Larynx

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Larynx

The larynx (plural larynges), or 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 at the point where the upper tract splits into the trachea and the esophagus.

Structure

The structure of the larynx is mainly composed of cartilage bound by ligaments and muscle. At the front is the thyroid cartilage, creating the prominence of the Adam's apple in humans. The inferior horns (protrusions at the bottom rear of the thyroid cartilage) of the thyroid cartilage rest on a ring-shaped cartilage called the cricoid cartilage which connects the larynx to the trachea. The cricoid cartilage resembles a signet ring (narrow in front, broader in back). Above the larynx is the hyoid bone, by which (via various muscles and ligaments) the larynx is connected to the jaw and skull. These muscles move the larynx during swallowing. The hyoid is the only floating bone in the body; it is not 'attached' to any other bones. The epiglottis is another cartilage that extents upwards behind the back of the tongue and projects down through the hyoid bone. It connects to the thyroid cartilage just beneath the thyroid notch (the Adam's apple).

The space defined by these main cartilages can be divided roughly into the supraglottis at the top and the glottis.

The glottis is defined as the space between the vocal folds (more commonly known as vocal cords), which are located at the upper rim of the cricoid cartilage. They attach to the thyroid cartilage at the front, and to the Arytenoid cartilages at the back. These are two roughly tetrahedral cartilages responsible for pulling the vocal folds together and apart (adduction and abduction — see Anatomical terms of location). The glottis is the laryngeal area of most interest to speech researchers, as it is widely believed to be where most of the control of phonation and pitch goes on. The vocal folds are muscular masses coated with a mucous membrane which protects much of the respiratory tract from foreign particles. Their inner edges contain the vocal ligament.

The supraglottis is that part of the pharynx above the glottis. It contains the ventricle of the larynx (laryngeal sinus), the ventricular folds (or false vocal folds), the epiglottis, and the aryepiglottal folds — two folds of connective tissue that connect the epiglottis to the arytenoid cartilages. Muscles in the aryepiglottal folds can pull the leaf-shaped epiglottis down, sealing the larynx and protecting the trachea below from foreign objects.

Muscles

Cricothyroid muscle tensor for vocal ligament
Posterior cricoarytenoid muscle abductor for vocal ligament
Lateral cricoarytenoid muscle adductor for vocal ligament
Oblique Arytenoid muscle closes the larynx orfice
Transverse Arytenoid muscle adductor for vocal ligament
epiglottic Arytenoid muscle closes the larynx orfice
Thyroarytenoid muscle make vocal ligament relaxes

Function

During swallowing, the larynx (at the epiglottis and at the glottis) closes to prevent swallowed material from entering the lungs; there is also a strong cough reflex to protect the lungs. Sensation is transferred by the superior laryngeal nerve (glottis and supraglottis - Sensory and Autonomic Innervation) and the recurrent laryngeal nerve (subglottis and muscles - Sensory and Motor Innervation), both branches of the vagus nerve.

While articulation of the sound (the fine manipulation that creates the many different vowel and consonant sounds of the world's languages) is achieved by the use of the teeth, tongue, palate, and lips, sound is generated in the larynx, and that is where pitch and volume are manipulated.

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.

Descended larynx

In most animals, including infant humans, 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 cannot raise the larynx enough to directly couple it to the nasal passage.

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.

Disorders

There are several things that can cause a larynx to not function properly. Some symptoms are hoarseness, loss of voice, pain in the throat or ears, and breathing difficulties. Acute laryngitis is the sudden inflammation and swelling of the larynx. It is caused by the common cold or by excessive shouting. It is not serious. Chronic laryngitis is caused by smoking, dust, frequent yelling, or prolonged exposure to polluted air. It is much more serious than acute laryngitis.

Presbylarynx is a condition in which age-related atrophy of the soft tissues of the larynx results in weak voice and restricted vocal range and stamina. Bowing of the anterior portion of the vocal cords is found on laryngoscopy.

Ulcers may be caused by the prolonged presence of an endotracheal tube. Polyps and nodules are small bumps on the vocal cords caused by prolonged exposure to cigarette smoke and vocal overuse, respectively. Finally, two related types of cancer of the larynx, namely squamous cell carcinoma and verrucous carcinoma, are strongly associated with repeated exposure to cigarette smoke and alcohol.

References

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

Head and neck | Human voice | Respiratory system | Phonetics