1) Voluntary Stage : bolus is passed into the oropharynx
2) Pharyngeal Stage : the involuntary passage of the bolus through the pharynx into the esophagus. The bolus stimulates receptors in the oropharynx to the deglutition centre which sends returning impulses causing the soft palate to move upward to close off the nasopharynx preventing food and liquid from entering the nasal cavity. In addition, the epiglottis closes off the opening to the larynx preventing the bolus from entering the respiratory tract. 3) Esophageal Stage : the involuntary passage of the bolus through the esophagus into the stomach. During this stage Peristalsis (a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis) pushes the bolus onward. Peristalsis also occurs in other parts of the GI tract and the ureters, bile ducts, and uterine tubes.
Esophagus
Collapsible muscular tube, 125 – 150 cm long Contains: nonkertinized stratified squamous epithelium, lamina propria (areolar CT), and a muscularis mucosae (smooth muscle) The muscular wall of the esophagus increases gradually in thickness from cranial to caudal Two sphincters: 1) Upper Esophageal Sphincter which consists of skeletal muscle and regulates the movement of food from the pharynx into the esophagus 2) Lower Esophageal Sphincter which consists of smooth muscle and regulates the movement of food from the lower esophagus into the stomach Superficial layer of esophagus is known as the Adventitia not the serosa as in the stomach and intestines Esophagus secretes mucus and transports food into the stomach. It does not produce digestive enzymes, and it does not carry on absorption. At its origin, the esophagus is located dorsal to the trachea; in the distal 1/3 rd of the neck it deviates to the left and lays lateroventrally to the trachea from C4 onward. The esophagus passes through the thoracic inlet and again reaches the dorsal surface of the trachea. It continues caudally in the mediastinum, dorsal to the base of the heart and trachea and crosses the aorta on the right. The esophagus passes through the esophageal hiatus of the diaphragm and joins the stomach at an acute angle, ventral to T12.
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