The Larynx: Structure and Function
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The muscles of the larynx consist mainly of muscles that change the opening of the glottis, as well as the tenseness of the vocal folds, thereby keeping the glottis open during respiration or more closed during vocalization. The extrinsic muscles connect the thyroid, cricoid, and arytenoid cartilages to other structures of the head and neck, while the smaller muscles within the structure of the larynx, the intrinsic muscles, move the vocal folds in reference to each other. The intrinsic muscles of the larynx abduct (move apart), adduct (bring together), alter vocal fold shape or change the longitudinal tension of the folds.
The cricothyroid muscle lengthens and stretches the vocal folds. It lies anterior and external to the larynx, and is part of the muscular triangle of the neck. It arises from the cricoid cartilage and attaches to the inferior horn and lower margin of the thyroid cartilage. When it contracts, it pulls the thyroid cartilage forward around the axis through the cricothyroid joint, increasing the distance between the thyroid and arytenoid cartilages and stretching and tensing the vocal fold. (The action of the cricothyroid tilts the thyroid cartilage down, increasing the front-to-back distance of the larynx, and thus placing the vocal folds under increased tension. This action is often referred to as the 'laryngeal tilt' or ‘rocking/pivoting the larynx’, and is important in being able to sing higher pitches.) Unlike all the other muscles of the larynx, which are innervated by the recurrent laryngeal nerve (branch) of the vagus, the cricothyroid muscle is the only muscle that is supplied by the external laryngeal branch of the vagus (superior laryngeal nerve).
The cricothyroid ligament is the larger part of the laryngeal membrane, continuing inferiorly as a median or anterior part and twin lateral ligaments. The median cricothyroid ligament is a flat band of white tissue joining the cricoid and thyroid cartilages, while the lateral cricothyroid ligament (also known as the cricothyroid membrane) keeps the cricoid and thyroid from traveling too far.
The lateral cricoarytenoid muscle arises from the upper border of the cricoid cartilage and inserts into the muscular process of the arytenoid cartilages. The lateral cricoarytenoid muscles adduct and internally rotate the arytenoid cartilages to close the rima glottides (glottis).
The posterior cricoarytenoid muscles arise from the expanse of the cricoid lamina and insert into the muscular process of the arytenoid cartilages. These muscles adduct and externally or laterally rotate the arytenoid cartilages, causing the vocal folds to separate from one another, thus opening the rima glottidis. These are the only muscles that are capable of opening the space between (abducting) the vocal folds to allow for normal breathing. If this muscle is incapacitated on both sides, as in a bilateral injury to the recurrent laryngeal nerve, the inability to pull the vocal folds apart will cause difficulty in breathing.
The transverse arytenoid muscle is a single muscle that arises from the posterior surface and lateral border of one arytenoid cartilage and is inserted into the corresponding parts of the opposite (arytenoid) cartilage, and fills up the posterior concave surfaces of the arytenoid cartilages. The transverse arytenoid muscle pulls the arytenoids toward each other when they contract, which results in adducted vocal folds. This action closes the rima glottides (the opening of the glottis), especially at its back part, to eliminate the posterior commissure - the point, angle, or surface where two parts join or connect - of the vocal folds.
The aryepiglottic fold (or aryepiglottis) is a fold of mucous membrane, enclosing ligamentous and muscular fibres, that extends from the side of the epiglottis to the apex of the arytenoid cartilage, forming the borders of the opening of the larynx.
Located in the upper part of the aryepiglottic fold is the aryepiglottic muscle (aryepiglotticus or recurrent laryngeal nerve of the vagus). It is attached to the lateral border of the epiglottis and becomes the oblique arytenoid muscle, which then attaches into the arytenoid cartilage. This muscle works as a sort of purse string to close the opening of the larynx when swallowing, protecting the larynx. The oblique arytenoids narrow the laryngeal inlet by constricting the distance between the arytenoid cartilages and the epiglottis. (When the aryepiglottis contracts, it causes the arytenoids to appose each other - it closes the laryngeal aditus by bringing the aryepiglottic folds together – and draws the epiglottis down to bring its lower half into contact with the arytenoids, thus closing the aditus.)
The thyroarytenoid muscle is a broad, thin muscle that lies parallel with and lateral to the vocal fold and extends from the lower half of the back of the thyroid cartilage to the front side of the arytenoid cartilage, to the middle cricothyroid ligament. It is variously described as being divided into the thyroarytenoid and vocalis muscle (or the thyromuscularis and the thyrovocalis), depending on the source. The thyroarytenoid muscle pulls the arytenoid cartilages forward toward the thyroid when it contracts, thereby loosening (relaxing and shortening) the vocal ligament (see below).
