larynx

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

The larynx (plural larynges), commonly called the voice box, is an organ in the neck of amphibians, reptiles, birds and mammals (including humans) involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume. The larynx houses the vocal folds (commonly but improperly termed the "vocal cords"), which are essential for phonation. The vocal folds are situated just below where the tract of the pharynx splits into the trachea and the esophagus.

Anatomical position and description

In adult humans, the larynx is found in the anterior neck at the level of the C3–C6 vertebrae. It connects the inferior part of the pharynx (hypopharynx) with the trachea. The laryngeal skeleton consists of nine cartilages: three single (epiglottic, thyroid and cricoid) and three paired (arytenoid, corniculate, and cuneiform). The hyoid bone is not part of the larynx, though it is connected to it. The larynx extends vertically from the tip of the epiglottis to the inferior border of the cricoid cartilage. Its interior can be divided in supraglottis, glottis and subglottis.

In newborn infants, the larynx is initially at the level of the C2–C3 vertebrae, and is further forward and higher relative to its position in the adult body.^[1] The larynx descends as the child grows.^[2][3]

Function

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.^[4]

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 as well as tone, certain realizations of stress and other types of linguistic prosody. The larynx also has a similar function as the lungs in creating pressure differences required for sound production; a constricted larynx can be raised or lowered affecting the volume of the oral cavity as necessary in glottalic consonants.

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 (either directly by contracting the cricothyroids or indirectly by changing the vertical position of the larynx), by manipulating the tension of the muscles within the vocal folds, and by moving the arytenoids forward or backward. This causes the pitch produced during phonation to rise or fall. In most males the vocal folds are longer and with a greater mass, producing a deeper pitch.

The vocal apparatus consists of two pairs of mucosal folds. These folds are false vocal folds (vestibular folds) and true vocal folds (folds). The false vocal folds are covered by respiratory epithelium, while the true vocal folds are covered by stratified squamous epithelium. The false vocal folds are not responsible for sound production, but rather for resonance. The exceptions to this are found in Tibetan Chant and Kargyraa, a style of Tuvan throat singing. Both make use of the false vocal folds to create an undertone. These false vocal folds do not contain muscle, while the true vocal folds do have skeletal muscle.

During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent swallowed material from entering the larynx which leads to 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.

Innervation

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 folds cannot be tightened. Injury to one of the recurrent laryngeal nerves produces hoarseness, if both are damaged the voice may or may not be preserved, but breathing becomes difficult.

Intrinsic muscles associated with the larynx

• Cricothyroid muscles lengthen and stretch the vocal folds.
• Posterior cricoarytenoid muscles abduct and externally rotate the arytenoid cartilages, resulting in abducted vocal folds.
• Lateral cricoarytenoid muscles adduct and internally rotate the arytenoid cartilages, which can result in adducted vocal folds.
• Transverse arytenoid muscle adducts the arytenoid cartilages, resulting in adducted vocal folds.^[5]
• Oblique arytenoid muscles narrow the laryngeal inlet by constricting the distance between the arytenoid cartilages.
• Vocalis muscles increase the thickness of the chords changing the tone.
• Thyroarytenoid muscles - sphincter of vestibule, narrowing the laryngeal inlet.

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 folds 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 superior laryngeal nerve (a branch of the vagus).

Extrinsic muscles associated with the larynx

• Thyrohyoid muscles
• Sternothyroid muscles
• Omohyoid muscles
• Inferior constrictor muscles
• Digastric
• Stylohyoid
• Mylohyoid
• Geniohyoid
• Hyoglossus

Descended larynx

In infant humans and most animals, 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. Adult humans cannot raise the larynx enough to directly couple it to the nasal passage. Despite its presence in deer, proponents of the aquatic ape hypothesis claim that the similarity between the descended larynx in humans and aquatic mammals supports their theory.^[6]

