arrow arrow arrow Airway Physiology and Reflexes

Airway Physiology and Reflexes
Kelly D. Sweeney, M.D.Byron J. Bailey, M.D. Francis B. Quinn, Jr., M.D.   

I. Anatomy of the Larynx - consists of a framework of cartilages held in position by an intrinsic and an extrinsic musculature. lined by mucous membrane arranged in characteristic folds

. lies in front of the fourth, fifth, and sixth cervical vertebrae. upper portion is continuous with the pharynx and is triangular in shape, while the lower portion is continuous with the trachea and in circular.

A. Skeletal Framework

1. Hyoid Bone

a. serves as an attachment for the thyroid cartilage via the thyrohyoid membrane and the extrinsic muscles of the larynx.
b. U-shaped bone consisting of the body and paired greater and lesser horns.
c. embryologically- derived from mesodermal cells that form cartilaginous masses in the second and third branchial arches (lesser horn- second arch, greater horn and body- third arch).

2. Thyroid Cartilage

a. largest of the laryngeal cartilages.
b. shield-shaped and encloses the larynx anteriorly and laterally.
c. composed of two alae which are fused anteriorly in the midline after birth (angle of 90' in men, 120' in women).
d. superior cornu extend posteriorly and laterally from each wing and attach to the hyoid bone by means of the thyrohyoid ligament.
e. inferior cornu articulate with the cricoid cartilage forming the cricothyroid joint, a synovial joint which alters the angle between the thyroid and cricoid cartilages.

3. Cricoid Cartilage

a. only complete cartilaginous ring in the airway and is the strongest of the laryngeal cartilages.
b. lies directly below the thyroid cartilage at the C6- C7 level in adults and C3-C4 level in children.
c. shaped like a signet-ring with a broad arch posteriorly, which extends upward to form the posterior border of the larynx, tapering anteriorly to a narrow arch.
d. composed of hyaline cartilage, but may become partially ossified later in life and can be fractured in blunt laryngeal trauma.

4. Epiglottis

a. leaf-shaped structure composed of fibroelastic cartilage.
b. petiole-inferior stem of the cartilage which is attached to the posterior surface of the angle of the thyroid cartilage by the thyroepiglottic ligament.
c. anterior midportion is attached to the tongue by three mucosal folds: median glossoepiglottic fold, and paired lateral glossoepiglottic folds.
d. lingual valleculae- paired fossae formed by these folds between the base of the tongue and the anterior surface of the epiglottis.
e. mucosal lining of the laryngeal surface contains abundant mucosal glands which pit the surface of the cartilage and can serve as a potential route of spread of tumors from the laryngeal epiglottis to the pre- epiglottic space.
f. aryepiglottic folds- formed from the mucosal continuation from the epiglottis to the surface of the arytenoids.

5. Arytenoid Cartilages

a. small, paired, pyramid-shaped structures which are primarily responsible for the opening and closing of the larynx.
b. articulate with facets on the superior surface of the posterior arch of the cricoid cartilage at the cricoarytenoid joint.
c. vocal process- anterior projection of each arytenoid which received the attachment of the posterior, mobile end of each vocal cord.
d. muscular process-lateral prominence which contains the attachment for the posterior and lateral cricoarytenoid muscles.
e. Aryepiglottic fold attaches to the apex of the pyramid with the vestibular ligament and fold inserting inferior to this attachment.
f. arytenoid movement results in rapid and diverse changes in aryepiglottic, vestibular, and vocal folds.

6. Corniculate Cartilages

a. tiny, paired fibroelastic cartilages, also known as the cartilages of Santorini, which lie in the aryepiglottic folds and articulate with the medially directed apex of the arytenoids.
b. may provide a stiffening and elastic recoil function for the posterior A-E folds.

7. Cuneiform Cartilages

a. tiny, paired, elongated pieces of yellow elastic cartilage, also known as the cartilages of Wrisberg, which lie above the corniculates in the A-E folds.
b. not functionally significant.

8. Triticeous Cartilages

a. small, elastic cartilage in the lateral thyrohyoid ligament.
b. when calcified can be mistaken for a foreign body on soft tissue X-ray films.

