Glottis

Structure

The glottis is composed of the vocal folds and the opening between them called the rima glottidis. It is located in the interior of the larynx between the supraglottis and subglottis. The supraglottis refers to the region of the larynx between the base of the tongue and the vocal cords, while the subglottis is the part of the larynx that lies just inferior to the vocal cords down to the tracheal opening. These terms are clinically important for indicating the exact location of lesions within the laryngeal cavity.

Specifically, the glottis lies just inferior to the paired vestibular folds at the entrance of the middle part of the laryngeal cavity. The vestibular folds are folds of the mucous membrane which cover the vestibular ligaments. The vestibular folds are also called “false vocal cords” because they do not play a role in sound production, but instead protect the true vocal cords. The triangular opening between the two adjacent vestibular folds is called the rima vestibuli, and is continuous with the rima glottidis. The lateral walls of the laryngeal cavity between the vestibular and vocal folds bulge outward to form lateral pouches called laryngeal ventricles.

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Vocal folds

The vocal folds are wedge-shaped folds of mucous membrane that project medially from the lateral walls of the middle part of the laryngeal cavity, inferior to the vestibular folds. Each vocal fold consists of a vocal ligament and a vocalis muscle and the overlying mucosa which covers it. The vocal ligament is the thickened, medial free edge of the conus elasticus (cricothyroid membrane) that extends from the posterior surface of the thyroid cartilage to the vocal processes of the arytenoid cartilages. The vocalis muscle arises from the vocal process of the arytenoid cartilage and runs parallel to and inserts along the length of the vocal ligament. Its function is to adjust the tension in the vocal folds.

The vocal folds receive both sensory and motor innervation from the inferior laryngeal nerve, the terminal part of recurrent laryngeal nerve, from the vagus nerve (CN X). The arterial supply to the vocal folds is by the inferior laryngeal artery, a branch of the inferior thyroid artery, which accompanies the inferior laryngeal nerve.

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Rima glottidis

The rima glottidis refers to the narrow, triangular opening between the two adjacent vocal folds. This opening is continuous with the rima vestibuli above and leads into the infraglottic cavity below. The shape of this opening changes depending on the type of activity the larynx is involved in, which determines the position of the vocal folds. Rotational movements of the arytenoid cartilages at the cricoarytenoid joints can separate (abduct) the vocal folds, widening the rima glottidis or appose (adduct) the vocal folds and narrow the rima glottidis.

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Functions

The vocal folds are called the “true vocal cords” because they are responsible for sound production. This is achieved by vibrations of the free edges of the vocal folds when air is forced through the rima glottidis. The quality of sound produced depends on the length, tension and position of the vocal folds. The vocal folds also act as the primary inspiratory laryngeal sphincter when they are closed, to block the passage of air.

During regular breathing at rest, the rima glottidis is a narrow wedge shape, but during forced respiration it is a wide triangular shape and the vocal cords are as far apart as they can extend. During phonation, the vocal cords close and the rima glottis is slit-like.