Intrinsic Muscles of Larynx Anatomy
1. Cricothyroid muscle
Origin: Arises from the anterolateral part of the cricoid cartilage.
Insertion: Inserts into the inferior aspect and inferior horn of the thyroid cartilage.
Action: Stretches and tenses the vocal folds.
Innervation: External branch of the superior laryngeal nerve of the vagus.
Comment: This muscle is innervated by the small, external branch of the superior laryngeal nerve of the vagus. Most of the superior laryngeal nerve continues as an internal branch that pierces the thyrohyoid membrane to provide sensory innervation above the vocal folds.
This muscle, similar to the other muscles of the larynx, is derived embryologically from the 4th through 6th pharyngeal (branchial) arches. All of these laryngeal muscles are innervated by the vagus nerve.
Clinical: Damage on 1 side to the superior laryngeal nerve, a branch of the vagus nerve (CN X), will paralyze the ipsilateral cricothyroid muscle. Consequently, the voice will be affected because the ipsilateral vocal fold cannot be fully stretched and tensed. Additionally, the ipsilateral laryngeal mucosa above the level of the vocal folds will be anesthetized (the superior laryngeal nerve is sensory to the laryngeal mucosa above the vocal folds), somewhat compromising the protective gag reflex that would normally keep foreign objects from being aspirated into the larynx.
1. Posterior crico-arytenoid muscle
Origin: Arises from the posterior surface of the laminae of the cricoid cartilage.
Insertion: Attaches to the muscular process of the arytenoid cartilage.
Action: Abducts the vocal folds and widens the rima glottidis, the space between the vocal folds.
Innervation: Recurrent (inferior) laryngeal nerve of the vagus.
Comment: The posterior crico-arytenoid muscles are extremely important because they are the only muscles that abduct the vocal folds.
Clinical: Damage to the recurrent laryngeal nerve during neck surgery (e.g., resection of the thyroid gland) can cause the vocal folds to adduct, causing hoarseness or closure of the rima glottidis, or both. This occurs because the posterior cricoarytenoid muscles are the only laryngeal muscles that abduct the vocal folds and keep the rima glottidis open. The vocal folds are controlled by the laryngeal muscles, all of which are innervated by the vagus nerve (CN X). During quiet respiration, the vocal folds are gently abducted to open the rima glottidis (space between the folds). In forced inspiration (taking a rapid, deep breath), the folds are maximally abducted by the posterior crico-arytenoid muscles, further enlarging the rima glottidis. During phonation, the folds are adducted and tensed to create a reed-like effect (similar to a reed instrument), causing vocal fold mucosal vibrations that produce sound that is then modified by the upper airway (pharynx, oral cavity, tongue, lips, nose, and paranasal sinuses). Closure of the rima glottidis occurs when holding your breath or when lifting something heavy (the Valsalva maneuver), and the folds are completely adducted.