- The line differentiating the esophagus from the stomach is extremely clear
- Esophageal epithelium is stratified squamous non-keratinizing epithelium
- Stomach epithelium is simple columnar
- The esophagus is divided into three sections, which can be distinguished depending on the muscle type in the muscularis externa:
- Upper section: Predominantly striated muscle
- Intermediate section: Both striated and smooth muscle
- Lower section: Predominantly smooth muscle
- This division is consistent with the need to voluntarily swallow in the upper section of the esophagus, but not in the lower section
Upper esophageal sphincter
The upper esophageal sphincter (aka UES) is the more superior of the two sphincters, situated at the proximal end of the esophagus. It is composed of striated muscle (cricopharyngeus muscle).
During a swallow the UES opens, which is primarily due to contraction of the surrounding muscles (i.e., passive displacement of the cricopharyngeus muscle and therefore opening the UES). At least part of UES closure is due to tonic impulses from the CNS. Swallowing causes cessation of these impulses and subsequent relaxation of the UES.
Lower esophageal sphincter
The lower esophageal sphincter (aka LES) is the distal sphincter of the esophagus, located at the gastroesophageal border. The LES is opened as a result of vagal stimulation. Failure of the LES to relax during swallowing results in the failure of food to enter the stomach, a condition called achalasia. Failure of the LES to maintain adequate closure results in reflux of gastric contents into the esophagus, which results in a syndrome called gastroesophageal reflux disease.
When peristalsis is preceeded by a swallow, it is referred to as primary peristalsis. When it is not preceeded by a swallow, but instead is the result of distension of the esophagus, it is referred to as secondary peristalsis. Secondary peristalsis is involuntary.