B. Laryngeal Ligaments and Membranes

1. Extrinsic Ligaments- bind the cartilages to the adjoining structures and to one another, and round out the laryngeal framework.

a. Thyrohyoid Membrane- attaches the thyroid cartilage to the hyoid bone. The thickened median portion is known as the median thyrohyoid ligament and is pierced on either side by the superior laryngeal vessels and the internal branch of the superior laryngeal nerve. Lateral thyrohyoid ligament forms the thickened posterior border of the thyrohyoid membrane on each side and connects the tips of the superior cornua of the thyroid cartilage to the posterior and greater cornua of the hyoid bone.
b. Cricothyroid Membrane- connects the cricoid and thyroid cartilages and is the site of entrance for an emergency cricothyroidotomy.
c. Cricotracheal Ligament- attaches the cricoid to the first tracheal ring.
d. Hyoepiglottic Ligament- forms the roof of the preepiglottic space and floor of the vallecula.

2. Intrinsic Ligaments- unite the cartilages of the larynx and perform and important role in closure of this organ. The elastic membrane is the fibrous framework of the larynx which lies beneath the mucosa and is divided into upper and loser parts by the ventricle.

a. Quadrangular Membrane- upper part of the elastic membrane of the larynx, extending from the lateral margin of the epiglottis to the arytenoid and corniculate cartilages, and inferiorly to form the vestibular ligament (false cord). Forms part of the wall between the upper pyriform sinus and the laryngeal vestibule.
b. Conus Elasticus- lower part of the elastic membrane which is attached to: inferiorly- superior border of the cricoid cartilage; superoanteriorly- deep surface of the angle of the thyroid cartilage; superoposteriorly- vocal process of the arytenoid. The free upper edge forms the vocal ligament (true vocal cords).

C. Cavity of the Larynx -
Extends from the laryngeal inlet of the pharynx to the lower border of the cricoid cartilage. -Divided into three parts- vestibule, ventricle, and subglottic space- by the false cords and the true cords.

1. Vestibule

a. lies between the inlet and the edges of the false cords.
b. anterior border- posterior surface of the epiglottis.
c. posterior border- interval between the arytenoids.
d. lateral border- inner surface of the A-E folds and upper surfaces of the false cord.

2. Ventricle (of Morgagni)

a. deep, spindle-shaped recess between the false and true cords.
b. lined by a mucous membrane that is covered externally by the thyroarytenoid muscle.
c. Saccule: conical pouch that ascends from the anterior part of the ventricle and lies between the inner surface of the thyroid cartilage and the false cord. Numerous mucous glands open onto the surface of its lining mucosa for lubricating the vocal cords.
d. Laryngocele-abnormal development and penetration of the saccule into the thyrohyoid membrane forming and air sac.

3. Glottis (rima glottidis)

a. space between the free margin of the true vocal cords.
b. wide and triangular when the vocal cords are abducted (respiration), but assumes a slit-like appearance during adduction of the cords (phonation).
c. posterior glottic chink in adults is 18-19mm, in the newborn is 4mm.

4. Subglottic Space -between the true vocal cords and the lower border of the cricoid cartilage.

5. Preepiglottic Space

a. wedge-shaped space lying in front of the epiglottis.
b. boundaries: anterior- thyrohyoid membrane, anterosuperior- hyoid bone, superior- vallecula, posterior- epiglottis, lateral- hyoepiglottic ligament, inferior- petiole of the epiglottis.
c. filled with fat and loose areolar tissue and is a common site of invasion of epiglottic tumors through the small glandular perforations in the cartilage.

6. Pyriform Fossa

a. small recesses on either side of the larynx.
b. bounded medially by A-E fold and laterally by the thyroid cartilage and thyrohyoid membrane.
c. branches of the internal laryngeal nerve and the communicating sensory branch of the RLN lie within the mucous membrane of this fossa.
d. considered part of the hypopharynx.

7. Paraglottic Space -bounded anterolaterally by the inner perichondrium of the thyroid, medially by the conus elasticus, quadrangular membrane, and ventricle, and posteriorly by reflection of the pyriform mucosa.

