Chapter 28. Larynx

Big Picture

The larynx forms the air passageway from the hyoid bone to the trachea. The larynx is continuous with the laryngopharynx superiorly and with the trachea inferiorly. The larynx provides a patent (open) airway and acts as a switching mechanism to route air and food into the proper channels. The larynx is commonly known as the voice box and provides the cartilaginous framework for vocal fold and muscle attachments, which vibrate to produce sound.

Hyoid Bone

The hyoid bone consists of a body, two greater horns, and two lesser horns, and is the only bone that does not articulate with another bone. The hyoid bone is U-shaped and is suspended from the tips of the styloid processes of the temporal bones by the stylohyoid ligaments (Figure 28-1A and C). The hyoid bone is connected to the thyroid cartilage and supported by the suprahyoid and infrahyoid muscles and by the middle pharyngeal constrictor muscle. In addition, the hyoid bone supports the root of the tongue.

Laryngeal Cartilages

The framework of the larynx is an intricate arrangement of nine cartilages connected by membranes and ligaments (Figure 28-1A–C).

• Thyroid cartilage. The thyroid cartilage forms a median elevation, called the laryngeal prominence (“Adam's apple”), and lies inferior to the hyoid bone. The thyroid cartilage typically is larger in males than in females because the male sex hormones stimulate its growth during puberty. The superior horn of the laryngeal cartilage is attached to the tip of the greater horn of the hyoid bone, whereas its inferior horn articulates with the cricoid cartilage, forming the cricothyroid joint.
  • The thyrohyoid membrane, as its name implies, stretches between the thyroid cartilage and the hyoid bone. The superior laryngeal vessels and the internal laryngeal nerve pierce the membrane en route to providing vascular supply and sensory information, respectively, to the mucosa superior to the vocal folds.
• Cricoid cartilage. The cricoid cartilage is shaped like a signet ring, with the broad part of the ring facing posteriorly. The lower border marks the inferior limits of the larynx and pharynx. The function of the cricoid cartilage is to provide attachments for laryngeal muscles, cartilages, and ligaments involved in opening and closing of the airway to produce sound.
• Epiglottis. The epiglottis is elastic cartilage, shaped like a spoon, that is posterior to the root of the tongue. The lower end of the epiglottis is attached to the deep surface of the thyroid cartilage. When only air is flowing into the larynx, the inlet to the larynx is open wide, with the free edge of the epiglottis projecting superiorly and anteriorly. During swallowing, the larynx is pulled superiorly and the epiglottis tips posteriorly to cover the laryngeal inlet. As a result, the epiglottis acts as a deflector to keep food out of the larynx (and trachea) during swallowing.
• Arytenoid cartilages. The arytenoid cartilages are shaped like a pyramid. Their base articulates with the cricoid cartilage. Each arytenoid cartilage has a vocal process, which gives attachment to the vocal ligaments and vocalis muscle, and a muscular process, which gives attachment to the thyroarytenoid and the lateral and posterior cricoarytenoid muscles.
• Cuneiform and corniculate cartilages. These tiny cartilages lie on the apices of the arytenoid cartilages and are enclosed within the aryepiglottic folds.

Vocal Ligaments and Their Movements

From the vocal process of each arytenoid cartilage, a fibrous band extends anteriorly to attach to the deep surface of the thyroid cartilage (Figure 28-1C). These two fibrous ligaments are composed largely of elastic fibers and form the core of the mucosal folds called the vocal folds, or true vocal cords. Consequently, the vocal folds vibrate, producing sound as air rushes up from the lungs through the rima glottis, the opening between the vocal cords. Superior to the true vocal folds are a similar pair of mucosal folds, called the vestibular (false) vocal folds, which play no part in the production of sound because the false vocal folds are not opposable.

Big Picture

Laryngeal muscles are innervated by the vagus nerve [cranial nerve (CN) X] and move the laryngeal skeleton. In turn, this movement changes the width and the tension on the vocal folds so that air passing between the vocal folds causes them to vibrate, producing sound.

Laryngeal Muscles

The intrinsic laryngeal muscles move the laryngeal framework, altering the size and shape of the rima glottidis and the length and tension of the vocal folds (Figure 28-2). The actions of the laryngeal muscles are best understood when considered in the following functional groups: adductors and abductors and tensors and relaxers.

Adductors and Abductors

Rotation of the cricoarytenoid cartilages results in medial or lateral displacement of the vocal folds, thereby decreasing or increasing, respectively, the aperture of the rima glottis.

• Adduction (closing) of the vocal folds. The lateral cricoarytenoid muscles pull the muscular processes anteriorly, rotating the arytenoids so their vocal processes swing medially. When this action is combined with that of the transverse arytenoid muscles, which pull the arytenoid cartilages together, the gap between the vocal folds is decreased. Air pushed through the rima glottidis causes vibration of the vocal ligaments.
• Abduction (opening) of the vocal folds. The posterior cricoarytenoid muscles pull the muscular processes posteriorly, rotating the vocal processes laterally and thus widening the rima glottidis.

Gliding Movement of the Arytenoid Cartilages

Horizontal gliding action of the arytenoid cartilages permits the bases of these cartilages to move side to side. Medial gliding and medial rotation of the arytenoid cartilages occur simultaneously, as do the two lateral movements.

