cartilage near the midline and inserts onto the arytenoid cartilage. Its contraction shortens and relaxes the vocal folds, producing a lower pitch. The medial portion of this muscle, closely associated with the vocal ligament, is often distinguished as the vocalis muscle, which allows fine adjustments to vocal fold tension and is responsible for subtle pitch control.

<image>Panel A: Posterior cricoarytenoid (green) shown as the only abductor with arrow showing direction of arytenoid rotation. Panel B: Lateral cricoarytenoid (blue) and transverse/oblique arytenoids (purple) as adductors with action arrows. Panel C: Cricothyroid muscle (orange) shown externally with arrow indicating its tilting action to tense vocal folds. Panel D: Thyroarytenoid/vocalis (red) within the vocal fold with innervation indicated -- cricothyroid by external laryngeal nerve, all others by recurrent laryngeal nerve.</image>

Innervation of the Larynx

The larynx receives its motor and sensory innervation from branches of the vagus nerve (CN X). Two branches are particularly important: the superior laryngeal nerve and the recurrent laryngeal nerve.

The superior laryngeal nerve arises from the inferior (nodose) ganglion of the vagus in the upper neck. It descends along the pharynx and divides into internal and external branches. The internal laryngeal nerve pierces the thyrohyoid membrane along with the superior laryngeal artery to enter the larynx. It provides sensory innervation to the laryngeal mucosa above the level of the vocal folds and is responsible for the afferent limb of the cough reflex when foreign material contacts this region. The external laryngeal nerve descends along the inferior pharyngeal constrictor muscle to reach the cricothyroid muscle, which it innervates. Damage to the external laryngeal nerve results in weakness of voice and inability to produce high-pitched sounds, because the vocal folds cannot be tensed.

The recurrent laryngeal nerve has a different course on each side due to the asymmetric development of the aortic arches. On the right, it loops posteriorly around the subclavian artery and ascends in the tracheoesophageal groove to enter the larynx. On the left, it loops under the arch of the aorta (ligamentum arteriosum), posterior to the aorta, and then ascends in the tracheoesophageal groove. Both nerves enter the larynx by passing deep to the lower border of the inferior constrictor muscle. The recurrent laryngeal nerve provides motor innervation to all intrinsic laryngeal muscles except the cricothyroid and sensory innervation to the laryngeal mucosa at and below the level of the vocal folds.

Injury to the recurrent laryngeal nerve produces characteristic findings depending on whether the lesion is unilateral or bilateral. Unilateral injury causes the ipsilateral vocal fold to assume a paramedian position (slightly abducted from midline) because it cannot be either abducted or adducted. The patient experiences hoarseness because the paralyzed fold cannot meet the functioning opposite fold during phonation, but the airway is usually adequate because the other fold can still abduct. Bilateral injury is far more serious: both vocal folds assume the paramedian position and can neither abduct nor adduct. The patient has a weak, breathy voice, but more critically, airway obstruction occurs because neither fold can open during inspiration. This may require emergency tracheostomy.

The left recurrent laryngeal nerve is more commonly injured than the right because of its longer course and its relationship to the aortic arch and mediastinal structures. Causes of recurrent laryngeal nerve injury include thyroid surgery (the most common cause), thoracic surgery, lung cancer (particularly left apical tumors), aortic aneurysm, and mediastinal masses.

<image>Panel A: Anterior view of the neck and thorax showing the vagus nerves with the superior laryngeal nerve dividing into internal and external branches. Panel B: Internal laryngeal nerve (blue for sensory) piercing thyrohyoid membrane with vessel, providing sensation above vocal folds, and external branch (green) supplying cricothyroid. Panel C: Recurrent laryngeal nerves with right looping around subclavian artery and left looping under aortic arch, both ascending in the tracheoesophageal groove. Panel D: Motor (red) and sensory (purple) components of the recurrent laryngeal nerves below the vocal folds with key surgical danger zones highlighted.</image>

Blood Supply of the Larynx

The arterial supply to the larynx comes from the superior and inferior laryngeal arteries.

The superior laryngeal artery is a branch of the superior thyroid artery (itself from the external carotid artery). It pierces the thyrohyoid membrane along with the internal laryngeal nerve to supply the supraglottic larynx, including the vestibule and ventricular structures.

The inferior laryngeal artery is a branch of the inferior thyroid artery, which arises from the thyrocervical trunk (a branch of the subclavian artery). The inferior laryngeal artery accompanies the recurrent laryngeal nerve as it ascends to the larynx and supplies the subglottic larynx and the intrinsic muscles.