D. Laryngeal Musculature

1. Extrinsic Muscles

a. Elevators of the larynx (suprahyoid group): digastric, geniohyoid, stylohyoid, mylohyoid, and stylopharyngeus. Innervated by the ansa hypoglossi (C1-C3) and CN V, VII, and IX.
b. Depressors (infrahyoid group): sternothyroid, sternohyoid, omohyoid, and thyrohyoid. Innervated by the ansa hypoglossi.

*The action of these groups of muscles determines the position of the larynx in the upper aerodigestive tract. There is a slight upward movement of the larynx during the normal expiratory cycle and an upward and forward motion of the larynx toward the base of tongue during swallowing which results in passive bending and displacement of the epiglottis which closes off the laryngeal vestibule.*

2. Intrinsic Muscles

*These are directly concerned with phonation and the protective functions of the larynx and consist of four paired muscles and one unpaired muscle which act on the cricothyroid and cricoarytenoid.*

a. Cricothyroid -arises from the arch of the cricoid anterior and laterally and inserts in the inferior cornu and body of the thyroid cartilage. -innervated by the external branch of the superior laryngeal nerve. -action: elongation and tension of the cords. -contraction of this muscle in expiration results in vocal elongation which increases glottic size, reduces airway resistance, and shortens expiratory duration.
b. Thyroarytenoid (Vocalis Muscle) -arises from the inner aspect of the thyroid angle anteriorly and inserts into the anterolateral surface of the vocal process of the arytenoid cartilage. -consists of three parts: externus- draws the arytenoid forward and adducts the vocal fold by rotating the arytenoid toward the midline; ventricle, and posteriorly by reflection of ventricle, and posteriorly by reflection of ventricle, and posteriorly by reflection of ventricle, and posteriorly by reflection of the pyriform mucosa.

D. Laryngeal Musculature

1. Extrinsic Muscles

a. Elevators of the larynx (suprahyoid group): digastric, geniohyoid, stylohyoid, mylohyoid, and stylopharyngeus. Innervated by the ansa hypoglossi (C1-C3) and CN V, VII, and IX.
b. Depressors (infrahyoid group): internus- foreshortens the vocal fold and created a variable tension on the fold, varying the vocal frequency (pitch) produced by the larynx; thyroepiglotticus- originates with the external thyroarytenoid muscle anteriorly and courses posteriorly and superiorly to insert into the A-E fold and the margin of the epiglottis and functions to widen the laryngeal entrance by action on the A-E fold.
c. Posterior Cricoarytenoid -arises from the posterior surface of the cricoid cartilage. -apex of the fibers insert into the muscular process of the arytenoid. -action: lateral rotation of the arytenoid and abduction of the vocal cords (main abductor).
d. Lateral Cricoarytenoid -smaller, band-like muscle which arises from the anterior aspect of the cricoid arch, passes obliquely upward and posteriorly, and inserts into the anterior portion of the muscular process of the arytenoid. -action: medial rotation of the arytenoid and adduction of the vocal folds.
e. Interarytenoid (unpaired) -consists of transverse and oblique fibers which arise from the posterior and lateral surface of the arytenoid, crossing in a horizontal manner to the opposite arytenoid cartilage. -innervated bilaterally by the recurrent laryngeal nerve, so not affected by unilateral RLN disease. -action: transverse fibers- closure of the posterior glottis and adduction of vocal fold; oblique fibers- adduction of the arytenoid and slight rotational abduction of the vocal fold.

E. Laryngeal Mucosa

1. Continuous with the mucosa of the mouth and pharynx and the trachea.
2. Consists of stratified squamous epithelium over the true vocal cord, superior A-E fold, upper half of the posterior surface and anterior surface of the epiglottis, and pseudostratified ciliated columnar epithelium over the remainder of the cavity.
3. Mucous glands are found in the ventricles and the sacculi, posterior surface of the epiglottis, and the margins of the A-E fold.
4. Mucosa is attached loosely to the anterior surface of the epiglottis and in the vallecula, but is firmly attached to the posterior surface of the epiglottic and the laryngeal surface of the cuneiform and the arytenoid cartilages.
5. The TVC have a very thin mucosal layer with no mucous glands. Therefore, mucus secreted superiorly must flow downward to cover the surface of the cords.

