The esophagus is a hollow muscular tube, closed proximally and distally by muscular sphincters.
The esophageal wall is composed of an inner mucosal layer of squamous epithelium and connective tissue, within which lies a longitudinally oriented muscle layer called the muscularis mucosa. The function of this muscle layer is unclear, but it likely is involved in mucosal movement. The outer muscular coat, known as the muscularis propria, is involved in bolus transport and consists of an inner layer of circularly oriented muscle fibers and an outer layer of longitudinally oriented fibers.
In between these two muscle layers lies the myenteric plexus, which controls the motor function of these muscles.
The upper esophageal sphincter (UES) and proximal one third of esophageal body is composed of striated muscle. There is then a transition zone where striated and smooth muscle intermix. The lower esophageal sphincter (LES) and the distal one half to two thirds of the esophageal body are composed of smooth muscle.
The main function of the esophagus is to propel swallowed food or fluid into the stomach. This occurs through sequential or "peristaltic" contraction of circular muscle in the esophageal body, in concert with appropriately timed relaxation of the upper and lower esophageal sphincters. The esophagus also must clear any refluxed gastric contents back into the stomach and takes part in vomiting and belching.
Esophageal peristalsis can be initiated by deglutition ("primary" peristalsis) or local distention ("secondary" peristalsis). Deglutition is one of the most complex reflex neural activities. The initial phase is voluntary when food is chewed, mixed with saliva and formed into a bolus before being pushed to the posterior pharynx by the tongue. Receptors in the posterior pharynx are then activated to initiate the involuntary phase of deglutition, which involves carefully sequenced contraction of numerous head and neck muscles. The food bolus is rapidly pushed toward the esophagus by the pharyngeal constrictor muscles. Concurrently, there is activation of muscles that lift the palate and close off and elevate the larynx in order to prevent misdirection of the bolus into the nasopharynx and larynx, respectively. The UES opens almost immediately upon activation of the deglutition reflex, allowing the food bolus to pass through. It then rapidly shuts to prevent retrograde passage of the bolus. Once this oropharyngeal phase has served to propel the bolus into the esophagus, the esophageal phase of deglutition takes over. This involves two major phenomena, namely the sequential contraction of circular muscle of the esophageal body, which results in a peristaltic wave that pushes the food toward the stomach, and relaxation and opening of the LES.
Diseases of the esophagus include:
- Esophageal foreign body
- Helicobacter spp-associated gastroesophageal reflux
- Spirocerca lupi infection
- Vantrappen G & Hellemans J (1974) Diseases of the Esophagus. New York: Springer-Verlag
- Lux CN et al (2012) Gastroesophageal reflux and laryngeal dysfunction in a dog. J Am Vet Med Assoc 240(9):1100-1103