Anatomy of larynx
The larynx, often called the “voice box” lies in the midline of the neck and extends from the laryngeal inlet to the inferior border of the cricoid cartilage. Situated in front of the hypopharynx, it lies opposite the third to sixth cervical vertebrae, although it is positioned slightly higher in children and adult females. Its anteroposterior dimension varies, averaging around 36 mm in males and approximately 26 mm in females. During swallowing and speaking (phonation), the larynx moves both vertically and in the front-to-back (anteroposterior) directions. It can also be shifted passively from side to side, resulting in a distinctive grating sensation known as “laryngeal crepitus”.
Framework of the Larynx
The structural framework of the larynx is composed of several cartilages and the hyoid bone. These cartilages are interconnected by various ligaments, joints, and membranes and are mobilized by both intrinsic and extrinsic muscles. Internally, the larynx is lined with a mucous membrane that continues upward to connect with the pharynx and downward to merge with the trachea. This framework supports not only the laryngeal structures involved in voice production but also the critical functions of airway protection and ventilation.
1. Hyoid Bone
The hyoid bone is a U-shaped bone suspended by several suprahyoid muscles and ligaments. The hyoid provides the upper attachment point for many extrinsic laryngeal muscles, effectively suspending the larynx within the neck.
2. Laryngeal cartilages
The larynx is constructed of three unpaired and three paired cartilages, which collectively support its complex functions:
- Unpaired cartilages: Thyroid, cricoid and epiglottis.
- Paired cartilages: Arytenoid, corniculate and cuneiform.
- Thyroid cartilage. It is a V-shaped cartilage and it is the largest of all laryngeal cartilages. The thyroid cartilage is composed of right and left laminae which are roughly quadrilateral in shape. The lower part of the anterior border of the right and left laminae are fused in the midline giving rise to the laryngeal prominence or “Adam’s apple”. The angle of fusion is more pronounced in men at about 90° (reason for loud voice and low pitch) while in females it is at 120° (reason for soft voice and high pitch). The upper part of the anterior border of the right and left lamina does not fuse and remains separated by the thyroid notch. The posterior borders of the right and left laminae are far apart and extend upwards and downwards as the superior and inferior cornua. The superior cornua links to the hyoid bone via the lateral thyrohyoid ligament. The inferior cornua articulates with the cricoid cartilage to form the cricothyroid joint and it is connected to the cricoid cartilage by the conus elasticus. The vocal cords attach to the middle of the thyroid angle, which is also where foreign objects are commonly lodged, often above the vocal cords. In emergencies, an airway can be opened by puncturing the cricothyroid membrane – a procedure known as cricothyrotomy.
- Cricoid cartilage. It is the only cartilage forming a complete ring, encircling the larynx below the thyroid cartilage. It forms the structural base of the larynx, with a broad lamina at the back and a narrow arch in the front, supporting both the thyroid and arytenoid cartilages. The arch connects superiorly with the thyroid cartilage via the cricothyroid ligament and connects inferiorly with the 1st tracheal ring via the cricotracheal ligament. The lamina articulates superiorly with the arytenoid cartilage and inferior cornua of the thyroid cartilage.
- Epiglottis. It is a thin, leaf-like, yellow hue, elastic cartilage forming the anterior wall of the laryngeal inlet. It is attached to the tongue, hyoid bone and thyroid cartilage via the median glossoepiglottic fold, hyoepiglottic and thyroepiglottic ligaments respectively The attachment of hyoepiglottic ligament divides the epiglottis into suprahyoid and infrahyoid epiglottis. There is a stalk also called as petiole present in the epiglottis which attaches and secures the epiglottis to the thyroid angle just above the attachment of vocal cords. The anterior surface of the epiglottis is separated from the thyrohyoid membrane and upper part of thyroid cartilage by a potential fat-filled space – the pre-epiglottic space that can be invaded by carcinoma of supraglottic larynx or the base of tongue. The posterior surface of the epiglottis is concavoconvex—concave above but convex below forming a bulge called tubercle of the epiglottis, which obstructs the view of the anterior commissure during laryngoscopic examination. Epiglottic cartilage contains mucous gland pits and may feature perforations that enable cancer to spread into the pre-epiglottic space. Although the epiglottis has a significant role in protecting the airway, it is not essential for swallowing and can be removed in carcinoma, with minimal risk of aspiration.
