Laryngeal and Pharyngeal Function PDF

PART V Laryngology and Bronchoesophagology 53 Laryngeal and Pharyngeal Function Gayle Ellen Woodson KEY POINTS sphincter to protect the lungs from water.5 Consequently, du...

PART V Laryngology and Bronchoesophagology 53 Laryngeal and Pharyngeal Function Gayle Ellen Woodson KEY POINTS sphincter to protect the lungs from water.5 Consequently, during embryologic development, the foregut is the common origin of The upper aerodigestive tract serves the competing the larynx, trachea, and esophagus. functions of respiration and swallowing. Normal function of the larynx and pharynx requires precise timing and coordination of competing functions of this system. The anterior ends of the vocal folds are fixed at the Thus, function is easily disrupted by structural or neurologic anterior commissure. pathology. Further, the treatment of any disease or disorder of All motion of the vocal folds is caused by muscles that this region may have an impact on more than one function. For move the arytenoid cartilage. example, surgery to improve the glottic airway can impair the The posterior cricoarytenoid muscle is the only muscle voice or lead to aspiration during swallowing. It is, therefore, that actively opens the larynx. imperative for otolaryngologists to understand the function of the upper aerodigestive tract. This chapter focuses on the functions The recurrent laryngeal nerve supplies all intrinsic of breathing and speech; swallowing is addressed in Chapter 56. laryngeal muscles except for the cricothyroid muscle, which is supplied by the motor branch of the superior laryngeal nerve. LARYNGEAL MOTION The internal branch of the superior laryngeal nerve carries sensory information from the larynx. Applied Anatomy Vocal sound is produced by passive vibration of the In the illustrations of many textbooks, the membranous vocal vocal folds, powered by exhaled air. folds are depicted as moving solely in the axial plane, with rotational motion similar to that of a windshield wiper. Details of motion of the posterior, cartilaginous portions of the larynx have been largely ignored. The reason is that early concepts of motion were based on observing laryngeal motion with a mirror and recording The upper aerodigestive tract serves the competing functions of the observations with two-dimensional (2D) freehand drawings. respiration and swallowing. The nose is the primary respiratory However, with the advent of flexible endoscopy, stroboscopy, video orifice, and the mouth is the portal for ingestion of food. Both recording, and computerized imaging, it has become clear that open into a common cavity, the pharynx. The patency of the upper laryngeal motion is more complex than previously recognized. airway must be actively supported during breathing, yet total and Vocal folds move in three dimensions and undergo conformational forceful collapse is required to propel food into the esophagus changes in length, shape, and volume (Fig. 53.2).6 The terms during swallowing. The airway must be protected during a swallow cadaveric and paramedian have been commonly used to describe so that ingested food or water does not spill into the trachea. the position of paralyzed vocal folds. These terms are inadequate Aspiration of food or foreign material can lead to serious conse- to describe the three-dimensional (3D) changes in configuration quences, such as asphyxia or lung infection. In humans, the function of the glottis in paralysis.7 Motion of the larynx is best understood of the upper aerodigestive tract is considerably more complex, as the net result of the interaction of its component parts. owing to the demand for speech as well as a significant structural The laryngeal skeleton consists of the hyoid bone and a series difference. In infants and in all nonhuman mammals, the pharynx of cartilages. The hyoid bone is a U-shaped structure that opens is functionally compartmentalized into separate passages for posteriorly and is suspended from the base of the skull and mandible breathing and alimentation. The epiglottis interdigitates with the by muscles and ligaments. The thyroid cartilage, the largest cartilage uvula to form a respiratory channel from the nose into the larynx in the larynx, is suspended from the hyoid bone. The word thyroid and two lateral pathways from the mouth to the esophagus through means “shield”—an appropriate name, because the structure is the piriform sinuses.1 During postnatal development in humans, not only shaped like a shield but also provides support and protec- enlargement of the cranium with flexion of the base of the skull tion for the vocal folds. In the axial plane, the thyroid cartilage results in a downward displacement of the larynx that elongates looks like the letter V with two wings that project posteriorly. the pharynx and distracts the uvula and epiglottis so that they are Like the hyoid bone, the thyroid cartilage is open posteriorly; the no longer in contact. The result is a common pharyngeal cavity vocal folds attach to the anterior inner surface of the thyroid for breathing and swallowing (Fig. 53.1).2,3 The larynx begins its cartilage; and the posterior ends of the vocal folds are anchored descent at the age of about 18 to 24 months. Two positive outcomes to the arytenoid cartilages, the chief moving parts of the larynx. are that vocal power is greater because of increased resonance, The arytenoid cartilages sit atop the posterior rim of the cricoid and articulatory diversity is expanded.4 cartilage and articulate via shallow ball-and-socket joints. The This complicated and potentially hazardous configuration of cricoid cartilage is the only complete rigid ring within the airway. the upper airway results from embryology and reflects