Laryngeal Function and Speech Production
The larynx is an air valve located within the front of the neck. This valve is positioned vertically between the trachea (windpipe) and pharynx (throat) and can be adjusted to vary the amount of coupling between the two. The larynx serves a variety of functions, including speech production (inclusive of voice production).
This chapter begins with description of the anatomy of the laryngeal apparatus followed by discussion of its forces and movements, control variables, neural control, and general functions. Focus then turns to laryngeal function during speech production and variables that influence that function.
ANATOMY OF THE LARYNGEAL APPARATUS
The skeleton of the larynx, its joints, and its internal topography are considered in this section. The muscles of the larynx are discussed in the next section, Forces of the Laryngeal Apparatus.
Figure 3–1 depicts the skeletal framework of the laryngeal apparatus. This framework consists of cartilage, bone, ligament, and tendon. The flexibility of this framework changes with age, being soft and pliable in childhood and hard and more rigid in adulthood.
The thyroid cartilage is the largest of the laryngeal cartilages and forms most of the front and sides of the laryngeal skeleton. This cartilage provides a shield-like housing for the larynx and offers protection for many of its structures.
Figure 3–2 shows the thyroid cartilage from four different views. Two quadrilateral plates, called the thyroid laminae, are fused together at the front of the thyroid cartilage and diverge widely (more so in women than in men) toward the back. The configuration of the two thyroid laminae resembles the bow of a ship. The line of fusion between the two plates is called the angle of the thyroid. The upper part of the structure contains a prominent V-shaped depression termed the thyroid notch that can be palpated at the front of the neck. This notch is located just above the most forward projection of the cartilage, an outward jutting called the thyroid prominence or Adam’s apple.
Sizing Things Up
There’s a tendency when viewing anatomical drawings, photographs, and video images of structures of the larynx to overestimate their size. The trachea looks long and large in cross-section. The glottis seems to be a big hole. The vocal folds appear to be massive lips. The vocal ligaments look like pencils. And the vibratory movements of the vocal folds (when slowed down) give the impression of a flag blowing in a stiff wind. Some calibration may be helpful. Your trachea is about as long and big around as your middle finger. Your wide-open glottis is about the size of a dime. Your approximated vocal folds have a surface area about the size of your thumbnail (well trimmed). The vocal ligaments are about as thick as wooden matchsticks. And your vocal folds only move about the length of the cuticle on your thumbnail. If you’re like us, you’ll find this to be surprisingly small. Yes?
Figure 3–1. Skeletal framework of the laryngeal apparatus. This framework is composed of two paired cartilages (arytenoid cartilages and corniculate cartilages), three unpaired cartilages (thyroid cartilage, cricoid cartilage, and epiglottis), and one bone (hyoid bone).
The back edges of the thyroid laminae extend upward into two long horns, called the superior cornua, and downward into two short horns, called the inferior cornua. The superior cornua are coupled to the hyoid bone. The inferior cornua have facets (areas where other structures join) on their lower inside surfaces that form joints with the cricoid cartilage. The inferior cornua straddle the cricoid cartilage like a pair of legs (see Figure 3–1).
The cricoid cartilage forms the lower part of the laryngeal skeleton. It is a ring-shaped structure located above the trachea. As shown in Figure 3–3, the cricoid cartilage has a thick plate at the back, the posterior quadrate lamina, which resembles a signet on a finger ring. A semicircular structure, called the anterior arch, forms the front of the cricoid cartilage and is akin to a band on a finger ring.
Four facets are located on the cricoid cartilage. The lower two facets, one on each side at the same level, are positioned near the junction of the posterior quadrate lamina and anterior arch. Each of these facets articulates with a facet on one of the inferior cornua of the thyroid cartilage. The upper two facets of the cricoid cartilage, one on each side at the same level, are located on the sloping rim of the posterior quadrate lamina. Each of these facets articulates with a facet on the undersurface of one of the arytenoid cartilages.
Figure 3–2. Thyroid cartilage from four different views. The thyroid prominence is also called the Adam’s apple.
There are two arytenoid cartilages. Each is located atop one side of the sloping rim of the posterior quadrate lamina of the cricoid cartilage. As shown in Figure 3–4, each arytenoid cartilage has a complex shape that includes an apex, base, and three sides. The apex of each cartilage is capped with another small cone-shaped cartilage called a corniculate cartilage that is often fused to the arytenoid cartilage. The base of each arytenoid cartilage has a flexible pointed projection that extends toward the front and is designated the vocal process. The base also includes a rounded stubby projection that extends toward the back and side and is referred to as the muscular process. The undersurface of each muscular process has a facet that articulates with one of the upper facets of the cricoid cartilage.
Figure 3–3. Cricoid cartilage from three different views. This cartilage is often described as having the shape of a signet ring.
Figure 3–5 depicts the epiglottis. The epiglottis is a single cartilage that is positioned behind the hyoid bone and root of the tongue. The upper part of the epiglottis, its body, is broad and resembles the distal end of a forward-curving shoehorn. The front and back surfaces of this part of the structure are referred to as its lingual (tongue) and laryngeal (larynx) surfaces, respectively. The lingual surface attaches to the hyoid bone. The lower part of the cartilage tapers downward into a stalk called the petiolus (little leg) and attaches to the inside of the thyroid cartilage just below the thyroid notch.
Figure 3–6 depicts the hyoid bone (tongue bone). Technically, the hyoid bone is not a part of the larynx. Nevertheless, it serves as an integral component in many laryngeal functions. Thus, it is commonly afforded a prominent place in discussion of the laryngeal skeleton.
The hyoid bone is described as free-floating in the sense that it is not attached to any other bone. It is a U-shaped structure that is positioned horizontally within the neck, its open end facing toward the back. The hyoid bone consists of a body and two pairs of greater and lesser horns (cornua) that project upward. The greater cornua are located toward the back of the structure and join with the superior cornua of the thyroid cartilage. The lesser cornua extend from the body of the structure and may be capped by tiny cone-shaped cartilages. The hyoid bone is positioned at the top of the larynx and suspends it from above through various connections.
There are two pairs of joints in the larynx. One pair is between the cricoid and thyroid cartilages on each side. The other pair is between the cricoid and arytenoid cartilages on each side. Movements at these joints are conditioned by the nature of the facets on their articulating cartilages and the arrangement of surrounding ligaments.
Figure 3–4. Arytenoid and corniculate cartilages, both paired cartilages, shown from four different views.
Figure 3–6. Hyoid bone from three different views. The hyoid bone, often described as horseshoe-shaped, has the special status of being the only free-floating bone in the body, meaning that it does not directly articulate with any other bones. The hyoid bone is not technically part of the larynx.
Figure 3–7 depicts the cricothyroid joints. These joints are positioned on the sides of the larynx and involve articulations between facets on the inner surfaces of the inferior cornua of the thyroid cartilage (see A in the figure) and the outer surfaces of the lower part of the cricoid cartilage (see B in the figure). The cricothyroid joints are encapsulated by membranes that secrete synovial fluid. This fluid serves as a lubricant.
