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Anatomy and development and physiology of the larynx
The larynx serves to protect the lower airways, facilitates respiration, and plays a key role in phonation. 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.
Sensory nerves to the larynx are derived from the internal branch of the superior laryngeal nerve (iSLN) and the recurrent laryngeal nerves (RLNs); both are branches of the vagus nerve. The iSLN and RLN innervate the mucosa above and below the level of the true vocal cord, respectively.
Water aerosol inhalation stimulation in partial upper airway obstruction activates water chemoreceptors on the epiglottis and causes reflex respiratory slowing and increase in tidal volume.
The RLN innervates all intrinsic laryngeal muscles except the cricothyroid muscle, which is innervated by the external division of the SLN (eSLN).
Abduction of the vocal cords during respiration is brought about by the posterior cricoarytenoid muscle, whereas their adduction involves all the intrinsic muscles, particularly the thyroarytenoid and cricoarytenoid muscles.
Reflexive glottic closure is achieved by simultaneous adduction of both vocal cords.
Tracheostomy itself leads to centrally mediated impaired reflexes and decannulation (adductor failure and vocal cord fusion).
False vocal cords, even if denervated,van cleef and arpels fake necklace, resist air flow from the lower respiratory tract and serve expectorative function. The true vocal cords resist air flow from outside and play a protective role in respiration, thus explaining the difficulty in overcoming laryngospasm by abrupt pressure peaks from above.
IntroductionThe larynx serves three important functions in humans. In order of functional priority, they are protective,van cleef replica necklaces, respiratory, and phonatory. A sound understanding of these functional priorities appears essential to the management of the myriad diseases besetting this complex organ. This review addresses these three categories of function in terms of phylogeny, morphology, and neuromuscular reflexes, followed by a discussion of clinical events that often threaten to disrupt the anatomy and function of this organ. Original experimental data from the Yale Larynx Laboratory are included here to support physiologic performance,van cleef fake necklaces, which is important to our understanding of clinical behavior.
Phylogeny and FunctionLaryngeal function may be best understood by an appreciation of its origin determined by primitive needs. In this regard, Negus's1 masterful contributions are most illuminating. On an evolutionary scale, as animals migrated from an aquatic to a terrestrial existence, a major change in respiratory requirements became necessary. According to Negus, these accomplishments were reflected in certain contemporary species of fish that developed unique respiratory modifications to allow intermittent sojourns on dry land. Notably, the climbing perch (Anabas scandens) possessed a respiratory diverticulum located above its gills (Figure 1a). The Indian siluroid fish (Saccobranchus) also acquired a long diverticulum leading into an internal air reservoir. These structures, however, contained no valves to prevent the entrance of water when an aquatic existence was resumed.
Figure 1: Structure and function of the larynx viewed phylogenetically (according to Negus)
Full size image
(49 KB)
Download Power Point slide
(698 KB)
The most primitive larynx may be found in the bichir lungfish (Polypterus), which inhabits the Nile River. The larynx of this fish consists simply of a muscular sphincter to guard against the entrance of water (Figure 1b). On the other hand, the African lungfish (Protopterus) and Australian lungfish (Neoceratodus) both possess, in addition to sphincteric musculature, discrete muscle fibers that effectively draw the valvular margins apart to produce active dilatation (Figure 1c). The muscular sphincter, therefore, remains contracted when the fish is in the water, but during periods of drought the sphincter is actively opened to allow air to be gulped into the lungs by a swallowing maneuver. This capacity provides obvious advantages of respiratory survival when the supply of water is limited or undependable.
The primitive larynx, therefore, basically functioned as a simple sphincter to protect the lower airway from the intrusion of foreign matter. However, as respiratory requirements in a terrestrial environment grew, the need for adequate ventilation also grew, accompanied by the necessary potential for active dilatation at the primitive sphincter.
To enhance ventilatory flow requirements through the laryngeal aperture, the acquisition of lateral cartilages may be noted in certain amphibians such as the Mexican axolotl (Amblystoma). These lateral cartilages form bars on either side of the glottis to which the dilator muscles insert (Figure 1d). 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 (Figure 1e). Therefore, although the lateral cartilages of amphibians share a structural similarity to the arytenoid bodies of humans, the origin of the cartilaginous ring in reptiles can be compared to the cricoid cartilages in other mammals, including humans.