• Adduction of the vocal folds also is aided by horizontal medial gliding of the arytenoid cartilages. This action is caused by bilateral contraction of the lateral cricoarytenoid, the transverse and oblique arytenoids, and the aryepiglottic muscles.
• Abduction of the vocal folds also is aided by horizontal lateral gliding of the arytenoid cartilages. This action is caused by bilateral contraction of the posterior cricoarytenoid muscles.

Length and Tension of the Vocal Folds

The cricothyroid joint is a synovial articulation between the side of the cricoid cartilage and the inferior horn of the thyroid cartilage. This joint enables the thyroid cartilage to tilt back and forth upon the cricoid cartilage, altering the length and tension of the vocal folds.

• Tensing of the vocal folds. The principal tensors are the cricothyroid muscles, which tilt the thyroid cartilage anteriorly and inferiorly, increasing the distance between the thyroid cartilage and the arytenoid cartilage. This movement elongates and tightens the vocal ligaments, raising the pitch of the voice.
• Relaxing the vocal folds. The principal relaxers are the thyroarytenoid muscles, which pull the arytenoid cartilages anteriorly toward the thyroid angle (prominence), thereby relaxing the vocal ligaments.
  • Vocalis muscles. Produce minute adjustments of the vocal ligaments, selectively tensing and relaxing parts of the vocal folds during animated speech and singing.

Voice Production

To understand how we speak and sing, we first need to understand how sound is produced, and then we need to understand how that sound is articulated.

Phonation

Phonation, or the production of sound, involves the intermittent release of expired air coordinated with opening and closing the rima glottis. The length of the true vocal folds and the size of the rima glottis are altered by the action of the intrinsic laryngeal muscles, most of which move the arytenoid cartilages. As the length and tension of the vocal folds change, the pitch of the sound is altered. Generally, the more tense the vocal folds, the faster they vibrate and thus the higher the pitch. The rima glottis is wide when deep tones are produced and narrows to a slit when high-pitched sounds are produced. As a young boy's larynx enlarges during puberty, the vocal folds become both longer and thicker, causing them to vibrate more slowly and thus the voice becomes deeper.

Loudness of the voice depends on the force with which air rushes across the vocal folds. The greater the force of air across the vocal folds, the stronger the vibration, which results in louder sounds. The vocal folds do not move at all when we whisper, but they vibrate vigorously when we yell.

Although the vocal folds produce sounds, the quality of the voice depends on the coordinated activity of many other structures. For example, the pharynx acts as a resonating chamber to amplify and enhance the quality of the sound, as do the oral and nasal cavities and the paranasal sinuses.

Articulation

Articulation, or the production of intelligible sounds, involves the actions of the pharyngeal muscles (vagus nerve, CN X), the tongue (hypoglossal nerve, CN XII), the muscles of facial expression (facial nerve, CN VII), mandibular movements (mandibular branch of the trigeminal nerve, CN V-3), and the soft palate (CN X). Each of these structures modifies the crude sounds that are produced by the larynx and convert them into recognizable consonants and vowels.

Big Picture

Blood supply to the larynx is derived principally from the superior and inferior laryngeal arteries. Innervation is both sensory and motor (CN X).

Laryngeal Vessels

The blood supply to the larynx is derived from the superior and inferior laryngeal arteries (Figure 28-3A and B). Rich anastomoses exist between the corresponding contralateral and ipsilateral arteries.

• Superior thyroid artery. Branching from the external carotid artery, the superior thyroid artery descends en route to the thyroid gland. In its course, the superior thyroid artery gives rise to the superior laryngeal arteries, which penetrate the thyrohyoid membrane to gain entrance to the interior of the larynx. The artery supplies most of the tissues of the larynx above the vocal folds, including almost all of the laryngeal muscles.
• Inferior laryngeal artery. Branching from the thyrocervical trunk, the inferior laryngeal artery supplies the region inferior to the vocal folds.

Venous return from the larynx occurs via the superior and inferior laryngeal veins, which are tributaries of the superior and inferior thyroid veins, respectively. The superior thyroid vein drains into the internal jugular vein, whereas the inferior thyroid vein drains into the brachiocephalic vein.

Laryngeal Innervation

Innervation of the larynx is from CN X via the superior and recurrent laryngeal nerves (Figure 28-3A and B).

Superior Laryngeal Nerve

The superior laryngeal nerve arises from the inferior vagal ganglion and divides into a smaller external laryngeal branch and a larger internal laryngeal branch.

• External laryngeal nerve. The external laryngeal nerve courses laterally to the external surface of the larynx to provide motor innervation to the cricothyroid muscle.
• Internal laryngeal nerve. The internal laryngeal nerve, along with the superior thyroid artery and vein, pierces the thyrohyoid membrane and provides general sensory innervation to the mucosa superior to the vocal folds.

Recurrent Laryngeal Nerve

All intrinsic laryngeal muscles are supplied by CN X. The recurrent laryngeal nerve of CN X innervates all of the intrinsic muscles, except for the cricothyroid muscle, which is supplied by the external laryngeal nerve of CN X. The recurrent laryngeal nerve also provides visceral sensory innervation from the mucosa inferior to the vocal folds. The superior portion of the recurrent laryngeal nerve has a close but variable relationship to the inferior thyroid artery and vein.

• Hoarseness is the most common symptom of disorders of the larynx, including inflammation or carcinoma of the larynx.
• Paralysis of the superior laryngeal nerve causes anesthesia of the laryngeal mucosa superior to the vocal folds. As a result, the protective mechanism designed to keep food out of the larynx (the sensory limb of the cough reflex) is inactive.