Muscles of the larynx
There are many muscles that either make up a certain part of the laryngeal structure inside the neck, or that sit adjacent to it and aid in its function. These muscles produce the movements of the larynx and its cartilages, thus enabling the proper air conduction, speech, movements of the epiglottis and airways protection.
The muscles of the larynx are divided into two groups:
- Extrinsic muscles, which produce the movements of the hyoid bone. These are the infrahyoid (sternohyoid, omohyoid, sternothyroid, thyrohyoid) and suprahyoid muscles (stylohyoid, digastric, mylohyoid, geniohyoid)
- Intrinsic muscles, which move the vocal cords in order to produce speech sounds. They are functionally divided into adductors (lateral cricoarytenoid, transverse arytenoid), abductors (posterior cricoarytenoid), sphincters (transverse arytenoid, oblique arytenoid, aryepiglottic), muscles that tense the vocal cords (cricothyroid), and muscles that relax the vocal cords (thyroarytenoid, vocalis).
This article will discuss the anatomy and function of the laryngeal muscles.
- Extrinsic muscles
- Intrinsic muscles
- Clinical notes
The extrinsic muscles of the larynx are those that are somehow attached to the hyoid bone, be it via origin or insertion and thus move the thyroid cartilage. The infrahyoid muscles are part of and attach to the lower larynx as well as the inferior aspect of the hyoid bone. This muscle group includes the sternohyoid muscle, the omohyoid muscle, the sternothyroid muscle and the thyrohyoid muscle which work to lower the larynx and the hyoid bone.
The suprahyoid muscles are attached to the superior aspect of the hyoid bone and function to fixate the hyoid bone as well as elevate it along with the larynx. The muscles in this group include the stylohyoid muscle, the digastric muscle and the mylohyoid muscle. The stylopharyngeus muscle is not attached directly to the hyoid bone, however it acts indirectly to elevate both the hyoid bone and the larynx.
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The intrinsic muscles of the larynx alter both the length and the tension placed upon the vocal cords as well as the rima glottidis. The adductor muscle group is made up of the lateral cricoarytenoid muscles and the transverse arytenoid muscles. The muscles that govern abduction are the posterior cricoarytenoid muscles. The sphincter muscles are the transverse arytenoid muscles, the oblique arytenoid muscles and the aryepiglottic muscles. The cricothyroid muscles are responsible for tensing the vocal cords whilst the thyroarytenoid muscles and the vocalis muscles are responsible for relaxing them.
This muscle originates on the anteriolateral part of cricoid cartilage and inserts into the inferior border of the thyroid cartilage and its inferior horn. It is innervated by the external branch of the superior laryngeal nerve and is supplied by the superior and inferior thyroid arteries, as are all the intrinsic laryngeal muscles. Upon contraction, it lengthens and tenses the vocal ligaments.
Posterior cricoarytenoid muscle
The proximal attachment of this muscle is on the posterior surface of the lamina of the cricoid cartilage and its corresponding insertion point is on the muscular process of the arytenoid cartilage. The recurrent laryngeal nerve innervates this muscle, as it does all the other intrinsic muscles of the larynx, with the exception of the cricothyroid muscle. Its function is to abduct the vocal folds.
Lateral cricoarytenoid muscle
Ailing from the arch of the cricoid cartilage, this muscle distally attaches itself to the muscular process of the arytenoid cartilage. It acts as an adductor of the vocal folds.
The thyroarytenoid muscle originates from the angle of thyroid cartilage and adjacent cricothyroid ligament. It inserts into the anterolateral surface of arytenoid cartilage, just as the posterior and lateral cricoarytenoid muscles do. As for function, the muscle shortens and relaxes the vocal cords.
The proximal attachment of the vocalis muscle is upon the vocal process of the arytenoid cartilage. It inserts distally upon the vocal ligament and acts by tensing the anterior vocal ligament and relaxing the posterior vocal ligament.
Transverse and oblique arytenoid muscles
Lastly, the arytenoid cartilage acts as a point of origin for both the transverse and oblique arytenoid muscles, which run between the two arytenoid cartilages, as they distally attach to the opposing arytenoid cartilage. Due to their points of attachment, they are able to close the intercartilaginous portion of the rima glottidis.
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Vocal fold paresis
The recurrent laryngeal nerve is responsible for innervating all muscles of the larynx except the cricothyroid muscle. The clinical term to describe when one or two of the recurrent laryngeal nerves are injured is vocal fold paresis (also known as recurrent laryngeal nerve paralysis or vocal fold paralysis).
The recurrent laryngeal nerves are involved in breathing, swallowing and vocalization. So if one of the nerves is injured (unilateral injury), it usually results in hoarseness due to the reduced movement of one of the vocal folds. Unilateral injury may also cause minor shortages of breath, and aspiration problems especially with liquids.
A bilateral injury, or when the two recurrent laryngeal nerves are injured, causes the vocal folds to weaken the air flow, which results in breathing problems, snoring sounds, stridor and fast physical exhaustion. When the vocal folds are billaterally paralyzed, hoarseness rarely occurs.