Anatomy and development and physiology of the larynx

Clarence T. Sasaki, M.D.About the contributor

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Key Points

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.

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The protective function is entirely reflexive and involuntary, whereas the respiratory and phonatory functions are initiated voluntarily but regulated involuntarily.

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.

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. Anesthesia and sedation impair reflexive vocal cord closure and predispose to aspiration.

Laryngeal denervation leads to vocal dysfunction and aspiration during swallowing.

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Tracheostomy itself leads to centrally mediated impaired reflexes and decannulation (adductor failure and vocal cord fusion).

False vocal cords, even if denervated, 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.


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Introduction

The larynx serves three important functions in humans. In order of functional priority, they are protective, 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, which is important to our understanding of clinical behavior.


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Phylogeny and Function

Laryngeal 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.


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The density of sensory innervation appears greatest in the laryngeal inlet, an observation consistent with the concept that the aditus serves as a protective zone for the more distal respiratory system. When nerve staining techniques are used, the laryngeal surface of the epiglottis appears to contain the most compact innervation, whereas the true cords exhibit lesser degrees of sensory density.7 Specifically, the posterior half of the true cords is more heavily furnished with touch receptors than its anterior portion. On the other hand, chemical and thermal sensors appear limited to the supraglottic larynx. In this regard, water chemoreceptors on the epiglottis have been experimentally implicated in the production of prolonged apnea.8 Furthermore, it has been demonstrated that the respiratory response to water-aerosol inhalation for treatment of croup and other upper airway obstruction may be related to the exquisite water sensitivity of these epiglottic receptors. The effect produced consists of respiratory slowing with concurrent increase in tidal volume, certainly an effect beneficial to partial upper airway obstruction. It may be of further interest that this centrally mediated respiratory response appears greater early in life than in adulthood.9 It is generally agreed that sensory components of the superior laryngeal nerve include representation from mucosal touch receptors, epiglottic chemoreceptors, joint receptors, aortic baroreceptors, and stretch receptors from the intrinsic laryngeal muscles.10 Afferent impulses are delivered through ganglion nodosum to the brainstem nucleus tractus solitarius.

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