Chewing and Deglutition

Introduction

Chewing and swallowing are coordinated processes that prepare food for safe passage through the digestive tract. While swallowing begins voluntarily, it proceeds reflexly. Effective mastication enhances salivation and digestion, while airway closure during deglutition prevents aspiration and choking.

Chewing (Mastication)

• Mastication is primarily a voluntary process, often initiated reflexly when food enters the mouth. It breaks large food particles into smaller fragments, increasing the surface area for digestion.
• Chewing stimulates salivary secretion, which moistens and softens food. It ensures proper mixing of food with saliva, forming a cohesive bolus.
• Lubrication facilitates smooth and safe swallowing.
• Salivary enzymes begin carbohydrate digestion, improving taste perception of starchy foods.
• Adequate chewing supports oral hygiene and maintains dental health.
• Mastication is produced by coordinated action of several muscles.
• The masseter elevates the mandible and enables strong biting.
• The temporalis assists in elevation and retraction of the mandible.
• The pterygoid muscles enable opening, protrusion, and side-to-side grinding movements.

• The buccinator prevents food accumulation between the cheek and teeth.

Swallowing (Deglutition)

• Deglutition is the process by which food passes from the oral cavity into the esophagus. It begins voluntarily but continues mainly as an involuntary reflex.
• During swallowing, respiration is temporarily inhibited, preventing aspiration into the airway.
• Sensory receptors near the pharyngeal region initiate the reflex.
• Afferent signals are transmitted to swallowing centers in the medulla and pons.
• Efferent impulses travel through cranial nerves to coordinate muscles of the pharynx and upper esophagus. This coordinated activity ensures safe and efficient transport of the bolus.

Stages of Deglutition

Deglutition occurs in three coordinated phases: oral, pharyngeal, and esophageal.

Oral Phase

• The oral phase is voluntary, while the pharyngeal and esophageal phases are involuntary.
• In the oral phase, the tongue forms a bolus and presses it against the hard palate.
• Sequential tongue movements push the bolus backward into the pharynx.
• Contact with pharyngeal receptors initiates the swallowing reflex.

Pharyngeal Phase

• The pharyngeal phase ensures safe transfer of food into the esophagus while protecting the airway.
• Sensory signals travel via cranial nerves to swallowing centers in the medulla.
• Motor impulses coordinate muscles of the pharynx and tongue.

Events

• The soft palate elevates, preventing entry of food into the nasopharynx.
• The laryngeal inlet closes, and the larynx moves upward, preventing aspiration.
• Pharyngeal constrictor muscles contract, pushing the bolus toward the esophagus.
• The upper esophageal sphincter relaxes, allowing entry of the bolus.
• During this phase, respiration is temporarily inhibited, a phenomenon called deglutition apnea.
• The esophageal phase involves peristaltic waves that propel the bolus toward the stomach.
• Coordinated muscular activity ensures efficient and safe transport of food through the gastrointestinal tract.

Esophageal Phase

• The esophageal phase begins when the bolus enters the esophagus after upper esophageal sphincter relaxation.
• A primary peristaltic wave propels the bolus downward.
• The upper esophageal sphincter then contracts to prevent regurgitation.
• The wave travels at about 3 to 5 centimeters per second, moving the bolus in nearly 10 seconds.
• If transport is incomplete, secondary peristalsis is triggered to ensure effective clearance of the esophagus.

Disorders of Deglutition

• Disorders of deglutition impair safe transfer of food from the mouth to the esophagus.
• Loss or reduction of the swallowing reflex can lead to regurgitation into the nasal cavity or aspiration into the airway. This may occur in lesions affecting cranial nerves supplying the pharynx or after local anesthesia of the pharyngeal mucosa.
• Aerophagia refers to involuntary swallowing of air during eating or drinking. It is common in individuals with reduced tone of the upper esophageal sphincter or heightened anxiety.
• Swallowed air may cause belching or pass into the intestine, producing flatus.
• Dysphagia is defined as difficulty in swallowing. It may result from inflammation, obstruction, or motor dysfunction of the pharynx or esophagus. ·  Prompt evaluation is essential to prevent complications such as aspiration and malnutrition.