Musculature of
Esophagus
The musculature of the esophagus consists of an outer longitudinal
muscle layer and an inner muscular layer, generally described for
convenience as the circular muscle layer, although, strictly speaking,
the term “circular” is not properly accurate, as will be seen below. The outer
longitudinal muscle layer originates principally from a stout tendinous band
that is attached to the upper part of the vertical ridge on the dorsal aspect
of the cricoid cartilage.
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| Musculature of Esophagus |
From this tendon two muscle bands originate and diverge
as they descend and sweep around the right and left sides of the esophagus.
They meet and interdigitate somewhat in the posterior midline, leaving a
Vshaped gap superior to and between them. This gap is known as the V-shaped
area (of Laimer), and the base of the area is formed by the underlying
circular muscle. Superiorly, it is bounded by the cricopharyngeus muscle. A few
sparse fibers of the longitudinal muscle spread over this area, as do some
accessory fibers from the lower margin of the cricopharyngeus. The longitudinal
muscle fibers are not uniformly distributed as they descend over the surface of
the upper esophagus. Instead, the fibers gather into thick lateral
longitudinal muscle masses on each side of the esophagus, but they remain
considerably thinner over other parts of the tube. The muscle is thinnest on
the anterior wall (i.e., the wall that is applied to the posterior surface of
the trachea). Indeed, high up on the esophagus’s anterior surface, the
longitudinal muscle is said to be entirely lacking, and this portion of the
esophagus is designated as the “bare” area. The longitudinal muscle of
the esophagus also usually receives additional contributions by way of accessory
muscle slips on each side, which originate from the posterolateral aspect
of the cricoid cartilage and also from the contralateral side of the deep
portion of the cricopharyngeus muscle. As the longitudinal muscle descends, it
progressively forms a more uniform sheath over the entire circumference of the
esophagus. The anterior wall of the esophagus is firmly applied to the
posterior tendinous wall of the trachea in its upper portion where the two
organs are attached to each other by fibroelastic membranous tissue containing
some muscle fibers.
The inner, socalled circular
layer of esophageal muscle underlies the longitudinal muscle layer.
Although a definite layer, it is
slightly thinner than the longitudinal coat. This ratio of longitudinal and
circular muscle coat is unique for the esophagus and is reversed in all other
parts of the alimentary tract. The circular layer in the upper esophagus is not
truly circular but rather elliptical, with the anterior part of the ellipse at
a lower level than the posterior part. The inclination of the ellipses becomes
less as the esophagus descends, until, at about the junction of the upper and
middle thirds, the fibers run in a truly horizontal plane. Here, for a segment
of about 1 cm, they may be said to be truly circular. Below this point they
again become elliptical, but the inclination is reversed from that of the
higher fibers (i.e., the posterior part of the ellipse now assumes a lower
level than the anterior part). In the lower third of the esophagus, the course
of the fibers again changes to a screwshaped or spiral course, winding
progressively inferiorly as they pass around the esophagus. It should be noted
also that the elliptical, circular, and spiral fibers of this layer are not
truly uniform and parallel but may overlap and cross or even have clefts
between them. Some fibers in the lower two thirds of the esophagus occasionally
leave the elliptical or spiral fibers at one level, to pass diagonally or even
perpendicularly upward and downward to join the fibers at another level, but
they never form a continuous layer. They may be threadlike or 2 to 3 mm in
width and from 1 to 5 cm in length; they are usually branched. The musculature
of the esophagogastric junction will be discussed in the next section.
Spontaneous rupture of the esophagus almost invariably occurs in the lower 2 cm
of the esophagus. A linear tear may occur through the entire thickness of the
esophageal wall. Severe vomiting predisposes to rupture of this region,
releasing gastric juice into the mediastinum.
The cricopharyngeus muscle, although
strictly speaking a muscle of the pharyngeal wall, being the lowermost portion
of the inferior constrictor of the pharynx, is nevertheless of great
importance in the function and mal function of the esophagus. This narrow band
of muscle fibers originates on each side from the posterolateral margin of the
cricoid cartilage and passes slinglike around the posterior aspect of the
pharyngoesophageal junction. Its superiormost fibers ascend to join the median
raphe of the inferior constrictor muscle posteriorly. The cricopharyngeus also
has muscle fibers that run horizontally to encircle the pharyngoesophageal
junction, acting as the superior pharyngeal constrictor. This cricopharyngeal
constriction is felt when an esophagoscope is introduced, because even at rest
the muscular tonus felt within the esophageal lumen is greater at the level of
the cricopharyngeus than in other parts of the esophagus, and the relaxation of
this muscle is an integral part of the act of swallowing.
Superior to the cricopharyngeus (between this muscle and the main part of the
inferior constrictor) the musculature is somewhat weaker and sparser
posteriorly. It is through this sparse area that most Zenker diverticula are
believed to originate.
The musculature of the upper
portion of the esophagus is striated, whereas that of the lower portion is made
up almost entirely of smooth muscle. The level at which this
transition takes place varies. In general, it may be said that the upper fourth
of the esophagus contains purely striated muscle, the second fourth is a
transitional zone in which both striated and smooth muscle are present, and the
lower half contains purely smooth muscle. Between the longitudinal and circular
coats of the esophageal muscles is a narrow layer of connective tissue where
the myenteric ganglia and plexus (of Auerbach) can be found.