Facets on the cricoid and thyroid cartilages vary from larynx to larynx and from side to side within the same larynx (Dickson & Maue-Dickson, 1982). Those on the cricoid cartilage generally face upward, toward the side, and backward. They are usually round or oval in shape and are concave. Facets on the thyroid cartilage typically face downward, toward the midline, and forward. They are usually round in shape and are convex. Occasionally a larynx will have cricothyroid joint facets that are rudimentary. Then the articulation between the cricoid and thyroid surfaces is formed by fibrous connective tissue (Zemlin, 1998).
Three ligaments extend between the side and back surfaces of the cricoid cartilage and the lower outside surfaces of the inferior cornu of the thyroid cartilage on each side. These three ligaments encircle most of the corresponding cricothyroid joint and are referred to as the anterior, lateral, and posterior ceratocricoid (cerato meaning horn) ligaments. The anterior ligament extends backward into the front surface of the inferior cornu. The lateral ligament extends upward into the lower surface of the cornu. And the posterior ligament extends forward into the back surface of the cornu. Together the three ligaments bind the corresponding cricothyroid joint and place restrictions on the movements of the cricoid and thyroid cartilages.
The movements at the cricothyroid joints are of two types—rotating and sliding—as portrayed in Figure 3–8. The most significant movements are rotational and occur about a lateral axis extending through the two joints. Either or both the cricoid and thyroid cartilages can rotate about this axis (Mayet & Muendnich, 1958; Takano & Honda, 2005; Vennard, 1967; Zemlin, 1998). One consequence of rotation is a change in the distance between the top of the anterior arch of the cricoid cartilage and the bottom of the laminae of the thyroid cartilage at the front. This is analogous to rotating the chin guard (representing the anterior arch of the cricoid cartilage) and the visor (representing the laminae of the thyroid cartilage) on a motorcycle helmet.
Figure 3–7. Cricothyroid joints. These two joints, one on each side, are created by articulations between convex facets on the inner surface of the inferior cornu of the thyroid cartilage (A) and concave facets on the outer surface of the cricoid cartilage (B). The anterior, lateral, and posterior ceratocricoid ligaments secure the joints and restrict their movements.
Secondary movements at the cricothyroid joints are of a sliding nature and can occur in those larynges that have oval-shaped cricoid facets. These movements are small and occur along the long axes of the cricoid facets (Takano & Honda, 2005; Titze, 1994; van den Berg, Vennard, Berger, & Shervanian, 1960).
The cricoarytenoid joints are illustrated in Figure 3–9. These joints are positioned near the top of the larynx and involve articulations between the facets on the undersurfaces of the arytenoid cartilages (A in the figure) and sloping rims of the cricoid cartilage (B in the figure). Synovial membranes encapsulate these joints and lubricate them.
Figure 3–8. Cricothyroid joint movements. These movements are primarily rotational. They can also be of a sliding nature.
Facets on the cricoid and arytenoid cartilages are relatively uniform in characteristics from larynx to larynx and from side to side within a larynx (Dickson & Maue-Dickson, 1982). Facets on the cricoid cartilage face upward, toward the side, and forward. These facets are usually oval in shape and are convex. Facets on the arytenoid cartilages face downward, toward the midline, and backward. They are usually round in shape and are concave (Frable, 1961; Zemlin, 1998).
Two ligaments, the anterior and posterior cricoarytenoid ligaments, influence the function of each cricoarytenoid joint by binding it and restricting its movements. The anterior cricoarytenoid ligament extends from the side of the cricoid cartilage to the front and side of the arytenoid cartilage. The ligament runs upward and backward and limits the degree to which the arytenoid cartilage can be moved backward. The posterior cricoarytenoid ligament extends upward and toward the side from the back of the cricoid cartilage to the back of the arytenoid cartilage. This ligament limits the degree to which the arytenoid cartilage can be moved forward.
Figure 3–10 depicts the movements at the cricoarytenoid joints. These can be of two types, rocking and sliding, the most significant of which is rocking. Rocking movements involve the arytenoid cartilages moving at right angles to the long axes of their articulating cricoid facets (Ardran & Kemp, 1966; Selbie, Zhang, Levine, & Ludlow, 1998; Sellars & Keen, 1978; Sonesson, 1959; von Leden & Moore, 1961). This means that as the arytenoid cartilages rock on the cricoid cartilage their vocal processes move either upward and outward or downward and inward.
Figure 3–9. Cricoarytenoid joints. These two joints, one on each side, are created by articulations between concave facets on the arytenoid cartilages (A) and convex facets on the cricoid cartilage (B). The anterior and posterior cricoarytenoid ligaments secure the joints and restrict their movements.
Being One with Your Larynx
Many aspects of laryngeal measurement show an uncanny “oneness” with the metric system. Titze (1994) suggested that this realization is helpful in making calculations off the top of your head when you don’t have references at hand. A partial listing of items that he recommends be committed to memory are that (a) the mass of a vocal fold is about 1 g, (b) the length of a vibrating vocal fold is about 1 cm, (c) the excursion of a vibrating vocal fold is about 1 mm, (d) the shortest period of vocal fold vibration is about 1 ms, (e) the surface wave velocity on a vibrating vocal fold is about 1 m/s, (f) the maximum peak-to-peak airflow through a vibrating larynx is about 1 L/s, (g) the maximum acceleration of airflow through a vibrating larynx is about 1 m3/s2, and (h) the maximum aerodynamic power generated by a vibrating larynx is about 1 J/s. We suggest you make it a goal to memorize these before you go to sleep tonight.
Figure 3–10. Cricoarytenoid joint movements. These are primarily rocking movements, although they can also be of a sliding nature.
Limited sliding movements can also occur along the cricoid facets. These movements involve small upward and inward or downward and outward adjustments of the arytenoid cartilages that follow the courses of the long axes of the cricoid facets (Fink, Basek, & Epanchin, 1956; Pressman, 1942; von Leden & Moore, 1961; Wang, 1998).
The interior of the larynx defines the boundaries of the laryngeal airway. Figure 3–11 depicts the structures that form these boundaries and lie immediately deep to them.
The laryngeal cavity extends from a lower opening formed by the base of the cricoid cartilage to an upper opening designated as the laryngeal aditus. The laryngeal aditus forms a collar at the top of the larynx. The rim of this collar comprises the tops of the arytenoid cartilages (and corniculate cartilages), sides of the epiglottis, and the aryepiglottic folds. The aryepiglottic folds run between the arytenoid cartilages and the epiglottis and envelop the aryepiglottic muscles (discussed below) and a pair of small cuneiform (wedge-shaped) cartilages. The cuneiform cartilages stiffen the aryepiglottic folds and help to maintain the upper opening (collar entrance) into the larynx.
The upper region of the laryngeal cavity (sometimes called the supraglottal region) is bounded below by the ventricular folds and above by the laryngeal aditus (upper opening into the larynx). This region is also called the laryngeal vestibule (cavity approaching a cavity). The configuration of the vestibule is roughly that of a funnel, the lumen of which increases in size toward its upper end. The lower region of the laryngeal cavity (sometimes called the subglottal region) is bounded below by the lower margin of the cricoid cartilage and above by the vocal folds. This region is cone-shaped and converges toward the undersurface of the vocal folds.
