Intrinsic muscles of the larynx
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The intrinsic muscles of the larynx can be considered in two groups:
- muscles that control the inlet of the larynx
- muscles that move the vocal ligaments
Muscles of the inlet
- aryepiglottic muscle: lies within the aryepiglottic fold, runs from the side of the epiglottis and inserts onto the muscular process and posterior surface of the arytenoid cartilage on the same side
- oblique arytenoid muscle: this paired muscle runs from the muscular process of one arytenoid cartilage to the opposite aryepiglottic muscle and corniculate cartilage, therefore creating an X-shape superficial to the transverse arytenoid muscle
- thyroepiglottic muscle: originates from the upper border of the lamina of the thyroid cartilage and inserts into the side of the epiglottis
These muscles act to close the inlet of the larynx by drawing the epiglottis down so that its lower half makes contact with the arytenoids. This creates an effective protection from swallowed material. The inlet is opened by air pressure from the respiratory tract below.
Muscles of the vocal folds
- cricothyroid muscle: this triangular muscle runs from the anterolateral part of the cricoid cartilage, fans out and inserts onto the inferior margin and inferior horn of the thyroid cartilage
- thyroarytenoid muscle: like the thyroepiglottic muscle, the thyroarytenoid originates from the upper border of the lamina of the thyroid cartilage, however, this muscle runs to insert onto the anterolateral arytenoid surface
- posterior cricoarytenoid muscle: arises from the posterior surface of the lamina of the cricoid cartilage and inserts onto the vocal process of the arytenoid cartilage
- lateral cricoarytenoid muscle: arises from the upper border of the cricoid arch and its fibers pass backwards and upwards to insert onto the vocal process of the arytenoid cartilage
- transverse arytenoid (or interarytenoid) muscle: consists of a mass of transverse fibers, which connects the posterior surfaces of the arytenoid cartilages together
- vocalis muscle: originates at the lateral surface of the vocal process of the arytenoid cartilage; it inserts onto the ipsilateral vocal ligament
Actions of the intrinsic muscles
These muscles act in opposite pairs in order to open and close, or lengthen and shorten the vocal folds:
Adductors and abductors: The lateral cricoarytenoid muscles pull the muscular processes anteriorly which rotates the arytenoid cartilages so that the vocal processes swing medially. They are the principal adductors. When they act with the transverse and oblique arytenoid muscles, which pull the arytenoid cartilages together, this brings about phonation. The only muscles that abduct the vocal folds are the posterior cricoarytenoid muscles. They pull the muscular processes posteriorly, which rotates the vocal processes laterally. The posterior cricoarytenoid muscles open the rima glottidis.
Tensors: The cricothyroid muscles pull the angle of the thyroid cartilage anteriorly and inferiorly toward the arch of the cricoid cartilage, which causes elongation and tightening of the vocal ligaments. This brings about a higher pitch of phonation.
Relaxers: The thyroarytenoid muscles pull the arytenoid cartilages anteriorly which reduces the vocal ligaments and therefore lowers the pitch of the voice.
Fine adjustments: The vocalis muscles bring about this by selectively tensing and relaxing the anterior and posterior parts of the vocal folds. This action only occurs when the vocal folds are in contact for phonation, and therefore contributes to changes in pitch during animated speech or singing.
Many of these muscles also help contribute to closing and protecting the laryngeal inlet. The muscles involved are the lateral cricoarytenoids, transverse and oblique arytenoids, and aryepiglottic muscles. Contraction of these muscles brings the aryepiglottic folds together and pulls the arytenoid cartilages toward the epiglottis. The transverse arytenoid closes the posterior, intercartilaginous portion of the rima glottidis, to produce whispering.
All but one intrinsic muscle of the larynx is supplied by the inferior laryngeal nerve, which is the terminal part of the recurrent laryngeal nerve, itself from the vagus nerve (CN X). The only exception is the cricothyroid muscle, which is supplied by the external laryngeal nerve from the superior laryngeal nerve, a branch of the vagus nerve (CN X).
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