Velopharyngeal-Nasal Function and Speech Production
The velopharyngeal-nasal apparatus is located within the head and neck and comprises a system of valves and air passages that interconnects the pharynx (throat) and the atmosphere through the nose. Although most textbooks focus on the velopharyngeal part of this system, this chapter covers the complete velopharyngeal-nasal apparatus as a single functional entity. This is because the nasal part of the apparatus can have a significant influence on speech production, especially when velopharyngeal function is impaired.
This chapter covers the anatomy of the velopharyngeal apparatus, forces and movements of the apparatus, control variables, neural substrates, various functions of the apparatus, especially speech production, and variables that may influence speech production. The chapter concludes with a review.
ANATOMY OF THE VELOPHARYNGEAL-NASAL APPARATUS
The valves and air passages of the velopharyngeal-nasal apparatus are linked together, some arranged in mechanical series (one after another) and some arranged in mechanical parallel (side by side). The superstructure of the velopharyngeal apparatus is the skeletal framework that supports it and this is discussed in the first part in this section. From there, discussion moves to the anatomy of the pharynx, velum, nasal cavities, and outer nose. The muscles of the velopharyngeal-nasal apparatus are covered in the section on forces.
The skeletal framework of the velopharyngeal-nasal apparatus consists of the first six cervical vertebrae and various bones of the skull. The bones of the skull include cranial (braincase) bones and facial (forehead, eyes, nose, mouth, and upper throat) bones. These bones are individually intricate structures that are rigidly joined together and contribute to formation of the walls, floor, and roof of the velopharyngeal-nasal apparatus, as well as provide anchors to which many of the velopharyngeal-nasal muscles attach. The velopharyngeal-nasal apparatus and the pharyngeal-oral apparatus share many bones of the skull. Therefore, the bones discussed in this section pertain to Chapter 5 as well.
Figure 4–1 depicts the cranial bones. Some of these bones are paired and some are not. The eight cranial bones are the temporal (two), parietal (two), occipital (one), frontal (one), sphenoid (one), and ethmoid (one) bones. Each temporal bone includes a narrow prominence called the styloid process to which muscle and ligament attach (other relevant features of the temporal bone are discussed in Chapter 13). The sphenoid bone, a double-winged structure, sits behind the eyes and forms the back wall of the nasal cavities. The ethmoid bone forms the upper sidewalls of the nasal cavities and the upper part of their medial wall.
Figure 4–2 depicts the facial bones, most of which are paired. These 14 bones are the maxillary (two), palatine (two), vomer (one), inferior nasal conchae (two), lacrimal (two), nasal (two), and zygomatic bones (two) and mandible (one). The bones most closely associated with structure of the velopharyngeal-nasal apparatus are the maxillary bones, which form the front of the floor of the nasal cavities; the palatine bones, which form the back of the floor of the nasal cavities; the vomer bone, which forms the lower part of the medial wall of the nasal cavities; the inferior nasal conchae (plural for concha), which form the lower sidewalls of the nasal cavities; and the nasal bones, which form the bridge of the outer nose. The zygomatic bones form the prominences of the cheeks and are also called the cheekbones. The small and fragile lacrimal bones form part of the orbit and articulate with the inferior nasal concha, ethmoid, frontal, and maxillary bones. The mandible is a movable facial bone that is discussed in detail in Chapter 5.
Figure 4–1. The cranial bones of the skull, two of which are paired. They include the temporal (paired), parietal (paired), occipital, frontal, sphenoid, and ethmoid bones.
Figure 4–2. The facial bones of the skull, most of which are paired. They are the maxillary (paired), palatine (paired), vomer (single), inferior nasal conchae (paired), lacrimal (paired), nasal (paired), and zygomatic bones (paired) and mandible (single).
Duane C. Spriestersbach (1916–2011)
Spriestersbach had a distinguished career as a clinical investigator of the communication problems of children with cleft palate and craniofacial disorders. “Sprie,” as he was affectionately called, served for many years as the program director of a large federally funded research grant on cleft palate at the University of Iowa. His leadership fostered much of the research done over two decades on normal velopharyngeal function for speech production and on the mechanisms involved in control of the velopharyngeal apparatus in individuals with velopharyngeal incompetence. Many of the names in the reference list to this chapter cut their research teeth under his guidance. Spriestersbach was an exceptional thinker. He had an enormous impact on translating the products of research into practical clinical applications for those with speech disorders caused by cleft palate. In his spare time, he took to the stage, where he performed in the Iowa City Community Theatre, and to the card table, where he played a legendary mean hand of poker.
