Overview
The vocal folds, also known as vocal cords, are located within the larynx (also colloquially known as the voice box) at the top of the trachea. The larynx plays a critical role in respiration, phonation, and airway protection to prevent aspiration. [3] The vocal folds are open during inhalation and come together to close during swallowing and phonation. When closed, the vocal folds may vibrate and modulate the expelled airflow from the lungs to produce speech and singing. Studies have challenged the conventional belief that laryngeal closure during swallowing occurs from inferior to superior, which means that the vocal folds adduct first, and the epiglottis covers the arytenoids and glottis last. Studies using simultaneous electromyography and fiberoptic endoscopic evaluation of swallowing have reported that the aryepiglottic folds close before vocal fold adduction during swallowing. [3]
A video displaying the anatomy of the vocal cords and voice box is provided below.
Gross Anatomy
The vocal cords are composed of mucous membrane infoldings that stretch horizontally across the middle laryngeal cavity. They are attached anteriorly at the angle, often referred to as the anterior commissure, on the interior surface of the thyroid cartilage and project posteriorly to the arytenoid cartilages on either side.
The vestibular folds, or false vocal cords, are formed by the superior layer of infolded membrane; the vocal folds, or true vocal cords, are formed from the inferior layer of infolded membrane. The laryngeal ventricle sits between the vestibular fold and the thyroid cartilage and opens to the airway between the inferior edge of the vestibular fold and the true vocal fold. [1, 2]
Vestibular folds (false vocal cords)
The vestibular folds are mucosal folds that play a primarily protective role within the larynx. [3] The quadrangular membrane extends between the lateral aspects of the epiglottis and the anterolateral surface of the arytenoid cartilages on each side. The free lower inferior margin of this ligament is thickened to form the vestibular ligament, which forms the vestibular folds (false vocal cords) and is covered by mucosa.
The vestibular folds lie superior and lateral to the true vocal cords and are separated from the true vocal folds by the opening of the laryngeal ventricle. [4] The lower border of the vestibular folds forms the upper boundary of the laryngeal ventricle.
Histologically, they are composed of connective tissue, elastic fibers, and seromucous glands that contribute to lubrication and antimicrobial defense of the laryngeal mucosa. [4] Their glandular density is significantly higher than that of the true vocal cords, aiding in maintaining viscoelastic properties and reducing friction during phonation. [5]
Functionally, while the vestibular folds are not directly involved in sound production, they contribute to phonation in specific scenarios such as deep tones, throat singing, or compensatory mechanisms during vocal fold paralysis. They also assist in airway protection by closing during swallowing and reflexive actions such as coughing or gagging. During phonation, they are typically held apart to prevent unwanted vibration. [4]
Laryngeal ventricles
On either side of the middle laryngeal cavity, in between the vestibular and vocal folds, the mucosa bulges laterally to form troughs known as the laryngeal ventricles. The laryngeal saccules are tubular extensions of each ventricle anterosuperiorly between the vestibular fold and the thyroid cartilage. The walls of these saccules are thought to contain many mucous glands that lubricate the vocal folds.
Vocal folds (true vocal cords)
The conus elasticus extends superiorly from the anterior arch of the cricoid cartilage and attaches to the thyroid cartilage anteriorly and the vocal processes of the arytenoid cartilages posteriorly. The free superior margin of this submucosal membrane is thickened to form the vocal ligament, which forms the vocal folds (true vocal cords) once covered by mucosa. [6] The vocal folds also contain muscle fibers originating from the vocalis part of the thyroarytenoid muscle, which lies deep and inferior, parallel with the vocal ligament to which it is attached at the posterior end. The primary function of this muscle is to make fine adjustments in tension, length, and thickness of the vocal folds, enabling precise tonal control during phonation. Contraction of this muscle can selectively cause the different portions of the vocal ligament to relax or become tense, influencing pitch and voice quality. [6]
The adult vocal fold has a layered structure consisting of stratified squamous epithelium (approximately 0.05 mm thick in the midportion of the vocal fold); a three-layered lamina propria (about 1 mm thick), subdivided into the superficial, intermediate, and deep layers; and the vocalis muscle. [7]
The vocal folds differ in size and color between males and females. Adult males typically have larger, longer folds (due to the sexually dimorphic laryngeal prominence), resulting in a lower-pitched voice. The vocal folds appear more pearly white in females than in males. In adult males, they typically measure 17-21 mm in length, while in adult females, they measure 11-15 mm in length. [8]
Functional anatomy of the vocal cords
The larynx, or voice box, has multiple intrinsic and extrinsic muscles that control movement of the vocal folds. All of these muscles are innervated by the recurrent laryngeal branch of the vagus nerve (cranial nerve X) except the cricothyroid muscles, which are innervated by the external branch of the superior laryngeal nerve, which is also a branch of the vagus nerve.
The cricothyroid muscles function to elevate the anterior arch of the cricoid cartilage and depress the posterior portion of the thyroid cartilage lamina. This produces tension and elongation of the vocal cords, resulting in higher-pitch phonation.