F. Innervation of the Larynx-CN X (Vagus)

1.Superior Laryngeal Nerve

a. divides into an internal (sensory) branch and an external (motor and sensory) branch.
b. internal branch supplies sensation to the false vocal cord, pyriform sinus, and the epiglottis.
c. external branch provides motor innervation to the cricothyroid muscle and sensory innervation to the anterior infraglottic larynx at the level of the cricothyroid membrane.

2. Recurrent Laryngeal Nerve

a. supplies motor innervation to all the intrinsic muscles of the larynx on the same side except for cricothyroid, and to the interarytenoid muscle of both sides.
b. supplies sensory innervation to the laryngeal mucosa from the glottis and below.
c. left RLN has a much longer course, turns around the aortic arch, and is much more likely to be injured.
d. right RLN turns around the subclavian artery.

G. Laryngeal Blood Supply

1. Superior Laryngeal Artery

a. branch of the superior thyroid artery which is a branch off of the external carotid artery.
b. enters the larynx through the thyrohyoid membrane along with the superior laryngeal nerve.
c. runs downward beneath the mucosa of the lateral wall and floor of the pyriform fossa to supply the muscles and mucous membranes in the superior portion of the larynx.
d. anastomoses freely with the contralateral and inferior blood supply.

2. Inferior Laryngeal Artery

a. branch of the thyrocervical trunk.
b. ascends in the tracheoesophageal groove along with the recurrent laryngeal nerve and in its course crosses anterior to the ansa cervicalis and middle cervical sympathetic ganglion.
c. enters the larynx below the inferior margin of the inferior constrictor muscle.
d. supplies the muscles attaching to the arytenoid cartilages as well as the laryngeal mucosa which is adjacent to the false vocal cord and the ventricle.

3. Cricothyroid Artery

a. small branch of the superior thyroid artery.
b. pierces the cricothyroid membrane to anastomose with the superior laryngeal artery.

H. Lymphatic Drainage of the Larynx

*extensive- with the exception of the surface of TVC*

1. Supraglottic

a. extensive lymphatic network.
b. channels collect in a pedicle at the anterior end of the A-E fold, pass laterally anterior to the anterior wall of the pyriform fossa and leave the larynx with the neurovascular bundle through the thyrohyoid membrane to end in the deep cervical nodes and superior and middle jugular group.

2. Infraglottic a.more variable.

a. anterior group- pass through the cricothyroid membrane, then many vessels end in prelaryngeal (delphian) nodes in the region of the thyroid isthmus. Channels then leave these nodes with the remaining anterior channels and travel to the deep cervical lymph nodes.
b. postero-lateral group- exit through the cricotracheal membrane to paratracheal nodes or infrajugular chain.
c. these lymphatics exhibit a greater potential for contralateral spread because of cross-over of channels in the cricoid cartilage area and around the cricothyroid membrane.

I. Laryngeal Joints

1.Cricothyroid Joint

a. synovial joint with a capsular ligament between the inferior cornu of the thyroid cartilage and the facet on the cricoid cartilage at the junction of the arch and the lamina.
b. Cricoid rotates on the inferior horns of the thyroid cartilage about an axis passing transversely through the joints; some gliding movement of the cricoid can also occur in different directions.

2. Cricoarytenoid Joint

a. synovial joint with a capsular ligament between the base of the arytenoid cartilage and the facet on the upper border of the lamina of the cricoid cartilage.
b. movements include gliding motion for abduction and adduction, ventral and dorsal inclining (tilting), and verticle axis rotation.

*Through contraction and relaxation of the intrinsic laryngeal musculature, the arytenoids with the true vocal folds attached can make s sweeping motions in a medial and lateral direction to facilitate protection of the airway, phonation, and respiration.*