Venous drainage parallels the arterial supply. The superior laryngeal vein drains to the superior thyroid vein and thence to the internal jugular vein. The inferior laryngeal vein drains to the inferior thyroid vein, which empties into the brachiocephalic vein.

Lymphatic drainage of the larynx follows a pattern that has implications for the spread of laryngeal cancer. The supraglottic region has abundant lymphatics that drain to the upper deep cervical nodes. In contrast, the glottic region (the vocal folds themselves) has sparse lymphatic drainage, which is why vocal fold cancers tend to spread late and present early with hoarseness before lymph node involvement. The subglottic region drains to the deep cervical, pretracheal, and paratracheal nodes.

<image>Panel A: Lateral view showing superior laryngeal artery from superior thyroid entering with internal laryngeal nerve through thyrohyoid membrane (labeled supraglottic supply). Panel B: Inferior laryngeal artery from inferior thyroid artery ascending with recurrent laryngeal nerve (labeled subglottic supply). Panel C: Venous drainage shown in blue paralleling the arterial supply. Panel D: Lymphatic drainage zones indicated with arrows to respective node groups for supraglottic, glottic, and subglottic regions.</image>

Trachea

The trachea is a cartilaginous and membranous tube that serves as the conduit for air between the larynx and the bronchi. It extends from the lower border of the cricoid cartilage (at the level of C6) to its bifurcation at the carina (at the level of T4-T5 vertebrae, or the sternal angle anteriorly). Its length is approximately ten to eleven centimeters, and its diameter is approximately two to two and a half centimeters.

The tracheal wall is supported by sixteen to twenty C-shaped hyaline cartilage rings that maintain airway patency. These rings are incomplete posteriorly, where the gap is bridged by the trachealis muscle, a band of smooth muscle whose contraction narrows the airway during coughing. The posterior wall, lacking cartilage, lies against the esophagus, allowing the esophagus to expand during swallowing. The interior of the trachea is lined by pseudostratified ciliated columnar epithelium (respiratory epithelium), with goblet cells producing mucus that traps inhaled particles, which are then swept upward by the ciliary action toward the pharynx (the mucociliary escalator).

In the neck (cervical trachea), the trachea is related anteriorly to the isthmus of the thyroid gland (which usually overlies tracheal rings two through four), the sternothyroid and sternohyoid muscles, the pretracheal fascia, and superficially, the jugular venous arch. Posteriorly lies the esophagus. Laterally are the lobes of the thyroid gland, the carotid sheath (containing the common carotid artery, internal jugular vein, and vagus nerve), and the recurrent laryngeal nerves ascending in the tracheoesophageal grooves.

In the thorax (thoracic trachea), the anterior relations include the manubrium of the sternum, the brachiocephalic artery (which crosses from left to right in front of the trachea), the left brachiocephalic vein, the thymus (or its remnant), and the origins of the sternohyoid and sternothyroid muscles. Posteriorly remains the esophagus. On the right are the azygos vein, the right vagus nerve, and the pleura. On the left are the aortic arch, the left common carotid and left subclavian arteries, and the left recurrent laryngeal nerve.

The carina is the cartilaginous ridge at the tracheal bifurcation, visible at bronchoscopy as a sharp sagittal ridge separating the origins of the right and left main bronchi. It is exquisitely sensitive to touch, and contact with it provokes vigorous coughing. The right main bronchus is wider, shorter (approximately 2.5 cm), and more vertical than the left, which is narrower, longer (approximately 5 cm), and more horizontal. Consequently, aspirated foreign bodies and misplaced endotracheal tubes are more likely to enter the right main bronchus.

<image>Panel A: Anterior view of the trachea from larynx to carina showing C-shaped cartilage rings with posterior membranous wall containing trachealis muscle. Panel B: Cervical trachea relations with thyroid isthmus overlying rings 2-4, recurrent laryngeal nerves in tracheoesophageal grooves, and esophagus posteriorly. Panel C: Carina at T4-5 showing right main bronchus as wider, shorter, and more vertical compared to the left main bronchus. Panel D: Cross-section inset showing C-shaped cartilage, trachealis muscle, and posterior relation to the esophagus.</image>

Blood Supply of the Trachea

The trachea receives its blood supply from multiple sources. The cervical trachea is supplied primarily by branches of the inferior thyroid artery, which arise from the thyrocervical trunk of the subclavian artery. The thoracic trachea receives blood from the bronchial arteries (which arise from the thoracic aorta) and from branches of the inferior thyroid and internal thoracic arteries. These arteries reach the trachea and divide into longitudinal anastomoses that give off branches to supply the tracheal wall.

Venous drainage is to the inferior thyroid veins (draining to the brachiocephalic vein) and the bronchial veins.

