22.2. Vocal Systems

The Human Larynx

The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs. The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.

The epiglottis, attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea. When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds. A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea.

Muscles

The muscles of the larynx are divided into intrinsic and extrinsic muscles.

The intrinsic muscles are divided into respiratory and the phonatory muscles (the muscles of phonation). The respiratory muscles move the vocal cords apart and serve breathing. The phonatory muscles move the vocal cords together and serve the production of voice. The extrinsic, passing between the larynx and parts around; and intrinsic, confined entirely. The main respiratory muscles are the posterior cricoarytenoid muscles. The phonatory muscles are divided into adductors (lateral cricoarytenoid muscles, arytenoid muscles) and tensors (cricothyroid muscles, thyroarytenoid muscles).

Intrinsic

The intrinsic laryngeal muscles are responsible for controlling sound production.

• Cricothyroid muscle lengthen and tense 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, increase medial compression. 

• Transverse arytenoid muscle adduct the arytenoid cartilages, resulting in adducted vocal folds.

• Oblique arytenoid muscles narrow the laryngeal inlet by constricting the distance between the arytenoid cartilages. 

• Thyroarytenoid muscles – sphincter of vestibule, narrowing the laryngeal inlet, shortening the vocal folds, and lowering voice pitch. The internal thyroarytenoid is the portion of the thyroarytenoid that vibrates to produce sound. 

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

The extrinsic laryngeal muscles support and position the larynx within the trachea.

• Sternothyroid muscles depress the larynx.

• Omohyoid muscles depress the larynx.

• Sternohyoid muscles depress the larynx.

• Thyrohyoid muscles elevates the larynx.

• Digastric elevates the larynx.

• Stylohyoid elevates the larynx.

• Mylohyoid elevates the larynx.

• Geniohyoid elevates the larynx.

• Hyoglossus elevates the larynx.

• Genioglossus elevates the larynx
  
  

Vocal folds

Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium transitioning into pseudostratified ciliated columnar epithelium that contains goblet cells. Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus.

The vocal folds are located within the larynx at the top of the trachea. They are attached posteriorly to the arytenoid cartilages and anteriorly to the thyroid cartilage. They are part of the glottis which is the point at which the airflow is normally interrupted. Their outer edges are attached to muscle in the larynx while their inner edges, or margins, are free forming the opening called the rima glottidis.

The tissues of the vocal folds have three major layers: cover, vocal ligament and body.

• The cover is a mucosa formed by stratified squamous epithelium bordered by ciliated pseudostratified epithelium with goblet cells. It is covered with two layers of mucus: a mucinous layer and serous layer. Both mucus layers provide viscous and watery environment for cilia beating posteriorally and superiorly. The mucociliary clearance keeps the vocal folds essentially moist, clean and lubricated. The epidermis layer is secured to the deeper connective tissue by basement membrane. The mucosa layer vibrates at a frequency range of 100–1000 Hz and displacements of about 1mm.

• The vocal ligament is enclosed within the vocal folds and is responsible for strain during phonation. It is a band of yellow elastic tissue attached anteriorly to the angle of the thyroid cartilage and posteriorly to the vocal process of the arytenoid cartilage. Within the vocal ligament, fibrous proteins such as elastin and collagen are key in maintaining the proper elasticity while collagen is responsible for the resistance and resilience of the vocal fold to tensile strength. The normal strain level of vocal ligament ranges from 0–15% during phonation (voicing).

• The body of the vocal fold contains the thyroarytenoid (vocalis) muscle. This muscle shortens the vocal ligament letting the vocal fold relax and lower the pitch of voice. This muscle is a flat triangular band and is pearly white in color. It is actually divided into bundles with different attachments and secondary roles have been proposed for them.
  
  

False vocal folds

The vocal folds are sometimes called 'true vocal folds' to distinguish them from the 'false vocal folds' known as vestibular folds or ventricular folds. These are a pair of thick folds of mucous membrane that protect and sit slightly superior to the more delicate true folds. They have a minimal role in normal phonation, but are often used to produce deep sonorous tones in Tibetan chant and Tuvan throat singing, as well as in musical screaming and the death growl vocal style.

Innervation

The larynx is innervated by two branches of the vagus nerve (CN X): the superior and recurrent laryngeal nerves.

The superior laryngeal nerve consists of two branches: the internal laryngeal nerve (sensory), which supplies sensory fibers to the laryngeal mucosa, and the external laryngeal nerve (motor), which innervates the cricothyroid muscle.