evolution. It is shaped like a signet ring and is broadest posteriorly. Inferiorly The lower respiratory tract has evolved as an offshoot of the and just lateral to the cricoid joints, the inferior cornua of the digestive tract, first appearing in the lungfish as a simple muscle thyroid cartilage articulates with the cricoid in two hinge-like 799 Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 799.e1 Abstract Keywords 53 This chapter presents the anatomy and physiology of the larynx Larynx and pharynx. The human upper aerodigestive tract is uniquely anatomy configured among mammals, as the larynx is lower in the neck. physiology This evolution is favorable for the function of speech but com- respiration plicates the competing functions of respiration and swallowing. phonation The pharynx must be patent during breathing but collapses forcefully during swallow. The larynx protects the lower airway from aspiration and regulates the flow of air in and out of the lungs. Phonation is produced by exhalation against a closed glottis, which induces passive vibration of the vocal folds. Our understand- ing of the actions of laryngeal muscles has been greatly advanced by 3D modeling of laryngeal motion. Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 800 PART V Laryngology and Bronchoesophagology A A B Fig. 53.1 Postnatal descent of the larynx. Migration of epiglottis away from the uvula results in loss of pharyngeal compartmentalization for breathing and feeding. (A) Sagittal view through the head and neck of a neonate. (B) Sagittal view through an adult head and neck. PC A B Fig. 53.3 Three-dimensional effects of posterior cricoarytenoid muscle contraction. (A) Sagittal view. (B) Posterior view. PCA, Abduction Neutral Adduction Posterior cricoarytenoid. muscles has been greatly advanced by 3D modeling of laryngeal motion.9,10 The arytenoid rotates inwardly to close the glottis, and upward Fig. 53.2 Three-dimensional motion of the arytenoid cartilage and and outward to open it. The LCA is the primary adductor of the vocal fold. (From Hirano M: Anatomy and behavior of the vocal vocal fold, primarily via a rocking motion of the arytenoid in the process. In Baer T, Sasaki C, Harris K, editors: Laryngeal function in coronal plane about the cricoid facet, with very horizontal sliding phonation and respiration, Boston, 1987, College-Hill Press.) or rotation about a vertical axis.10 The LCA pulls the muscular process of the arytenoid anteriorly and caudally, which moves the vocal process medially. While it is the strongest adductor, the joints that create a visor-like or “bucket handle” structure, with LCA alone does not completely close the anterior glottis. Complete motion that controls the space between the anterior rims of the glottic closure requires simultaneous action of all the adductors. thyroid and cricoid cartilages.8 There are two functional subunits of the TA muscle, with distinct The epiglottic cartilage is a leaf-shaped structure attached actions on motion of the vocal fold.9 The lateral, muscularis portion inferiorly to the anterior–interior surface of the thyroid cartilage. (TAm) adducts the vocal fold, and its action is necessary for The upper margin is free and projects into the hypopharynx above complete closure of the anterior glottis. The medial portion, also the glottic opening. The mucosa that covers the epiglottis spreads known as the vocalis muscle (TAv), shortens and thickens the laterally on both sides and is continuous with the mucosa over vocal fold. Because IA connects the arytenoid cartilages, it is the arytenoid to create the aryepiglottic folds, the lateral borders logically implicated in adduction, although 3D modeling suggests of the supraglottis. Muscle fibers within each aryepiglottic fold that isolated contraction of the IA actually abducts the vocal folds. contribute to constriction of the supraglottis; in addition, two The PCA muscle abducts the vocal fold and provides posterior small sesamoid cartilages, the corniculate and cuneiform cartilages, support during phonation. PCA contraction pulls the muscular sit just above each arytenoid within the aryepiglottic fold. process of the arytenoid posteriorly and caudally. The structure The membranous vocal folds are suspended between the thyroid of the cricoarytenoid joint prevents the entire arytenoid from cartilage and the arytenoid cartilages. Abduction and adduction being pulled along this vector. Instead, the arytenoid rotates, which of the vocal folds are accomplished by the actions of the intrinsic displaces the vocal process upward and laterally and, hence, abducts laryngeal muscles that connect the arytenoid cartilages to either the vocal fold (Fig. 53.3).9 The human PCA has two compart- the cricoid or thyroid cartilages. These include the thyroarytenoid ments supplied by separate nerve branches; they differ in fiber muscle (TA), lateral cricoarytenoid muscle (LCA), interarytenoid type and insert on opposite sides of the muscular process (Fig. muscle (IA), and the posterior cricoarytenoid (PCA) muscle. Our 53.4).11,12 The functional significance of this compartmentalization understanding of the individual and combined actions of laryngeal is unknown. Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 801 53 A Fig. 53.4 Posterior view of a cadaver larynx shows compartmentalization of the human posterior cricoarytenoid muscle. The most recently recognized intrinsic muscle is a small bundle of fibers in the aryepiglottic fold that can constrict the supraglottis. This muscle is implicated in Valsalva and swallowing and is likely active in patients with vocal hyperfunction.13 The cricothyroid muscles connect the anterior edges of the thyroid and cricoid cartilages. CT contraction pulls the thyroid and cricoid cartilages closer together, which increases the distance between the anterior commissure and the cricoid. The result is a stretching of the vocal fold and an increase in its length and tension (Fig. 53.5). Because both vocal folds insert on the anterior commissure, contraction of either CT muscle affects both ipsilateral and contralateral vocal folds—that is, unilateral CT contraction does not result in rotation of the glottis.8 In contrast to other intrinsic laryngeal muscles, the CT muscle does not insert on the arytenoid cartilage and, therefore, does not adduct or abduct the vocal fold. Aerodynamic studies, cadaveric anatomic studies, and computational models of motion have indicated that CT contraction does not increase glottic area or reduce translaryngeal resistance during inspiration.8,9,14 Extrinsic laryngeal muscles connect the larynx to other structures B and exert traction on the laryngeal cartilages. The sternohyoid, thyrohyoid, and omohyoid muscles pull the larynx caudally. Muscles Fig. 53.5 Effects of (A) thyroarytenoid and (B) cricothyroid muscle that exert a cephalad force include the geniohyoid, anterior belly contraction on the thickness of the vocal fold. of the digastric, mylohyoid, and stylohyoid muscles. In patients with hyperfunctional dysphonia, excess activity can usually be palpated in the extrinsic laryngeal muscles. The nerve supply to the larynx is via the superior and recurrent a vestigial remnant of embryonic connection between the aorta laryngeal nerves, both of which are branches of the vagus nerves. and pulmonary artery. On the right side, the recurrent nerve The superior laryngeal nerve leaves the vagus nerve high in the travels only as far caudally as the subclavian artery before returning neck at the nodose ganglion; its internal branch exits the lateral cephalad. thyrohyoid membrane and carries sensory afferent fibers from The laryngeal mucosa is richly supplied with sensory receptors. the supraglottis and vocal folds, whereas the external branch of In fact, there are many more sensory receptors in the larynx than the superior laryngeal nerve supplies motor fibers to the CT muscle. in the lungs, which have a vastly larger area of surface mucosa. All other intrinsic laryngeal muscles are supplied by the recurrent Laryngeal sensory receptors respond to a variety of stimuli, laryngeal nerve, which leaves the vagus nerve in the chest and including mechanical, thermal, chemical, and taste. These receptors then travels back up to enter the larynx near the cricothyroid provide important input for protection of the larynx and information joint. The course of the recurrent laryngeal nerve is much longer on the movement of air in and out of the lungs. The receptors on the left side, because it runs under the ligamentum arteriosum, provide the afferent limbs of a variety of reflexes. Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 802 PART V Laryngology and Bronchoesophagology as 10 L/s. Cough plays an important role in cleaning the tracheo- LARYNGEAL FUNCTION IN BREATHING bronchial tree and in maintaining patency of the lower airways. The primary and most primitive function of the larynx is to protect Abnormal cough can be a serious clinical problem that interferes the lower airway. In evolution, the larynx first appeared as a with normal daily function and impairs quality of life. sphincter to prevent the ingress of water into the airway of the lungfish.5 Subsequently, dilator muscles evolved to permit active opening of the larynx. In more evolved animals, the larynx is not Control of Ventilation just an open or shut valve; rather, it is a variable resistor capable The role of the larynx as an active organ of respiration is not of regulating airflow. Other laryngeal functions are the Valsalva widely recognized. The larynx is located at the entrance into the maneuver and coughing. The larynx is also a sensory organ that trachea; not only is it capable of opening and closing rapidly, it provides information about airway function and the purity of can also create sudden alterations in resistance. Hence, the larynx inhaled air and serves in the afferent limbs of many reflexes. is better adapted than any other portion of the respiratory tract to regulate airflow. Abduction and adduction of the larynx in phase with respiration plays an important role in regulating respiration. Protection The resistance changes that result from laryngeal responses to When the larynx is mechanically stimulated, it closes abruptly respiratory stimuli, such as negative airway pressure and blood and respiration ceases. Apnea can also occur in response to such gas changes, have a beneficial effect on ventilation.17 diverse chemical agents as ammonia, phenyl diguanide, and cigarette Widening of the glottis during inspiration is a primary action smoke. These are appropriate and beneficial responses that prevent of ventilation that ceases only during deep anesthesia or sleep. the entry of foreign matter into the lower airway, although strong The PCA muscle, the only active dilator of the larynx, begins to laryngeal stimulation may result in responses that appear to be contract with each inspiration before activation of the diaphragm.18 maladaptive, such as laryngospasm and prolonged bronchoconstric- The level of PCA activity, and, hence, laryngeal movement with tion.15 These reflexes may be produced in experimental animals breathing varies. Laryngeal motion may be imperceptible during by electrical stimulation of the superior laryngeal nerve and unlabored, quiet breathing; however, with increasing respiratory probably represent an oversaturation of pathways that serve a drive, peak inspiratory PCA activity increases proportionately with useful function at lower levels