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.