The fibres of the thyroarytenoid pass backward and lateralward, to be inserted into the base and anterior surface of the arytenoid cartilage. A considerable number of the fibres of the thyroarytenoid are prolonged into the aryepiglottic fold, where some of them become lost, while others are continued to the margin of the epiglottis. These fibres are called the thyreoepiglotticus or thyroepiglottic, and are sometimes described as a separate muscle. A few fibres extend along the wall of the ventricle from the lateral wall of the arytenoid cartilage to the side of the epiglottis and constitute the ventricularis muscle.
The lower and deeper fibres - the fine and most medial fibres - of the thyroarytenoid muscle can be differentiated as a triangular band originating from the depression between the two laminae of the thyroid cartilage and inserted into the vocal process of the arytenoid cartilages as well as portions of the vocal ligament, and into the adjacent portion of its anterior surface. This band is termed the vocalis or vocalis muscle, which lies parallel with the vocal ligament to which it is adherent (attached).
The vocalis muscle, as the name implies, is an important muscle for speech (and thus singing). The main function or action of the vocalis muscle is to adjust or alter the tension of small segments of the vocal folds in order to vary tonal qualities and pitches of the voice. It is a sphincter of vestibule that tightens the front part of the ligament near to the thyroid cartilage, thus narrowing the laryngeal inlet. It also supports the wall of the ventricle and its appendix. The vocalis muscle is innervated (supplied) by the recurrent laryngeal nerve.
The vocal ligaments, or inferior thyroarytenoid (thyroartenoideus internus), are two strong bands enclosed within the vocal folds. Each ligament consists of a band of yellow elastic tissue, attached in front to the angle of the thyroid cartilage, and behind to the vocal process of the arytenoid. They are constructed from epithelium (a tissue composed of cells that line the cavities and surfaces of structures throughout the body), but they have a few muscle fibres in them, namely the vocalis muscle. Owing to the connection of the deeper portion of the thyroarytenoid with the vocal fold, this part, if acting separately, is supposed to modify the fold’s elasticity and tension, while the lateral portion rotates the arytenoid cartilage inward, and thus narrows the rima glottidis by bringing the two vocal folds together.
Various parts of the larynx are closed by connective tissue membranes, which include the cricothyroid membrane, the thyrohyoid membrane and the quadrangular membrane.
The cricothyroid membrane (or conus elasticus) extends from the upper margin of the cricoid cartilage and attaches to the back of the thyroid cartilage anteriorly and the arytenoid cartilage posteriorly. Its upper free margin is the vocal ligament (true vocal fold).
The thyrohyoid membrane is a tough, fibro-elastic ligament (or membrane) that connects the thyroid cartilage with the hyoid bone. It extends from the superior margin (upper border) of the thyroid cartilage below and the upper margin of the posterior surface of the body and greater cornua of the hyoid bone above. Essentially, it fills the gap between the hyoid bone and the thyroid cartilage. It is pierced by the internal laryngeal nerve and superior laryngeal artery.
The quadrangular membrane is free at the top and bottom but attached posteriorly to the arytenoid cartilage and anteriorly to the side of the epiglottis. The lower free margin forms the vestibular fold (false vocal fold).
The extrinsic muscles are larger muscles, located outside the larynx, that position and support the larynx. They may move the cartilages, which in turn, stretch or compress the vocal folds.
The thyrohyoid muscle is a small, quadrilateral muscle (belonging to the infrahyoid muscles group) that appears like an upward continuation of the sternothyroid muscle (next paragraph). It originates at the oblique line on the lamina of the thyroid cartilage and inserts into the inferior (lower) border of body and greater cornu (horn) of the hyoid bone. It depresses the hyoid and elevates the larynx.
The sternothyroid muscle is an infrahyoid muscle that originates from the posterior surface of the manubrium sterni (or episternum) and the first and sometimes second costal cartilages (the elastic cartilages that connect the sternum – breastbone - and the ends of the ribs, and allow the chest to move during respiration), with insertion into the oblique line of the lamina of the thyroid cartilage. (The manubrium sterni is the broad, upper part of the sternum that articulates with the clavicle – collar bone - and the first two ribs.) This muscle is shorter and wider than the sternohyoid muscle (paragraph below), beneath which it is situated. Its nerve supply comes from the upper cervical nerve through the cervical ansa. The sternothyroid muscle depresses the larynx and the thyroid cartilage for mastication (chewing) and swallowing.
The sternohyoid muscle is a thin, narrow muscle that arises from the posterior border of the medial end of the clavicle, the posterior sternoclavicular ligament, and the upper and posterior part of the manubrium sterni. Passing upward and medially, it is inserted by short tendinous fibres into the lower border of the body of the hyoid bone. The sternohyoid muscle attaches the hyoid bone to the sternum. It is one of the paired strap muscles of the infrahyoid muscles group, and it serves to depress the hyoid bone.