The vocal folds are two prominent shelf-like structures that extend from the sidewalls of the laryngeal cavity into the laryngeal airway. Each vocal fold has a front attachment near the midline of the thyroid cartilage and a rear attachment to the vocal process of the arytenoid cartilage on the same side.
The vocal folds are not structurally homogeneous, but are made up of muscle covered by several layers of tissue. Figure 3–12 shows a frontal section through the midlength of an adult vocal fold that exemplifies this characteristic layering. Up to five layers are recognized (Hirano, 1974; Hirano & Sato, 1993) and include: (a) a thin stiff capsule of squamous epithelium that determines the outer shape of the vocal fold, (b) a superficial layer of lamina propria (subflooring) that consists of loose fibrous matrix that resembles soft gelatin and is anchored to the epithelium through a region called the basement membrane zone, (c) an intermediate layer of lamina propria that contains elastic fibers and is likened to a bundle of soft rubber bands, (d) a deep layer of lamina propria that contains collagen fibers and bears analogy to a bundle of cotton thread, and (e) muscle fibers that form the inner vocal fold and are the equivalent of a bundle of stiff rubber bands. The combined epithelium and superficial layer of the lamina propria make up what is called the mucosa. The combined intermediate and deep layers of the lamina propria make up what is called the vocal ligament, a ligament that runs along the inner edge of the vocal folds from front to back.
Figure 3–11. Structures of the interior of the larynx from three different views.
Although the layering shown in Figure 3–12 is typical at the midlength of the vocal folds, it may be quite different at other locations because the layers of the lamina propria change in their relative proportions along the length of the vocal fold. For example, toward the ends of each vocal fold, elastic fibers and then collagenous fibers predominate. These masses of elastic and collagenous fibers act to cushion and protect those areas of the vocal folds from stresses. There are also differences in the cellular structure and concentration of other constituents at other locations within the lamina propria (Catten, Gray, Hammond, Zhou, & Hammond, 1998; de Melo et al., 2003; Ishii, Zhai, Akita, & Hirose, 1996; Obrebowski, Wojnowski, & Obrebowski-Karsznia, 2006; Strocchi et al., 1992).
Figure 3–12. Frontal section through the mid-length of the membranous adult vocal fold. This section shows the five layers of the vocal folds. The epithelium and superficial layer of the lamina propria make up the mucosa and the intermediate and deep layers of the lamina propria make up the vocal ligament. From Histological Color Atlas of the Human Larynx, (1st ed., p. 45), by M. Hirano and K. Sato, 1993, Belmont, CA: Delmar Learning. Copyright 1993 by Delmar Learning, a division of Thomson Learning: www.thomsonrights.com. Fax: 800-730-2215. Modified and reproduced with permission.
It is common practice to subgroup the five layers of the vocal folds into a so-called body and cover. The vocal fold body comprises muscle fibers and the deep layer of the lamina propria; the vocal fold cover comprises the intermediate and superficial layers of the lamina propria and the epithelium. This two-layered scheme, depicted in Figure 3–13, is significant because it helps to explain certain aspects of the mechanical behavior of the vocal folds during phonation.
When viewed from above, the medial borders of the vocal folds diverge from front to back when at rest, as illustrated in Figure 3–14. Between the vocal folds is a triangularly shaped opening called the glottis. The front part of the glottis is termed the membranous glottis and occupies about 60% of the length of the vocal folds. It lies between the thyroid cartilage and the tips of the vocal processes of the arytenoid cartilages and courses along the vocal ligaments. The back part of the glottis is called the cartilaginous glottis. It occupies about 40% of the length of the vocal folds and lies between the tips of the vocal processes of the arytenoid cartilages and the most rearward points on their medial surfaces.
Figure 3–13. Subgroupings of the five layers of the vocal folds into the body and the cover. The body of the vocal folds consists of the muscle and deep layer of the lamina propria (L. P.) and the cover consists of the intermediate and superficial layers of the lamina propria and the epithelium. This subgrouping is helpful to understanding the mechanical behavior of the vocal folds during phonation.
Figure 3–14. The vocal folds as viewed from above (with the front of the larynx at the top). The glottis (opening between the vocal folds) is bounded in the front by the membranous vocal folds and in the back by the vocal processes of the arytenoid cartilages.
As shown in Figure 3–11 (upper images), another set of shelf-like structures extends from the sidewalls of the laryngeal cavity into the laryngeal airway. These structures lie above the vocal folds (but are less prominent) and are referred to as the ventricular folds or false vocal folds. These folds attach to the thyroid cartilage at the front and to the fronts and sides of the arytenoid cartilages at the back. Each fold contains a ventricular ligament that runs from front to back near its medial edge. Muscular tissue is sparse within the ventricular folds. The opening between the ventricular folds is referred to as the false glottis and is nearly always wider than the glottis between the vocal folds.
The vocal folds and ventricular folds have a sinus (depression) between them (see Figure 3–11, upper left image). This sinus is called the laryngeal ventricle and constitutes a horizontal pouch in each sidewall of the laryngeal tube. The laryngeal ventricles extend most of the length of the vocal folds. Toward the front of the larynx they course upward into saccules that are richly endowed with mucous glands. These glands contain mucus that lubricates the vocal folds.
Laryngeal ligaments and membranes help to bind the laryngeal structures to one another as well as to structures outside the larynx. The ligaments that bind the joints of the larynx are discussed above and depicted in Figures 3–7 and 3–9. These are the anterior, lateral, and posterior ceratocricoid ligaments, for the cricothyroid joints, and the anterior and posterior cricoarytenoid ligaments, for the cricoarytenoid joints. Most of the other intrinsic and extrinsic ligaments and membranes are depicted in Figure 3–15 and discussed below.
Intrinsic Ligaments and Membranes. The intrinsic ligaments and membranes of the larynx are those that connect laryngeal cartilages to one another. These ligaments and membranes are important in regulating the extent and direction of movement of the laryngeal cartilages in relation to one another. Most of the intrinsic ligaments and membranes of the larynx arise from a common sheet of connective tissue called the elastic membrane. This sheet lines the entire laryngeal airway, except for the part that lies between the vocal and ventricular ligaments on each side. This discontinuity enables the mucous glands in the laryngeal saccules to be expressed into the laryngeal cavity as lubricant.
The part of the elastic membrane that lines the region between the lower margin of the cricoid cartilage and the vocal folds connects the cricoid, arytenoid, and thyroid cartilages to one another and is designated as the conus elasticus. This membrane gives rise to a middle cricothyroid ligament, two lateral cricothyroid membranes, and two vocal ligaments. The middle cricothyroid ligament extends between the top of the anterior arch of the cricoid cartilage and the bottom of the thyroid cartilage in the region of the angle of the thyroid cartilage. This ligament limits the degree to which the cricoid cartilage and thyroid cartilage can be separated vertically at the front. The two lateral cricothyroid membranes are thinner than the middle cricothyroid ligament and extend upward from the upper border of the anterior arch of the cricoid cartilage at the sides. They, like the middle cricothyroid ligament, restrict the separation of the cricoid and thyroid cartilages toward the front. The lateral cricothyroid membranes thicken significantly toward the top of the conus elasticus and are continuous with the paired vocal ligaments. The vocal ligaments extend between the angle of the thyroid cartilage and the vocal processes of the arytenoid cartilages and lie near the free margins of the vocal folds. These ligaments restrict the degree to which the thyroid and arytenoid cartilages can be separated from front to back.