The pharynx is a tube of tendon and muscle that extends from the base of the skull to the cricoid cartilage in the front and to the sixth cervical vertebra in the back. The pharyngeal tube is widest at the top and narrows down its length and is oval in cross section, being larger side to side than front to back. As shown in Figure 4–3, the front wall of the pharynx is partially formed by the back surfaces of the velum (defined below), tongue, and epiglottis. Otherwise, the structure is open at the front and connects, from top to bottom, with the nasal cavities, oral cavity, and laryngeal aditus (upper entrance to the larynx).
Figure 4–3. Salient features of the pharynx as revealed from a back view in which the posterior pharyngeal wall is opened from behind. The skull and mandible are shown for reference.
The mix of tendon and muscle varies along the length of the pharynx. The upper part is made up solely of connective tissue, called the pharyngeal aponeurosis, which effectively suspends the pharyngeal tube from above (the way the rim of a basketball goal suspends the net). Muscular tissue increases in proportion down the length of the pharynx until it predominates. Muscle tissue encircles the pharynx, making its architecture resemble that of a sphincter. In fact, its overall arrangement is similar to that of the gut. This should come as no surprise, given that the pharynx is an active component of the digestive system and its lower part is continuous with the esophagus (gullet), where its front and back walls are in contact. This contact is broken during activities such as swallowing and regurgitation.
The pharynx comprises three cavities that are designated, from top to bottom, as the nasopharynx, oropharynx, and laryngopharynx. The boundaries of these cavities are shown in Figure 4–4. The nasopharynx lies behind the nose and above the velum. Because the velum is mobile, the lower boundary of the nasopharynx is somewhat arbitrary. Thus, a common convention is to specify this boundary by a reference line extending between the upper surface of the hard palate and the most forward point on the uppermost vertebra.
The nasopharynx always remains patent, a feature that distinguishes it from the other subdivisions of the pharynx. The pharyngeal ends of the paired auditory tubes (also called the eustachian tubes) are located on the lateral walls of the nasopharynx. When these tubes open, they allow the pressure to equilibrate between the middle ears and atmosphere. Across the back surface of the nasopharynx, between the pharyngeal orifices of the auditory tubes, lies a large mass of lymphoid tissue called the pharyngeal tonsil. This tissue is also referred to as the nasopharyngeal tonsil and, when abnormally enlarged, is designated as adenoid tissue (or just the adenoids). At the front, the nasopharynx connects to the nasal cavities through the nasal choanae (funnel-like openings), also called the posterior nares (nostrils) or internal nares. These are two oval-shaped apertures that are about twice as long (top to bottom) as they are wide (side to side) and are oriented in the vertical plane (see Figure 4–3).
Figure 4–4. Boundaries of the nasopharynx, oropharynx, and laryngopharynx. The boundary between the nasopharynx and oropharynx can be arbitrary; in this figure it is defined by an imaginary line extending backward at the level of the hard palate. The boundary between the oropharynx and laryngopharynx is the hyoid bone, and the lower boundary of the laryngopharynx is the base of the cricoid cartilage.
The oropharynx forms the middle part of the pharyngeal tube. Its upper boundary is coextensive with the lower boundary of the nasopharynx and its lower boundary is the hyoid bone. As shown in Figure 4–5, the front of the oropharynx opens into the oral cavity through the faucial isthmus (the narrow passage situated between the velum and the base of the tongue). This isthmus is bounded on the left and right sides by the anterior and posterior faucial pillars, pairs of muscular bands that resemble pairs of legs. The palatine tonsils are located between the anterior and posterior faucial pillars on each side of the isthmus. They are also often called the faucial tonsils and are “the” tonsils most often referred to colloquially. The back surface of the tongue is the site of yet another tonsil, the so-called lingual tonsil. This tonsil is a broad aggregate of lymph glands distributed across much of the posterior (root) part of the tongue. The oropharynx is the only subdivision of the pharynx that can be visualized without special equipment. The back wall of the oropharynx is best viewed when the velum is elevated, as in “open your mouth wide and say ‘ah.’”
The laryngopharynx constitutes the lowermost part of the pharynx. The upper boundary of the laryngopharynx is the hyoid bone and the lower boundary is the base of the cricoid cartilage, where the pharynx is continuous with the esophagus. At the front, the laryngopharynx is bounded by the back surface of the tongue (and the lingual tonsil), the laryngeal aditus (the opening into the larynx formed by the epiglottis and aryepiglottic folds), and the pyriform sinuses (pear-shaped cavities located lateral to the aryepiglottic folds; see Figure 4–3).
