SPPA 2050 Speech Anatomy and Physiology

SPPA 2050 Speech Anatomy and Physiology

CHAPTER v

PHONATION

SPPA 2050 Speech Anatomy and Physiology

The larynx is a funnel-shaped arrangement of cartilage, connective tissue, and muscle, slightly wider above than below, that spans a space in the neck stretching from roughly the 3rd to the 6th cervical vertebra. The larynx is ventral to the spine and esophagus, caudal to the pharynx, and sits on top of the trachea, forming a valve whose main function (from a phylogenetic or evolutionary perspective) is to protect the lower airway. As we did with the respiratory system, we will begin by learning the skeletal structure of the larynx. We can reasonably break this down into the (1) cartilages, (2) joints that “connect” the cartilages, and (3) ligaments and membranes that hold everything together. In one sense, the larynx is an easier structure to study because it is more “self-contained” than the large and cumbersome respiratory system.

SPPA 2050 Speech Anatomy and Physiology

The Cartilages (and maybe a bone)

SPPA 2050 Speech Anatomy and Physiology

A lot of what you will learn later about laryngeal anatomy and physiology follows from this set of information, and especially the names of the cartilages. It is important that you memorize these early and use them often in conversations with your friends.

Cartilages, rather than bone, provide the skeleton of the larynx. At the outset of any description of the cartilaginous skeletal framework of the larynx, it is customary to mention the delicate, albeit important horseshoe-shaped hyoid bone that is rostral to the main body of the larynx. Key landmarks include the body, lesser cornu (cornu = horn) and greater cornu. The hyoid bone is a site of attachment for many key muscles of the head and neck. Many authorities do not include this bone as part of the larynx, proper, though we can reasonably include it (and all relevant muscles and connective tissues) as apart of something we might call the hyo-laryngeal complex. If the hyoid is part of the larynx, it is its only bone. If not, then the larynx is still safely bone-free.

We usually say that there are six cartilages (by name) in the larynx. Three are unpaired (only one of them) and three are paired (two of them, like lots of things in the body), making a total of nine cartilages. The first four (three unpaired and one paired = five total) are very important for understanding laryngeal structure and function. Know these cold. Learn them as soon as possible, since our knowledge literally builds on them. The other cartilages are included for completeness and you will not be responsible for them.

In descending order of their size, these six are:

(1) (unpaired) thyroid, shaped something like a butterfly:

The thyroid cartilage is the most rostral of the laryngeal cartilages. It has two laminae (the wings of our butterfly), joined ventrally in the midline at the angle (usually visible through the skin of the neck of a skinny guy), and superior and inferior cornu (i.e., horns). Distinctive thyroid landmarks include the oblique line, on either lamina, and the superior thyroid notch, easily palpated at the rostral margin of the thyroid angle (the so-called “Adam’s apple”). The thyroid is "closed" ventrally, at the angle where the two laminae meet, but open in the back like the ever-popular hospital gown.

(2) (unpaired) cricoid, shaped something like a signet ring:

The ring-like cricoid is the foundation of the larynx, and thus, is its most caudal component. It is tied to the rostral-most tracheal ring by a relatively stout cricotracheal membrane. Its dorsal "face" (i.e., the "signet part" of the ring) is sometimes referred to as the lamina, while its shorter ventral portion is often referred to as the arch. The lower horns of the thyroid laminae straddle the cricoid, joining its lateral margins at articular facets, forming a pivot-like cricothyoid joint.

(3) (unpaired) epiglottis, shaped something like a leaf:

The rostral margin of the epiglottis extends above all other laryngeal cartilages, though the cartilage itself "originates" below the rostral margin of the thyroid, where it is tied to the thyroid by the thyroepiglottic ligament, at a point-like thyroepiglottic joint, just caudal and dorsal to the superior thyroid notch. It is possible to glimpse the upper margin of the epiglottis with the nekkid eye, especially in young babies, but even in adults, given suitable oral gymnastics (wild tongue-pulling, and the like).

(4) (paired) arytenoids, shaped something like four-sided pyramids:

These articulate with the cricoid, at cricoarytenoid joints formed about saddle-shaped facets that run laterally, caudally, and ventrally along the rostrolateral margins of the cricoid lamina. Each facet, on either side of the cricoid, is convex upward from the point of view of the cricoid, and matches a corresponding concave upward facet on the under surface of the arytenoid. Each arytenoid has four vertices or corners. The lateral-most is typically called the muscular process; the ventral-most is typically called the vocal process; and the corner that points in the rostral direction is typically called the apex.

The remaining cartilages are not critical to our understanding of laryngeal function. I include them simply for completeness, but will not be included in any testing.

(5) (paired) corniculates, shaped something like miniature arytenoids:

Each corniculate joins the apex of its corresponding arytenoid, though there is no motion about these joints.