The posterior cricoarytenoid muscle functions to rotate the arytenoid cartilages laterally, thereby abducting the vocal cords. Its action opposes that of the lateral cricoarytenoid muscles. It is the only abductor of the true vocal cords. By laterally rotating the arytenoid cartilages, it separates the vocal folds and opens the rima glottidis for breathing. It is also active during phonation to fine-tune glottic control. [6, 9]
The lateral cricoarytenoid muscles function to rotate the arytenoid cartilages medially, thereby adducting the vocal cords and closing the rima glottidis to facilitate phonation and protect the airway during swallowing. [6]
The main function of the transverse arytenoid muscle is adduction of the vocal cords. The oblique arytenoid muscles assist in similar actions, while also contributing to narrowing of the laryngeal inlet. [6]
The thyroarytenoid muscles function to draw the arytenoid cartilages forward, thereby relaxing and shortening the vocal cords, while also rotating the arytenoid cartilage inward, thus adducting the vocal folds and narrowing the rima glottis. The vocalis part of each thyroarytenoid muscle is contained within the vocal folds on each side which fine-tunes tension within the vocal folds for precise modulation of pitch during phonation. [6]
Of note, the only muscles capable of separating the vocal cords for normal breathing are the posterior cricoarytenoid muscles. Bilateral injury to the recurrent laryngeal branches of the vagus nerve (CN X) results in the inability to abduct the vocal folds and causes difficulty in breathing.
Microscopic Anatomy
The upper vestibular folds are covered with a pseudostratified ciliated columnar epithelium with goblet cells. The underlying lamina propria contains an abundance of mixed serous and mucous glands, from which excretory ducts open onto the epithelial surface.
The ventricle separating the vestibular folds and vocal folds is the site where the epithelium transitions from respiratory epithelium to stratified squamous epithelium on the vocal folds. This transition zone is crucial for maintaining structural integrity and function in areas subject to varying mechanical stresses. [3]
The lamina propria within the laryngeal ventricles blends with the perichondrium of the hyaline thyroid cartilage. No distinct submucosa exists.
The lower vocal folds are lined with a thick stratified squamous epithelium, which functions to protect the mucosa from abrasion caused by the rapid movement of air when breathing and during phonation.
A thicker layer of connective tissue is located beneath the vocal fold epithelium and is subdivided into three layers: the superficial lamina propria, the intermediate lamina propria, and the deep lamina propria.
The superficial lamina propria is composed of few elastic or collagenous fibers, resulting in increased pliability; the intermediate lamina propria is mainly composed of elastic fibers; and the deep lamina propria is composed of more collagenous fibers. These collagen fibers provide the necessary structural support and preserve tissue integrity of the vocal folds against the repeated stress of high-frequency stretching and recoiling. [10] The elastic and collagenous fibers within the intermediate and deep layers form the vocal ligament. Beneath the deep lamina propria, the skeletal muscle fibers of the vocalis muscle form the innermost layer and body of the vocal folds.
Natural Variants
The structure of vocal folds in adults differs from that in newborns. The infant lamina propria is only composed of a single layer without a vocal ligament. The vocal ligament begins to appear at about 4 years of age. Formation of the three defined lamina propria layers occurs between the ages of 6-12 years, and they are fully mature at the end of adolescence.
In old age, thinning of the superficial layer of the lamina propria and atrophy of the vocalis muscle occurs, which contributes to the difficulty of speech seen with presbylarynx.
Pathophysiological Variants
Spasmodic dysphonia
Spasmodic dysphonia is a voice disorder in which involuntary movements of one or more muscles of the larynx occur during speech. Three types exist: adductor, abductor, and mixed.
In adductor spasmodic dysphonia, the vocal folds involuntarily slam together and stiffen. Words are often cut off or difficult to start, causing speech to be choppy and sound similar to stuttering.
In abductor spasmodic dysphonia, the vocal folds open involuntarily. As a result, the voice sounds weak and whispery.
In mixed spasmodic dysphonia, elements of both adductor and abductor spasmodic dysphonia are present because of the involuntary opening and closing of the vocal folds.
Vocal cord nodules and polyps
Vocal cord reactive nodules, also called polyps, are masses of tissue that grow on the vocal cords, typically on the anterior and middle two thirds of the vocal fold. The nodules usually appear as symmetrical swellings on both sides of the vocal cords and develop most commonly in heavy smokers or individuals who use strenuous or abusive voice practices.
Reinke edema
Swelling from abnormal accumulation of fluid that occurs within the superficial lamina propria (Reinke's space) may cause the vocal fold mucosa to appear floppy with excessive movement.
Vocal fold paresis
Spontaneous or traumatic unilateral loss of one of the recurrent laryngeal nerves leads to paresis of one side of the larynx. As a result, the voice becomes breathy, and the patient is prone to aspiration due to incomplete closure of the laryngeal inlet during swallowing.
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Airway endoscopy demonstrating the larynx of a 3-year-old child. The larynx is situated at the level of the base of tongue. Separating the tongue from the larynx is the flaplike structure called the epiglottis. The larynx itself consists of 2 glistening white vocal folds, which form a V-shaped structure. The glottic airway is between the vocal folds. Just superior to the vocal folds are the false vocal folds. The posterior aspect of the larynx is lined by the arytenoids. The arytenoids form 2 "bumps" at the posterior aspect of the vocal folds and are separated by the intra-arytenoid notch. Posterior to the larynx is the esophageal inlet. (Video courtesy of Dr. Ravi Elluru)