The velum, which means curtain, is a pendulous flap consisting of the soft palate and uvula (meaning little grape). In this case, the velum is the curtain that hangs down from the back of the roof of the mouth, as illustrated in Figures 4–4 (side view) and 4–5 (front view). A broad sheet of connective tissue, the palatal aponeurosis, forms a fibrous skeleton for the velum.
Despite a similar surface appearance throughout, the velum is not structurally homogeneous. Four tissue layers have been identified in the velum (Kuehn & Kahane, 1990). These include: (a) a layer toward the under surface (oral surface) that is glandular (secreting) tissue with adipose (fat) tissue at the sides, (b) a middle layer of muscle tissue in which fibers run side to side in the central portion of the structure and front to back in its more superficial portion toward the upper surface (nasal surface), (c) an upper front layer consisting of connective tissue (tendon), and (d) a lower back layer consisting largely of glandular tissue.
Figure 4–5. The oropharynx as seen from the front. The oropharynx is best viewed when instructed to “open your mouth wide and say ‘ah.’” The narrow opening between the velum and the tongue (top to bottom) and between the anterior and posterior faucial pillars (side to side) is called the faucial isthmus.
They were twin girls. Each had speech that was a dead ringer for the other and was characterized by multiple misarticulations and hypernasality. What was the cause? Had they developed some sort of twin speech? Did one have a problem and the other was imitating it? Oral examinations revealed identical structural anomalies. Each girl had a short velum. Nasoendoscopic examinations further revealed that, for each girl, the velum elevated only occasionally during speech production, but never came close to the posterior pharyngeal wall. The girls’ parents were with them and being interviewed by a student clinician and her supervisor. The moment the mother spoke there were suspicions. She had a severe speech disorder characterized by multiple misarticulations and hypernasality, and exhibited pronounced nasal grimacing when speaking. She allowed an oral examination. She had a short velum. It was three of a kind.
Patterns of muscle fiber distribution differ along the length of the velum (Kuehn & Moon, 2005). These include: (a) a front portion that is void of muscle fibers, (b) a middle one-third that is rich with muscle fibers that course in various directions (including across the midline) and include insertions into the lateral margins of the structure, (c) a proportioning of muscle fibers that tapers off toward the front and back of the structure, and (d) a uvular (back) portion that is sparsely interspersed with muscle fibers. The uvula also has a richer vascular system than the soft palate, perhaps to prevent excessive cooling of this region (Moon & Kuehn, 2004).
The nasal cavities, also called the nasal fossae (pronounced like posse), lie behind the outer nose. They are two large chambers that run side by side and are separated from each other by the nasal septum (which is often not perfectly vertical). As shown in Figure 4–6, this partition has: (a) a front part composed of cartilage, (b) an upper back part that is the perpendicular plate of the ethmoid bone, and (c) a lower back part that is the vomer bone. The floor of the nasal cavities is broad and slightly concave and is formed by two sets of bones that constitute the hard palate. The palatine processes of the maxillary bones (left and right upper jaws) form the front three-fourths and the horizontal processes of the palatine bones form the back one-fourth of the hard palate (this can be seen in the middle image in Figure 4–2). The roof of the nasal cavities, in contrast to the floor, is quite narrow and formed by part of the ethmoid bone called the cribriform plate. The configuration of the two cavities is similar to the roofline of an A-frame house.
Figure 4–6. Components of the nasal septum (partition between the two nasal cavities). The nasal septum consists of cartilage at the front and bone (ethmoid and vomer) in the back. Selected other bones and teeth are shown for reference.
By far the most complex formations within the nasal cavities are located on its lateral walls. These formations are convoluted and labyrinthine and contain many nooks and crannies. Three shell-like structures give rise to this complexity. These structures are portrayed in Figure 4–7 and include the superior, middle, and inferior nasal conchae, also called the nasal turbinates. The nasal conchae extend along the length of the nasal cavities and have corresponding meatuses (passages) named for the conchae with which they are associated. The enfolding structure of the nasal cavities provides a large surface area to the inner nose and has a rich blood supply. Near the front of each nasal cavity is the nasal vestibule, a modest dilation just inside the aperture of the anterior naris.
Figure 4–7. Superior, middle, and inferior nasal conchae (also called nasal turbinates). These conchae contain many nooks and crannies and create a large surface area to the inner nose.
There are four sinuses (hollows) that surround the nasal cavities. Called the paranasal sinuses, they include the maxillary, frontal, ethmoid, and sphenoid sinuses, each located within the bone of corresponding name. Three of these are shown in Figure 4–8. The sphenoid, not pictured, is located behind and above the superior nasal conchae within the sphenoid bone. They are usually air filled but can become liquid filled when infected. Their relevance to speech is primarily related to their effects on the resonance characteristics of the acoustic signal during nasal sound production (see Chapter 9).
