Definition of Swallowing: Also known as deglutition, it is the process of moving food from the mouth through the digestive tract.
Deglutition Reflex: Required to achieve swallowing, involves several anatomical structures starting from the mouth, through the pharynx, and into the esophagus.
Narrator: Dr. Stephen Sullivan, Professor of Anatomy and Physiology.
Anatomy of the Esophagus
Structure: The esophagus is a long muscular tube that extends from the laryngopharynx to the stomach.
**Pathway: **
Begins at the laryngopharynx
Passes through the posterior thoracic cavity
Pierces the diaphragm through an opening called the esophageal hiatus
Ends at the stomach at a location known as the cardiac orifice
Cardia of the Stomach: The region of the stomach nearest the esophagus is referred to as the cardia.
Lower Esophageal Sphincter:
A circular band of smooth muscle that regulates the passage of the bolus through the cardiac orifice and is sometimes termed the cardiac sphincter.
Histological Structure of the Esophagus
Mucosa: Consists of non-keratinized, stratified squamous epithelium.
Esophageal Glands: Extend from submucosa to secrete lubricating mucus into the lumen of the esophagus.
Muscularis Externa: Unique characteristics based on its superior, middle, and inferior thirds:
Superior One-Third: Composed only of skeletal muscle.
Middle One-Third: A combination of skeletal muscle and smooth muscle.
Inferior One-Third: Composed only of smooth muscle.
Key Transition: Motility transitions from voluntary to reflexive to involuntary.
Adventitia: Surrounds the muscularis externa; it is the connective tissue covering for the esophagus, continuous with adjacent structures like the aorta and trachea.
Phases of Deglutition
General Overview
Phases of Swallowing: Deglutition is a three-phase process, involving both voluntary and involuntary actions, utilizing approximately 24 different muscles.
1. Oral Phase
Control: This phase is under voluntary control.
Process: The bolus is moved to the fauces so it can enter the oropharynx.
2. Pharyngeal Phase
Initiation: Begins with the sensory receptors surrounding the fauces being mechanically stimulated by the bolus.
Sensory Signals: These stimuli generate sensory nerve signals that travel to the swallowing center in the brainstem's medulla oblongata.
Motor Signals: The medulla processes these signals and sends motor nerve signals to the pharyngeal muscles (effectors of the reflex):
Draw the bolus through the fauces into the oropharynx.
Elevate the soft palate and uvula to close off the nasopharynx, preventing food from entering.
Infrahyoid muscles contract to elevate the larynx, meeting with the epiglottis, covering the larynx and preventing aspiration.
Aspiration Definition: The occurrence when food or liquids enter the airway.
Aspiration Pneumonia: A potential infection caused by aspiration.
3. Esophageal Phase
Involuntary Control: The final phase is involuntary.
Initiation: Occurs as the bolus passes through the upper esophageal sphincter, stimulating stretch receptors in the myenteric plexus of the esophageal wall.
Peristalsis: A sequential wave of contractions occurs from proximal to distal, pushing the bolus through the esophagus to the stomach:
Mechanism:
Circular muscles contract to push the bolus downward.
Longitudinal muscles contract to shorten and widen the segment of the esophagus where the bolus enters.
Result: The bolus reaches the relaxed lower esophageal sphincter and passes through the cardiac orifice into the stomach.
Summary of Key Terminology
Deglutition: Swallowing process.
Deglutition Reflex: Reflex action that initiates swallowing.
Esophageal Hiatus: Opening in the diaphragm for the esophagus.
Cardiac Orifice: Opening between the esophagus and stomach.
Peristalsis: Wave-like contraction that moves bolus through the esophagus.
Aspiration: Entry of food or liquid into the trachea leading to potential complications.
Muscularis Externa: Layer of muscle involved in movement through the esophagus.