The part of the elastic membrane that lines the region from the ventricular folds to the laryngeal aditus connects the epiglottis, thyroid cartilage, arytenoid cartilages, and corniculate cartilages to one another and is referred to as the quadrangular membrane. This membrane is paired left and right and thickens significantly toward the bottoms of the pair to form the ventricular (false vocal fold) ligaments. These ligaments extend the length of the ventricular folds near their free margins and attach to the thyroid and arytenoid cartilages. The ventricular ligaments place limits on the degree to which the thyroid and arytenoid cartilages can be separated from front to back. The remaining intrinsic ligament of the larynx is the thyroepiglottic ligament. This ligament extends between the bottom of the epiglottis and the inside of the angle of the thyroid cartilage, just beneath the thyroid notch, and functions as a fastener.
Extrinsic Ligaments and Membranes. Extrinsic ligaments and membranes of the larynx connect laryngeal cartilages to structures outside the larynx and provide support and stability for the laryngeal housing. The cricotracheal membrane (sometimes called the cricotracheal ligament) comprises the lowermost extrinsic connection to the larynx. This membrane extends around the bottom of the larynx between the first tracheal ring and the lower margin of the cricoid cartilage. The cricotracheal membrane is somewhat more extensive than the connective tissue between successive tracheal rings, the first tracheal ring being somewhat larger than the rest. The hyoepiglottic ligament extends between the upper back surface of the body of the hyoid bone and the lingual surface of the epiglottis. This ligament limits the degree to which these two structures can be separated from front to back. The hyothyroid (also called the thyrohyoid) ligaments and membrane form a large interconnection between the hyoid bone and the upper margin of the thyroid cartilage of the larynx. This interconnection gives the appearance that the laryngeal housing proper is suspended from the hyoid bone. The hyothyroid membrane thickens toward the midline of the larynx at the front and is designated in that location as the middle hyothyroid ligament. The same membrane also thickens toward the back in the space between the greater cornua of the hyoid bone and the superior cornua of the thyroid cartilage. These thickenings are referred to as the lateral hyothyroid ligaments. Often embedded within each of these lateral ligaments is a small triticeal (grain of wheat) cartilage.
Figure 3–15. Intrinsic and extrinsic laryngeal ligaments and membranes. The intrinsic ligaments (middle cricothyroid, vocal, ventricular, and thyroepiglottic ligaments) and membranes (conus elasticus, lateral cricothyroid, and quadrangular membranes) connect laryngeal cartilages to one another. Extrinsic ligaments (hyoepiglottic and middle and lateral hyothyroid ligaments) and membranes (cricotracheal and hyothyroid membranes) connect laryngeal cartilages to structures outside the larynx. Mucous membrane (not shown) lines the entire laryngeal cavity.
Mucous Membrane. The entire internal laryngeal cavity is lined by a mucous membrane, like the trachea below it and the pharynx above it. This lining is covered by columnar epithelium, except for the inner edges of the vocal folds and ventricular folds and the upper half of the epiglottis, which are covered with squamous epithelium.
FORCES OF THE LARYNGEAL APPARATUS
Two types of force, passive and active, operate on the larynx. Passive force is inherent within the apparatus. Active force is applied in accordance with the will and ability of the individual.
The passive force of the larynx comes from several sources. These include the natural recoil of muscles, cartilages, and connective tissues (ligaments and membranes), the surface tension between structures in apposition (vocal folds, ventricular folds, epiglottis and aryepiglottic folds, and/or tongue), and the pull of gravity. The distribution, sign, and magnitude of passive force depend on the mechanical milieu, including the positions, deformations, and levels of muscle activity (if applicable) of different parts of the laryngeal apparatus.
The active force of the laryngeal apparatus results from activation of laryngeal muscles. Laryngeal muscles can be categorized as intrinsic, extrinsic, or supplementary. Muscles categorized as intrinsic have both ends attached within the larynx, whereas muscles categorized as extrinsic have one end attached within the larynx and one end attached outside the larynx. Muscles categorized as supplementary do not attach to the larynx directly but influence it by way of attachments to the neighboring hyoid bone.
The function described below for individual muscles assumes that the muscles of interest are engaged in shortening (concentric) contractions, unless otherwise specified as being engaged in lengthening (eccentric) contractions or fixed-length (isometric) contractions. The influence of individual muscle actions may also be conditioned by whether or not other muscles are active.
Technical and colloquial definitions of terms can sometimes be quite different. Take, for example, the term “elastic.” In a physical sense, something that’s elastic returns to its original shape following deformation. Throw a golf club on the floor and it will deform and then return to its original shape. And you can predict what that shape will be. But consider the waistband of your underwear. Throw your underwear on the floor and its waistband will not assume a shape you can predict. That’s because, in a physical sense, it’s inelastic—it doesn’t return to its original shape following deformation. So, although you might think of the waistband of your underwear as being elastic, a physicist would think just the opposite. Both of you are right in your uses of the term “elastic,” but you need to know which world you’re talking in (colloquial or technical) before you consider something to be “elastic” or not.
Probably as a child you played “motorboat” with friends by rapidly pounding your fists on their chests or backs while they sustained “ah.” The variation in loudness that sounded to you like an idling motorboat was caused by rapid changes in alveolar pressure. Pound on someone’s chest or back and with each blow their lungs compress a small amount and the air pressure inside them goes up momentarily. Pound at different rates and you change the perceived speed of your imaginary motor noise. The basis of all this fun is that the laryngeal apparatus and the voice are sensitive to adjustments in the breathing apparatus. You don’t necessarily need to have a friend pounding on you to appreciate this sensitivity. Try to talk while driving a car down a cross-rutted dirt road or while sitting atop a trotting horse. The road and the horse will effectively do the pounding for you by bouncing your gut up and down and changing your alveolar pressure.
Figure 3–16 depicts the intrinsic muscles of the larynx. These muscles are responsible for changing the position and mechanical status of structures that form the walls of the laryngeal cavity. They are the thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, arytenoid, and cricothyroid muscles.
The thyroarytenoid muscle forms most of each vocal fold. This muscle extends between the inside surface of the thyroid cartilage (near the angle) and the arytenoid cartilage on the corresponding side. The front attachment of the muscle lies to the side of the front attachment of the corresponding vocal ligament. Fibers run generally parallel to the vocal ligament to insert on the front and outer sides of the arytenoid cartilage. Upper fibers run a straight course from front to back, whereas lower fibers twist in their course and swing off in an outward, backward, and upward direction (Broad, 1973; Zemlin, 1998). A small number of fibers toward the side of the muscle depart from the predominant front-to-back orientation of the others and course upward to the aryepiglottic fold, the side of the epiglottis, and into the region of the ventricular fold on the same side (Zemlin, 1998). The effects of contraction of the thyroarytenoid muscle are portrayed in Figure 3–17. Contraction of its longitudinal fibers shortens it and reduces the distance between the thyroid and arytenoid cartilages. The reduction in distance between the two cartilages is typically effected as a forward pull on the arytenoid cartilage that rocks it toward the mid-line. Fixed-length (isometric) or lengthening (eccentric) contractions of the thyroarytenoid muscle (with other intrinsic muscles opposing) increase its internal tension (force per unit length). Contraction of vertical fibers of the thyroarytenoid muscle near the sidewall of the larynx may have an influence on the position and configuration of the corresponding ventricular fold (Reidenbach, 1998; not illustrated in Figure 3–17).
