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. They 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.
A video displaying the anatomy of the vocal cords and voice box is provided below.
The vocal cords are composed of mucous membrane infoldings that stretch horizontally across the middle laryngeal cavity. They are attached anteriorly at the angle 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 ventricles extend laterally and are located between the vestibular and vocal folds. [1, 2]
Vestibular folds (false vocal cords)
The quadrangular membrane extends between the lateral aspects of the epiglottis and the anterolateral surface of the arytenoid cartilages on the 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) once covered by mucosa. The lower border of the vestibular folds forms the upper boundary of the laryngeal ventricle.
The primary function of the vestibular folds is protection of the more delicate vocal folds that lie beneath. The vestibular folds have a minimal role in phonation but may be used in the production of deep tones and screaming or throat singing.
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. 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 main function of the muscle is fine tonal control of the vocal cords.
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 women than in men.
Functional anatomy of the vocal cords
The larynx, or voice box, has multiple intrinsic muscles that control movement of the vocal folds. All of these muscles are innervated by the recurrent laryngeal branch of the vagus nerve (CN 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 muscles function to rotate the arytenoid cartilages laterally, thereby abducting the vocal cords. Their action opposes that of the lateral cricoarytenoid muscles.
The lateral cricoarytenoid muscles function to rotate the arytenoid cartilages medially, thereby adducting the vocal cords.
The main function of the transverse arytenoid muscle is adduction of the vocal cords.
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.
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 an inability to abduct the vocal folds and causes difficulty breathing.
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 the stratified squamous epithelium on the vocal folds. 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 3 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 an increased pliability; the intermediate lamina propria is mainly composed of elastic fibers; and the deep lamina propria is composed of more collagenous fibers. These 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.
The structure of adult vocal folds differs from the vocal fold structure 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. The formation of the 3 defined lamina propria layers occurs between the ages of 6 and 12 and 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.
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.
Swelling from abnormal accumulation of fluid that occurs within the superficial lamina propria (Reinke space) may cause the vocal fold mucosa to appear floppy with excessive movement.