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.
|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.