Figure 3–16. Intrinsic muscles of the larynx. Intrinsic muscles have both ends attached within the larynx. They are the thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, arytenoid (transverse and oblique), and cricothyroid muscles.
The thyroarytenoid muscle is sometimes described as having two distinct parts (Dickson & Maue-Dickson, 1982; van den Berg & Moll, 1955; Wustrow, 1953): the thyromuscularis muscle (sometimes called the external thyroarytenoid) and the thyrovocalis or vocalis muscle (sometimes called the internal thyroarytenoid). As depicted schematically in Figure 3–18, the thyromuscularis muscle lies nearest the laryngeal wall and to the side of the thyrovocalis muscle. The thyrovocalis muscle and the vocal ligament are sometimes referred to as the vocal cord, as distinguished from the vocal fold (which also includes the thyromuscularis muscle); however, the terms “vocal cord” and “vocal fold” are also used interchangeably. The term “vocal fold” is more descriptive of the entire shelf-like structure and is generally preferred (Hall, Cobb, Kapoor, Kuchai, & Sandhu, 2017).
Figure 3–17. Effects of contraction of the thyroarytenoid muscles. Contraction of the longitudinal fibers shortens the muscle (straight green arrows) and reduces the distance between the thyroid and arytenoid cartilages. This contraction also rocks the arytenoid cartilages toward the midline (curved green arrows). If the contraction is isometric (fixed length) and is opposed by other muscle forces, the internal tension of the muscle is increased (yellow arrows).
The notion of a two-part thyroarytenoid muscle, although embraced by many, is not accepted universally. Some argue that dissections have failed to reveal a separating fascial sheath within the thyroarytenoid muscle that would support the notion of two distinct anatomical parts (Mayet, 1955; Zemlin, 1998). Others argue that, with or without such a separating fascial sheath, the thyroarytenoid muscle is capable of differential actions in what are conceptualized to be its thyromuscularis and thyrovocalis subdivisions (Broad, 1973; Sonesson, 1960). These presumed differential actions (discussed below in another section) are believed by some to have salience in the control of voice production (Kahane, 2007; Orlikoff & Kahane, 1996; Sanders, Rai, Han, & Biller, 1998; Titze, 2006a).
The posterior cricoarytenoid muscle is fan-shaped and located on the back surface of the cricoid cartilage. The muscle originates on the cricoid lamina and courses upward and toward the side in a converging pattern to insert on the upper and back surfaces of the muscular process of the arytenoid cartilage. As illustrated in Figure 3–19, contraction of the posterior cricoarytenoid muscle rocks the arytenoid cartilage away from the midline. This rocking is effected mainly by fibers located laterally within the muscle and that insert on the upper surface of the muscular process. Forceful contraction of these fibers may also slide the arytenoid cartilage upward and backward along the sloping rim of the cricoid cartilage. Fibers in the medial part of the posterior cricoarytenoid muscle insert on the back surface of the muscular process and contract to stabilize the arytenoid cartilage against other forces that are directed forward (Zemlin, Davis, & Gaza, 1984).
Figure 3–18. Thyromuscularis and thyrovocalis (or vocalis) subdivisions of the thyroarytenoid muscle. The vocal ligament runs along the internal edge of the thyrovocalis muscle.
The lateral cricoarytenoid is a small fan-shaped muscle that originates from the upper rim of the cricoid cartilage. Fibers of this muscle extend upward and backward to insert on the muscular process and front surface of the arytenoid cartilage. As depicted in Figure 3–20, contraction of the lateral cricoarytenoid muscle rocks the arytenoid cartilage toward the midline. Activation of the lateral cricoarytenoid muscle may also slide the arytenoid cartilage forward and toward the side along the downward sloping path of the long axis of the cricoid facet of the cricoarytenoid joint.
Figure 3–19. Effects of contraction of the posterior cricoarytenoid muscles. Their contraction rocks the arytenoid cartilages rock away from the midline and also moves the vocal folds (which contain the thyroarytenoid muscles) away from the midline.
The arytenoid (also called the interarytenoid) muscle extends from the back surface of one arytenoid cartilage to the back surface of the other arytenoid cartilage. The arytenoid muscle has two distinct and separate subdivisions: the transverse arytenoid muscle and the oblique arytenoid muscle. The transverse arytenoid muscle arises from the back surface and side of one arytenoid cartilage and courses horizontally to insert on the back surface and side of the other arytenoid cartilage. Those muscle fibers that insert on the sides of the arytenoid cartilages interdigitate with fibers of the thyroarytenoid muscles. The oblique arytenoid muscle overlies the transverse component of the muscle and diagonally crosses the back surface of the two arytenoid cartilages. The muscle originates from the back and side surface and muscular process of one arytenoid cartilage and courses upward to insert near the apex of the other arytenoid cartilage. Some muscle fibers of the oblique arytenoid muscle extend around the side of the apex of the arytenoid cartilage and course upward and forward to insert into the side of the epiglottis. This part of the muscle is given its own name, the aryepiglottic muscle. As illustrated in Figure 3–21, contraction of different components of the arytenoid muscle has different effects. Contraction of the transverse arytenoid muscle pulls the arytenoid cartilages toward one another. This is manifested through an upward, inward, and backward sliding movement along the long axis of each cricoarytenoid joint. Contraction of the oblique arytenoid muscle pulls one arytenoid cartilage toward the other in a tipping action that occurs in accordance with the movement permitted at the cricoarytenoid joint. And contraction of the aryepiglottic muscle pulls the epiglottis backward and downward to cover the upper opening into the larynx.
Figure 3–20. Effects of contraction of the lateral cricoarytenoid muscles. Their contraction rocks the arytenoid cartilages toward the midline and moves the vocal folds (which contain the thyroarytenoid muscles) toward the midline.
Figure 3–21. Effects of contractions of different parts of the arytenoid muscles. Contraction of the transverse arytenoid muscle pulls the arytenoid cartilages toward one another. Contraction of the oblique arytenoid muscle tips the arytenoid cartilages toward one another. Contraction of the aryepiglottic muscle (an extension of the oblique arytenoid muscle) pulls the epiglottis backward and downward.
