Esophagus (review of anatomy & physiology)

Anatomy

•  The esophagus develops from the cranial portion of the foregut and is recognizable by the 3rd week of gestation. 
• The normal esophagus is a hollow, highly distensible muscular tube that extends from the epiglottis in the pharynx, at about the level of C6 vertebra, to the gastroesophageal junction at the level of the T11 or T12. 
• Measuring between 10 and 11 cm in the newborn, it grows to a length of about 25 cm in the adult.
• For the endoscopist, the esophagus is recorded as the anatomic distance  between  15 and 40 cm from the incisor teeth, with the gastroesophageal junction located at the 40-cm point.
• Several points of luminal narrowing can be identified along its course-proximally at the cricoid cartilage, midway in its course alongside the aortic arch and at the anterior crossing of the left main bronchus and left atrium, and distally where it pierces the diaphragm.
• Although the pressure in the esophageal lumen is negative compared with the atmosphere, manometric recordings of intraluminal pressures have identified two higher-pressure areas that remain relatively contracted in the resting phase.
• A 3-cm segment in the proximal esophagus at the level of the cricopharyngeus muscle is referred to as the upper esophageal sphincter (UES).
• The 2- to 4-cm segment just proximal to the anatomic gastroesophageal junction, at the level of the diaphragm, is referred to as the lower esophageal sphincter (LES). Both 'sphincters' are physiologic, in that there are no anatomic landmarks that delineate these higher-pressure regions from the intervening esophageal musculature.

Wall of esophagus

Taken from www.mc.vanderbilt.edu

• The wall of the esophagus consists of a mucosa, submucosa, muscularis propria/externa, and adventitia, reflecting the general structural organization of the gastrointestinal tract.
• The mucosa has a smooth, glistening , and pink-tan surface. It has three components;
1. a non-keratinizing stratified squamous epithelial layer,
2. lamina popria,
3. muscularis mucosa

1. The epithelial layer has mature squamous cells overlying  basal cells. The basal cells, constituting 10% to 15% of the mucosal thickness, are reserve cells with great proliferative potential.
   
   
   
   
   
   
   
   • A small number of specialized cell type, such as melanocytes, endocrine cells, dendritic cells, and lymphocytes, are present in the deeper  portion of the epithelial layer.
2.  The lamina propria is the non-epithelial portion of the epithelial layer, above the muscularis  mucosae.
•  It consists of areolar connective tissue and contain vascular structures and scattered leukocytes.
•  Finger-like extensions of the lamina propria, called papillae, extend into the epithelial layer. 
3. The muscularis mucosa is a delicate layer of longitudinally oriented smooth-muscle bundles.

• The submucosa consists of loose connective tissue containing;
• blood vessels, 
• a rich network of lymphatics, 
• a sprinkling of leukocytes with occasional lymphoid follicles, 
• nerve fibers (including the ganglia of Meissner plexus), and
  
• submucosal glands.
• Submucosal glands connected to the lumen by squamous epithelium-line ducts are scattered along the entire esophagus but are more concentrated in the upper and loweer portions. Their mucin-containing fluid secretions help lubricate the esophagus
• As is true throughout the alimentary tract, the muscularis propria/externa consists of an inner circular and an outer longitudinal coat of smooth muscle with an intervening, well-developed myenteric plexus (Auerbach plexus).
• The muscularis propria striated muscle fibers from the cricopharyngeus muscle.
• Besides creating a unique histologic interplay of smooth muscle and skeletal muscle fibers, this feature explains why skeletal muscle disorder can cause upper esophageal dysfunction.
  

• In sharp contrast to the rest of the gastrointestinal tract, the esophagus is mostly devoid of a serosal coat. Only small segments of the intra-abdominal esophagus are covered by serosa; the thoracic esophagus is surrounded by fascia that condenses around the esophagus to form a sheath like structure.
• In the upper mediasternum, the esophagus is supported by this fascial tissue, which forms a similar sheath around adjacent structures, the great vessels and the tracheobronchial tree. This intimate anatomic proximity to important throracic viscera is of significance in permitting the ready and widespread dissemination of infections and tumors of the esophagus into the posterior mediasternum. The rich network of mucosal and submucosal lymphatics that runs longitudinally along the esophagus further facilitates spread.
• As a summary, wall of esophagus composed of;
1. Mucosa
1. epithelial cells (non-keratinized stratified columnar epithelium)
2. lamina popria
3. muscularis mucosa (longitudinal smooth muscle)
2. Submucosa
1. submucosal plexus (Myentric's)
3. Muscularis externa/popria
1. inner circular smooth muscle
1. Meissner plexus (Auerbach's)
2. outer longitudinal muscle
   
4. Adventitia
   

Physiology of esophagus

• The main functions of the esophagus are;
1. to conduct food and fluids from the pharynx to the stomach, 
2. to prevent passive diffusion from the  pharynx to the stomach, 
3. to prevent passive diffusion of substances from the food into the blood, and 
4. to prevent reflux of gastric contents into the esophagus.
• These functions require motor activity  coordinated with swallowing, namely a wave of peristaltic contraction, relaxation of the LES in anticipation of the peristaltic wave, and closure of the LES after the swallowing reflex. The mechanisms governing this motor function are complex, involving both extrinsic and intrinsic innervation, humoral regulation and properties of the muscle wall itself.
• The control of the lower esophageal sphincter (LES) is critical to esophageal function. Maintenance of sphincter tone is necessary to prevent reflux of gastric contents, which are under positive pressure relative to the esophagus.
• During deglutition, both active inhibition of the muscularis propria muscle fibers by inhibitory nonadrenergic/noncholinergic neurons and cessation of tonix excitation by cholinergeic neurons enable the LES to relax. 
• Many chemical agents (eg gastrin, acetylcholine, serotonin, prostaglandin F20, motilin, substance P, histamine and pancreatic polypeptide)l decrease the tone. However, their precise roles in normal esophageal function remain unclear.