of input. diaphragmatic activity. Important differences have been noted The larynx occupies a protected position in the body, and it between PCA activity and diaphragmatic behavior (Fig. 53.6). is rarely subject to direct stimulation. Therefore, laryngospasm When the upper airway is partially occluded, inspiration generates and apnea are not everyday occurrences. Severe laryngeal reflexes negative airway pressure, which is a potent stimulus to the PCA are most often encountered in patients in the operating room in and several other muscles that dilate the upper airway.17,19 In response to direct stimulation during intubation, endoscopy, or contrast, the diaphragm responds by actually decreasing inspiratory extubation. These reflexes most likely occur in patients during force and by increasing the duration of inspiration.19 This response light anesthesia and in those who are well oxygenated. occurs because during partial airway obstruction, the PCA and Recurrent paroxysmal laryngospasm is occasionally encountered the diaphragm have opposing effects on patency of the lumen. in clinical practice. In some patients, it is caused by gastroesophageal Increasing PCA contraction dilates the airway. However, increasing reflux, which responds to acid-suppressing medication. In other the strength of diaphragmatic contraction increases negative patients, the pathophysiology appears to be a hypersensitive intrathoracic pressure, which tends to collapse the intrathoracic laryngeal closure reflex, because such patients report some trig- airway. During strong respiratory demand, the PCA continues to gering event, such as eating or inhaling steam or odors. The onset contract during expiration after the diaphragm has relaxed; this frequently occurs during an upper respiratory infection, but it can delays expiratory adduction and facilitates the outflow of air. During also occur after surgical trauma to the recurrent laryngeal nerve. panting, the glottis sustains an abducted posture to ensure maximal Most often the condition resolves spontaneously within a few airflow. Because of these physiologic differences, the phrenic nerve months, but it may become a permanent and debilitating problem. is not an ideal choice for reinnervation of the PCA in patients The laryngeal closure reflex is particularly sensitive in infants and with laryngeal paralysis. can be elicited by a stimulus as weak as water. During early infancy, Closure of the larynx during expiration during sleep is passive the strength of this reflex increases and then decreases, and it due to relaxation of abductor muscles.20,21 But during wakefulness, does so along a time course similar to that of the incidence of adductor muscles are variably activated to prolong the duration sudden infant death syndrome, which suggests that laryngeal reflexes of exhalation. Expiratory duration is also modulated by diaphrag- may play a role in its cause.16 matic activity.22,23 Simultaneous recordings of TA electromyography (EMG), upper airway pressure, and airflow have documented that expiratory airflow is inversely proportional to the level of laryngeal Cough adductor activity, while transglottal airflow resistance and duration Another important protective reflex that involves the larynx is the of expiration are increased by laryngeal adduction (see Fig. 53.7).24,25 cough, which ejects mucus and foreign material from the lungs.15 Laryngeal regulation of breathing is not essential for life, as Cough can be a voluntary action or a reflexive response to stimula- evidenced by the fact that patients ventilate satisfactorily through tion of the larynx or receptors in the lungs. The cough reflex is a tracheotomy, although the inability to breathe and speak normally suppressed during sleep, so that a greater stimulus is required through natural orifices has a devastating impact on quality of with progressive stages of sleep. During deep sleep, a cough cannot life. Optimal function of the upper aerodigestive tract requires be elicited unless the stimulus first results in arousal to a lighter normal laryngeal function. level of sleep. The first phase of a cough is inspiratory. The larynx opens widely to permit rapid and deep inhalation. In voluntary cough, Sensory Receptors the extent of inspiratory effort is varied according to the intended The larynx is not only a motor organ, it is also the location of a strength of the cough. The second phase is compressive and involves variety of sensory receptors that exert influences on breathing tight closure of the glottis and strong activation of expiratory and cardiovascular function. The larynx is densely supplied with muscles; thus, the effectiveness of the cough is impaired by glottic sensory receptors, which are several times more numerous than incompetence. Finally, the larynx suddenly opens widely, which those of the lungs. This finding is remarkable given that the internal results in a sudden and rapid outflow of air at speeds of as high surface area of the lungs is several square meters, whereas the Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 803 Upper Airway Occlusion Tracheal Occlusion A B C SLN Intact 53 TA EMG (raw) DIA EMG DIA EMG TA EMG (MA) PCA EMG Expir V 0 Inspir PCA EMG Pes 0 (cm H2O) 10 0 5 sec Pua -1 (kPa) -2 Fig. 53.7 Laryngeal adductor activity during breathing as shown 0 on electromyography. (A) Plateau in thyroarytenoid muscle activity (TA Pos -1 -2 EMG) correlates with decreasing flow. (B) Progressive increase in (kPa) -3 thyroarytenoid activity correlates with flattened airflow trace and longer exhalation. (C) Decreasing activity during expiration correlates with SLN Out shorter exhalation. Expir., expiration; Inspir, Inspiration; MA, averaged; Pes, esophageal (intrathoracic) pressure; v̇ airflow. (From Kuna ST, DIA EMG Insalaco G, Woodson GE: Thyroarytenoid muscle activity during wakefulness and sleep in normal adults, J Appl Physiol 63:1332, 1985.) DIA EMG PCA EMG can result in bradycardia. The direct result of experimental laryngeal stimulation on blood pressure is hypertension,27 although in the PCA EMG clinical situation, the effects of bradycardia or ectopy usually prevail and result in hypotension. In patients with obstructive sleep apnea, Pua 0 -1 negative airway pressure may stimulate receptors in the larynx so (kPa) -2 strongly that cardiac arrhythmias occur. Animal experiments show Pos 0 that the afferent limb is the superior laryngeal nerve, because -1 (kPa) -2 transection of this nerve abolishes cardiovascular responses (Fig. -3 53.8). The efferent limb for bradycardia is the vagus nerve, and the efferent limb for blood pressure elevation is via sympathetic Fig. 53.6 Effects of upper airway and tracheal occlusions in an nerve fibers but intervening central connections have not been anesthetized dog before and after a section of the superior laryngeal identified.28 nerve (SLN). In each panel, the top trace is time in seconds, and the second and third traces are integrated electromyographic activity of the diaphragm (DIA EMG) and posterior cricoarytenoid muscle (PCA PHARYNGEAL FUNCTION IN BREATHING EMG). The lower two traces in each panel are pressure in the upper The upper airway is a conduit with several points at which the airway (Pua) and esophagus (Pos), the intrathoracic pressure. Tracheal shape and cross-sectional area can be dynamically altered. The occlusion affects only intrathoracic pressure, whereas upper airway pharynx is the largest but most compliant region, and it is sus- occlusion also affects upper airway pressure, which is sensed by the ceptible to passive collapse. Maintenance of patency requires the SLN. (From Sant’Ambrogio FB, Mathew OP, Clark WD, Sant’Ambrogio action of striated upper airway muscles in coordination with the G: Laryngeal influences on breathing pattern and posterior activity of respiratory pump muscles.29 Upper airway muscles also cricoarytenoid muscle activity, J Appl Physiol 58:1298, 1985.) determine whether air is inspired through the nose or the mouth. The anatomy and intrinsic properties of upper airway muscles indicate that they are primarily adapted for nonrespiratory functions larynx is a small orifice. Single nerve fiber recordings from the but can be used in respiratory tasks. Most of the muscles that act superior laryngeal nerve have identified three major types of on the pharynx show no respiratory activity during awake tidal laryngeal respiratory receptors—negative pressure, airflow, and breathing; rather, they are recruited by increased respiratory drive “drive” receptors26—activated by the process of breathing, and or upper airway obstruction (Fig. 53.9).30 these influence the central control of breathing. Airflow receptors In healthy patients, the nose is the preferred route of breathing, actually respond to a decrease in temperature, because the larynx because the relaxed position of the mandible closes the mouth, is cooled by inspired air; thus, airflow receptors do not respond and the relaxed palate occludes the oropharyngeal inlet. Further- to air that has been warmed and humidified by the nose; rather, more, the nose warms, humidifies, and filters inspired air. The they are activated by air that comes in through the mouth, par- selection of oral or nasal breathing is performed primarily by ticularly in cold and dry weather. Drive receptors are probably the soft palate, a band of moveable soft tissue suspended from the proprioceptors that respond to the respiratory motion of the larynx. posterior bony palate. Oral breathing requires activation of the Laryngeal sensations of touch and chemical stimuli are not activated levator veli palatini, to elevate the soft palate, and activation of during normal breathing conditions, but stimulation can profoundly the musculus uvula. Nasal breathing is favored by constriction of affect ventilation. the oropharyngeal passage, which is accomplished primarily through activation of the palatoglossal muscles, medialization of the faucial arches, and elevation of the base of tongue. This activity is the Circulatory Reflexes highest during forced nasal breathing with the mouth open.31 Stimulation of the larynx can alter heart rate and blood pressure. Little objective information is available regarding the respiratory During induction of general anesthesia, endotracheal intubation function of the pharyngeal constrictors. It is widely assumed that Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 804 PART V Laryngology and Bronchoesophagology SLN Intact SLN Cut BP 20 (kPa) 10 0 DIA EMG  DIA EMG PCA EMG  PCA EMG 0 Pes 1 (kPa) 2 3 Fig. 53.8 The effects of upper airway occlusion on arterial blood pressure (BP) in an anesthetized dog before and after transection of the superior laryngeal nerve (SLN). In each panel, the top trace marks time in seconds. The third and fifth traces are raw and integrated electromyographic activity of the diaphragm (DIA EMG) and posterior cricoarytenoid muscle (PCA EMG). The bottom trace is intraesophageal pressure (Pes) as an indicator of respiratory effort. (From Sant’Ambrogio FB, Mathew OP, Clark WD, Sant’Ambrogio G: Laryngeal influences on breathing pattern and posterior cricoarytenoid muscle activity, J Appl Physiol 58:1298, 1985.) 