The recurrent laryngeal nerves (RLN) branch from the vagus nerve. As the name indicates, they run down into the thorax and back up to the larynx. The vagus nerves exit the skull at the jugular foramen and travel alongside the carotid arteries through the neck. The left RLN branches off the vagus at the aortic arch and the right at the right subclavian artery. The left RLN passes in front of the arch and then wraps underneath and behind it. The right RLN does the same around the right subclavian artery. Both nerves then typically ascend in a groove at the junction of the trachea and esophagus. They extend behind the posterior lobes of the thyroid gland and enter the larynx deep to the inferior constrictor muscle, just posterior to the cricothyroid joint. The terminal branch is called the inferior laryngeal nerve.

The recurrent laryngeal nerves control all intrinsic muscles of the larynx except for the cricothyroid muscle. These muscles act to open, close, and adjust the tension of the vocal cords, and include the posterior cricoarytenoid muscles, the only muscle to open the vocal cords. These nerves also carry sensory information from the mucous membranes of the larynx below the lower surface of the vocal fold, as well as sensory, secretory and motor fibers to the cervical segments of the esophagus and the trachea.

The extreme detour of the recurrent laryngeal nerves, about 4.6 m in the case of giraffes, became famous as evidence of evolution, as opposed to Intelligent Design. The nerve's route would have been direct in the fish-like ancestors of modern tetrapods, traveling from the brain, past the heart, to the gills, as it does in modern fishes. Over the course of evolution, as the neck extended and the heart became lower in the body, the laryngeal nerve was caught on the wrong side of the heart’s main vessels. Natural selection gradually lengthened the nerve by tiny increments to accommodate, resulting in the circuitous route now observed. Sauropod dinosaurs were the vertebrates with the longest necks in record. The total length of the vagus nerve and recurrent laryngeal nerve (brain to larynx) would have been up to 28 m in Supersaurus.

Development

In all other mammals and 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. The epiglotis rests against the soft palate, forcing food to press the epiglottis closed during swallowing and therefore preventing choking. The larynx descends later as the child grows, especially in males with the rise of testosterone levels during puberty.

Summary

The human larynx is protects the entrance of the trachea and produces voice. It is structured by three large unpaired cartilages (thyroid, cricoid and epiglottis) and three paired cartilages (arytenoid, corniculate and cuneiform). Internally it contains a pair of true vocal folds and a second, more superior pair false vocal folds with protective role. The intrinsic laryngeal muscles control the abduction of the vocal folds for respiration and adduction for voice or for protection during swallowing. These muscles are innervated by the superior and recurrent laryngeal nerves, which are branches of the vagus nerve (CN X). The vocal folds are formed by cover of mucus, epithelium and connective tissue, a vocal ligament rich in elastin and collagen and a body formed by the thyroarytenoid muscle.

Key terms

Larynx, laryngopharynx, oropharynx, thyroid cartilage, arytenoid cartilage, epiglottis cartilage, cricoid cartilage, corniculate cartilage, cuneiform cartilage, posterior cricoarytenoid muscle, lateral cricoarytenoid muscle, cricothyroid muscle, transverse arytenoid muscle, oblique arytenoid muscle, thyroarytenoid muscle, vocalis muscle, sternothyroid muscle, omohyoid muscle, sternohyoid muscle, inferior constrictor muscle, thyroihyoid muscle, digastric muscle, stylohyoid muscle, mylohyoid muscle, geniohyoid muscle, hyoglossus muscle, genioglossus muscle, glottis, trachea, cover, vocal ligament, body, true vocal fold, false vocal fold, ventricular vocal fold, vestibular vocal fold, vagus nerve, superior laryngeal nerve, recurrent laryngeal nerve, inferior laryngeal nerve.

Figure credits

Figure 1 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148362

Figure 2 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 958, Public Domain, https://commons.wikimedia.org/w/index.php?curid=244426

Figure 3 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 959, Public Domain, https://commons.wikimedia.org/w/index.php?curid=244427

Figure 4 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 960, Public Domain, https://commons.wikimedia.org/w/index.php?curid=244428

Figure 5 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 386, Public Domain, https://commons.wikimedia.org/w/index.php?curid=519732

Figure 6 by melvil - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=52972195

Figure 7 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148363

Figure 8 by Fred the Oyster, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35176865

Figure 9 by Jkwchui - Based on drawing by Truth-seeker2004, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30998747

Figure 10 by Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=56197715