The cricothyroid muscle extends between the outer front and side of the anterior arch of the cricoid cartilage and the outer front and side of the lower border of the lamina and inferior cornu of the thyroid cartilage. The muscle is fan-shaped with its fibers diverging as they course from the cricoid cartilage to the thyroid cartilage. Two subdivisions of the muscle are most often recognized, a vertical component toward the front called the par rectus, and an upward sloping component toward the back called the pars oblique (Zemlin, 1998). A third subdivision, the pars media, is occasionally noted. Its fibers lie underneath and closer to the midline than the fibers of the pars rectus (Charpied & Shapshay, 2004), although some consider the pars media to be an anatomical variation of the cricothyroid muscle proper (Kucinski, Okrazewska, & Piszcz, 1979). As illustrated in Figure 3–22, contraction of the cricothyroid muscle increases the distance between the thyroid and arytenoid cartilages and decreases the distance between the upper border of the cricoid cartilage and the lower border of the thyroid cartilage at the front of the larynx (decreases the angle formed between the two cartilages). These distance changes result from a rotation of the thyroid cartilage on the cricoid cartilage and/or a rotation of the cricoid cartilage on the thyroid cartilage. Rotation is effected through activation of both the pars rectus and pars oblique components of the cricothyroid muscle. Activation of the pars oblique component also results in a secondary movement that increases the distance between the thyroid and arytenoid cartilages. This movement causes a limited forward sliding of the thyroid cartilage, backward sliding of the cricoid cartilage, or both (Arnold, 1961; Takano & Honda, 2005; van den Berg et al., 1960).
Figure 3–23 depicts the extrinsic laryngeal muscles. These muscles have a role in supporting and stabilizing the larynx and in changing its position within the neck. They include the sternothyroid, thyrohyoid, and inferior constrictor muscles.
The sternothyroid is a long muscle located toward the front and side of the larynx. It originates from the back surface of the top of the sternum (breastbone) and the first costal (rib) cartilage. Fibers of the muscle course upward and slightly toward the side to insert on the outer surface of the thyroid cartilage. Contraction of the sternothyroid muscle pulls the thyroid cartilage downward. This action may also enlarge the pharynx by drawing the larynx forward and downward (Zemlin, 1998).
Figure 3–22. Effects of contractions of the cricothyroid muscles. Contraction of the cricothyroid muscles (pars rectus and pars oblique) rotates the thyroid cartilage on the cricoid cartilage (or vice versa). This rotation increases the distance between the thyroid and arytenoid cartilages at the back and decreases the distance between the thyroid and cricoid cartilages at the front. The pars oblique component can also cause the thyroid cartilage to slide forward and/or the cricoid cartilage to slide backward.
Figure 3–23. Extrinsic muscles of the larynx. Extrinsic muscles have one attachment inside the larynx and one attachment outside the larynx. The extrinsic laryngeal muscles are the sternothyroid, thyrohyoid, and inferior constrictor muscles.
The thyrohyoid muscle is located on the front and side of the larynx. It extends between the outer surface of the thyroid cartilage and the lower edge of the greater cornu of the hyoid bone. The course of its fibers is essentially vertical. Contraction of the thyrohyoid muscle decreases the distance between the thyroid cartilage and the hyoid bone. Relative fixation of the thyroid cartilage and hyoid bone determines the extent to which the structures may move toward one another.
The inferior constrictor muscle (discussed in more detail in Chapter 4) is the lowest of the group of three muscles that forms the back and sidewalls of the pharynx. Fibers of the inferior constrictor muscle extend forward from the median raphe (seam) at the back of the pharynx to insert on the sides of the cricoid and thyroid cartilages. Contraction of the inferior constrictor muscle moves the sidewall of the lower pharynx inward and decreases the size of the pharyngeal lumen (tubular cavity). Its activation also serves to stabilize the position of the laryngeal housing.
Supplementary Laryngeal Muscles
Some muscles do not attach on the larynx, but are nonetheless important in influencing its position and stability. These muscles, depicted in Figure 3–24, are referred to as supplementary muscles of the larynx. Most of them attach to the hyoid bone and are subdivided into those that originate below the hyoid bone, the so-called infrahyoid muscles, and those that originate above the hyoid bone, the so-called suprahyoid muscles.
The infrahyoid muscles include the sternohyoid and omohyoid muscles. These two muscles apply forces that can influence the positioning of the hyoid bone from below.
The sternohyoid muscle is a flat structure that courses vertically along the front surface of the neck and overlies the sternothyroid muscle (an extrinsic laryngeal muscle). The sternohyoid muscle originates from the back surface of the top of the sternum and the inner end of the clavicle (collar bone). Fibers course upward and insert on the lower edge of the body of the hyoid bone. Contraction of the sternohyoid muscle places a downward pull on the hyoid bone. This downward pull lowers the hyoid bone, or it can anchor the hyoid bone in position if the downward pull is counterbalanced by other forces.
Figure 3–24. Supplementary muscles of the larynx. Supplementary muscles do not attach directly to the larynx, but exert indirect influences through attachments to the hyoid bone. Infrahyoid muscles (sternohyoid and omohyoid muscles) originate below the hyoid bone, and suprahyoid muscles (digastric, stylohyoid, mylohyoid, geniohyoid, hyoglossus, and genioglossus muscles) originate above the hyoid bone.
They Didn’t Quite Get It
She had a beautiful coloratura soprano voice and after years of formal training was just about to begin an operatic career. It ended abruptly on a ski slope when a careless youngster crashed into her and slammed her headfirst into a tree. She suffered facial lacerations, blunt trauma to the larynx, and temporomandibular joint damage. She never again had full singing ability. Jaw movement was difficult and very painful. She could no longer meet the demands of operatic roles. Forensic testimony concluded that she was 100% impaired because she could not perform a full operatic role. The career for which she had prepared was lost. The jury decided otherwise and awarded her little more than her medical expenses. The twisted logic was revealed in an interview with the foreman of the jury following the trial. “We didn’t see why she couldn’t just sing country songs instead. They’re short and not as demanding.” They didn’t quite get it.
The omohyoid (shoulder-to-hyoid bone) muscle is located on the front and side of the neck. It is a narrow muscle that has two long bellies. The posterior (lower) belly arises from the upper edge of the scapula (shoulder blade) and courses horizontally inward and forward to attach to an intermediate tendon positioned near the sternum. The anterior (upper) belly arises from the opposite end of the same intermediate tendon and runs vertically and toward the midline to attach to the lower edge of the greater cornu of the hyoid bone. Contraction of the omohyoid muscle places a downward and backward pull on the hyoid bone. Contraction also tenses the supporting fascia in the region and prevents the neck from being sucked inward during forceful inspiration.
The suprahyoid muscles apply forces that can influence the positioning of the hyoid bone from above. They are the digastric, stylohyoid, mylohyoid, geniohyoid, hyoglossus, and genioglossus muscles.
The digastric is a two-bellied sling of muscle in which the two bellies are joined end-to-end by an intermediate tendon that attaches to the top of the hyoid bone. The anterior belly originates inside the lower border of the mandible (jaw) and courses downward and backward to the intermediate tendon. The posterior belly originates from the mastoid process of the temporal bone of the skull and courses downward and forward to the intermediate tendon. Contraction of the digastric muscle pulls upward on the hyoid bone and/or downward on the mandible. The relative movement of the hyoid bone and mandible is dependent on the degree to which the two structures are fixed in position by other muscles. Of interest here are influences on the hyoid bone. Contraction of the anterior belly of the muscle moves the hyoid bone upward and forward, whereas contraction of the posterior belly of the muscle moves the hyoid bone upward and backward. Contraction of the two bellies of the digastric muscle at the same time pulls the hyoid bone upward and forward or upward and backward at any angle, depending on the forces generated by the two bellies.