Unlike the other components of the velopharyngeal-nasal apparatus, the outer nose is familiar to everyone. The outer nose is hard to ignore because it is in the center of the face and projects outward and downward conspicuously. The more prominent surface features of the outer nose include the root, bridge, dorsum, apex, alae, base, septum, and anterior nares, as shown in Figure 4–9.
Figure 4–8. The paranasal sinuses. Shown in this figure are the maxillary, frontal, and ethmoid sinuses. Not shown are the paired sphenoid sinuses, which are located behind and above the superior nasal conchae.
Mucus (a slimy substance) is formed in the nose to the tune of about half a pint a day (more when you have a cold). Particles filtered by the nose are collected in a blanket of mucus and moved through the nose by the action of cilia (tiny hair cells that collectively form a fringe). Things that get trapped are moved along toward the back of the throat and then swallowed into the stomach. Some material dries before reaching the back of the throat and fractionates into pieces containing filtered particles. This happens at different spots within the nose and in residues of various consistencies. Prim and proper folks refer to these residues as nasal exudates. Most of us refer to them as “boogers.” They are best gently blown into a tissue to rid them from the nose, but we all know other manual methods that are commonly practiced.
The root (point of attachment) of the outer nose is to the bottom of the forehead. Following downward along the center line are the bridge (upper bony part), dorsum (prominent upper surface), and apex (tip). The alae (wings) form much of the sides of the nose and contribute significantly to its general shape. The base of the nose is partitioned down the middle (more or less) by the lowermost part of the nasal septum and includes the anterior nares (nostrils), also called the external nares. The anterior nares are apertures that are somewhat pear-shaped, typically about twice as long (front to back) as they are wide (side to side). Margins of the anterior nares contain stiff hairs, called vibrissae. These hairs arrest the passage of particles riding on air currents.
FORCES OF THE VELOPHARYNGEAL-NASAL APPARATUS
Both passive and active forces operate on the velopharyngeal-nasal apparatus. Passive force is inherent and always present (although subject to change) and arises from the natural recoil of muscles, cartilages, and connective tissues, the surface tension between structures in apposition, the pull of gravity, and aeromechanical forces. Active force is applied by muscles and depends on the will and ability of the individual. Active force within the velopharyngeal-nasal apparatus comes from muscles of the pharynx, velum, and outer nose.
Figure 4–9. Surface features of the outer nose.
Figure 4–10 portrays the muscles of the pharynx and Figure 4–11 summarizes their actions. They are the superior constrictor, middle constrictor, inferior constrictor, salpingopharyngeus, stylopharyngeus, and palatopharyngeus muscles. These muscles influence the size and shape of the lumen (cavity) of the pharyngeal tube. Of course, other structures along the front side of the pharynx can also influence the lumen of the pharynx through their adjustments (velum, tongue, and epiglottis).
The superior constrictor muscle is located in the upper part of the pharynx. It is a complex muscle with multiple origins that arise from the front of the pharyngeal tube. Front points of attachment include the medial pterygoid plate of the sphenoid bone, the pterygomandibular ligament (a tendinous inscription between the superior constrictor muscle and the buccinator muscle, described in Chapter 5), the mylohyoid line (site of attachment of the mylohyoid muscle, described in Chapter 5, on the inner surface of the body of the mandible), and the side of the back part of the tongue. These multiple points of origin are sometimes used as a basis for conceptualizing the superior constrictor muscle as a cluster of four individual muscles. From top to bottom, these four are designated as the pterygopharyngeus, buccopharyngeus, mylopharyngeus, and glossopharyngeus muscles. Fibers from the multiple origins of the superior constrictor muscle course backward, toward the midline, and upward to insert into the fibrous median raphe (seam) of the posterior pharyngeal wall. There, they join with fibers of the paired muscle from the opposite side. The uppermost fibers of the superior constrictor muscle are horizontal and located at the level of the velum. When the superior constrictor muscle contracts, it reduces the regional cross section of the pharyngeal lumen by forward movement of the posterior pharyngeal wall and forward and inward movement of the lateral pharyngeal wall on the same side. The paired superior constrictor muscles encircle the posterior and lateral walls of the upper pharynx so that their simultaneous contraction constricts the lumen of that part of the pharyngeal tube in the manner of a sphincter.
Figure 4–10. Muscles of the pharynx. The superior constrictor, middle constrictor, inferior constrictor, salpingopharyngeus, and palatopharyngeus muscles constrict the pharynx, whereas the stylopharyngeus muscle dilates the pharynx. Some of these muscles can also move the pharynx in other ways (see Figure 4–11).
