I. Introduction
The esophagus is a tubular organ approximately 25 cm in length and 2–3 cm in diameter. Its main function pertains to the food transport from mouth to stomach preventing the gastric reflux. The esophagus wall is composed of four main layers, as mucosa (the most inner one), submucosa, muscularis and adventitia (the most outer one). Mostly striated muscular fibers make up the muscular layer in proximal esophageal region, while smooth muscular fibers are mainly found in the distal region. The two regions are connected by a graded transition zone. The muscular component is distributed within two separate layers, where fibers are oriented according to circumferential and longitudinal directions, as the inner and the outer layers, respectively [1]. The coordination of esophageal longitudinal contractions, due to the longitudinal fibers, and lumen-occluding contractions, due to the circumferential fibers, is known as peristalsis, which is a mostly involuntary reflex that occurs when the bolus enters the esophagus during swallowing [1] –[5]. The upper esophageal sphincter (UES) is located at the proximal extremity of the esophagus and is made of several striated muscles, while the lower esophageal sphincter (LES) is located at the distal extremity of the esophagus and is composed entirely of smooth muscle.