The intrinsic muscles of the larynx can be divided into three groups based on on their actions. All are innerveated by the recurrent laryngeal nerve, except for cricothyroid which is innervated by the external branch of the superior laryngeal nerve (all branches of CN X)

Laryngeal inlet muscles

The inlet of the larynx is closed by aryepiglotticus, which runs in the aryepiglottic folds of the quadrangular membrane between the apex of the arytenoids and lateral border of the eppiglottis as an extension of the oblique arytenoids. This action is opposed by thyroepiglotticus.

The arytenoid muscle (consisting of the oblique and transverse arytenoids) helps constrict the laryngeal inlet by running between the arytenoid cartilages.

Vocal folds abduction & adduction

Abduction (opening) and adduction (closing) of the vocal folds result from movements at the cricoarytenoid joints. The posterior cricoarytenoid, which arises from the posterior surface of the cricoid lamina to inserts on the muscular process of the ipsilateral arytenoid, is the sole abductor of the vocal cords and required to open the airway for breathing. The lateral arytenoids (pulling the arytenoids from the lateral parts of the cricoid arch) and interarytenoids (between the arytenoids) act as adductors.

CLINICAL CORNER

Bilateral injury to the recurrent laryngeal nerve could prevent abduction of the vocal cords by the posterior cricoarytenoid, causing difficulty breathing.

Vocal fold tension

Vocal fold tension changes pitch during phonation. The cricothyroid muscle causes the thyroid cartilage to move forwards or the cricoid cartilage up and backwards to tighten the vocal folds, resulting in a lower pitch. The thyroarytenoid runs alongside each vocal cord and opposes this action to reduce tension in the folds. Vocalis, the medial part of thyroarytenoid within each vocal fold, specifically reduces cord length and alters its thickness.