80 The dilator muscles of the pharynx are located anteriorly and laterally. The best-studied and probably most important pharyngeal dilator is the genioglossus, a fan-shaped muscle that originates from the anterior mandible and spreads out to insert into the 60 tongue. Although no studies have conclusively established what TA Peak Height (U) muscles are responsible for pulling the base of the tongue forward to dilate the airway, evidence strongly supports the role of the genioglossus. In animal experiments, increased EMG activity of 40 the genioglossus is associated with a greater capacity of the pharynx to withstand negative collapsing pressure.33 Genioglossus EMG activity increases reflexively in response to negative upper airway 20 pressure (Fig. 53.10).19 In humans with obstructive sleep apnea, decreased genioglossus EMG activity has been noted during obstructive events, whereas obstruction is relieved with recovery of genioglossus activity.34 0 The hyoid bone supports the hypopharynx. In humans and in 1.5 2.0 2.5 3.0 3.5 other primates, the hyoid does not articulate with any other skeletal Expiratory Time (sec) element; rather, it is suspended by muscles and ligaments. Contrac- tion of muscles attached to the hyoid has been shown in animal Fig. 53.9 The peak electromyography activity of the thyroarytenoid experiments to increase the size and stability of the upper airway.35 muscle (TA peak height) as a function of expiratory time in an awake Muscles attached to the hyoid are also believed to resist the human. Correlation coefficient = 0.680. (From Kuna ST, Insalaco G, downward traction exerted on the airway by the trachea during Woodson GE: Thyroarytenoid muscle activity during wakefulness and inspiration.36 sleep in normal adults, J Appl Physiol 63:1332, 1988.) FUNCTION IN SPEECH some active tone in these muscles increases their stiffness and, Human speech requires coordinated interaction of the mouth, hence, decreases the tendency of the pharynx to collapse with the pharynx, larynx, lungs, diaphragm, and abdominal and neck muscles. negative pressure generated during inspiration. Conversely, flac- The three fundamental components in the process are phonation, cidity of the constrictors is considered to destabilize the airway, resonance, and articulation: phonation is the generation of sound and thus it promotes collapse, although no physical data are available by vibration of the vocal folds, resonance is the induction of vibration to support this concept.5 On the contrary, contraction of the in the rest of the vocal tract to modulate laryngeal output, and constrictor muscles actively collapses the pharyngeal lumen. articulation is the shaping of the voice into words. Spontaneous activity in the respiratory cycle has been detected in the superior constrictor muscle, but this activity occurs during the expiratory phase and disappears with bronchoconstriction, Phonation which suggests that it plays a role in modulating expiratory airflow The role of the larynx in sound production has been recognized for resistance.32 centuries,37 although the mechanism of how the larynx generates Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 805 10 mVc laterally, which produces a sudden decrease in subglottic pressure. GG EMG The forces that contribute to the return of the vocal folds to the 53 midline include this pressure decrease, elastic forces in the vocal fold, and the Bernoulli effect on airflow. When the vocal folds return to the midline, pressure in the trachea builds again, and the cycle is repeated. Vocal fold structure determines whether the resulting vibration is periodic or chaotic. GG EMG Actual phonation is more complex than the previous model’s description, because the vocal fold is not a homogeneous structure I Vc and it vibrates in three dimensions.40 Moreover, the pattern of DIA EMG vibration varies with pitch and vocal register. The “body-cover” concept of phonation is that vibration of the mucosa does not correspond directly to that of the rest of the vocal fold.41 Instead, DIA EMG the “body” of the vocal fold is relatively static, whereas the wave is propagated in the mucosal “cover.” This mucosal wave begins on the inferomedial aspect of the vocal fold and moves rostrally Tracheal 10 (Fig. 53.11). As the superior edges of the vocal fold begin to separate, pressure 0 the lower edges close, and this temporal relationship is accounted (cm H2O) for by the two-mass model proposed by Ishizaka and Flanagan.42 10 As the superior edges of the vocal folds separate, airflow through the divergent glottis generates greater negative pressure at the lower edge of the vocal folds, which accelerates closure of the 1 sec inferior glottis. The body-cover theory and the two-mass model Fig. 53.10 The diaphragm and genioglossus (GG) muscle responses are consistent with most of the observed motion during modal to nasal occlusion (beginning at arrow) in an anesthetized phonation (e.g., chest register the middle range of pitch), although vagotomized rabbit. The upper trace is raw electromyographic (EMG) the mucosal wave decreases at higher pitches and is not visible activity of the genioglossus in millivolts; the second trace is integrated during falsetto, which suggests that the motions of the mucosa genioglossus activity in volts. The third and fourth traces are raw and the underlying tissue become coupled. In this mode, elastic and integrated EMGs of the diaphragm (DIA). (From Mathew OP, recoil, rather than the Bernoulli effect, is the primary force that Abu-Osba YK, Thach BT: Influence of upper airway pressure changes drives the closing phase of phonation; the closing phase is much in respiratory frequency, J Appl Physiol 52:483, 1982.) shorter, and only the superior edges of the vocal folds make contact. The vibratory characteristics of falsetto have been attributed to increased tension and decreased thickness of the vocal fold. During phonation at low pitches, the vocalis muscle is relaxed so that the BOX 53.1 Five Requirements for Phonation “body” of the fold participates in oscillation. 