Figure 4–11. Summary of force vectors of the muscles of the pharynx. The muscles that constrict the pharynx are the superior constrictor (1), middle constrictor (2), and inferior constrictor (3) muscles. Muscles that pull both upward and inward on the pharynx are the salpingopharyngeus (4) and palatopharyngeus (6). The stylopharyngeus muscle (5) pulls upward and outward on the pharynx.
The middle constrictor muscle is a fan-shaped structure located midway along the length of the pharyngeal tube. Fibers of the muscle arise from the greater and lesser horns of the hyoid bone and the stylohyoid ligament (which runs between the downward and forward projecting styloid process of the temporal bone and the lesser horn of the hyoid bone) and radiate backward and toward the midline where they insert into the median raphe of the pharynx. The middle constrictor muscle is also sometimes conceptualized as comprising two muscles designated as the chondropharyngeus and ceratopharyngeus muscles. The uppermost fibers of the middle constrictor muscle course obliquely upward and overlap the lower fibers of the superior constrictor muscle, whereas the lowermost fibers of the muscle run obliquely downward beneath the fibers of the inferior constrictor muscle. The middle fibers of the middle constrictor muscle run horizontally. The overlapping arrangement of the muscle fibers between the middle constrictor and superior constrictor muscles and between the inferior constrictor and middle constrictor muscles is akin to the way in which roof shingles partially overlap. When the middle constrictor muscle contracts, it decreases the cross section of the pharynx regionally, by virtue of forward movement of the posterior pharyngeal wall and forward and inward movement of the lateral pharyngeal wall. When the middle constrictor muscle acts in conjunction with its paired mate on the opposite side, the pharyngeal lumen is regionally constricted in the manner of a sphincter.
The inferior constrictor muscle is the most powerful of the three constrictor muscles of the pharynx. The fibers of this muscle arise from the sides of the thyroid and cricoid cartilages. The inferior constrictor muscle is sometimes thought of as consisting of two muscles. These are referred to as the thyropharyngeus and cricopharyngeus muscles. Fibers of the inferior constrictor muscle diverge from their origins in a fanlike configuration and course backward and toward the midline. There, they interdigitate with fibers from the inferior constrictor muscle of the opposite side at the median raphe of the pharyngeal tube. The middle and upper fibers of the inferior constrictor muscle ascend obliquely, whereas the lowermost fibers run horizontally and downward and are continuous with those of the esophagus. When the inferior constrictor muscle contracts, it draws the lower part of the posterior wall of the pharynx forward and pulls the lateral walls of the lower pharynx forward and inward. This action, in conjunction with that of the inferior constrictor muscle on the opposite side, constricts the lumen of the lower pharynx.
The salpingopharyngeus is a narrow muscle that arises from near the lower border of the pharyngeal orifice of the auditory tube. The fibers of the muscle course downward vertically and insert into the lateral wall of the lower pharynx where they blend with fibers of the palatopharyngeus muscle (discussed below). When the salpingopharyngeus muscle contracts, it pulls the lateral wall of the pharynx upward and inward. Acting simultaneously with its paired muscle from the opposite side, the effect is to decrease the width of the pharynx.
The stylopharyngeus is a slender muscle that runs a relatively long course. It originates from the styloid process of the temporal bone and runs downward, forward, and toward the midline. Most fibers of the muscle insert into the lateral wall of the pharynx at and near the juncture of the superior constrictor and middle constrictor muscles. Some fibers extend lower in the pharyngeal wall and insert into the thyroid cartilage. When the stylopharyngeus muscle contracts, it pulls upward on the pharyngeal tube and draws the lateral wall of the pharynx toward the side. Together with similar action of its paired mate from the opposite side, it widens the lumen of the pharynx in the region where the muscle fibers insert into the lateral walls of the pharynx. There is also an upward pull placed on the pharynx (and larynx) when the stylopharyngeus muscles contract.
