The larynx (Latin: larynx) is a flexible passageway for air between the oropharynx and the trachea. It plays an essential role in sound production and protects the lower airway against food inspiration.
The larynx is a part of the upper respiratory tract. The larynx is mostly built of a cartilaginous skeleton and muscles. The inner surface of the larynx is lined with a mucosal membrane.
Cartilages of larynx
The cartilaginous skeleton of the larynx is formed by nine irregularly shaped plates of hyaline and elastic cartilage - three large unpaired (cricoid, thyroid, epiglottis) and three pairs of smaller cartilages (arytenoids, corniculate, cuneiform). The most important two are the epiglottis and the arytenoid cartilage.
During swallowing, the pharynx and larynx elevates due the contraction of the extrinsic laryngeal muscles, and the epiglottis moves down and forms a lid over the glottis closing it off, thereby protecting airways against blockade from foods and drinks. The arytenoid cartilages are able to influence the position and tension of the vocal folds.
The muscles of the larynx (laryngeal muscles) are skeletal muscles participating in forming the walls of the larynx and providing movements involved in breathing, phonation, and swallowing processes. The laryngeal muscles can be divided into two groups: the extrinsic or external muscles which act to elevate or depress the larynx during swallowing, and the intrinsic or internal muscles which act to move the individual components of the larynx - playing a vital role in breathing and phonation.
The extrinsic muscles of the larynx move the larynx upwards and downwards. The extrinsic muscles of the larynx include suprahyoid and infrahyoid muscles of the neck, and also the stylopharyngeus, a muscle of the pharynx.
The intrinsic muscles of the larynx belong to a group of muscles which activate individual components of the larynx, in general, controlling the shape of the rima glottis and the length and tension of the vocal cords. These muscles are the cricothyroid, thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, and transverse and oblique arytenoids.
Mucosa of larynx
The mucosa of the larynx forms the inner layer of the laryngeal wall, and is continuous with mucosa of the pharynx above and the trachea below.
The laryngeal mucosa is the chief component of the vestibular folds, where it is the thickest. It is thinner over the vocal folds, where it is firmly fixed to the underlying vocal ligaments. It is loosely attached to the anterior surface of the epiglottis, but firmly attached to the anterior surface and the floor of the valleculae. On the aryepiglottic folds the mucosa is strengthened by large amount of fibrous connective tissue, and it attaches closely to the laryngeal surfaces of the cuneiform and arytenoid cartilages.
The epithelium of the laryngeal mucosa is mainly ciliated, pseudostratified epithelium called respiratory epithelium, providing a mucociliary clearance mechanism like in other parts of the respiratory tract. However, there are specific areas of the larynx requiring a different functional type of epithelium. The vocal folds are covered by non-keratinized, stratified squamous epithelium, which is much more durable and can protect the tissue from the effects of mechanical stresses that affect the surfaces of the vocal folds. The non-keratinized, stratified squamous epithelium also covers the exterior surfaces of larynx, which merge with the laryngopharynx and oropharynx, because these surfaces are affected by the abrasive effects of swallowed food.
The lamina propria of the laryngeal mucosa (connective tissue layer) contains numerous mucous glands, especially over the epiglottis, and along the margins of the aryepiglottic folds anterior to the arytenoid cartilages (arytenoid glands). There are many large glands in the saccules of the larynx that secrete periodically over the vocal folds during phonation. The free edges of the folds don`t have glands and the stratified squamous epithelium here requires the secretions of the neighbouring glands to keep the vocal folds lubricated.
Note that taste buds, like those in the tongue, can be found in the laryngeal mucosa covering the posterior epiglottic surface, aryepiglottic folds, but less often in other regions of the larynx.
Blood supply of larynx
The arterial blood supply to the larynx comes mainly from the superior and inferior laryngeal arteries, with rich anastomoses between the corresponding contralateral, and between the ipsilateral arteries.
Innervation of larynx
The innervation of the larynx is provided by the vagus nerve (CN X) on each side, with its branches: superior laryngeal nerve, recurrent laryngeal nerve. The internal branch of the superior laryngeal nerve provides general somatic sensory innervation to the glottis, laryngeal vestibule and the vocal fold, but the external branch of the superior laryngeal nerve innervates the cricothyroid muscle. The recurrent laryngeal nerve provides sensory innervation to the subglottis and motor innervation to all intrinsic muscles of the larynx, except the cricothyroid muscle.
Note, that the extrinsic muscles of larynx receive separate innervation than the intrinsic structures of the larynx, mostly from the facial nerve and the cervical spinal nerves.
The sensory nerve supply to the mucosa of the larynx is provided by two nerves arising from the vagus nerve, namely, the inferior branch of the superior laryngeal nerve and the recurrent laryngeal nerve.
The internal laryngeal nerve is the internal branch of the superior laryngeal nerve, arising from the inferior ganglion of the vagus nerve (CN X), and it innervates mucosa of the larynx down to the vocal cords. The internal laryngeal nerve divides into three branches. The superior branch supplies the mucosa of the piriform fossa. The middle branch innervates the mucosa of the laryngeal ventricle, specifically the quadrangular membrane, and probably is involved in the cough reflex. The inferior branch mainly supplies the mucosa of the ventricle and subglottic cavity. The nerve also shares many communicating branches with the recurrent laryngeal nerve.
The recurrent laryngeal nerve is a mixed branch of the vagus nerve (CN X), and it supplies sensory innervation (mainly with its posterior branch) to the mucosa of the larynx below the vocal cords. It also gives branches that anastomose with the internal laryngeal nerve. Thus, at the vocal cords there is an overlap between the territories innervated by the two nerves.