Viewed phylogenetically through Negus's eyes the primary function of the larynx is its use as a sphincter, protecting the lower airway from the intrusion of liquids and food. 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.
Although the simple primitive larynx of Polypterus possesses the mechanism of airway constriction, thus exhibiting the potential for sound production, phonation using the larynx as a flutter valve is only seen in vertebrates possessing the respiratory requirements of an effective bellows. Such is possible only in the group of vertebrates possessing thoracoabdominal diaphragms, the group we call mammals. Nonetheless, among all mammals, humans alone have acquired the potential for complex sound production using the laryngeal sphincter as a vibratory source. According to Negus, phonation is therefore considered the least significant of the three basic functions ascribed to this complex organ, whose primary role remains that of a guardian to the lower airway.
Structure and FunctionThe upper airway in adult humans traverses the digestive tract in the region of the pharynx, complicating its sphincteric protection of the lower airway. By sharing a common passageway with the upper digestive system, the larynx is also compromised in its respiratory performance by resultant ventilatory turbulence and, therefore, resistance. Thus, the anatomic configuration in adult humans that benefits phonatory purposes of the larynx simultaneously serves to compromise its sphincteric and respiratory functions. This functional dilemma is resolved at the laryngopharyngeal level by two important organic modifications: structural adaptation and delicate coordination among the three basic laryngeal functions as determined by precisely organized brainstem reflexes.
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 (Figure 2a). 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. This structural modification is most obvious among certain cetaceans and herbivores but appears to a lesser degree among carnivores that use an elongated epiglottis to effect nasolaryngeal connection during deglutition. In this regard, Negus considers the epiglottis to serve secondarily in an olfactory capacity, ensuring that inspired air enters exclusively through the nose. By a series of anatomic demonstrations in macrosmatic animals, this contention appears very convincing and is supported by later histologic work identifying epiglottic chemoreceptors similar in structure to taste buds of the oral cavity,van cleef alhambra replica necklace, implying epiglottic participation in chemosensory perception as well.2
The larynx serves to protect the lower airways, facilitates respiration, and plays a key role in phonation. 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.
Sensory nerves to the larynx are derived from the internal branch of the superior laryngeal nerve (iSLN) and the recurrent laryngeal nerves (RLNs); both are branches of the vagus nerve. The iSLN and RLN innervate the mucosa above and below the level of the true vocal cord, respectively.
Water aerosol inhalation stimulation in partial upper airway obstruction activates water chemoreceptors on the epiglottis and causes reflex respiratory slowing and increase in tidal volume.
The RLN innervates all intrinsic laryngeal muscles except the cricothyroid muscle, which is innervated by the external division of the SLN (eSLN).
Abduction of the vocal cords during respiration is brought about by the posterior cricoarytenoid muscle, whereas their adduction involves all the intrinsic muscles, particularly the thyroarytenoid and cricoarytenoid muscles.
Reflexive glottic closure is achieved by simultaneous adduction of both vocal cords.
Tracheostomy itself leads to centrally mediated impaired reflexes and decannulation (adductor failure and vocal cord fusion).
False vocal cords, even if denervated,van cleef and arpels fake necklace, resist air flow from the lower respiratory tract and serve expectorative function. The true vocal cords resist air flow from outside and play a protective role in respiration, thus explaining the difficulty in overcoming laryngospasm by abrupt pressure peaks from above.
IntroductionThe larynx serves three important functions in humans. In order of functional priority, they are protective,van cleef replica necklaces, respiratory, and phonatory. A sound understanding of these functional priorities appears essential to the management of the myriad diseases besetting this complex organ. This review addresses these three categories of function in terms of phylogeny, morphology, and neuromuscular reflexes, followed by a discussion of clinical events that often threaten to disrupt the anatomy and function of this organ. Original experimental data from the Yale Larynx Laboratory are included here to support physiologic performance,van cleef fake necklaces, which is important to our understanding of clinical behavior.