- Arytenoid cartilages. They are paired and each arytenoid cartilage is pyramidal in shape. Arytenoid cartilage has a base that articulates with the lamina of the cricoid cartilage; a muscular process that is directed laterally to give attachment to intrinsic laryngeal muscles; a vocal process that is directed anteriorly, giving attachment to the vocal cord; and an apex that is flattened for articulation with the corniculate cartilage, which sits atop it. These pyramidal cartilages articulate with the posterior cricoid cartilage, enabling movement of the vocal folds.
- Corniculate cartilages (of Santorini) (Corn = horn). They are paired and each corniculate cartilage is conical in shape. Each articulates (synovial joint) with the apex of the arytenoid cartilage. Sitting atop the arytenoids, they help extend the arytenoids and are involved in the movement of the aryepiglottic folds.
- Cuneiform cartilages (of Wrisberg). They are paired and each cuneiform cartilage is rod-shaped. Each is situated in the aryepiglottic fold in front of corniculate cartilage and provides passive support to the fold.
Most of the laryngeal framework – including thyroid, cricoid and most of the arytenoid cartilages are hyaline cartilages whereas epiglottis, corniculate, cuneiform and tip of arytenoid near the corniculate cartilage are elastic fibrocartilage and generally do not ossify. Hyaline cartilage may undergo ossification; it begins at the age of 25 years in thyroid, a little later in cricoid and arytenoids, and is complete by 65 years of age. This natural calcification in these cartilages can sometimes be mistaken for foreign bodies of the oesophagus or larynx in X-ray imaging.
Laryngeal joints
- Cricoarytenoid joint. It is formed between the base of the arytenoid and a facet on the upper border of the cricoid lamina. It is a synovial joint and allows the arytenoids to rotate around a vertical axis and glide movements in all directions, effectively opening or closing the vocal folds. These movements enable the production of sound and the regulation of airflow.
- Cricothyroid joint. It is formed between the inferior cornua of the thyroid cartilage and the facet on the cricoid cartilage. It is also a synovial joint and allows rotatory movements around a transverse axis passing through both cricothyroid joints permitting tension and relaxation of vocal cords: adjusting the pitch. There is some gliding movement also in different directions
Laryngeal membranes and ligaments
The membranes and ligaments of the larynx are classified as extrinsic or intrinsic depending upon their points of attachment. Extrinsic membranes attach the larynx to surrounding structures, such as the hyoid bone or trachea, while intrinsic membranes connect components within the larynx itself.
- Extrinsic membranes and ligaments
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- Thyrohyoid membrane. This connective tissue membrane connects the thyroid cartilage to the hyoid bone. It is pierced by superior laryngeal vessels and internal laryngeal nerves.
- Cricotracheal membrane. This membrane connects the cricoid cartilage to the first tracheal ring, stabilizing the larynx at its lower end.
- Hyoepiglottic ligament. This ligament anchors the epiglottis to the hyoid bone.
- Intrinsic membranes and ligaments
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- Cricovocal membrane. It is a triangular, fibroelastic membrane with a free upper border stretching from the middle of the thyroid angle to the vocal process of the arytenoid, forming the vocal ligament. Its lower border attaches to the arch of cricoid cartilage, and, together with its counterpart, forms the conus elasticus, a site where foreign objects may become lodged.
- Quadrangular membrane. Situated beneath the mucosa of the aryepiglottic folds, this membrane spans between the epiglottic and arytenoid cartilages. Its lower border forms the vestibular ligament, found in the false vocal cord.
- Cricothyroid ligament. The anterior section of the cricothyroid membrane, thickened to form this ligament, with its lateral portion forming the cricovocal membrane.