1. Adequate breath support 2. Approximation of vocal folds Expiratory Force 3. Favorable vibratory properties The force available to drive phonation depends on the volume of 4. Favorable vocal fold shape air in the lungs, the elastic recoil in the chest wall and diaphragm, 5. Control of length and tension and the strength in abdominal and intercostal muscles. Normally, passive expiration is adequate to power conversational speech. Shouting and singing require deeper prephonatory inspiration for larger lung volume and for active expiratory effort. Because sound from exhaled air was not clear until the mid-20th century. the amount of breath support required for normal voice use is In 1950, Husson38 presented the neurochronaxic hypothesis, which small compared with lung capacity, voice loss generally is not a held that glottic vibrations were caused by rhythmic impulses in presenting complaint of pulmonary disorders. Breath support the nerves to the larynx, synchronous with the frequency of the available for phonation becomes a clinical issue in two situations. sound produced, so that each vibratory cycle was caused by a First, in patients with functional dysphonia, insufficient prephona- separate neural impulse—a physiologically impossible hypothesis. tory inspiration requires excessive glottic pressure to produce an In the 1950s, van den Berg39 used high-speed motion pictures acceptable volume, which may lead to stress-induced injury of the to document the motion of the vocal folds during vibration and vocal folds. Second, in a patient with an organic voice disorder subsequently reported his theory of the mechanism of phonation; and impaired pulmonary function, the capacity to compensate for now widely accepted, the myoelastic-aerodynamic theory holds the glottic defect is limited. For example, a patient with laryngeal that the interaction of aerodynamic forces and the mechanical paralysis is more symptomatic with coexisting emphysema. An properties of the laryngeal tissues are responsible for inducing important component of vocal training is instruction on control vocal fold vibration and generating vocal sound. of breathing to maximize the power of vocal output. Normal phonation requires that five conditions be satisfied; these are listed in Box 53.1. Breath support should be adequate to provide power, and the vibratory edges of the vocal folds should Vocal Fold Positioning be aligned and separated by an appropriately small gap. The physical Phonation requires a critical relationship between the gap between properties of the vocal fold should be conducive to vibration, and the medial surface edges of the vocal folds and the expiratory its 3D contour should be favorable. Finally, a normal voice requires airflow. The folds should be close enough together so that airflow volitional control of glottic length, tension, and shape. entrains oscillations; if the gap is too wide, the voice is breathy The process of phonation begins with the inhalation of air, or aphonic, with only turbulent airflow noise and no periodic and then glottic closure positions the vocal folds near the midline. sound. The gap may be wider if airflow is greater; conversely, low A simplified explanation of phonation is that exhalation causes airflow requires a narrower gap. If the vocal folds are too tightly subglottic pressure to increase until the vocal folds are displaced apposed, excessive pressure is required, and phonation sounds Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 806 PART V Laryngology and Bronchoesophagology undulates freely over the underlying vocal ligament and vocalis muscle. Hirano’s43 important histologic studies showed that this 1 undulation is possible because the mucosa and muscle are separated by a specialized layer of connective tissue that serves as a shock absorber. This highly specialized tissue is characterized by stratified concentrations of elastin and collagen. The most superficial layer, 2 also known as the Reinke space, is made up of loosely connected fibers of collagen and elastin; the intermediate layer is composed predominantly of elastic fibers, and the deep layer is constructed of densely arranged collagen fibers. Together, the intermediate and deep layers form the vocal ligament. 3 Vocal Fold Shape In falsetto mode, only the superior edges of the vocal folds make 4 contact during the closing phase, although during modal phonation, which is more efficient, the mucosal wave begins on the inferior surface of the vocal fold. This requires a favorable configuration of the glottis in the coronal plane, with the medial surfaces of the vocal folds nearly parallel. If the vocalis muscle is atrophic or 5 paralyzed, the medial surface of the vocal fold is convex, and the glottal tract is too convergent for optimal phonation. A divergent glottis is also unfavorable. 6 Pitch Control Upper Upper Changes in vocal fold length and tension are used to control the lip lip fundamental frequency of vocal fold vibration to produce dynamic inflections of the voice. Such adjustments involve fine motor control. 