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.
