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Larynx
Anatomy of the larynx, anterolateral view
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Endoscopic image of larynx

The larynx (plural larynges), colloquially known as the voicebox, is an organ in the neck of mammals involved in protection of the trachea and sound production. The larynx houses the vocal cords, and is situated just below where the tract of the pharynx splits into the trachea and the esophagus.

FunctionEdit

Sound is generated in the larynx, and that is where pitch and volume are manipulated. The strength of expiration from the lungs also contributes to loudness.[1]

Fine manipulation of the larynx is used to generate a source sound with a particular fundamental frequency, or pitch. This source sound is altered as it travels through the vocal tract, configured differently based on the position of the tongue, lips, mouth, and pharynx. The process of altering a source sound as it passes through the filter of the vocal tract creates the many different vowel and consonant sounds of the world's languages.

During swallowing, the backward motion of the tongue forces the epiglottis over the laryngeal opening to prevent swallowed material from entering the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong cough reflex to protect the lungs.

The vocal folds can be held close together (by adducting the arytenoid cartilages), so that they vibrate (see phonation). The muscles attached to the arytenoid cartilages control the degree of opening. Vocal fold length and tension can be controlled by rocking the thyroid cartilage forward and backward on the cricoid cartilage, and by manipulating the tension of the muscles within the vocal folds. This causes the pitch produced during phonation to rise or fall. In most males the vocal cords are longer, producing a deeper pitch.

The vocal apparatus consists of two pairs of mucosal folds. These folds are false vocal cords (vestibular folds) and true vocal cords (folds). The false vocal cords are covered by respiratory epithelium, while the true vocal cords are covered by stratified squamous epithelium. The false vocal cords are not responsible for sound production, but rather for resonance. These false vocal cords do not contain muscle, while the true vocal cords do have skeletal muscle.

InnervationEdit

The larynx is innervated by branches of the vagus nerve on each side. Sensory innervation to the glottis and laryngeal vestibule is by the internal branch of the superior laryngeal nerve. The external branch of the superior laryngeal nerve innervates the cricothyroid muscle. Motor innervation to all other muscles of the larynx and sensory innervation to the subglottis is by the recurrent laryngeal nerve. While the sensory input described above is (general) visceral sensation (diffuse, poorly localized), the vocal fold also receives general somatic sensory innervation (proprioceptive and touch) by the superior laryngeal nerve.

Injury to the external laryngeal nerve causes weakened phonation because the vocal cords cannot be tightened. Injury to one of the recurrent laryngeal nerves produces hoarseness, if both are damaged the voice is completely lost, and breathing becomes difficult.

Muscles associated with the larynxEdit

Notably, the only muscle capable of separating the vocal cords for normal breathing is the posterior cricoarytenoid. If this muscle is incapacitated on both sides, the inability to pull the vocal cords apart (abduct) will cause difficulty breathing. Bilateral injury to the recurrent laryngeal nerve would cause this condition. It is also worth noting that all muscles are innervated by the recurrent laryngeal branch of the vagus except the cricothyroid muscle, which is innervated by the external laryngeal branch of the vagus.

Descended larynx Edit

In most animals, including infant humans and apes, the larynx is situated very high in the throat—a position that allows it to couple more easily with the nasal passages, so that breathing and eating are not done with the same apparatus. However, some aquatic mammals, large deer, and adult humans have descended larynges. An adult human, unlike apes, cannot raise the larynx enough to directly couple it to the nasal passage. Proponents of the aquatic ape hypothesis claim that the similarity between the descended larynx in humans and aquatic mammals further supports their theory.[2]

Some linguists have suggested that the descended larynx, by extending the length of the vocal tract and thereby increasing the variety of sounds humans could produce, was a critical element in the development of speech and language. Others cite the presence of descended larynges in non-linguistic animals, as well as the ubiquity of nonverbal communication and language among humans, as counterevidence against this claim.


CartilagesEdit

There are six in all, three unpaired and three paired.The cartilages of the larynx are the thyroid, cricoid, epiglottis, arytenoids, corniculate, and the cuneiforms.

ImagesEdit

See also Edit

ReferencesEdit

  1. Titze, I.R. (1994). Principles of Voice Production, Prentice Hall, ISBN 978-0137178933.
  2. Morgan, Elaine (1997). The Aquatic Ape Hypothesis, Souvenir Press Ltd.

Speech and Hearing Science: Anatomy and Physiology 3rd edition. Willard R. Zemlin. 1988. Prentice-Hall, Inc. Englewood Cliffs, New Jersey. ISBN 0-13-827429-0


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