The palatopharyngeus muscle runs the length of the pharynx. It is a pharyngeal muscle as well as a muscle of the soft palate (in that context it is called the pharyngopalatine muscle; see sidetrack on this page). The muscle is considered here from the pharyngeal perspective. The palatopharyngeus muscle arises mainly from the soft palate. The uppermost fibers are directed horizontally and intermingle with fibers of the superior constrictor muscle. A major fiber course is downward and toward the side through the posterior faucial pillar. Below the pillar, the fibers continue into the lower half of the pharynx and spread to the lateral wall of the structure and the thyroid cartilage. Some have suggested that the portion of the muscle that attaches to the thyroid cartilage be given recognition of its own as the palatothyroideus muscle (Cassell & Elkadi, 1995), whereas others disagree (Moon & Kuehn, 2004). When the velum is relatively stable, contraction of the palatopharyngeus muscle results in two movements. The uppermost fibers of the muscle draw the lateral pharyngeal wall inward to complement the action of the superior constrictor muscle of the pharynx, whereas the lowermost fibers of the muscle pull upward on the lateral pharyngeal wall and elevate the pharynx (attachments to the thyroid cartilage also effect an upward and forward pull on the larynx).
A muscle is usually thought of as having an origin and an insertion. The origin is its anchored end and the insertion is its movable end. This is all well and good in textbooks, but in real life things are a bit more complicated. What may be the anchored end of a muscle for one activity may be the movable end of that muscle for another activity. A lot of it has to do with what neighboring muscles are doing. Thus, a muscle’s function may change from time to time because various forces cause the mobility of its two ends to change in relation to one another. The convention adopted in this book is to reflect such change by alternately labeling a muscle in accordance with its perceived primary function in a given context. Some purists may not embrace this convention, but it carries instructive power and simply points out that in the busy world of the muscle, turnabout is fair play.
The muscles of the velum are shown in Figure 4–12. They are the palatal levator, palatal tensor, uvulus, glossopalatine, and pharyngopalatine muscles. These muscles influence the positioning, configuration, and mechanical status of the velum. Their force vectors are illustrated in Figure 4–13.
The palatal levator muscle (also called the levator veli palatini muscle) forms much of the bulk of the velum. The palatal levator is a flattened cylindrical muscle that arises from the petrous (hard) portion of the temporal bone and from the cartilaginous portion of the auditory tube. From there, it courses downward, forward, and toward the midline, passing on the outside of the posterior naris. Fibers of the palatal levator muscle insert into the side of the velum and spread out where they join those of the palatal levator muscle from the opposite side. The spread of muscle fibers in each of the palatal levator muscles is to the midline and beyond to the other side of the velum (Kuehn & Moon, 2005). Fibers extend from behind the hard palate to the front of the uvula, encompassing approximately the middle 40% of the velum (Boorman & Sommerlad, 1985) or more (Kuehn & Kahane, 1990). The paired palatal levator muscles form a muscular sling from their cranial attachments through the velum. Each palatal levator muscle inserts into the velum at an angle of about 45°. When the palatal levator muscle contracts, it draws the velum upward and backward. Simultaneous contraction of the paired palatal levator muscles lifts the velum toward the posterior pharyngeal wall along an angular trajectory. The velum and the posterior pharyngeal wall come into contact frequently, sometimes with significant contact force. The upper surface of the velum can withstand such forces, in part, because of the stratified squamous epithelium that covers it. Frictional forces can also result from the sliding of the velum up and down the posterior pharyngeal wall. These are mitigated by glandular secretions of the velum which lubricate the contact areas (Kuehn & Moon, 2005).
Figure 4–12. Muscles of the velum. They are the palatal levator, palatal tensor, uvulus, glossopalatine, and pharyngopalatine muscles. Most of these muscles act primarily to move the velum upward and backward and downward and forward. Their individual actions are shown schematically in Figure 4–13.
Figure 4–13. Summary of force vectors of the muscles of the velum. The palatal levator muscle (1) pulls the velum upward and backward. The uvulus muscle (2) shortens, lifts, and increases the thickness of the velum. The glossopalatine muscle (3) pulls the velum downward and forward and the pharyngopalatine muscle (4) pulls the velum downward and backward. The palatal tensor muscle is not included in this figure because it is not thought to have a significant effect on the velum.
The palatal tensor muscle (also termed the tensor veli palatini muscle) lies on the outer side of the palatal levator muscle. It arises from the pterygoid and scapular fossae and angular spine of the sphenoid bone as well as the cartilaginous portion of the auditory tube. From there, fibers course vertically downward to terminate in a tendon and insert into the hook-shaped hamulus of the medial pterygoid plate of the sphenoid bone. The tendon of the palatal tensor muscle (along with a sparse number of palatal tensor muscle fibers) courses inward and inserts into the hard palate and the velum (Barsoumian, Kuehn, Moon, & Canady, 1998). The palatal tensor muscle plays an important role in dilating the auditory (eustachian) tube, possibly in conjunction with other velopharyngeal muscles (Okada et al., 2018). Earlier conceptions of the function of the palatal tensor muscle also suggested that its contraction would tense the velum, because it was thought that the muscle itself wrapped around the hamulus to contribute to the horizontal portion of the structure. However, the fact that the palatal tensor muscle is now known to insert on the hamulus, with only a few fibers continuing on to insert into the velum, indicates that it does not have the mechanical means to tense the velum to any significant degree. In contrast, the tendon does seem to play an important mechanical role. The prominent size of this tendon suggests that it may relieve stress at the junction between the hard and soft palates, stress induced by frequent up-and-down movements of the velum. The stress-relief function can be thought of as analogous to a reinforced collar at the junction between an electrical plug and the wire extending from it (Kuehn, 1990).
