The thyroarytenoid muscle is one of the intrinsic muscles, or by some accounts set of muscles, of the larynx. It has two main muscle bellies (the internus and the externus) and a number of fibers attaching to various parts of the larynx. It originates at the lamina of thyroid cartilage just below the thyroid notch; the externus belly inserts the muscular process of the arytenoid cartilage, and the internus belly attaches to the vocal process of the arytenoid cartilage. It is innervated by the recurrent laryngeal nerve, a branch of the Vagus nerve (CN X).
The primary functions of the thyroarytenoid are to tense and relax the vocal folds; the vocal folds run along side of and are attached to the thyroarytenoid, and along with the cricothyroid muscle (the thyrovocalis's antagonist) helps to fine-tune the tension in the vocal folds for glottal closure, phonation and pitch control.
The thyroarytenoid has multiple names, depending on which part of the muscle you are talking about. In addition each part may be known by more than one name, as is always the case in anatomy. The list below may not be comprehensive, but it is as close as I can make it.
- The Thyrovocalis: The thyroarytenoid
internus, thyrovocalis or vocalis muscle is one of the major bellies of the thyroarytenoid; it is the foundation and tensor of the vocal folds. When contracted the vocal folds are shortened and thickened. This changes the fundamental frequency of the voice. To a close approximation, the paired thyrovocalis muscles are the vocal folds, although they are actually only the 'body' of the vocal folds; they are covered by the vocal ligaments and the mucosa, which is the actual location of the mucosal wave that produces voicing.
- The Thyromuscularis: The thyromuscularis, thyroarytenoid externus, or muscularis is the other belly of the thyroarytenoid. When it contracts the vocal folds are shortened and adducted (Although the cricoarytenoid is the primary vocal fold adductor). Contractions of the medial fibers of the thyromuscularis relax the vocal folds, and as such it is sometimes referred to as the laryngeal relaxer.
- The Thyreoepiglotticus : The thyreoepiglotticus or thyroepiglottic muscle are those portions of the thyroarytenoid that attach to the epiglottis. These fibers aid in epiglottal inversion during the swallow reflex.
- The Ventricularis: The ventricularis are the few fibers that run from the lateral wall of the arytenoid cartilage to the false vocal folds. The false vocal folds close during swallowing to prevent the bolus from entering the lungs, as do the true vocal folds. However the false vocal folds do not have sufficient muscle fibers to close on their own as the true vocal folds do; instead they are caused to approximate or overlap do to constriction of the larynx as a whole. As such the ventricularis is rarely referred to except as a landmark.
References:
Essentials of Anatomy and Physiology for Communication Disorders by J. Anthony Seikel, David Drumright, and Paula Seikel. Thomas Delmar Learning, 2004.
Speech Science: An Integrated Approach to Theory and Practice, by Carole T. Ferrand. Pearson Education Inc., 2007. This is a very poorly written book; I do not recommend it.
Manual of Voice Treatment: Pediatrics Through Geriatrics Third Edition, by Moya L. Andrews. Thomas Delmar Learning, 2006.
Operative Techniques in Laryngology by
Clark A. Rosen, C. Blake Simpson. Springer,
2008.
Note: The reason I wrote this writeup was because none of these sources is complete. Or even good. I found it necessary to synthesize all the various bits of information, and thought I would share it with you.