Musculature of Pharynx
Much of the framework of the lateral and posterior walls of the pharynx is formed by an outer and inner layer of musculature. These layers are not completely separable throughout, because in some places they are definitely intermingled and overlap. The outer layer is more nearly arranged in a circular fashion and is made up of the three constrictor muscles of the pharynx, designated as superior, middle, and inferior pharyngeal constrictors, which overlap each other. The inner layer, which falls far short of being a complete layer, is longitudinally arranged and is composed of the stylopharyngeus, palatopharyngeus, and salpingopharyngeus muscles plus some other variable and irregular bundles of muscle fibers.
The superior pharyngeal constrictor muscle is quadrilateral in shape and somewhat thin. It originates from the posteroinferior edge of the medial pterygoid plate, the hamulus of the medial pterygoid plate, the pterygomandibular raphe (which runs from the hamulus to the lingula of the mandible), the posterior one fifth or so of the mylohyoid line and the adjacent part of the alveolar process of the mandible, and the side of the root of the tongue (the glossopharyngeus muscle). From this extensive line of origin, the fibers course posteriorly, with the lower fibers passing somewhat inferiorly and medially to meet the ones from the opposite side in the median pharyngeal raphe. This raphe extends most of the length of the posterior wall of the pharynx, being attached superiorly to the basilar part of the occipital bone at the pharyngeal tubercle, to which the uppermost fibers of the superior constrictor are also attached. The curved upper edge of the muscle passes deep to the auditory tube and is thus separated by a short distance from the base of the skull except at the midline posteriorly. At this gap the only framework of the pharynx is the pharyngobasilar fascia. The buccinator muscle runs anteriorly from the pterygomandibular raphe, which serves as part of its origin, and this muscle and the superior constrictor thus form a continuous sheet, which is therefore the continuous framework of the lateral wall of the oral and oropharyngeal cavities. A slip of the superior part of the superior constrictor muscle blends into the palatine aponeurosis, forming the socalled palatopharyngeal sphincter, contraction of which produces a ridge (Passavant ridge) against which the soft palate is raised. A triangular gap filled with fibrous connective tissue can be noted between the lower border of the superior pharyngeal constrictor muscle, the posterior border of the hyoglossus muscle, and the upper border of the middle pharyngeal constrictor muscle. The stylopharyngeus muscle inserts into this gap between the superior and middle constrictors. The stylohyoid ligament and glossopharyngeal nerve also cross this gap.
The middle pharyngeal constrictor muscle has a V-shaped line of origin, with the V resting on its side and the angle pointing anteriorly. The superior arm of this V is formed by the terminal portion of the stylohyoid ligament and the lesser horn of the hyoid bone, whereas the inferior arm of the V is formed by the entire length of the greater horn of the hyoid bone. From this rather narrow origin the fibers fan out widely, with the upper fibers coursing superiorly while curving posteriorly and medially. The middle fibers course horizontally and curve posteriorly and medially. The inferior fibers course inferiorly and curve posteriorly and medially. The medial pharyngeal constrictor’s superior fibers overlap the superior constrictor, and the inferior fibers are overlapped by the inferior constrictor and reach quite far inferiorly in the posterior wall of the pharynx, to about the level of the superior border of the cricoid cartilage. The middle constrictor’s fibers fuse and blend with those of the other side in the median raphe. Between the lower border of the middle constrictor and the upper border of the inferior constrictor, a triangular gap is noted, which is bounded anteriorly by the thyrohyoid muscle.