The uvulus muscle is the only intrinsic muscle of the velum (both ends of its fibers are within the velum). Fibers of the uvulus muscle originate to the side of the posterior nasal spine formed by the palatine bones and behind the hard palate near the sling formed by the palatal levator muscles and about a fourth of the way along the length of the soft palate from the front. The muscle courses downward and backward, extending through much of the length of the soft palate. Very few fibers of the uvulus muscle actually enter the uvula proper, from which the muscle historically derived its name (Azzam & Kuehn, 1977; Huang, Lee, & Rajendran, 1997). This has prompted some to argue (and seemingly rightfully so) that the designation of this muscle as the uvulus muscle is both a misnomer and anatomically misleading (Moon & Kuehn, 2004). The location of the so-called uvulus muscle is above the sling formed by the palatal levator muscles. Structurally, the paired uvulus muscles account for the longitudinal convexity of the upper surface of the velum. This is true even in the region of the uvula, where muscle fibers are sparse, if they exist at all. One reason is that the encapsulating sheath that surrounds each uvulus muscle persists into the uvula, providing some cohesiveness between the soft palate and uvula and a girder for the uvula (Kuehn & Moon, 2005). When the uvulus muscle contracts, it has several effects that can be realized alone or in combination. These include that it (a) shortens the velum, (b) lifts the velum, and (c) increases the thickness (bulk) of the velum in the third quadrant of its length. Classic thought about the function of the paired uvulus muscles focused on the first of these effects, shortening of the velum. More recent conceptualizations, however, have focused on possible effects that involve the control of the stiffness of the upper part of the velum (Kuehn, Folkins, & Linville, 1988). Stiffness effected in the back part of the velum may counteract the deformation imposed on the velum by contraction of the palatal levator muscles. This avoids stretching of the top layer of the velum upward rather than moving the overall mass of the structure. It may also be that the uvulus muscles act to exert force within the upper part of the velum that causes the curvilinear structure to behave like a flexible beam that rotates the back half of the structure toward the posterior pharyngeal wall (somewhat analogous to extending flexed fingers with the palm of the hand facing downward). In this case, the uvulus muscles function as muscles that shorten the distance between the velum and the posterior pharyngeal wall or facilitate contact and/or force of contact between the two.
The glossopalatine muscle is both a muscle of the tongue (called the palatoglossus muscle in that context) and a muscle of the velum, and is discussed here as a muscle of the velum. Fibers of the glossopalatine muscle arise from the side of the tongue where they are closely blended with longitudinal fibers of the dorsum of the tongue. They course upward and inward, forming the substance of the anterior faucial pillar, and insert into the lower surface of the palatal aponeurosis. The location of attachment to the soft palate is reported to vary across individuals, with some having insertions forward near the hard palate and others having insertions rearward near the uvula (Kuehn & Azzam, 1978). When the dorsum of the tongue is relatively fixed, contraction of the glossopalatine muscle places a downward and forward pull on the velum. Although the glossopalatine muscle has force potential on the velum, that potential is limited in comparison to the force potential of the pharyngopalatine muscle (Moon & Kuehn, 2004).
The pharyngopalatine muscle (discussed above as the palatopharyngeus muscle in the context of the pharynx) is considered here in the context of the velum. Its fibers arise from the lower half of the lateral wall of the pharynx and thyroid cartilage and course upward and toward the midline where they pass through the posterior faucial pillar and insert into the soft palate (also the superior constrictor muscle). Its fibers do not approach or cross the midline of the soft palate, but insert more laterally within the structure (Kuehn & Kahane, 1990). One notion of mechanical prominence is that there is a downward directed sling formed by the pharyngopalatine muscles that is antagonistic to the upward directed sling provided by the palatal levator muscles (Fritzell, 1969; Moon, Smith, Folkins, Lemke, & Gartlin, 1994b). This notion has intuitive appeal but has been questioned on anatomical grounds, given the observation that fibers of the pharyngopalatine muscles do not extend to the midline of the velum (Moon & Kuehn, 2004). Dismissal of the notion based on this criticism may be premature, however, because interconnection through other structures of the velum may still effect a functional, if not anatomical, sling that can direct force downward symmetrically as a consequence of action of the paired pharyngopalatine muscles. When the pharyngeal attachment of the pharyngopalatine muscle is relatively fixed, contraction of its fibers (especially those that are vertically oriented) places a downward and backward pull on the velum. This suggested action is founded on assumed muscle vector pulls inferred from anatomical observations, which may or may not be wholly correct.