(6) (paired and possibly inconsequential) cuneiforms, shaped something like little bullets:

The cuneiform cartilages are "lost in space," in the sense that they do not articulate with any other laryngeal cartilage(s). Instead, they are suspended in the rostral-most border of the quandrangular membrane, which extends caudally and dorsally from the lateral margins of the epiglottis to the lateral margin of each arytenoid on the same side of the larynx. The rostral border of this membrane is given a special name, the aryepiglottic fold. The open area medial to the left and right margins of the aryepiglottic fold, and dorsal to the surface of the epiglottis, is usually referred to as the laryngeal aditus (i.e., entryway or opening).

Three of the laryngeal cartilages (by name: thyroid, cricoid, and arytenoid) are formed from a type of cartilage we call hyaline. These will ossify (turn to bone) in time. The other three cartilages (by name: epiglottis, corniculate, and cuneiform) are merely (and always) elastic in nature.

SPPA 2050 Speech Anatomy and Physiology

Laryngeal Joints

SPPA 2050 Speech Anatomy and Physiology

In the larynx, there are a number of important joints, at the intersections (i.e., articulations) of some pairs of cartilages. After you’ve learned the names of the cartilages, understanding the joints is much easier, since their names are often derived from the structures that form the articulation. The thyroepiglottic joint is found at the point of articulation between the thyroid and the “stem” of the epiglottis. The cricothyroid joint is found where the lower horns of the thyroid straddle and contact the lateral margins of the cricoid. And, the (paired) cricoarytenoid joints are found where the arytenoids rest atop the upper lateral margins of the cricoid lamina.

The motions of laryngeal cartilages about their respective joints provide mechanisms for the principal functions of the human larynx. The epiglottis, for example, seems to pitch (see NOTE below) about the point-like thyroepiglottic joint, obstructing the laryngeal opening (technically called the aditus) during swallowing and protecting the airway from ingestion of foreign matter (foodstuffs, swords, fire!), and the attendant pneumonia (aspiration pneumonia) that comes from inspiring what should not be inspired. Similarly, the thyroid cartilage pitches about the two-point pivotlike cricothyroid joint, stretching the vocal folds attached to the dorsal surface of the thyroid angle, in turn regulating (perceived) pitch of the voice (for speech) and/or stiffness of the vocal folds (again, useful in preventing wrong things from going down the wrong tube). Finally, each arytenoid pitches and rolls about its corresponding cricoarytenoid joint, and may translate (slide) along the length of the articular facet. These motions of the arytenoids allow intricate control of the size of the glottis (the name for the space between the vocal folds), and also of the mechanical properties of the vocal folds.

NOTE: It is an interesting fact to remember that all laryngeal cartilages are three-dimensional objects. In principle, in a three-dimensional world, any object bigger than a (virtual) point is capable of movements that may involve up to six degrees of freedom. These include rotation, about any of the three axes associated with the three-dimensional reference frame: pitch, about a dextral-sinistral axis; roll, about a dorsal-ventral axis; yaw, about a rostral-caudal axis and/or translation, effectively "sliding" along lines parallel to any of the three reference axes. No movement of any laryngeal cartilage about its respective (mobile) joint involves more than three degrees of freedom. This general fact is due partly to the design of each joint (e.g., the placement, shape and/or location of relevant articular facets), and partly to the many ties that bind these cartilages together. These ties are the ligaments and membranes of the larynx and will be discussed next.

SPPA 2050 Speech Anatomy and Physiology

Ligaments and membranes (L & M)

SPPA 2050 Speech Anatomy and Physiology

Laryngeal cartilages are interconnected by a rich, complicated set of ligaments and membranes. For a variety of reasons, it is possible to argue that these ligaments and membranes (hereafter, L & M) are as important for laryngeal function, especially for speech, as are the cartilages they connect, and the muscles that move those cartilages. No doubt, Ls & Ms keep parts of the larynx from flying apart unexpectedly, while you sleep at night. Thank goodness for that! But, these ties also clearly constrain the dimensionality of movements that occur about the joints. Consider, for example, the L & M set that affects the epiglottis. Ventrally, this cartilage is bound to the tongue by the median and lateral glossoepiglottic folds, and to the body of the hyoid bone by the hyoepiglottic ligament. Dorsally and caudally, the epiglottis is bound to the arytenoid complex by the quadrangular membranes, which terminate rostrally in a thickened fold of tissue known as the aryepiglottic fold. Finally, the epiglottis is bound to the thyroid at the point of the joint by the thyroepiglottic ligament. Given these relatively many ties, it is clear that the epiglottis cannot translate with respect to the thyroid, and it is difficult to imagine that any roll or yaw rotation (about body axes) would be possible. About the only plausible epiglottic movement we might expect would involve a pitching motion about the point of contact between the epiglottis and thyroid. Apparently, something like this occurs during swallowing. In high-speed cineradiographic and video-fluorographic images of swallowing, the epiglottis appears to pitch rearward, covering the aditus, as the larynx as a whole is raised and forced against and under the lower rear of the tongue. A pitching motion of this type would be possible, as long as the epiglottic Ls & Ms are not stretched beyond their resting length. Ligaments, especially, cannot be stretched much before they rupture or tear loose from their bony and cartilaginous anchor points. Those who have suffered ACL (anterior cruciate ligament) injuries involving the knee develop an understanding of this feature of ligament in a personal and painful way. In effect, the general function of ligaments in the body is to anchor the components of joints, and limit their motion with respect to one another.

