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Physiology of Larynx. Three important functions. The larynx serves three important functions in humans. In order of functional priority, they are protective, respiratory, and phonatory. In humans the protective and respiratory functions are compromised in favor of its phonatory function.
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Three important functions • The larynx serves three important functions in humans. In order of functional priority, they are protective, respiratory, and phonatory. • In humans the protective and respiratory functions are compromised in favor of its phonatory function. • The protective function is entirely reflexive and involuntary, whereas the respiratory and phonatory functions are initiated voluntarily but regulated involuntarily.
Laryngeal function may be best understood by an appreciation of its origin determined by primitive needs. • On an evolutionary scale, as animals migrated from an aquatic to a terrestrial existence, a major change in respiratory requirements became necessary. • These accomplishments were reflected in certain contemporary species of fish that developed unique respiratory modifications to allow intermittent sojourns on dry land. • These structures, however, contained no valves to prevent the entrance of water when an aquatic existence was resumed.
Structure and function of the larynx viewed phylogenetically • The climbing perch possessed a respiratory diverticulum located above its gills. • The most primitive larynx may be found in the bichir lungfish. The larynx of this fish consists simply of a muscular sphincter to guard against the entrance of water. • The African lungfish & Australian lungfish both possess, in addition to sphincteric musculature, discrete muscle fibers that effectively draw the valvular margins apart to produce active dilatation.
Structure and function of the larynx viewed phylogenetically • To enhance ventilatory flow requirements through the laryngeal aperture, the acquisition of lateral cartilages may be noted in certain amphibians. • These lateral cartilages form bars on either side of the glottis to which the dilator muscles insert . • To augment the mechanical advantage of these muscles, a cartilaginous ring (giving origin to the dilator muscles) can be found between the glottis and trachea in other higher vertebrates. Such a configuration is apparent among reptiles such as the alligator
The primitive larynx, therefore, basically functioned as a simple sphincter to protect the lower airway from the intrusion of foreign matter. • Its secondary function, supported by the sequential phylogenetic acquisition of the cricoarytenoid complex, centers about its role in respiration governed by active muscular dilatation of the laryngeal aperture. • The third function of the larynx, phonation (best observed in mammals), appears to be a late phylogenetic acquisition.
From a structural point of view, protective function of the adult human larynx is admittedly precarious by virtue of its low position in the neck . • Other mammalian species are provided with a relatively high-riding larynx, affording it a close approximation with structures of the posterior nasal cavities. • The intranarial position of the larynx, securing a continuous airway from the nose to the bronchi, therefore decreases the risk of pulmonary contamination by swallowed matter.
It is of some interest that the human newborn exhibits similar nasolaryngeal connection by approximation of its epiglottis with the posterior surface of its palate, thus ensuring against aspiration by forming a continuous upper and lower airway . • The observation of obligate nasal breathing in the newborn period may be related to this anatomic configuration, which is lost between 4 and 6 months postnatally. The nasolaryngeal relationship.
In adult humans the characteristic flat, shield-like configuration of the epiglottis serves to direct swallowed food laterally into the pyriform fossae, away from the midline laryngeal aperture. • Elevation of the larynx toward the nasal cavity during the height of deglutition exaggerates this protective function.
Aryepiglottic folds act as ramparts to the larynx, allowing food to pass on either side of the epiglottis along the gutter produced between each fold and the lateral pharyngeal wall. • Primary role of the supraglottic larynx in adult humans lies in its protection of the lower airway.
In the human larynx the ability to perform as an effective valve depends on the unique shelf-like configuration of its superior and inferior folds bilaterally represented • The false cords, which are located superiorly, act as exit valves, preventing the escape of air from the lower respiratory tract. When positioned by muscular contraction, they seal even more tightly as tracheal pressure is increased from below. • On the other hand, the true cords behave as a one-way valve in the opposite direction, obstructing the ingress of air. Therefore, it is not surprising that expectorative functions of the larynx remain unimpaired in bilateral laryngeal paralysis.
Cough Reflex Cough ejects mucus and foreign matter from the lungs and helps maintain patency of the pulmonary alveoli. May be voluntary, but more often in response to stimulation of receptors in the larynx or lower respiratory tract. 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 6-10 liters/sec.