The inferior pharyngeal constrictor muscle is relatively thick and strong. It arises from the oblique line of the thyroid cartilage and the area just dorsal to that line, from a tendinous arch extending from the inferior end of the oblique line of the thyroid cartilage to the lateral surface of the cricoid cartilage. That portion arising from the cricoid cartilage is frequently referred to as the cricopharyngeus muscle. As do the other constrictor muscles, the inferior constrictor passes posteriorly and then medially to blend with its counterpart at the pharyngeal raphe. The cranial fibers pass more and more obliquely as they approach the raphe and overlap the middle constrictor, reaching nearly as far superiorly as the middle constrictor does. The fibers of the cricopharyngeus portion of the muscle course horizontally and form an annular bundle with no median raphe. It does blend to some extent with the related circular fibers of the esophagus, being referred to by some as the superior esophageal sphincter. A zone of sparse musculature is present between the cricopharyngeus muscle and the rest of the inferior constrictor muscle, which creates a weaker area in the posterior wall of the pharynx, where an instrument may accidentally pierce the wall. Just below the inferior border of the cricopharyngeus muscle, a triangular area (sometimes called the Laimer triangle) occurs in which the posterior wall of the esophagus is variably deficient, because the longitudinal muscle fibers of the esophagus tend to diverge laterally and pass around the esophagus to attach on the cricoid cartilage. It is thus seen that there is more than one weakened area in the posterior wall of the general region of the pharyngoesophageal junction, where diverticula may occur. The recurrent laryngeal nerve and accompanying inferior laryngeal vessels pass deep to the inferior constrictor muscle to travel superiorly behind the cricothyroid joint in entering the larynx.
As their names indicate, the major action of the superior, middle, and inferior pharyngeal constrictor muscles of the pharynx is to constrict the pharynx. They contract in sequence, grasping the bolus of food as it passes from the mouth to the esophagus. The nerve supply of the constrictor muscles of the pharynx is derived from the pharyngeal plexus.
The stylopharyngeus muscle is long, slender, and cylindrical superiorly but flattens near its insertion. It originates from the medial aspect of the base of the styloid process and then passes inferiorly and anteriorly, going between the external and internal carotid arteries and then entering the wall of the pharynx in the interval between the superior and middle constrictor muscles. As it spreads out internal to the middle constrictor muscle, the greater horn of the hyoid bone, and the thyrohyoid membrane, some of its fibers join the palatopharyngeus muscle and insert on the superior and posterior borders of the thyroid cartilage. Some fibers pass into the pharyngoepiglottic fold, and they are primarily responsible for the production of this fold. The remaining fibers of the stylopharyngeus muscle spread between the constrictor muscles and the mucous membrane (blending to some extent with the constrictors) and pass caudally in the posterolateral wall of the pharynx, until they fade out and attach to the fibrous aponeurosis of the pharynx a short distance above the cricopharyngeus muscle. The stylopharyngeus muscle receives its nerve supply from the glossopharyngeal nerve, which curves around the posterior border of the muscle onto the lateral aspect in its course toward its final distribution on the posterior third of the tongue. The salpingopharyngeus muscle consists of a slender bundle that produces the mucous membrane fold of the same name, which is rather variable in its degree of distinctness. This muscle arises from the inferior part of the cartilaginous part of the auditory tube, near its orifice, and passes into the wall of the pharynx, blending in part with the posteromedial border of the palatopharyngeus muscle. Some authors have described this muscle as a part of the levator veli palatini muscle, which gives a definite clue as to its action. The salpingopharyngeus muscle receives its nerve supply from the pharyngeal plexus.
The palatopharyngeus muscle, together with the mucous membrane covering it, forms the palatopharyngeal fold, also known as the posterior pillar of the fauces. This muscle takes its inferior origin from a narrow fasciculus on the dorsal border of the thyroid cartilage near the base of the superior horn and by a broad expansion from the pharyngeal aponeurosis in the area posterior to the larynx, just cranial to the cricopharyngeus muscle. As the fibers pass cranially, they form a rather compact muscular band that inserts into the aponeurosis of the soft palate by two lamellae, separated by the insertion of the levator veli palatini and the uvula. As indicated above, some of the fibers of the palatopharyngeus muscle intermingle with the stylopharyngeus muscle. The actions of the palatopharyngeus muscle include constriction of the pharyngeal isthmus by approximation of the palatopharyngeal folds, depression of the soft palate, and elevation of the pharynx and larynx. This muscle also receives its nerve supply from the pharyngeal plexus.
Additional muscle bundles are quite common, such as the one labeled accessory muscle bundle from petrous part of temporal bone (petropharyngeus muscle). Other additional muscles are brought about by the splitting of one of the usual muscles, quite commonly the stylopharyngeus. The majority of the additional muscles tend to run longitudinally.