Here we'll learn the intrinsic muscles of the larynx, which alter tension on the vocal folds, the dimension of the rima glottidis, and modify the laryngeal inlet.
In contrast, the extrinsic muscles anchor the larynx within the neck and move it vertically during speaking and swallowing.
Suprahyoid Muscles
Infrahyoid Muscles
From inferior to superior, show the following: a tracheal ring, cricoid cartilage, thyroid cartilage, and hyoid bone; we've also shown the membranes that hold these structures in place.
The cricothyroid muscle attaches to the external arch of the cricoid cartilage and passes posteriorly to attach to the thyroid cartilage.
Cricothyroid has oblique and straight parts, which allow it to pull the thyroid cartilage inferiorly towards the cricoid cartilage, altering the tension on the vocal folds and changing the pitch of our voices.
The cricothyroid muscle is the only intrinsic muscle innervated by the external branch of the superior laryngeal nerve (CN X). The rest are innervated by the recurrent branch of the laryngeal nerve.
Now, let's show the larynx in posterolateral view; pay special attention to the anterior/posterior orientation of this unique perspective.
We re-draw the cartilages of the larynx.
Then, we show the epiglottis extending superiorly from its attachment to the inner surface of the anterior thyroid cartilage, and cover it with mucosa.
The Quadrangular membrane extends from the lateral edges of the epiglottis to the arytenoid cartilages; its upper and lower margins are free.
The upper margin of the quadrangular membrane forms the aryepiglottic ligament, which contributes to the aryepiglottic fold.
The aryepiglottic fold comprises the lateral border of the laryngeal inlet; within the fold, we can see the underlying cuneiform cartilage.
Because of these mucosal attachments, the laryngeal inlet has a triangular shape: it is wider anteriorly, where the epiglottis is, and slopes inferiorly to a narrower posterior border at the arytenoid and corniculate cartilages.
The lower margin of the quadrangular membrane thickens and forms the vestibular ligament, which is part of the vestibular fold, aka, false vocal cord.
To show the true vocal ligament, draw the conus elasticus; its superior free edge thickens to form the vocal ligaments.
The conus elasticus is a portion of the cricothyroid membrane that passes between the inner surfaces of the cricoid and thyroid cartilages and the tips of the vocal processes of the arytenoid cartilages.
The lateral and posterior cricoarytenoids rotate the arytenoid cartilages in opposite directions.
The lateral cricoarytenoid attaches to the superior edge of arch of the cricoid cartilage and passes posteriorly to the muscular process of the arytenoid cartilage. This muscle adducts the vocal cords by medially (internally) rotating the arytenoid cartilages; these actions increase medial compression and subglottal pressure in preparation for speech.
The posterior cricoarytenoids arise from the cricoid laminae and pass laterally to insert on the muscular processes of the arytenoid cartilages. By laterally (externally) rotating the arytenoid cartilages, these muscles abduct the vocal folds and open the glottis to let air flow freely through the larynx. Note that they are the only abductor muscles.
The transverse arytenoid muscle spans the gap between the right and left arytenoid cartilages; it's the only unpaired muscle we'll draw.
Superficial to the transverse muscle, the oblique arytenoid muscles cross each other on their paths from one arytenoid cartilage to the other. Fibers from the oblique arytenoid muscles continue laterally and superiorly into the aryepiglottic fold, where they comprise the aryepiglottic muscle.
The transverse and oblique arytenoid muscles adduct the arytenoid cartilages, closing the rima glottis. The oblique arytenoid muscles also act as a sphincter around the laryngeal inlet by adducting the aryepiglottic folds. Both actions increase medial compression and increase subglottal pressure.
The thyroarytenoid muscle is a broad, thin muscle that passes laterally over the quadrangular membrane from the anterior thyroid cartilage to the anterolateral surface of the arytenoid cartilage, with some fibers continuing upwards into the aryepiglottic fold (these fibers constitute the thyro-epiglottic part of the muscle).
The thyroarytenoid muscles act as a sphincter of the laryngeal inlet and vestibule and alter tension of the vocal ligaments.
We re-draw the thyroid and cricoid cartilages, and show the arytenoid and corniculate cartilages so that the vocal processes are pointing anteriorly.
We show the vocal ligament and label the rima glottidis.
The cricothyroid muscle travels inferiorly from the thyroid cartilage to the cricoid cartilage.
The transverse arytenoid muscle spanning the gap between the arytenoid cartilages; this muscle adducts the arytenoids and selectively closes the posterior rima glottidis when we whisper. The transverse and oblique arytenoids are sometimes referred to collectively as the interarytenoids.
The posterior cricoarytenoid muscles travel from the posterior cricoid laminae to the arytenoid cartilages. In this view, it's easier to imagine how they laterally (externally) rotate the arytenoids and abduct the vocal folds.
The lateral cricoarytenoids do the opposite: they travel from the arch of the cricoid cartilage to the arytenoid cartilages, which they internally (medially) rotate to adduct the focal folds.
The thyroarytenoid muscles, with their medial vocalis portions, travel from the thyroid cartilage to the arytenoid cartilages. When these fibers contract, the vocal ligaments shorten. Vocalis fibers can selectively relax the posterior parts of the vocal ligaments to raise the pitch of the voice.
Be aware that some texts do not refer to a vocalis portion, and others consider it to be a separate muscle.