Pioneering work on the structure and evolution of the larynx was carried out in the 1920s by the British comparative anatomist Victor Negus, culminating in his monumental work The Mechanism of the Larynx (1929). Negus, however, pointed out that the descent of the larynx reflected the reshaping and descent of the human tongue into the pharynx. This process is not complete until age six to eight years. Some researchers, such as Philip Lieberman, Dennis Klatt, Brant de Boer and Kenneth Stevens using computer-modeling techniques have suggested that the species-specific human tongue allows the vocal tract (the airway above the larynx) to assume the shapes necessary to produce speech sounds that enhance the robustness of human speech. Sounds such as the vowels of the words see and do, [i] and [u], (in phonetic notation) have been shown to be less subject to confusion in classic studies such as the 1950 Peterson and Barney investigation of the possibilities for computerized speech recognition. In contrast, though other species have low larynges their tongues remains anchored in their mouths and their vocal tracts cannot produce the range of speech sounds of humans. The ability to lower the larynx transiently in some species extends the length of their vocal tract, which as Fitch showed creates the acoustic illusion that they are larger. Research at Haskins Laboratories in the 1960s showed that speech allows humans to achieve a vocal communication rate that exceeds the fusion frequency of the auditory system by fusing sounds together into syllables and words. The additional speech sounds that the human tongue enables us to produce, particularly [i], allow humans to unconsciously infer the length of the vocal tract of the person who is talking, a critical element in recovering the phonemes that make up a word. ^[7]

Disorders of the larynx

There are several things that can cause a larynx to not function properly.^[8] Some symptoms are hoarseness, loss of voice, pain in the throat or ears, and breathing difficulties. Larynx transplant is a rare procedure. The world's first successful operation took place in 1998 at the Cleveland Clinic, and the second took place in October 2010 at the University of California Medical Center in Sacramento.^[9]

• 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 folds 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 folds caused by prolonged exposure to cigarette smoke and vocal misuse, respectively.

• 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.

• Idiopathic laryngeal spasm.

• Laryngopharyngeal reflux is a condition in which acid from the stomach irritates and burns the larynx. Similar damage can occur with gastroesophageal reflux disease (GERD).^[10][11]

• Laryngomalacia is a very common condition of infancy, in which the soft, immature cartilage of the upper larynx collapses inward during inhalation, causing airway obstruction.

• Laryngeal perichondritis, the inflammation of the perichondrium of laryngeal cartilages, causing airway obstruction.

Cartilages

There are nine cartilages, three unpaired and three paired, that support the mammalian larynx and form its skeleton. The unpaired cartilages of the larynx are the thyroid, cricoid and epiglottis. The paired cartilages of the larynx are the arytenoids, corniculate, and the cuneiforms.

• Thyroid Cartilage: This forms the Adam's apple. It is usually larger in males than in females. The thyrohyoid membrane is a ligament associated with the thyroid cartilage that connects the thyroid cartilage with the hyoid bone.

• Cricoid cartilage: A ring of hyaline cartilage that forms the inferior wall of the larynx. It is attached to the top of trachea.

• Epiglottis: A large, spoon-shaped piece of elastic cartilage. During swallowing, the pharynx and larynx rise. Elevation of the pharynx widens it to receive food and drink; elevation of the larynx causes the epiglottis to move down and form a lid over the glottis, closing it off.

• Paired Arytenoid Cartilage: Of the paired cartilages, the arytenoid cartilages are the most important because they influence the position and tension of the vocal folds. These are triangular pieces of mostly hyaline cartilage located at the posterosuperior border of the cricoid cartilage.

• Paired Corniculate Cartilage: Horn-shaped pieces of elastic cartilage located at the apex of each arytenoid cartilage.

• Paired Cuneiform Cartilage: Club-shaped pieces of elastic cartilage located anterior to the corniculate cartilages.

Non-mammalian larynges

Most tetrapod species possess a larynx, but its structure is typically simpler than that found in mammals. The cartilages surrounding the larynx are apparently a remnant of the original gill arches in fish, and are a common feature, but not all are always present. For example, the thyroid cartilage is found only in mammals. Similarly, only mammals possess a true epiglottis, although a flap of non-cartilagenous mucosa is found in a similar position in many other groups. In modern amphibians, the laryngeal skeleton is considerably reduced; frogs have only the cricoid and arytenoid cartilages, while salamanders possess only the arytenoids.^[12]

Vocal folds are found only in mammals, and a few lizards. As a result, many reptiles and amphibians are essentially voiceless; frogs use ridges in the trachea to modulate sound, while birds have a separate sound-producing organ, the syrinx.^[12]

Shape

It is tube shaped and is 4.1 inches long.^{[citation needed]}

See also

Wikimedia Commons has media related to: Larynx

• Histology of the Vocal Folds
• Pharynx
• Jeffrey Laitman

References