II. Laryngeal Development

a. The infantile larynx is softer, more pliable, and proportionally smaller in relation to the size of other structures and lies in a relatively higher position in the neck than its adult counterpart.
b. Newborns exhibit a nasolaryngeal connection by approximation of the epiglottis against the soft palate.
c. This position insures against aspiration by forming a continuous upper and lower airway (a more efficient respiratory organ than the adult position) and helps to explain why newborns are obligate nasal breathers for the first 6 months of life.
d. During development, descent of the larynx causes separation of the soft palate and larynx. This results in greater vocal power and diversity in articulation, but makes the process of swallowing more complex.
e. In adults, the epiglottis serves a minor protective role during swallowing by acting as a laryngeal shield over the laryngeal inlet and directing food laterally into the pyriform fossa and away from the midline laryngeal aperture. This protective function is enhanced by the elevation of the larynx toward the nasal cavity during the height of swallowing. It also provides a mass of tissue, the petiole, against which the ventricular bands contract to form the supraglottic sphincter and forming valleculae on its lingual surface where liquids and food can collect after the initial swallow.

III. Laryngeal Physiology

* The three basic functions of the larynx in order of importance are: protection of the airway, respiration, and phonation.*

A. Protection

*The primary function of the larynx is its use as a sphincter protecting the lower airway from the entrance of liquid and food during swallowing, vomiting, and coughing.*

1.Tri-Sphincteric Mechanism

*refers to three protective tiers within the larynx: the A-E fold, false vocal cords, and the true vocal cords arranged in a unique shelf-like configuration.*

a. A-E fold: not a true valvular closure, but simple muscular closure for its effect. -closes during swallowing, diverting liquid and food away from the respiratory tract and into the pyriforms and esophagus. -because of the high position of these folds at the inlet of the larynx, closure at this level serves an important protective function in guarding against the entrance of ingested foreign bodies and vomitus during retching and gagging.
b. False Vocal Cord: -function as exit valves to prevent the egress of air from the trachea (expectorative function). -in the closed position, they seal together even more tightly when pressure from below increases. -this feature of adducted FVC occurs independently of muscle tone due to their unique shape and configuration (down-turned direction of their free margins). -expectorative functions of the larynx remain unimpaired in bilateral laryngeal paralysis. -passive closure of the false cord alone appears essential to effective cough production.
c. True Vocal Cords: -primary role is protection of the airway (most significant of the tiers against aspiration). -exhibit upturned margins and are capable of impeding the ingress of air by resisting pressure from above (protective function). -this valvular action is implicated in the trouble experienced in overcoming laryngeal spasm by abrupt pressure peaks of positive pressure ventilation that only serve to further seal the glottis.

*Valsalva Maneuver- -tight closure of both sets of folds allows the larynx to resist very strong expiratory forces resulting in high intrathoracic pressures. -this maneuver is important in defecation because the pressure is transmitted to the abdominal cavity and also serves to stabilize the thorax during heavy lifting of the arms.*

2. Neuromuscular Physiology

a. Afferent System -the density of sensory innervation is greatest at the laryngeal inlet, especially the laryngeal surface of the epiglottis consistent with it's protective function of the distal respiratory tract. The posterior half of the true cord has more touch receptors than the anterior portion. -the internal branch of the superior laryngeal nerve innervates the ipsilateral larynx from the superior boundary to the level of the true cords. -the external branch of the superior laryngeal nerve innervates a diamond shaped area of mucosa inside the cricothyroid membrane and carries afferent fibers from deep receptors located in the connective tissue and in the capsule of the cricothyroid joint. -the recurrent laryngeal nerve innervates ipsilaterally below the true cords. -these afferent impulses are delivered through the ganglion nodosum to the tractus solitarius in the brain stem.
b. Efferent System -motor distribution to the intrinsic laryngeal musculature originates in the medullary nucleus ambiguous. Each RLN innervates all muscles except for the cricothyroid, which is innervated by the SLN. The interarytenoid receives motor innervation from both RLN's. -the sole abductor of the larynx is the posterior cricoarytenoid. -the major laryngeal adductors are the lateral cricoarytenoid and the thyroarytenoid. -the cricothyroid muscle adducts and tenses the vocal cords. -the interarytenoid muscles close the posterior gap in the glottis. -paralysis of the SLN leads to denervation of the ipsilateral cricothyroid muscle, resulting in rotation of the posterior commissure toward the intact side from unopposed contraction of the contralateral cricothyroid. -RLN injury results in a paramedian vocal cord position because of the adductor action of the intact SLN contracting the ipsilateral cricothyroid. -stimulation of one SLN does not produce simultaneous activation of the contralateral adductor musculature (no crossed adductor reflex). Therefore, unilateral SLN paralysis may lead to aspiration (failure of ipsilateral cord closure) even with intact RLN's. -bilateral SLN stimulation results in protective adduction of the three muscular tiers within the trisphincteric mechanism.

