Structure of the esophagus and mechanism of swallowing

Structure of the esophagus

The esophagus is a hallow muscular tube, about 25 cm long connecting the pharynx to the stomach, it has no significant absorption or secretory function.
It is lined by stratified squamous epithelium except near the gastro- esophageal junction lined by columnar epithelium.
The esophagus is separated from the pharynx by the upper esophageal sphincter (UES) which is normally closed by the cricopharyngeus muscle contraction .
The lower esophageal sphincter (LOS) consists of an area of the distal end of the esophagus with a high resting tone to prevent reflux .
The relaxation and reduction of LOS tone that occurs during swallowing is under the control of vagus and other hormonal mechanisms. The presynaptic neurotransmitter is acetylcholine. The postsynaptic neurotransmitters which cause relaxation are nitric oxide and vasoactive intestinal peptide.

Mechanism of Swallowing 

The act of swallowing begins with propulsion of the chewed bolus into the posterior oropharynx by the tongue.
During the next phase of  swallowing several actions occur : 
- The soft palate elevates to close the nasopharynx .
- The epiglottis close over the larynx .
- The UES relaxes .
- The pharyngeal constrictors contract to propel the bolus into the eosophagus .

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