The stylohyoid muscle runs a course somewhat parallel to the posterior belly of the digastric muscle. The stylohyoid muscle originates from the back and side surfaces of the styloid process of the temporal bone of the skull and courses downward and forward to the hyoid bone. The muscle divides into two bundles that pass on either side of the intermediate tendon of the digastric muscle before inserting at the junction of the body and greater cornu of the hyoid bone. Contraction of the stylohyoid muscle places an upward and backward pull on the hyoid bone. The action is similar to that which results from contraction of the posterior belly of the digastric muscle.
The mylohyoid muscle contributes to the formation of the floor of the oral cavity. Fibers of this muscle originate along much of the inner surface of the body of the mandible and course inward, backward, and downward. They join with fibers of their paired mate of the opposite side at a tendinous midline raphe (a seam running down the center of the floor of the oral cavity). Fibers toward the rear of the oral cavity attach directly into the front surface of the body of the hyoid bone. Contraction of the mylohyoid muscle results in an upward and forward pull on the hyoid bone. Contraction can also result in elevation of the floor of the oral cavity and tongue. With the hyoid bone fixed in position, contraction of the mylohyoid muscle may lower the mandible.
The geniohyoid is a cylindrical muscle that lies above the mylohyoid muscle. This muscle extends from the inner surface of the front of the mandible to the front surface of the body of the hyoid bone. Its fibers extend backward and downward in a diverging pattern. The muscle bundle runs above and nearly parallel to the fiber course of the anterior belly of the digastric muscle. Contraction of the geniohyoid muscle pulls the hyoid bone upward and forward. Its functional potential is similar to that of the anterior belly of the digastric muscle.
The hyoglossus is an extrinsic muscle of the tongue (having attachments within and outside the tongue) that has the potential to exert force on the hyoid bone and move the housing of the larynx. Fibers of this muscle course vertically and extend between the side of the tongue toward the back and the body and greater cornu of the hyoid bone. When the hyoglossus muscle contracts, it retracts and depresses the tongue and/or elevates the hyoid bone. If the tongue is relatively more fixed than the hyoid bone, the hyoid bone will rise within the neck.
The genioglossus is also an extrinsic muscle of the tongue and is the largest and strongest of such extrinsic muscles. This muscle has the potential to exert force on both the tongue and the hyoid bone. Fibers of the genioglossus muscle extend from the inner surface of the mandible and course complexly to insert into the entire undersurface of the tongue and body of the hyoid bone. Contraction of the genioglossus muscle can have a variety of influences on the positioning of the tongue and/or hyoid bone. Its major influence on the hyoid bone is to draw it upward and forward.
Summary of the Laryngeal Muscles
The laryngeal muscles are categorized as intrinsic, extrinsic, and supplemental, depending on the locations of their attachments (inside or outside the larynx). Actions of the intrinsic laryngeal muscles (those with both attachments inside the larynx) have a direct and profound influence on the vocal folds. Specifically, they can abduct (move apart), adduct (move together), shorten, lengthen, and tense the vocal folds. Actions of the extrinsic and supplemental laryngeal muscles (those with at least one attachment outside the larynx) serve to stabilize the larynx and can change its position within the neck. In general, contractions of muscles with attachments below the larynx can lower the larynx, and contractions of those with attachments above the larynx can raise the larynx. These laryngeal movements and their associated forces are detailed in the next two sections.
Two interesting developments hold promise for those with severe laryngeal injury or disease. One is laryngeal transplantation, in which a donor larynx from another individual is transferred to an individual whose larynx is no longer viable. The results of initial efforts in transplantation are somewhat encouraging, although only two laryngeal transplants have been reported in the medical literature to date. More promising is the regeneration and reconstruction of new laryngeal tissue through the use of tissue engineering. Tissue engineering has advanced to the stage where successful growth of new laryngeal tissue has been accomplished in humans. New cell therapies combined with new techniques for scaffold construction, such as 3D printing, hold great promise for those with damaged or diseased larynges (Hertegård, 2016). Imagine, one day we may be able to grow an entirely new larynx!
MOVEMENTS OF THE LARYNGEAL APPARATUS
Movements of the laryngeal apparatus allow it to function as a valve. These movements include those of the vocal folds, ventricular folds, epiglottis, and laryngeal housing.
The vocal folds are movable and flexible. Changes can be effected in their vertical and side-to-side positioning and in their shape and length.
Each vocal fold can go through vertical and side-to-side position changes as its corresponding arytenoid cartilage rocks at a right angle to the long axis of its associated cricoid facet. When the rocking movement is downward and inward, the back end of the vocal fold moves downward and toward the midline. When rocking movement is upward and outward, the back end of the vocal fold moves upward and toward the side. Under normal circumstances, the two arytenoid cartilages move simultaneously and similarly so that the two vocal folds move in similar trajectories.
The cross-sectional shape of each vocal fold can change. Changes in shape mainly constitute a thinning or thickening of the vocal fold toward its free margin such that the free margin may be relatively sharp or blunt in cross section toward the airway. The vocal fold may also appear to be somewhat tilted in its configuration.
The vocal folds can be lengthened or shortened considerably. Lengthening is limited by the degree to which the covering tissue of the vocal fold, vocal ligament, and different parts of the thyroarytenoid muscle are distensible. Lengthening of the vocal folds is effected when either or both the thyroid and cricoid cartilages are moved away from one another. Shortening of the vocal folds occurs when either or both the thyroid and cricoid cartilages are drawn toward one another.
These movements have functional significance to speech production. Of particular relevance are the movements that effect vocal fold abduction, adduction, and length change.
Vocal fold abduction is the movement of a vocal fold away from the midline (although other meanings are possible; see Figure 3–25 for an example). Abduction is normally simultaneous and symmetric in the two vocal folds. As the vocal folds move toward the side, the glottis (space between them) increases in size. As portrayed in Figure 3–26, full abduction of the vocal folds results in a wide glottis and the condition in which air flows most freely in and out of the pulmonary apparatus. It is the vocal folds that abduct, not the glottis.
Abduction of the vocal folds and concomitant enlargement of the glottis are effected mainly by contractions of the posterior cricoarytenoid muscles. These pull on the muscular processes of the arytenoid cartilages to swing the vocal folds upward and outward. Vocal fold abduction can also result from a downward pull on the larynx which places a downward pull on the conus elasticus (lower elastic lining of the larynx). Such a pull tends to tug the free margin of the vocal fold downward and toward the side, thus dilating the laryngeal airway (Zenker, 1964). This can happen when the diaphragm contracts footward and pulls downward on the trachea; this is called tracheal tug. The contribution of the posterior cricoarytenoid muscles to vocal fold abduction is much greater than that of tracheal tug.
Figure 3–26. Vocal fold abduction and widening of the glottis. Vocal fold abduction is accomplished primarily by contraction of the paired cricoarytenoid muscles, though downward force exerted on the larynx can also exert an abductory force on the vocal folds. The image on the right is a photograph of an author’s vocal folds (courtesy of Robin Samlan).
Vocal fold adduction is the movement of a vocal fold toward the midline. During adduction the two vocal folds usually follow similar movement pathways and the glottis decreases in size. As illustrated in Figure 3–27, movement toward the midline may be sufficient to approximate (bring together) the entire free margins of the two vocal folds and close the laryngeal airway. Alternatively, movement toward the midline may be limited to the membranous part of the vocal folds (front 60%), resulting in closure of only that portion of the airway, while leaving the cartilaginous portion (back 40%) of the vocal folds abducted.