Phylogeny and FunctionLaryngeal function may be best understood by an appreciation of its origin determined by primitive needs. In this regard, Negus's1 masterful contributions are most illuminating. On an evolutionary scale, as animals migrated from an aquatic to a terrestrial existence, a major change in respiratory requirements became necessary. According to Negus, these accomplishments were reflected in certain contemporary species of fish that developed unique respiratory modifications to allow intermittent sojourns on dry land. Notably, the climbing perch (Anabas scandens) possessed a respiratory diverticulum located above its gills (Figure 1a). The Indian siluroid fish (Saccobranchus) also acquired a long diverticulum leading into an internal air reservoir. These structures, however, contained no valves to prevent the entrance of water when an aquatic existence was resumed.
Figure 1: Structure and function of the larynx viewed phylogenetically (according to Negus)
Full size image
(49 KB)
Download Power Point slide
(698 KB)
The most primitive larynx may be found in the bichir lungfish (Polypterus), which inhabits the Nile River. The larynx of this fish consists simply of a muscular sphincter to guard against the entrance of water (Figure 1b). On the other hand, the African lungfish (Protopterus) and Australian lungfish (Neoceratodus) both possess, in addition to sphincteric musculature, discrete muscle fibers that effectively draw the valvular margins apart to produce active dilatation (Figure 1c). The muscular sphincter, therefore, remains contracted when the fish is in the water, but during periods of drought the sphincter is actively opened to allow air to be gulped into the lungs by a swallowing maneuver. This capacity provides obvious advantages of respiratory survival when the supply of water is limited or undependable.
The primitive larynx, therefore, basically functioned as a simple sphincter to protect the lower airway from the intrusion of foreign matter. However, as respiratory requirements in a terrestrial environment grew, the need for adequate ventilation also grew, accompanied by the necessary potential for active dilatation at the primitive sphincter.
To enhance ventilatory flow requirements through the laryngeal aperture, the acquisition of lateral cartilages may be noted in certain amphibians such as the Mexican axolotl (Amblystoma). These lateral cartilages form bars on either side of the glottis to which the dilator muscles insert (Figure 1d). 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 (Figure 1e). Therefore, although the lateral cartilages of amphibians share a structural similarity to the arytenoid bodies of humans, the origin of the cartilaginous ring in reptiles can be compared to the cricoid cartilages in other mammals, including humans.
Viewed phylogenetically through Negus's eyes the primary function of the larynx is its use as a sphincter, protecting the lower airway from the intrusion of liquids and food. 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.
Although the simple primitive larynx of Polypterus possesses the mechanism of airway constriction, thus exhibiting the potential for sound production, phonation using the larynx as a flutter valve is only seen in vertebrates possessing the respiratory requirements of an effective bellows. Such is possible only in the group of vertebrates possessing thoracoabdominal diaphragms, the group we call mammals. Nonetheless, among all mammals, humans alone have acquired the potential for complex sound production using the laryngeal sphincter as a vibratory source. According to Negus, phonation is therefore considered the least significant of the three basic functions ascribed to this complex organ, whose primary role remains that of a guardian to the lower airway.
Structure and FunctionThe upper airway in adult humans traverses the digestive tract in the region of the pharynx, complicating its sphincteric protection of the lower airway. By sharing a common passageway with the upper digestive system, the larynx is also compromised in its respiratory performance by resultant ventilatory turbulence and, therefore, resistance. Thus, the anatomic configuration in adult humans that benefits phonatory purposes of the larynx simultaneously serves to compromise its sphincteric and respiratory functions. This functional dilemma is resolved at the laryngopharyngeal level by two important organic modifications: structural adaptation and delicate coordination among the three basic laryngeal functions as determined by precisely organized brainstem reflexes.
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 (Figure 2a). 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. This structural modification is most obvious among certain cetaceans and herbivores but appears to a lesser degree among carnivores that use an elongated epiglottis to effect nasolaryngeal connection during deglutition. In this regard, Negus considers the epiglottis to serve secondarily in an olfactory capacity, ensuring that inspired air enters exclusively through the nose. By a series of anatomic demonstrations in macrosmatic animals, this contention appears very convincing and is supported by later histologic work identifying epiglottic chemoreceptors similar in structure to taste buds of the oral cavity,van cleef alhambra replica necklace, implying epiglottic participation in chemosensory perception as well.2
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