The larynx acts as a transducer during phonation converting the aerodynamic forces generated by the lungs, diaphragm, chest and abdominal muscles into acoustic energy. • This energy transduction occurs precisely at the space between the two vocal folds. However subglottic and supra glottic pressures also play a role in this transformation of aerodynamic energy into sound energy.
The requirements of normal phonation are as follows: • Active respiratory support • Adequate glottic closure • Normal mucosal covering of the vocal cord • Adequate control of vocal fold length and tension.
Phonation • It is generally agreed that speech results from the production of a fundamental tone produced at the larynx and is modified by resonating chambers of the upper aerodigestive tract. • Intelligible speech, therefore, represents the combined effect of the larynx, tongue, palate, and related structures of the oral vestibule • The consonants of speech can be associated with particular anatomical sites responsible for their generation i.e. 'p' and 'b' are labials, 't' and 'd' are dentals and 'm' and 'n' are nasals.
The production of the fundamental tone is due to the vibration of the vocal folds against each other, generated by the passage of air between them. Vocal cord vibrations may be a passive phenomenon representing the basis of the aerodynamic theory of sound generation. • Such a theory finds support in the observation that the completely paralyzed larynx is capable of producing sound, as is the cadaver larynx when subglottic pressure is forcefully increased..
The aerodynamic theory of sound production therefore replaces the neurochronaxic theory which incorrectly advanced the notion that the central generation of recurrent laryngeal nerve impulses produced cord vibrations by active contraction of the thyroarytenoid muscles. Each vibration, therefore, represented the result of beat-by-beat impulses through the recurrent laryngeal nerve.
The cricothyroid muscle increases fundamental frequency by tensing the vocal fold. The vocal fold is stretched, elongated, thinned, and slightly adducted to the paramedian position as the vocal fold is lowered within the larynx. These changes reduce the cross-sectional area of the vocal fold, reducing vibratory mass and increasing fundamental frequency.
Vocalis muscle (Voc), on the other hand, generates the opposite effect as it loosens and thickens the vocal fold. In addition, as it increases glottal resistance, it contributes to vocal intensity as subglottal pressure is increased. • Vocal control, therefore, is achieved by the coordinated efforts of respiratory, laryngeal, and articulatory muscles capable of producing great variations of tonal qualities characterizing the human voice.
The Glottic Cycle The vocal folds alternately trap and release air; each trap/release is one cycle of vibration. This cycle is often referred to as the glottic cycle, and it is divided into phases: opening phase, open phase, closing phase, closed phase During the closed phase, the air pressure builds up below the vocal folds. When the glottis opens, the air explodes through the vocal folds, and that's the beginning of the sound wave. The strength of that explosion determines the loudness of the sound coming directly from the larynx.
First, the laryngeal muscles position the vocal cords in various degrees of adduction and place them under the appropriate longitudinal tension. • Next, muscular and passive forces of exhalation cause the subglottic air pressure to increase. • When this subglottic pressure reaches a point where it exceeds muscular opposition, the glottic chink is forced to open. • 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.
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). • 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. • 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.
Pitch • The faster the vocal folds vibrate, the higher the pitch. • In general, men's vocal folds can vibrate from 90 - 500 Hz, and they average about 115 Hz in conversation. • Women's vocal folds can vibrate from 150 -1000 Hz, and they average about 200 Hz in conversation. • Vocal folds vibrate faster as they're pulled longer, thinner, and more taut and vibrate more slowly when they're shorter, thicker, and floppier. • The cricothyroid muscle and thyroarytenoid muscle coordinate with each other to create different pitches
Swallowing: • During swallowing the sphincters of larynx stay contracted preventing aspiration of food into the air passage. • During the pharyngeal stage of swallowing the larynx is elevated towards the lower jaw, this elevation opens up the cricopharyngeal sphincter thus facilitating swallowing. • The hyoid bone rotates in such a way that the greater cornua becomes horizontal, producing a backward tilting of the epiglottis towards the posterior pharyngeal wall. This movement of hyoid bone effectively closes the laryngeal inlet.
Fixation of the Chest • This less known function of the larynx is important for increasing intra abdominal pressure. Closure of the vocal cords achieves fixation of the chest necessary to raise intra abdominal pressure required for daily activities like lifting weights, climbing and even for passing urine and stools.
