| 2. Anatomy | page 88 |
2.1 Muscular Anatomy
The esophagus is a hollow muscular tube closed proximally by the upper esophageal sphincter (UES) and distally by the lower esophageal sphincter (LES). The UES consists predominantly of the cricopharyngeus and the caudal fibers of the inferior pharyngeal constrictor muscles. The UES forms a transverse slit at the C5-C6 vertebral level due to surrounding bony structures and cartilage. In the proximal one-quarter to one-third of the esophagus, the muscle is striated. There is then a transition zone of variable length where there is a mixture of both smooth and striated muscle. The distal one-half to one-third of the esophageal body and LES are composed of smooth muscle. The LES is located at the junction between the esophagus and stomach, usually localized at or just below the diaphragmatic hiatus. Despite its distinct physiological function, it is not easily distinguished anatomically.
| 2.2 Innervation | page 89 |
The motor innervation of the esophagus is via the vagus nerves. The cell bodies of the vagal efferent fibers innervating the UES and the proximal striated-muscle esophagus arise in the nucleus ambiguus, whereas fibers destined for the distal smooth-muscle segment and the LES originate in the dorsal motor nucleus. The esophagus and LES also receive sympathetic nerve supply (both motor and sensory) arising from spinal segments T1-T10. Sensory innervation is also carried via the vagus and consists of bipolar nerves that have their cell bodies in the nodose ganglion and project from there to the brainstem.
| 2.3 Blood Supply | page 89 |
Arterial blood supply to the UES and cervical esophagus is via branches of the inferior thyroid artery. Most of the thoracic esophagus is supplied by paired aortic esophageal arteries or terminal branches of bronchial arteries. The LES and the most distal segment of the esophagus are supplied by the left gastric artery and by a branch of the left phrenic artery. Venous drainage is via an extensive submucosal plexus that drains into the superior vena cava from the proximal esophagus and into the azygous system from the mid-esophagus. In the distal esophagus, collaterals from the left gastric vein (a branch of the portal vein) and the azygos interconnect in the submucosa. This connection between the portal and systemic venous systems is clinically important; when there is hypertension, variceal dilation can occur in this area. These submucosal esophageal varices can be the source of major gastrointestinal hemorrhage.
| 2.4 Lymphatic Drainage | page 89 |
In the proximal third of the esophagus, lymphatics drain into the deep cervical lymph nodes, whereas in the middle third, drainage is into the superior and posterior mediastinal nodes. The distal-third lymphatics follow the left gastric artery to the gastric and celiac lymph nodes. There is considerable interconnection among these three drainage regions.
| 2.5 Histology | page 89 |
The wall of the esophagus consists of mucosa, submucosa and muscularis propria. Unlike other areas of the gut, it does not have a distinct serosal covering, but is covered by a thin layer of loose connective tissue. The mucosa consists of stratified squamous epithelium in all regions of the esophagus except the LES, where both squamous and columnar epithelium may coexist. Beneath the epithelium are the lamina propria and the longitudinally oriented muscularis mucosa. The submucosa contains connective tissue as well as lymphocytes, plasma cells and nerve cells (Meissner's plexus). The muscularis propria consists of an inner circular and an outer longitudinal muscle layer. The circular muscle layer provides the sequential peristaltic contraction that propels the food bolus toward the stomach. Between the circular and longitudinal muscle layers lies another nerve plexus called the myenteric or Auerbach's plexus, which mediates much of the intrinsic nervous control of esophageal motor function.
