Regional Anatomical Relationships

The larynx is located in the midline of the neck. It lies in front of the vertebral column and between the hyoid bone above and the trachea below. In adults, it lies between the third and sixth cervical vertebrae. The root, or pharyngeal portion, of the tongue is interconnected with the epiglottis of the larynx by three fibroelastic bands, the glossoepiglottic folds. The lowermost portion of the pharynx, the hypopharynx, surrounds the posterior aspect of the larynx. Muscle fibers of the inferior pharyngeal constrictor attach to the posterolateral aspect of the thyroid and cricoid cartilages. The esophagus lies inferior and posterior to the larynx. It is a muscular tube that interconnects the pharynx and the stomach. Muscle fibers originating from the cricoid cartilage form part of the muscular valve, which opens to allow food to pass from the pharynx into the esophagus.

Cartilaginous Skeleton

The larynx is composed of five major cartilages: thyroid, cricoid, one pair of arytenoids, and the epiglottis (Fig. 1). The hyoid bone, though intimately associated with the larynx, is not part of it. The cartilaginous components of the larynx are joined by ligaments and membranes. The thyroid and cricoid cartilages are composed of hyaline cartilage, which provides them with form and rigidity. They are interconnected by the cricothyroid joints and surround the laryngeal cavity. These cartilages support the soft tissues of the laryngeal cavity, thereby protecting this vital passageway for unencumbered movement of air into and out of the lower airway. The thyroid cartilage is composed of two quadrangular plates that are united at midline in an angle called the thyroid angle or laryngeal prominence. In the male, the junction of the laminae forms an acute angle, while in the female it is obtuse. This sexual dimorphism emerges after puberty. The cricoid cartilage is signet ring shaped and sits on top of the first ring of the trachea, ensuring continuity of the airway from the larynx into the trachea (the origin of the lower respiratory tract). The epiglottis is a flexible leaf-shaped cartilage whose deformability results from its elastic cartilage composition. During swallowing, the epiglottis closes over the entrance into the laryngeal cavity, thus preventing food and liquids from passing into the laryngeal cavity, which could obstruct the airway and interfere with breathing. The arytenoid cartilages are interconnected to the cricoid cartilage via the cricoarytenoid joint. These pyramid-shaped cartilages serve as points of attachment for the vocal folds, all but one pair of the intrinsic laryngeal muscles, and the vestibular folds.

Figure 1..  

Laryngeal cartilages shown separately (top) and articulated (bottom) at the laryngeal joints. The hyoid bone is not part of the larynx but is attached to it by the thyrohyoid membrane. (From Orlikoff, R. F., and Kahane, J. C. [1996]. Structure and function of the larynx. In N. J. Lass [Ed.], Principles of experimental phonetics. St. Louis: Mosby. Reproduced with permission.)

The thyroid, cricoid and arytenoid cartilages are interconnected to each other by two movable joints, the cricothyroid and cricoarytenoid joints. The cricothyroid joint joins the thyroid and cricoid cartilages and allows the cricoid cartilage to rotate upward toward the cricoid (Stone and Nuttal, 1974). Since the vocal folds are attached anteriorly to the inside face of the thyroid cartilage and posteriorly to the arytenoid cartilages, which in turn are attached to the upper rim of the cricoid, this rotation effects lengthening and shortening of the vocal folds, with concomitant changes in tension. Such changes in tension are the principal method of changing the rate of vibration of the vocal folds. The cricoarytenoid joint joins the arytenoid cartilages to the superolateral rim of the cricoid. Rocking motions of the arytenoids on the upper rim of the cricoid cartilage allow the arytenoids and the attached vocal folds to be drawn away (abducted) from midline and brought toward (adducted) midline. The importance of these actions has been emphasized by von Leden and Moore (1961), as they are necessary for developing the transglottal impedances to airflow that are needed to initiate vocal fold vibration. The effect of such movements is to change the size and shape of the glottis, the space between the vocal folds, which is of importance in laryngeal articulation, producing devoicing and pauses, and facilitating modes of vocal atttack.

Laryngeal Cavity

The laryngeal cartilages surround an irregularly shaped tube called the laryngeal cavity, which forms the interior of the larynx (Fig. 2). It extends from the laryngeal inlet (laryngeal aditus), through which it communicates with the hypopharynx, to the level of the inferior border of the cricoid cartilage. Here the laryngeal cavity is continuous with the lumen of the trachea. The walls of the laryngeal cavity are formed by fibroelastic tissues lined with epithelium. These fibroelastic tissues (quadrangular membrane and conus elasticus) restore the dimensions of the laryngeal cavity, which become altered through muscle activity, passive stretch from adjacent structures, and aeromechanical forces.