- Thyroepiglottic ligament. Connects the epiglottis to the thyroid cartilage.
Muscles of larynx
Laryngeal muscles are divided into two types: intrinsic muscles, which attach laryngeal cartilages to each other and control the movements of the laryngeal cartilages relative to each other, and extrinsic muscles, which link the larynx to adjacent structures.
Intrinsic Muscles
The intrinsic muscles of the larynx are responsible for modifying the position, shape, and tension of the vocal folds or the laryngeal inlet. These adjustments allow the vocal folds to be drawn together (adduction), moved apart (abduction), or stretched (tensors) by increasing longitudinal tension.
- Cricothyroid muscle – The cricothyroid muscle links the cricoid cartilage to the thyroid cartilage. When it contracts, it tilts the thyroid cartilage forward and downward. This motion lengthens and tightens the vocal ligaments, enabling the production of higher-pitched sounds by increasing vocal fold tension.
- Cricoarytenoid muscle – This muscle connects the cricoid and arytenoid cartilages and is divided into two main parts: the posterior cricoarytenoid and the lateral cricoarytenoid.
- Posterior Cricoarytenoid Muscle. Contraction of this muscle rotates the arytenoid cartilage laterally causing abduction of vocal ligaments. It is the only muscle that abducts the vocal folds and is crucial for maintaining an open airway during breathing.
- Lateral Cricoarytenoid Muscle. Contraction of the lateral cricoarytenoid rotates of arytenoid cartilage medially causing adduction of vocal ligaments), which is essential for voice production and controlling airflow during speech.
- Interarytenoid muscles – These connect the right and left arytenoid cartilages. Two muscles run between the cartilages.
- Transverse interarytenoid muscle – It runs transversely between the posterolateral surfaces of arytenoids. Contraction of this muscle moves the arytenoid cartilages medially causing adduction of vocal ligaments.
- Oblique interarytenoid muscle – This muscle runs obliquely from the posterior surface of the muscular process of the arytenoid on one side to the opposite arytenoid’s apex. When it contracts, along with the aryepiglottic muscle, it helps close the laryngeal inlet during swallowing to prevent aspiration.
- Thyroarytenoid muscle – There are two parts of the thyroarytenoid muscle – the vocalis muscle (internal part) and the thyroepiglottic muscle (external part).
- Vocalis muscle – It runs between the arytenoids and thyroid angle, lateral to the vocal ligaments. Contraction of the vocalis muscle causes shortening (relaxing tension) of vocal ligaments by bringing arytenoids close to the thyroid angle. It helps in changing the pitch and quality of voice.
- Thyroepiglottic muscle – This muscle aids in opening the laryngeal inlet by pulling the aryepiglottic folds outward, a movement that facilitates breathing and voice modulation.
Muscles Acting on the Vocal Cords:
- Abductors: Posterior cricoarytenoid muscle
- Adductors: Lateral cricoarytenoid muscle, transverse interarytenoid muscle, thyroepiglottic muscle
- Tensors: Cricothyroid muscle, vocalis muscle
Muscles Acting on the Laryngeal Inlet:
- Openers: Thyroepiglottic muscle
- Closers: Oblique interarytenoid muscle and aryepiglottic muscle
Extrinsic Muscles
These muscles connect the larynx to adjacent structures and are categorized as elevators and depressors.
- Elevators:
- Primary Elevators: Directly elevate the larynx by attaching to the thyroid cartilage, including the stylopharyngeus, salpingopharyngeus, palatopharyngeus, and thyrohyoid.
- Secondary Elevators: Elevate the larynx indirectly by attaching to the hyoid bone, including the mylohyoid, digastric, stylohyoid, and geniohyoid.
- Depressors: Lower the larynx and include the sternohyoid, sternothyroid, and omohyoid.
Lymphatic drainage
- Supraglottis: The area above the vocal cords is drained by lymphatic vessels that penetrate the thyrohyoid membrane and empty into the upper deep cervical lymph nodes.
