Larynx

The larynx, commonly called the voice box, is an organ in the top of the neck involved in respiration, producing sound and protecting the trachea against food aspiration. The opening of the larynx into the pharynx, known as the laryngeal inlet, is about 4–5 centimeters in diameter. The larynx houses the vocal cords, and manipulates pitch and volume, which is essential for phonation. It is situated just below where the tract of the pharynx splits into the trachea and the esophagus.

Structure

The triangle-shaped larynx consists largely of cartilages that are attached to surrounding structures by muscles or by fibrous and elastic tissue components. The larynx is lined by a ciliated columnar epithelium except for the vocal folds. The cavity of the larynx extends from its triangle-shaped inlet to the epiglottis, and to the circular outlet at the lower border of the cricoid cartilage, where it is continuous with the lumen of the trachea. The mucous membrane lining the larynx forms two pairs of lateral folds that project inward into its cavity. The upper folds are called the vestibular folds. They are also sometimes called the false vocal cords for the rather obvious reason that they play no part in vocalization. The Kargyraa style of Tuvan throat singing makes use of these folds to sing an octave lower, and they are used in Umngqokolo, a type of Xhosa throat singing. The lower pair of folds is known as the vocal cords, which produce sounds needed for speech and other vocalizations. The slit-like space between the left and right vocal cords, called the rima glottidis, is the narrowest part of the larynx. The vocal cords and the rima glottidis are collectively referred to as the glottis. The laryngeal cavity above the vestibular folds is called the vestibule. The very middle portion of the cavity between the vestibular folds and the vocal cords is the ventricle of the larynx, or laryngeal ventricle. The infraglottic cavity is the open space below the glottis.

Location

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

Cartilages

There are nine cartilages, three unpaired and three paired, that support the mammalian larynx and form its skeleton.
Unpaired cartilages:

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 it with the hyoid bone. It supports the front portion of the larynx.
Cricoid cartilage: A ring of hyaline cartilage that forms the inferior wall of the larynx. It is attached to the top of the trachea. The median cricothyroid ligament connects the cricoid cartilage to the thyroid cartilage.
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 cartilages:

Arytenoid cartilages: Of the paired cartilages, the arytenoid cartilages are the most important because they influence the position and tension of the vocal cords. These are triangular pieces of mostly hyaline cartilage located at the posterosuperior border of the cricoid cartilage.
Corniculate cartilages: Horn-shaped pieces of elastic cartilage located at the apex of each arytenoid cartilage.
Cuneiform cartilages: Club-shaped pieces of elastic cartilage located anterior to the corniculate cartilages.
Muscles

The muscles of the larynx are divided into intrinsic and extrinsic muscles. The extrinsic muscles act on the region and pass between the larynx and parts around it, but have their origin elsewhere; the intrinsic muscles are confined entirely within the larynx and have their origin and insertion there.
The intrinsic muscles are divided into respiratory and the phonatory muscles. 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 main respiratory muscles are the posterior cricoarytenoid muscles. The phonatory muscles are divided into adductors and tensors.

Intrinsic

The intrinsic laryngeal muscles are responsible for controlling sound production.

Cricothyroid muscle lengthens and tenses the vocal cords.
Posterior cricoarytenoid muscles abduct and externally rotate the arytenoid cartilages, resulting in abducted vocal cords.
Lateral cricoarytenoid muscles adduct and internally rotate the arytenoid cartilages, increasing medial compression.
Transverse arytenoid muscle adducts the arytenoid cartilages, resulting in adducted vocal cords.
Oblique arytenoid muscles narrow the laryngeal inlet by constricting the distance between the arytenoid cartilages.
Thyroarytenoid muscles narrow the laryngeal inlet, shortening the vocal cords, 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 cords apart 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.
Additionally, intrinsic laryngeal muscles present a constitutive Ca²⁺-buffering profile that predicts their better ability to handle calcium changes in comparison to other muscles. This profile is in agreement with their function as very fast muscles with a well-developed capacity for prolonged work. Studies suggests that mechanisms involved in the prompt sequestering of Ca²⁺ are particularly elevated in laryngeal muscles, indicating their importance for the myofiber function and protection against disease, such as Duchenne muscular dystrophy. Furthermore, different levels of Orai1 in rat intrinsic laryngeal muscles and extraocular muscles over the limb muscle suggest a role for store-operated calcium entry channels in those muscles' functional properties and signaling mechanisms.

Extrinsic

The extrinsic laryngeal muscles support and position the larynx within the mid-cervical cereal region.

Sternothyroid muscle depresses the larynx.
Omohyoid muscle depresses the larynx.
Sternohyoid muscle depresses the larynx.
Inferior constrictor muscles.
Thyrohyoid muscle 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.
Nerve supply

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 are by the recurrent laryngeal nerve. While the sensory input described above is visceral sensation, the vocal cords also receive general somatic sensory innervation by the superior laryngeal nerve.
Injury to the external branch of the superior 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 may or may not be preserved, but breathing becomes difficult.

Development

The larynx is derived from the mesoderm of the fourth and sixth pharyngeal arches. 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 larynx descends as the child grows.

Laryngeal cavity

The laryngeal cavity extends from the laryngeal inlet downwards to the lower border of the cricoid cartilage, where it is continuous with that of the trachea.
It is divided into two parts by the projection of the vocal folds, between which is a narrow triangular opening, the rima glottidis.
The portion of the cavity of the larynx above the vestibular folds is called the laryngeal vestibule; it is wide and triangular in shape, its base or anterior wall presenting, however, about its center, the backward projection of the tubercle of the epiglottis.
It contains the vestibular folds, and between these and the vocal folds are the laryngeal ventricles.
The portion below the vocal folds is called the infraglottic cavity. It is at first of an elliptical form, but lower down it widens out, assumes a circular form, and is continuous with the tube of the trachea.

Function

Sound generation

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.
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 to 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 cords can be held close together so that they vibrate. The muscles attached to the arytenoid cartilages control the degree of opening. Vocal cord length and tension can be controlled by rocking the thyroid cartilage forward and backward on the cricoid cartilage, by manipulating the tension of the muscles within the vocal cords, and by moving the arytenoids forward or backward. This causes the pitch produced during phonation to rise or fall. In most males, the vocal cords are longer and have a greater mass than most females vocal cords, producing a lower pitch.
The vocal apparatus consists of two pairs of folds, the vestibular folds and the true vocal cords. The vestibular folds are covered by respiratory epithelium, while the vocal cords are covered by stratified squamous epithelium. The vestibular folds are not responsible for sound production, but rather for resonance. The exceptions to this are found in Tibetan chanting and Kargyraa, a style of Tuvan throat singing. Both make use of the vestibular folds to create an undertone. These false vocal cords do not contain muscle, while the true vocal cords do have skeletal muscle.