7 Lower In the lower vocal range, contraction of the TA results in a lowering lip Lower lip of pitch, because it decreases tension in the vocal cover. During contraction of the CT muscle, which increases length and tension, strengthening TA contraction results in rising pitch. Contraction 8 of the CT muscle in the absence of TA activity is generally accepted to be the mechanism of falsetto. The size and physical properties of a larynx determine the range of pitch that can be produced. A child has a smaller larynx and, thus, a higher pitch range than an adult. During puberty in 9 boys, the rapid increase in size of the larynx results in unstable pitch control, until adaptation to the new anatomy occurs. Size is not the only determinant of pitch range, because age-related loss of elasticity and increasing ossification of the thyroid lamina result in an elevation of pitch. The lowest pitches are produced 10 by young men, whose vocal folds are longer and heavier than those of women and more compliant than those of older men. 5 mm Fig. 53.11 Movements of different portions of vocal folds during one Resonance cycle of vibration shown schematically in the coronal plane (left) and from above (right). Mucosal upheaval begins caudally (1) and then The raw sound produced by the glottis in isolation from the rest moves rostrally. The lower portion is closing as the upper margin is of the vocal tract does not sound like human voice but is harsh opening (5). (From Hirano M: Clinical examination of voice, New York, and sounds something like a goose call. Phonated sound acquires 1981, Springer-Verlag.) the characteristics of a human voice through the resonance of the chest, upper airway, and skull. Resonance is the prolongation, amplification, and filtering of sound by the induction of sympathetic vibration; the vocal frequencies enhanced by resonance are termed formants. The pharynx itself does not resonate, because its walls strained or might not even be possible. An analogy to phonation are too compliant to support sympathetic vibration; the primary is the sound generated by air released from a toy balloon. The resonating structure is actually the air column contained in the pitch and volume may be varied by adjustments in the tension pharynx. The length of the vocal tract and the locations of across the neck of the balloon. The sound is louder with more constricted segments confer characteristic resonant frequencies air in the balloon. If pressure on the neck is sufficient to interrupt (formants), which are excited by any sound fed into the tract, airflow, sound ceases. With decreasing closing pressure, sound whether from the glottis or from an artificial larynx. A speaker becomes increasingly turbulent. controls resonance of the voice by altering the shape and volume of the pharynx, by raising or lowering the larynx, by moving tongue or jaw position, or by varying the amount of sound transmis- Vibratory Capacity of the Vocal Folds sion through the nasopharynx and nose. Vocal training for singing, The physical properties of the vocal folds are crucial in determining acting, or public speaking concentrates heavily on refining and vocal function. During normal modal phonation, the mucosa maximizing resonance. The goal is to produce the loudest and Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 807 most pleasing sound possible with minimal strain or pressure on of the use of nonauditory cues for feedback. An example of the the larynx. importance of nonauditory cues in voice control is the ability of 53 well-trained singers to perform with good control of pitch and loudness, even when they cannot hear their own voices. Tactile Articulation sensations of induced vibration in the face, throat, and chest are The source-filter hypothesis of speech states that the larynx is the important cues. Many sensory receptors are found in the larynx source of a constant sound, which is shaped into words by the that respond to such cues as air pressure and flow and joint motion, upper vocal tract. In this generally accepted model, consonants but the degree to which laryngeal sensation influences vocal control and vowels are formed by the action of the lips, tongue, palate, is not known. and pharynx. Participation of the larynx during articulation is generally considered to be limited to the onset and offset of phonation, coordinating with upper articulators to produce voiced Central Control of Speech and unvoiced sounds. In computerized simulation, this model The motor neurons of all intrinsic laryngeal muscles are located seems to account for speech fairly well, although evidence now in the nucleus ambiguus (NA), in a clear rostrocaudal organization. suggests that the position and shape of the glottis may actually CT muscles are most rostral, while LCA neurons are the most vary with production of different vowels, so that the contribution caudal.45 Of note, PCA and CT neurons are significantly smaller of the larynx to phonation may be more complex than previously than adductor muscle neurons.46 The somatotopic organization recognized.44 of laryngeal motor neurons is significantly altered after RLN injury, even when the nerve is only crushed, rather than transected, which may explain why normal laryngeal motor function is rarely Sensory Input to Speech Control completely recovered after peripheral nerve injury.47 One obvious mechanism for controlling phonatory output during The human brain has separate pathways for the control of speech is auditory feedback. This sensory input is most important spontaneous and learned vocalization.48 Learned behaviors, such when a person is learning to speak and is not essential for everyday as speaking and singing, are controlled by direct corticobulbar use. Prelingually deaf people never develop completely normal- projections to the nucleus ambiguus, pathways that have not been sounding speech, although those who become deaf after language found in other animals. acquisition can maintain fairly normal speech patterns, in part because of ballistic speech production but probably also because For a complete list of references, visit ExpertConsult.com. Downloaded for Andrea Trigueros ([email protected]) at Francisco Marroquín University from ClinicalKey.com by Elsevier on December 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  CHAPTER 53 Laryngeal and Pharyngeal Function 807.e1 REFERENCES 24. 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