Pharyngeal constrictors are muscles that serve to constrict the pharynx. When the bolus of food (a ball of chewed food matter mixed with saliva) reaches the pharynx, the elevator muscles relax, allowing the pharynx to descend. The constrictors then contract upon the bolus, and convey it downward into the esophagus (the muscular tube through which food passes from the pharynx to the stomach). The pharyngeal constrictors include the superior pharyngeal constrictor muscle, the middle pharyngeal constrictor muscle (which arises from the whole length of the upper border of the greater cornu of the hyoid bone), and the inferior constrictor muscle.
The inferior pharyngeal constrictor is the thickest of the three pharyngeal constrictors. It arises from the thyroid cartilages (from the oblique line on the side of the lamina from the surface behind this, nearly as far as the posterior border, and from the inferior cornu) and the sides of the cricoid cartilage (in the interval between the cricothyroid muscle in front and the articular facet of the inferior cornu of the thyroid cartilage behind). The first (and more superior) part arising from the thyroid cartilage is called the thyropharyngeal part, and the second part arising from the cricoid cartilage is called the cricopharyngeal part. From these origins, the fibres spread backward and medialward to be inserted with the muscle of the opposite side into the fibrous pharyngeal raphe – a continuous ridge of tissue that serves as the origin and insertion for several of the pharyngeal constrictors - in the posterior median line of the pharynx. The fibres then diverge from their origin, with the lower fibres descending beneath the inferior constrictor, the middle fibres passing transversely, and the upper fibres ascending and overlapping the superior constrictor. The inferior fibres are horizontal and continuous with the circular fibres of the esophagus. The rest of the fibres ascend, increasing in obliquity, and overlap the middle constrictor from the lesser cornu and from the stylohyoid ligament (see below for definition).
The pharyngeal constrictors are all innervated by branches from the pharyngeal plexus (a network of nerve fibres innervating most of the palate, larynx and pharynx) and by neuronal branches from the recurrent (inferior) laryngeal nerve (a branch of the vagus nerve - tenth cranial nerve - that supplies motor function and sensation to the larynx).
Other extrinsic muscles associated with the larynx are the digastric, stylohyoid, mylohyoid, geniohyoid and hyoglossus muscles. These supplemental muscles are known as the laryngeal elevators (or the suprahyoid muscles) because they raise the larynx in the neck or support it. Some of these muscles are visible in images of the tongue, as they are also extrinsic muscles of the tongue.
The digastric is a small muscle located under the jaw and extending, in a curved form, from the mastoid process (the smooth, pyramidal or cone-shaped bone projections at the base of the skull on each side of the head just below and behind the ears) to the symphysis menti (the midline symphysis between the two halves of the mandible, or jaw) that acts to elevate the hyoid bone when it contracts. It consists of two fleshy bellies united by an intermediate rounded tendon. If the hyoid is being held in place (by the infrahyoid muscles), it will tend to depress the mandible and thus open the mouth.
The stylohyoid is a slender muscle lying anterior and superior to the posterior belly of the digastric muscle. It arises from the posterior and lateral surface of the styloid process of the temporal bone (a slender, pointed piece of bone located just below the ear that projects down and forward from the inferior surface of the temporal bone, and serves as an anchor point for several muscles associated with the tongue and larynx), near the base. Passing inferiorly and anteriorly, it is inserted into the body of the hyoid bone, at its junction with the greater cornu, and just superior the omohyoid muscle (an infrahyoid muscle located at the front of the neck, arising from the scapula and inserted into the body of the hyoid bone, that consists of inferior and superior bellies separated by an intermediate tendon, and acts to depress the hyoid bone during chewing and swallowing).
The mylohyoid is a flat and triangular muscle situated immediately above the anterior belly of the digastric muscle, and running from the mandible (lower jaw) to the hyoid bone, forming the floor of the oral cavity.
The geniohyoid is a narrow muscle situated superior to the medial border of the mylohyoid muscle, and is involved in driving food from the mouth into the pharynx and in depressing the mandible.
The hyoglossus is a thin and quadrilateral muscle that arises from the side of the body and from the whole length of the greater cornu of the hyoid bone, and passes almost vertically upward to enter the side of the tongue, between the styloglossus (a muscle with origin from the lower end of the styloid process, with insertion into the side and undersurface of the tongue, with nerve supply from the hypoglossal nerve, and whose action retracts the tongue) and longitudinalis inferior (a narrow band situated on the under surface of the tongue between the genioglossus and the hyoglossus). It depresses and retracts the tongue, making its dorsum (upper side) more convex. It is important in singing.