The thyroid cartilage is tied directly to three laryngeal cartilages --to the epiglottis (see above); to the cricoid cartilage, by (1) the conus elasticus, including its thickened medial and lateral portions often referred to as the median and lateral cricothyroid ligaments; and (2) the ceratocricoid ligaments at each of the cricothyroid joint articulations. The thyroid is also tied to the arytenoids, by yet another portion of the conus elasticus typically referred to as the vocal ligament. The ceratocricoid ligaments, in particular, would seem to prevent motions of the thyroid (e.g., roll, yaw, or translation in any direction) in any sense other than a simple pitching motion about an axis drawn through the right and left cricothyroid articulations. Finally, the thyroid is tied to the hyoid bone by the thyrohyoid membrane, whose thickened medial and dorsal-lateral portions are often referred to, naturally enough, as medial and lateral thyrohyoid ligaments. Together, these thyrohyoid Ls & M form a continuous sheet of connective tissue that spans the entire space between the horns and caudal and rostral margins of the hyoid and thyroid, wrapping from one side of the larynx to the other.

The cricoid cartilage is tied by L & M to the rostral-most tracheal ring by the cricotracheal membrane. Along its dorsal lamina, the cricoid is attached to the arytenoids by the posterior cricoarytenoid ligaments. These are not shown in any of the figures, though such ligaments are typically shown in other texts, running between the dorsal lamina of the cricoid, near the cricothyroid joints, and the dorsocaudal margin of each arytenoid. Presumably, these latter ligaments restrict motion of the arytenoids relative to the cricoid, at least in extent if not direction. However, the facets on the cricoid and arytenoid cartilages, at their articulation, probably exert a stronger influence on possible motion than do the connecting ligaments, limiting motion of the arytenoids to a type of pitching and rolling rotation, supplemented by some translation along the "length" of the joint surfaces. We will discuss these actions in detail later. Your burden is to simply know that such ligaments exist.

Finally, the ventricular folds and ventricles “cover” the inside of the larynx above the glottis. We will discuss these structures in more detail later.

Most inferences we can draw about possible motions of cartilages at the laryngeal joints must be evaluated in the context of relevant lines of action of muscles that act on or "across" the joint components. It is for this reason that each student of laryngeal anatomy must learn the names, origins, insertions, and actions of muscles affecting the larynx. Fortunately, this chore is not so difficult for the larynx as it may have seemed for the respiratory system. In general, muscles that affect the larynx are divided into two main classes, those we refer to as intrinsic (to the larynx), and those we refer to as either extrinsic or supplemental. The names of the large majority of muscles in both groups are transparent, in the sense that they reflect and indicate the names of cartilages they interconnect.

SPPA 2050 Speech Anatomy and Physiology

Intrinsic Laryngeal Muscles

SPPA 2050 Speech Anatomy and Physiology

The intrinsic laryngeal muscles get the intrinsic part of their name from the fact that they interconnect various pairs of laryngeal cartilages, and thus, have both their origins and insertions "inside" the larynx. The contractions of these muscles, as a group, regulate vocal fold position and tension. These two variables determine whether and how the vocal folds vibrate during speech and song. We will emphasize five main intrinsic laryngeal muscles, though some texts will identify more (e.g., by subdividing fibers "belonging" to one muscle, some say, into two or more different muscles, they say). The most common names given to the five most commonly identified intrinsic laryngeal muscles reflect directly the various pairs of cartilages they connect,

(1) cricothyroid (CT), joining the cricoid and thyroid

(2) lateral cricoarytenoid (LCA), joining the cricoid and arytenoids,

(3) posterior cricoarytenoid (PCA), joining the cricoid and arytenoids,

(4) thyroarytenoid (TA), joining the thyroid and arytenoids, medially by vocalis fibers, more laterally by thyroarytenoid,

(5) interarytenoid (IA) or simply arytenoideus, joining the arytenoids, with transverse and oblique fibers.

It is important to note that the names of the fourth and fifth muscles in the preceding list differ somewhat from the terminology in Netter. It is also important to note that some texts expand the list of intrinsic muscles, by illustrating and emphasizing relatively sparse fiber bundles. Two examples include the aryepiglottic muscle(s), whose fibers connect each arytenoid to the corresponding lateral margin of the epiglottis; and, the thyroepiglottic muscle(s), whose fibers connect the quadrangular membrane and/or lateral margin of the epiglottis to the medial surface of the corresponding thyroid lamina. We will not focus any attention on these muscles in lecture.