Figure 2..  

The laryngeal cavity, as viewed posteriorly. (From Kahane, J. C. [1988]. Anatomy and physiology of the organs of the peripheral speech mechanism. In N. J. Lass, L. L. McReynolds, J. L. Northern, and D. E. Yoder [Eds.], Handbook of speech-language pathology and audiology. Toronto: B. C. Decker. Reproduced with permission.)

The laryngeal cavity is conventionally divided into three regions. The upper portion is a somewhat expanded supraglottal cavity or vestibule whose walls are reinforced by the quadrangular membrane. The middle region, called the glottal region, is bounded by the vocal folds; it is the narrowest portion. The lowest region, the infraglottal or subglottal region, is bounded by the conus elasticus. The area of primary laryngeal valving is the glottal region, where the shape and size of the rima glottidis or glottis (space between the vocal folds) is modified during respiration, vocalization, and sphincteric closure. The rima glottidis consists of an intramembranous portion, which is bordered by the soft tissues of the vocal folds, and an intracartilaginous portion, the posterior two-fifths of the rima glottidis, which is located between the vocal processes and the bases of the arytenoid cartilages. The anterior two-thirds of the glottis is an area of dynamic change occasioned by the positioning and aerodynamic displacement of the vocal folds. The overall dimensions of the intracartilaginous glottis remain relatively stable except during strenuous sphincteric valving.

The epithelium that lines the laryngeal cavity exhibits regional specializations. Stratified squamous epithelium covers surfaces subjected to contact, compressive, and vibratory forces. Typical respiratory epithelium (pseudostratified ciliated columnar epithelium with goblet cells) is plentiful in the laryngeal cavity and lines the supraglottis, ventricles, and nonvibrating portions of the vocal folds; it also provides filtration and moisturization of flowing air. The epithelium and immediately underlying connective tissue form the muscosa, which is supplied by an array of sensory receptors sensitive to pressure, chemical, and tactile stimuli, pain, and direction and velocity of airflow (Wyke and Kirchner, 1976). These receptors are innervated by sensory branches from the superior and recurrent laryngeal nerves. They are essential components of the exquisitely sensitive protective reflex mechanism within the larynx that includes initiating coughing, throat clearing, and sphincteric closure.

Laryngeal Muscles

The larynx is acted upon by extrinsic and intrinsic laryngeal muscles (Tables 1 and 2). The extrinsic laryngeal muscles are attached at one end to the larynx and have one or more sites of attachment to a distant site (e.g., the sternum or hyoid bone). The suprahyoid and infrahyoid muscles attach to the hyoid bone and are generally considered extrinsic laryngeal muscles (Fig. 3). Although these muscles do not attach to the larynx, they influence laryngeal position in the neck through their action on the hyoid bone. The thyroid cartilage is connected to the hyoid bone by the hyothyroid membrane and ligaments. The larynx is moved through displacement of the hyoid bone. The suprahyoid and infrahyoid muscles also stabilize the hyoid bone, allowing other muscles in the neck to act directly on the laryngeal cartilages. The suprahyoid and infrahyoid muscles are innervated by a combination of cranial and spinal nerves. Cranial nerves V and VII supply all of the suprahyoid muscles except the geniohyoid. All of the infrahyoid muscles are innervated by spinal nerves from the upper (cervical) portion of the spinal cord.

Figure 3..  

The extrinsic laryngeal muscles. (From Bateman, H. E., and Mason, R. M. [1984]. Applied anatomy and physiology of the speech and hearing mechanism. Springfield, IL: Charles C Thomas. Reproduced with permission.)

The suprahyoid and infrahyoid muscles have been implicated in fundamental frequency control under a construct proposed by Sonninen (1956), called the external frame function. Sonninen suggested that the extrinsic laryngeal muscles are involved in producing fundamental frequency changes by exerting forces on the laryngeal skeleton that effect length and tension changes in the vocal folds.

The designation of extrinsic laryngeal muscles adopted here is based on strict anatomical definition as well as on research data on the action of the extrinsic laryngeal muscles during speech and singing. One of the most convincing studies in this area was done by Shipp (1975), who showed that the sternothyroid and thyrohyoid muscles systematically change the vertical position of the larynx in the neck, particularly with changes in fundamental frequency. Shipp demonstrated that the sternothyroid lowers the larynx with decreasing pitch, while the thyrohyoid raises it.

