Larynx

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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 just below where the tract of the pharynx splits into the trachea and the esophagus.

[edit] Function

The entrance to the larynx, viewed from behind.

During swallowing, the larynx (at the epiglottis and at the glottis) closes 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.

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 tongue, lips, mouth, and pharynx, 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, and is necessary for the vocal cords to produce speech.

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.

[edit] Innervation

The larynx is innervated by branches of the vagus nerve (CN X) on each side. Sensory innervation to the glottis and supraglottis is by the external branch of the superior laryngeal nerve. The inferior 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.

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.

[edit] Muscles

• Cricothyroid muscle lengthens and stretches the vocal cords.
• Posterior cricoarytenoid muscle abducts the vocal cords.
• Lateral cricoarytenoid muscle adducts the vocal cords.
• Thyroarytenoid muscle (also called vocalis muscle) shortens vocal cords.
• Transverse arytenoid muscle adducts the vocal folds.

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.

[edit] Descended larynx

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 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.

[edit] 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.

Vocal cord paresis is weakness of one or both vocal folds that can greatly impact daily life. Symptoms of paresis include: hoarseness, vocal fatigue, pain in the throat when speaking, shortness of breath, and aspiration (food or liquids going down the trachea).

Some of the primary causes of paresis include: viral infection, cancer or tumor compressing the recurrent laryngeal nerve, trauma, compression of the vocal cord nerve from intubation, or laryngopharyngeal reflux. Many patients may also experience paresis from an unknown cause (idiopathic).

Paresis can be diagnosed by a lack of motion of one or both cords on laryngoscopy, and more precisely by EMG (electromyography) which measures the level of electrical activity in the muscles of the larynx. This diagnosis can be made by a laryngologist, or by some otolaryngologists. In some cases, the nerves may heal and full motion of the vocal cords may return. In other cases, surgery may be required to provide temporary or permanent medialization of the vocal cords.

[edit] Additional images

Conducting passages

Bronchi, bronchial tree, and lungs

[edit] See also

• Laryngitis
• Epiglottitis

[edit] 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