All of the muscles of the outer nose can be used for facial expression to convey meaning. For the purposes of this chapter, however, interest in these muscles is in their potential to influence velopharyngeal-nasal function. Five muscles of the outer nose, shown in Figure 4–14, have this potential.
The levator labii superioris alaeque nasi muscle (the muscle with the longest name of any muscle in animals) is a thin structure located at the side of the outer nose between the orbit of the eye and the upper lip. Its origin is from the frontal process and infraorbital margin of the maxilla. From there, the muscle courses downward and toward the side, subdividing into two muscular slips. One slip inserts into the upper lip (blending with the orbicularis oris muscle, described in Chapter 5) and the other slip (of more interest here) inserts into the cartilage of the nasal ala (wing at the side of the nose). Contraction of this latter muscular slip draws the ala upward on the same side of the outer nose (like lifting a side flap on a tent) and enlarges the corresponding anterior naris.
The anterior nasal dilator muscle is a small muscle positioned on the lower lateral surface of the outer nose. It arises from the lower edge of the lateral nasal cartilage and runs downward and outward. Following a short course, it inserts into the deep surface of the skin near the outer margin of the naris on the same side. Contraction of the anterior nasal dilator muscle enlarges the anterior naris on that side of the outer nose.
The posterior nasal dilator is a small muscle located on the lower lateral surface of the outer nose. It lies behind the anterior nasal dilator muscle. Fibers of the posterior nasal dilator muscle originate from the nasal notch of the maxilla and adjacent cartilages of the outer nose. From this origin, they follow a short course and insert into the skin near the lower part of the alar cartilage along the outer margin of the naris on the same side. Contraction of the posterior nasal dilator muscle enlarges the corresponding anterior naris.
Figure 4–14. Muscles of the outer nose. Three muscles can dilate the nares (levator labii superioris alaeque nasi, anterior nasal dilator, and posterior nasal dilator muscles) and two can constrict the nares (nasalis and depressor alae nasi muscles) when activated.
The nasalis muscle is located on the side of the outer nose. It originates from the maxilla, above and lateral to the incisive fossa (a depression in the maxilla above the incisor teeth). Fibers run upward and toward the midline and insert into an aponeurosis that is continuous with its paired muscle from the opposite side. When the nasalis muscle contracts, it draws down the cartilaginous part of the outer nose on the same side (like pulling down a side flap on a tent) and decreases the aperture of the corresponding anterior naris. Strong contraction of this muscle and its counterpart from the opposite side may bring the two alae of the outer nose together or compress them against one another.
The depressor alae nasi is a short muscle that originates from the incisive fossa of the maxilla and radiates upward to insert into the back part of the ala and the cartilaginous septum of the outer nose. When the depressor alae nasi muscle contracts, it draws the ala of the outer nose downward on the side of action and decreases the aperture of the corresponding naris.
MOVEMENTS OF THE VELOPHARYNGEAL-NASAL APPARATUS
The velopharyngeal-nasal apparatus comprises several movable parts. Movements of the pharynx, velum, and outer nose are considered here.
The pharynx is a highly mobile tube. As illustrated in Figure 4–15, this mobility is vested in structures of the pharynx itself and in structures that comprise its lower and front boundaries. These movement capabilities are: (a) lengthening and shortening through downward and upward movements of the larynx, (b) inward and outward movements of the lateral pharyngeal walls, (c) forward and backward movements of the posterior pharyngeal wall, and (d) forward and backward movements of velum, tongue, and epiglottis. These movements, which dilate or constrict the pharynx at multiple sites, can change the size and shape of its internal cavity. In fact, one part of the pharynx may be constricted, another part dilated, and yet another part alternately constricted and dilated during the performance of a given activity. These size and shape changes can have a profound influence on the acoustic signal during speaking. They are also critical to certain phases of the swallowing process.
Figure 4–15. Movements of the pharynx. These movements can be downward and upward, inward and outward, and forward and backward and can lengthen, shorten, widen, and constrict the pharyngeal tube. Some of these movements are carried out by parts of the pharynx and others are carried out by nearby structures (velum, tongue, epiglottis, and larynx).
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