- Glottis: There are practically no lymphatics in the vocal cords themselves, so cancers originating here rarely metastasize via lymphatic pathways.
- Subglottis: Below the vocal cords, lymphatic vessels pass through the cricothyroid membrane to reach the prelaryngeal, pretracheal, and lower deep cervical lymph nodes. Some lymphatics also penetrate the cricotracheal membrane and drain directly into the lower deep cervical nodes.
Nerve supply
Both the motor supply and sensory supply of the larynx are provided by the recurrent laryngeal nerve and superior laryngeal nerve.
- Motor. All the muscles of the larynx are supplied by the recurrent laryngeal nerve, except for the cricothyroid muscle, which is supplied by the external branch of the superior laryngeal nerve.
- Sensory. Below the vocal cords, the sensory supply is provided by the recurrent laryngeal nerve. Above the vocal cords, the sensory supply is provided by the internal branch of the superior laryngeal nerve.
The course of the recurrent laryngeal nerve. The course of right and left recurrent laryngeal nerves are not symmetrical.
- The right recurrent laryngeal nerve arises from the vagus at the level of the subclavian artery, loops around it and then ascends into the neck in the tracheo-oesophageal groove and enters into the larynx at the level of the cricothyroid joint. In the upper part of the groove, it is intimately related to the inferior thyroid artery. As it approaches the thyroid, it runs near the bifurcation of the right inferior thyroid artery, where it takes an oblique and more lateral course compared to the left RLN. This positioning increases its susceptibility to injury during dissection or manipulation of the thyroid gland.
- The left recurrent laryngeal nerve has a longer course than the right. It arises from the vagus in the mediastinum at the level of the arch of aorta, loops around it and then ascends into the neck in the tracheo-oesophageal groove and enters into the larynx at the level of the cricothyroid joint. It lies posterior to the inferior thyroid artery.
The course of the superior laryngeal nerve. It arises from the inferior ganglion of the vagus, descends behind the internal carotid artery and, at the level of greater cornua of the hyoid bone, divides into the external laryngeal nerve and internal laryngeal nerve.
- The internal branch travels with the superior laryngeal artery, pierces the thyrohyoid membrane and provides sensory supply to the mucosa of the larynx above the vocal cords
- The external branch travels with the superior thyroid artery to provide motor supply to the cricothyroid muscle.
Blood supply
Arterial supply:
- Superior laryngeal artery branch of superior thyroid artery
- Inferior laryngeal artery branch of inferior thyroid artery
- Cricothyroid branch of the superior thyroid artery
Venous drainage: Veins accompanying the laryngeal arteries provide venous drainage.
- Superior Laryngeal Veins: Drain into the internal jugular vein via the superior thyroid or facial veins.
- Inferior Laryngeal Veins: Drain into the inferior thyroid veins, which then flow into the brachiocephalic vein.
- Additional drainage is via the middle thyroid vein, which also empties into the internal jugular vein.
The mucous membrane of the larynx
The laryngeal mucous membrane covers the entire larynx, adhering loosely except on the posterior surface of the epiglottis, the true vocal cords, and the corniculate and cuneiform cartilages. The epithelial lining is mostly ciliated columnar epithelium, although it becomes stratified squamous epithelium over the vocal cords and the upper supraglottis. Mucous glands are widely distributed throughout the membrane, being particularly dense on the posterior surface of the epiglottis, the posterior part of the aryepiglottic folds, and within the saccules. However, the vocal folds lack mucous glands.
———— End of the chapter ————
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Reference Textbooks.
- Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
- Cummings, Otolaryngology-Head and Neck Surgery.
- Stell and Maran’s, Textbook of Head and Neck Surgery and Oncology.
- Ballenger’s, Otorhinolaryngology Head And Neck Surgery
- Susan Standring, Gray’s Anatomy.
- Frank H. Netter, Atlas of Human Anatomy.
- B.D. Chaurasiya, Human Anatomy.
- P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
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