3. Glottic Closure Reflex

a. reflex laryngeal closure is produced by rapid contraction of the thyroarytenoid muscle in response to SLN stimulation.
b. the thyroarytenoid is one of the fastest striated muscle, approaching the speed of contraction of the medial rectus muscle of the eye.
c. Sensory stimuli other than those classically elicited by direct SLN stimulation are also capable of eliciting this reflex. Examples of these include stimulation of all major cranial afferent nerves and other special sensory and spinal somatic nerves.
d. the susceptibility of this reflex response to such diverse sensory excitation is unique and emphasizes its primitive role in respiratory protection of the organs from a wide variety of noxious stimuli.

4. Laryngospasm

a. physiologic exaggeration of the glottic closure reflex- forceful and prolonged closure of the larynx.
b. maintained well beyond the cessation of mucosal irritation.
c. most likely to occur when the patient is in the OR undergoing endotracheal intubation and is very well oxygenated and under very light anesthesia.
d. the obstructive apnea produced may cause death by acute hypoxia and hypercapnia.
e. the body's fail-safe mechanism for dealing with this phenomenon is that laryngospasm is inhibited by: increased arterial pCO2, decreased arterial pO2, positive intrathoracic pressure, and the inspiratory phase of respiration.
f. the most common causes are inhaled irritants, manipulation of the upper aerodigestive tract, foreign bodies, mucus, or blood in the glottic chink.

5. Cough Reflex

a. cough ejects mucus and foreign matter from the lungs and helps maintain patency of the pulmonary alveoli.
b. may be voluntary, but more often in response to stimulation of receptors in the larynx or lower respiratory tract.
c. three phases: inspiratory- larynx opens wide to permit rapid and deep inspiration; compressive- tight closure of the glottis and strong activation of expiratory muscles; expulsive- larynx opens widely and a sudden outflow of air in the range of 610 liters/sec.

6. Apnea Reflex

a. chemical and thermal sensors appear limited to the supraglottic larynx, and apnea may occur in response to diverse chemical and noxious stimuli (cigarette smoke, ammonia, etc.).
b. this reflex of apnea in response to laryngeal stimulation probably evolved because it prevents aspiration of the stimulating material into the lower airway.
c. water is notorious for initiating this reflex especially in infants in whom the reflex is much more prominent.
d. in normal conscious adults, the response to water in the larynx is forceful coughing. However, in infants, this can result in apnea.
e. laryngeal reflexes have been implicated in the pathogenesis of sudden infant death syndrome (SIDS) since respiratory reflex pathways go through a process of maturation in infancy that make laryngospasm and reflex apnea much more likely to occur during the interval of peak incidence of SIDS.
f. this reflex can be used in a therapeutic manner in the treatment of croup in which inhalation of aerosolized water droplets can result in respiratory slowing with concomitant increase in tidal volume- a beneficial effect in partial upper airway obstruction.

7. Circulatory Reflexes

a. stimulation of the larynx can produce changes in heart rate and blood pressure.
b. this effect is most evident during induction of anesthesia and can occur in natural circumstances such as obstructive sleep apnea.
c. the direct result of laryngeal stimulation on blood pressure is hypertension, but if laryngeal stimulation produces significant bradycardia by vagal stimulation, hypotension can indirectly occur.
d. afferent limb for these pathways is the SLN and transection of this nerve abolishes cardiovascular responses to laryngeal stimulation and elective SLN stimulation affects BP and HR.
e. the effect of SLN stimulation is modulated by blood levels of CO2 since no suppression is observed during hypocapnia.