The segmental nature of the tracheal blood supply has clinical relevance for tracheal surgery. Excessive mobilization of the trachea can compromise its blood supply, and anastomotic complications following tracheal resection may relate to ischemia of the tracheal ends.

<image>Panel A: Inferior thyroid artery supplying the cervical trachea with segmental branches. Panel B: Bronchial arteries supplying the thoracic trachea. Panel C: Longitudinal anastomotic vessels running along the tracheal surface connecting arterial sources. Panel D: Venous drainage to inferior thyroid veins and bronchial veins indicated in blue.</image>

Clinical Correlations

Laryngitis is inflammation of the laryngeal mucosa, most commonly due to viral infection (often accompanying an upper respiratory infection) but also caused by bacterial infection, voice overuse, or gastroesophageal reflux. The cardinal symptom is hoarseness or voice loss. The mucosa becomes edematous and erythematous, and the vocal folds may not approximate properly during phonation.

Vocal cord paralysis results from injury to the nerves supplying the larynx. Unilateral recurrent laryngeal nerve palsy causes the affected vocal fold to assume a paramedian position. The patient is hoarse because the paralyzed fold cannot meet the functioning opposite fold during phonation, allowing air to escape. The airway is usually adequate, although stridor may occur with exertion. Bilateral recurrent laryngeal nerve palsy is a medical emergency: both folds remain near the midline and cannot abduct during inspiration, causing severe stridor and respiratory distress that may require tracheostomy. External laryngeal nerve injury causes a more subtle deficit: the voice is weak and monotonous, and the patient cannot sing high notes, because the cricothyroid cannot tense the vocal folds.

Laryngeal carcinoma most commonly arises from the vocal folds (glottic carcinoma). Because the glottis has sparse lymphatic drainage, these tumors tend to present early with hoarseness before they have metastasized to lymph nodes. Supraglottic and subglottic tumors may present later with lymph node involvement, airway obstruction, or referred otalgia.

Cricothyrotomy is an emergency surgical procedure to establish an airway when endotracheal intubation is impossible (for example, due to upper airway obstruction, severe facial trauma, or inability to access the airway). A horizontal incision is made through the skin and the median cricothyroid ligament (cricothyroid membrane), and a tube is inserted into the airway. This procedure is faster and simpler than tracheostomy but is considered a temporary measure.

Tracheostomy is a surgical procedure in which an opening is made in the anterior tracheal wall, usually between the second and fourth tracheal rings (below the thyroid isthmus, which may be divided or retracted). A tracheostomy tube is inserted to provide a long-term artificial airway. Indications include prolonged mechanical ventilation, upper airway obstruction, and protection of the airway in patients with severe aspiration risk.

Foreign body aspiration occurs more commonly into the right main bronchus because of its wider diameter, shorter length, and more vertical orientation. Objects that pass the glottis are likely to enter the right bronchus, causing coughing, unilateral wheezing, and potentially atelectasis or pneumonia if the obstruction is not relieved. Diagnosis and treatment are by bronchoscopy.

<image>Panel A: Laryngoscopic view comparing normal vocal folds with paralyzed fold in paramedian position. Panel B: Cricothyrotomy procedure showing incision site at cricothyroid membrane between thyroid and cricoid cartilages. Panel C: Tracheostomy showing opening between rings 2-4 with tube in place. Panel D: Chest radiograph showing foreign body in right main bronchus with right-sided atelectasis.</image>

Summary

The larynx contains nine cartilages: the thyroid, cricoid, and epiglottis are unpaired; the arytenoid, corniculate, and cuneiform cartilages are paired. The cricothyroid membrane (conus elasticus) has its free upper edge as the vocal ligament and is the site of emergency airway access at its anterior thickening (median cricothyroid ligament). The vocal folds consist of the vocal ligament, vocalis muscle, and squamous epithelium; the rima glottidis is the opening between them. The posterior cricoarytenoid is the only muscle that abducts the vocal folds. The cricothyroid is the only intrinsic muscle innervated by the external laryngeal nerve; all other intrinsic muscles receive the recurrent laryngeal nerve. The recurrent laryngeal nerve supplies motor innervation to all intrinsic muscles except the cricothyroid and sensory innervation below the vocal folds. The trachea extends from C6 to T4-T5, supported by C-shaped cartilage rings open posteriorly; the right main bronchus is wider, shorter, and more vertical, making it the more common destination for aspirated foreign bodies. Unilateral recurrent laryngeal nerve injury causes hoarseness; bilateral injury causes airway obstruction and is a surgical emergency.

Key Terms

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