Adduction resulting in full approximation of the two vocal folds is caused by the combined contraction of the lateral cricoarytenoid muscles and arytenoid muscles (transverse and oblique components), whereas adduction resulting in approximation of only the membranous (front) portions of the vocal folds is caused by contraction of the lateral cricoarytenoid muscles alone. Action of the lateral cricoarytenoid muscles pulls forward on the muscular processes of the arytenoid cartilages, rocking them over the cricoid cartilage and swinging the vocal folds downward and inward. Action of the arytenoid muscles pulls the arytenoid cartilages toward the midline and approximates the cartilaginous (back) portion of the vocal folds.
The Daniel Boone you’ve heard of may or may not be this one. Both are pioneers. This one wrote a clinical textbook that represented the first comprehensive approach to the topic of voice disorders. In it, he proposed a set of facilitating techniques that were widely adopted by clinicians working with individuals with voice disorders and remain in use today throughout the world—his textbook is in its 9th edition! Boone is a master clinician and an outstanding clinical teacher. He has a knack for cutting to the heart of clinical matters quickly, and few equal him in his compassion for people with serious voice disorders. Boone is a past president of the American Speech-Language-Hearing Association and was instrumental in guiding that association to prominence. He is retired and lives in Tucson, Arizona, but still travels the world lecturing about voice disorders. Boone has a wonderful sense of humor. One of his passions is the card game Hearts.
Once the vocal folds are approximated (fully adducted), the vertical extent of their approximation (amount of contact) and the compressive force (force of contact) can be adjusted. The amount of contact of the cross-sectional thickness through the approximated vocal fold surfaces can be adjusted by contracting the thyrovocalis portion of the thyroarytenoid muscle. Contraction of the thyrovocalis muscle bulges the medial surfaces of the vocal folds toward the midline. The force of contact is adjusted by actions of the lateral cricoarytenoid muscles and the arytenoid muscles that squeeze the vocal folds together. The squeezing force exerted between the vocal processes of the arytenoid cartilages by the lateral cricoarytenoid muscles has been called medial compression (van den Berg et al., 1960). Medial compression can be applied even when the vocal folds are separated along their cartilaginous length.
Figure 3–27. Vocal fold adduction. The lateral cricoarytenoid muscles adduct the front (membranous) portion of the vocal folds and the arytenoid muscles adduct the back (cartilaginous) portion of the vocal folds. The amount of vocal fold contact in the vertical dimension can be adjusted by bulging of the medial surfaces through contraction of the thyrovocalis muscles. The image on the right is a photograph of an author’s vocal folds (courtesy of Robin Samlan).
The length of the vocal folds can be changed through a variety of external and internal adjustments. The length changes that are most important to speech production are mediated through the cricothyroid joints. Length changes can also be mediated through the cricoarytenoid joints.
The vocal fold length changes that are mediated through the cricothyroid joints are best understood in the context of approximated vocal folds, as illustrated in Figure 3–28. Vocal fold lengthening (upper images in Figure 3–28) is achieved by forward directed forces pulling on the front ends of the vocal folds at their points of attachment to the inside of the thyroid cartilage and/or by backward directed forces pulling on the back ends of the vocal folds at their points of attachment to the vocal processes of the arytenoid cartilages. Forward directed forces result from contraction of the cricothyroid muscles, which place pulls on the front ends of the thyroid and cricoid cartilages that tend to close the visor angle of the larynx by rocking the thyroid cartilage on the cricoid cartilage and/or rocking the cricoid cartilage on the thyroid cartilage. The consequence of any combination of rocking of the front ends of these two cartilages toward one another is an increase in the distance between the front of the thyroid cartilage and the vocal processes of the arytenoid cartilages and a forward stretching of the vocal folds. Actions of the cricothyroid muscles (especially the pars oblique portions) can also cause a sliding movement at the cricothyroid joints that increases the distance between the front of the thyroid cartilage and the vocal processes of the arytenoid cartilages and contributes to vocal fold lengthening.
Actions of the posterior cricoarytenoid muscles counteract those of the cricothyroid muscles and serve to anchor the arytenoid cartilages and their vocal processes from forward tilting and sliding during contractions of the cricothyroid muscles. The posterior cricoarytenoid muscles may also lengthen the vocal folds somewhat by pulling the arytenoid cartilages backward and upward along the facets on the slope of the cricoid rim. Thus, the cricothyroid muscles are responsible for stretching the vocal folds to a greater length in a forward direction, whereas the posterior cricoarytenoid muscles are responsible for securing the back ends of the vocal folds or for stretching them to a greater length in a rearward direction.
Shortening of the vocal folds (lower images in Figure 3–28) results from relaxation of the distending muscles just discussed (cricothyroid and posterior cricoarytenoid muscles) or from concentric contractions of the thyroarytenoid muscles. Because the thyroarytenoid muscles constitute the main mass of the vocal folds, their contraction shortens the vocal folds and their internal fibers. If unopposed by actions of other muscles of the larynx, thyroarytenoid muscle contractions serve to draw the thyroid and arytenoid cartilages toward one another (pull the two ends of the vocal folds toward their respective centers lengthwise).
Vocal fold length changes also occur during abduction and adduction (see Figures 3–26 and 3–27) and are mediated through the cricoarytenoid joints. These length changes are the result of rocking and sliding of the arytenoid cartilages on the cricoid cartilage, which carries the tips of the vocal processes of the arytenoid cartilages upward, backward, and outward or downward, forward, and inward. Upward, backward, and outward movement of the vocal processes, as mediated through activation of the posterior cricoarytenoid muscles, abducts the vocal folds and lengthens them; downward, forward, and inward movement of the processes, as mediated through activation of the lateral cricoarytenoid muscles, adducts the vocal folds and shortens them.
Movements of the Ventricular Folds
The ventricular folds can move and change shape. Although the ventricular folds are usually widely separated and rounded toward the airway, under certain circumstances they may extend well into the airway to form a roof over the vocal folds. The ventricular folds may also tilt downward toward the vocal folds and even come in contact with them.
The muscular mechanisms underlying ventricular fold movements are not well understood. One suggestion is that the actions of other muscles in the vicinity of the ventricular folds combine to effect sphincter-like folding and unfolding of the interior of the larynx that moves and shapes the passive ventricular folds (Fink, 1975; Zemlin, 1998). Fibers of the thyromuscularis portions of thyroarytenoid muscles that course upward along the sidewalls of the larynx above the vocal folds may serve a special role in the enfolding influence on the ventricular folds (Reidenbach, 1998).
Figure 3–28. Vocal fold length changes mediated through the cricothyroid joints. Active vocal fold lengthening (upper images) is accomplished primarily by forward directed forces through contraction of the cricothyroid muscles (with possibly backward-directed forces exerted by the posterior cricoarytenoid muscles). Active vocal fold shortening (lower images) is accomplished primarily by contraction of the thyroarytenoid muscles. The images on the right are photographs of actual vocal folds (courtesy of Robin Samlan).