B. Respiratory Function

1. CNS control of respiratory movement of the larynx is of a dual nature: medullary centers control involuntary respiratory movements, and higher cortical centers control voluntary respiratory movements.
2. Each of these centers can override the other if necessary; a person can breath hold voluntarily but as the CO2 accumulates, the medullary center overrides and the cords abduct as a result of a coordinated involuntary process.
3. Widening of the glottis occurs with rhythmic bursts of activity in the RLN. The glottis opens a fraction of a second before air is drawn in by the descent of the diaphragm.
4. EMG studies show that phasic inspiratory abduction, via muscular contraction of the posterior cricoarytenoids, is synchronous with respiration. The degree of abductor activity varies directly with the degree of ventilatory resistance (i.e. decreases with tracheotomy).
5. Phasic inspiratory contraction of the cricothyroid muscle (vocal cord adductor and isotonic tensor) increases the A-P diameter of the glottic chink.
6. The lungs and larynx influence each other through a highly refined neurologic feedback mechanism. the larynx can act as a breaking system during expiration by its ability to instantaneously alter the area of the glottic aperture.
7. Variations in respiratory rate results primarily from changing the duration of the expiratory phase rather than the inspiratory phase. These reflex changes are triggered by pressure receptors in the lungs, subglottic trachea, and larynx.
8. The receptors in the more peripheral part of the lung primarily influence glottic aperture during respiration (inspiratory contractions of the PCA can be abolished by inflating the lung through a tracheotomy canula).

C. Phonation

1. The third and least important function of the larynx which is best observed in mammals and appears to be a late phylogenetic acquisition.
2. The human voice results from the coordinated interaction of the larynx, lungs, diaphragm, abdominal muscles, throat, neck, lips, tongue, cheeks, and soft palate.
3. Speech results from the production of a fundamental tone produced at the larynx and modified by resonating chambers in the upper aerodigestive tract. The vocal cords are positioned near the midline by isotonic tensing of the cricothyroid muscles. The thyroarytenoid muscles provide finer isometric modifications.
4. As pitch increases, the true cords lengthen and tense isotonically through the action of the cricothyroid. Cord thinning is produced by thyroarytenoid action which increases the tension of the TVC. Extrinsic laryngeal muscles may also affect pitch.
5. Myoelastic-Aerodynamic Theory of Laryngeal Voice Production:

*During expiration the air current flowing through the glottis is unidirectional, and the vocal cords vibrate in an alternating mode.*

Sequence of Events:

a. First, the laryngeal muscles position the vocal cords in various degrees of adduction and place them under the appropriate longitudinal tension.
b. Next, muscular and passive forces of exhalation cause the subglottic air pressure to increase.
c. When this subglottic pressure reaches a point where it exceeds muscular opposition, the glottic chink is forced to open.
d. When the vocal cords start opening from complete closure, they open in a posterior to anterior direction with the posterior portion of the glottis opening first, reaching maximum excursion first, and recontacting each other at the end of the vibratory cycle prior to the anterior portion of the cords.
e. After release of the puff of air there is a reduction of subglottic pressure, and the vocal cords approximate each other again (myoelastic forces of the vocal cords have exceeded the aerodynamic forces).
f. The myoelastic forces are enhanced because air current flowing through a narrow channel exerts a negative pressure on the channel walls; This is the basis of Bernouilli's Principle.
g. The vocal cords are thus sucked back together in an adducted state until the subglottic air pressure can overcome the myoelastic forces of the reapproximated cords, and the cycle is then repeated.
h. The resulting waveform of the vocal cords is not sinusoidal but sawtooth in type and can be classified as a relaxation oscillator.

BIBLIOGRAPHY Bailey, Byron J., ed. Head and Neck Surgery-Otolaryngology. Philadelphia, PA.: J. B. Lippincott Co., 1993. Bailey BJ, Biller HF, ed. Surgery of the Larynx. Philadelphia, PA.: WB Saunders Company, 1985. Cummings, Charles, ed. Otolaryngology- Head and Neck Surgery. St. Louis, Missouri: Mosby-Year Book, Inc., 1993. Kassir R, Anatomy and Physiology of the Larynx. Grand Rounds Presentation, January 13, 1993. Lee KJ, ed. Essential Otolaryngology. New York, NY.: Elesvier Science Publishing Company, Inc., 1991. Paparella MM, Shumrick DA, ed. Otolaryngology. Philadelphia, PA.: WB Saunders Company, 1991.

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