The intrinsic muscles of the larynx (Fig. 4) are a collection of small muscles whose points of attachment are all in the larynx (to the laryngeal cartilages). The anatomical properties of the intrinsic laryngeal muscles are summarized in Table 2. The muscles can be categorized according to their effects on the shape of the rima glottidis, the positioning of the folds relative to midline, and the vibratory behavior of the vocal folds. Hirano and Kakita (1985) nicely summarized these behaviors (Table 3). Among the most important functional or biomechanical outcomes of the actions of the intrinsic laryngeal muscles are (1) abduction and adduction of the vocal folds, (2) changing the position of the laryngeal cartilages relative to each other, (3) transiently changing the dimensions and physical properties of the vocal folds (i.e., length, tension, mass per unit area, compliance, and elasticity), and (4) modifying laryngeal airway resistance by changing the size or shape of the glottis.

Figure 4..  

The intrinsic laryngeal muscles as shown in lateral (A), posterior (B), and superior (C) views. (From Kahane, J. C. [1988]. Anatomy and physiology of the organs of the peripheral speech mechanism. In N. J. Lass, L. L. McReynolds, J. L. Northern, and D. E. Yoder [Eds.], Handbook of speech- language pathology and audiology. Toronto: B. C. Decker. Reproduced with permission.)

The intrinsic laryngeal muscles are innervated by nerve fibers carried in the trunk of the vagus nerve. These branches are usually referred to as the superior and inferior laryngeal nerves. The cricothyroid muscle is innervated by the superior laryngeal nerve, while all other intrinsic laryngeal muscles are innervated by the inferior (recurrent) laryngeal nerve. Sensory fibers from these nerves supply the entire laryngeal cavity.

Histochemical studies of intrinsic laryngeal muscles (Matzelt and Vosteen, 1963; Rosenfield et al., 1982) have enabled us to appreciate the unique properties of the intrinsic muscles. The intrinsic laryngeal muscles contain, in varying proportions, fibers that control fine movements for prolonged periods (type 1 fibers) and fibers that develop tension rapidly within a muscle (type 2 fibers). In particular, laryngeal muscles differ from the standard morphological reference for striated muscles, the limb muscles, in several ways: (1) they typically have a smaller mean diameter of muscle fibers; (2) they are less regular in shape; (3) the muscle fibers are generally uniform in diameter across the various intrinsic muscles; (4) individual muscle fibers tend not to be uniform in their directionality within a fascicle but exhibit greater variability in the course of muscle fibers, owing to the tendency for fibers to intermingle in their longitudinal and transverse planes; and (5) laryngeal muscles have a greater investment of connective tissues.

Vocal Folds

The vocal folds are multilayered vibrators, not a single homogeneous band. Hirano (1974) showed that the vocal folds are composed of several layers of tissues, each with different physical properties and only 1.2 mm thick. The vocal fold consists of one layer of epithelium, three layers of connective tissue (lamina propria), and the vocalis fibers of the thyroarytenoid muscle (Fig. 5). Based on examination of ultra-high-speed films and biomechanical testing of the vocal folds, Hirano (1974) found that functionally, the epithelium and superficial layer of the lamina propria form the cover, which is the most mobile portion of the vocal fold. Wavelike mucosal disturbances travel along the surface during sound production. These movements are essential for developing the agitation and patterning of air molecules in transglottal airflow during voice production. The superficial layer of the lamina propria is composed of sparse amounts of loosely interwoven collagenous and elastic fibers. This area, also known as Reinke's space, is important clinically because it is the principal site of swelling or edema formation in the vocal folds following vocal abuse or in laryngitis. The intermediate and deep layers of the lamina propria are called the transition. The vocal ligament is formed from elastic and collagenous fibers in these layers. It provides resiliency and longitudinal stability to the vocal folds during voice production. The transition is stiffer than the cover but more pliant than the vocalis muscle fibers, which form the body of the vocal folds. These muscle fibers are active in regulating fundamental frequency by influencing the tension in the vocal fold and the compliance and elasticity of the vibrating surface (cover).

Figure 5..  

Schematic of the layered structure of the vocal folds. The leading edge of the vocal fold with its epithelium is at left. Co, collaginous fibers; Elf, elastic fibers; M, vocalis muscle fibers. (From Hirano, M. [1975]. Official report: Phonosurgery. Basic and clinical investigations. Otologia [Fukuoka], 21, 239–440. Reproduced with permission.)

See also voice production: physics and physiology.