Physiology of Swallowing (Deglutition)

Introduction

1. Oral Phase (voluntary)
2. Pharyngeal Phase (involuntary reflex)
3. Esophageal Phase (involuntary)

Figure: Swallowing mechanism - neural control via CN V, IX, X, XII with the swallowing center in the medulla (Guyton & Hall)

Phase 1: Oral Phase (Voluntary)

• Mastication of food with addition and mixing of saliva
• Bolus formation and control by lips, tongue, buccinator muscle, and palate
• Selection and verification of bolus safety (volume, texture, foreign bodies)
• Sensory input via mechanoreceptors (touch, pressure), proprioceptors, chemoreceptors, and taste/temperature receptors in tongue, teeth, soft palate, and hard palate
• Mechanoreceptors in the tongue and palate modulate muscles of mastication through brain-stem integrative pathways

• The bolus is "voluntarily" squeezed or rolled posteriorly into the pharynx by upward and backward pressure of the tongue against the palate
• The oral phase ends when the bolus is pressed against the faucial arches - pressure-sensitive receptors on the anterior tonsillar pillar (CN IX and X) trigger the next involuntary phase
• Duration: variable (under voluntary control, can be prolonged)

Phase 2: Pharyngeal Phase (Involuntary Reflex)

2a. Nasopharyngeal Closure

• Soft palate elevates (levator and tensor veli palatini muscles)
• Passavant's ridge forms by contraction of the superior pharyngeal constrictor
• Prevents bolus regurgitation into the nasopharynx/nasal cavity

2b. Palatopharyngeal Fold Approximation

• The palatopharyngeal folds on each side are pulled medially to form a sagittal slit
• Acts as a selective gate - only adequately masticated food passes; large objects are impeded
• Entire stage lasts less than 1 second

2c. Cessation of Respiration

• Respiration is inhibited, usually occurring during the expiratory phase
• The swallowing center in the medulla directly inhibits the respiratory center
• The entire pharyngeal stage lasts less than 6 seconds - interrupting respiration for only a fraction of the respiratory cycle

2d. Glottic Closure (Airway Protection - Multi-level)

• Sequence (in order): True vocal cords adduct first → false vocal cords close → arytenoids approximate against the epiglottis
• Adduction of the lateral cricoarytenoid muscles approximates the arytenoids
• Vocal fold adduction during swallowing averages approximately 2.3 seconds
• This is the most critical protective mechanism - destruction of vocal cord adductor function markedly increases aspiration risk

2e. Laryngeal Elevation and Epiglottic Retroflexion

• Suprahyoid muscles and the thyrohyoid muscle contract, pulling the hyolaryngeal complex superiorly and anteriorly
• Laryngeal elevation serves three functions:
  1. Protects the laryngeal vestibule from aspiration
  2. Causes epiglottic retroflexion (tilting backward over the laryngeal inlet)
  3. Actively dilates the upper esophageal sphincter (UES/cricopharyngeus)
• Epiglottic retroflexion: partly active (due to laryngeal elevation, pharyngeal constriction, tongue base retraction) and partly passive (pressure from the bolus itself)
• The epiglottis diverts food laterally into the pyriform sinuses (piriform fossae)
• Note: The epiglottis is NOT absolutely essential for airway protection - functional swallowing has been documented even with an absent epiglottis

2f. Bolus Propulsion

• Tongue base protrusion provides the primary "tongue driving force"
• Sequential contraction of pharyngeal constrictors from top to bottom (superior → middle → inferior) generates peristaltic propulsion
• The bolus passes through the now-open UES into the esophagus

2g. UES (Cricopharyngeal Sphincter) Opening

1. Relaxation of cricopharyngeus muscle - precedes bolus arrival (neurally mediated)
2. Active dilation - contraction of suprahyoid and thyrohyoid muscles pulls the hyolaryngeal complex forward, mechanically opening the sphincter (most important component)
3. Pressure of descending bolus - assists opening from above

Phase 3: Esophageal Phase (Involuntary)

Two Types of Peristalsis:

• Direct continuation of the peristaltic wave that began in the pharynx
• Passes from the pharynx to the stomach in approximately 8 to 10 seconds
• In the upright position, gravity assists, delivering the bolus in 5 to 8 seconds

• Triggered when the primary wave fails to clear all retained food
• Initiated by esophageal distension from the retained bolus
• Mediated by: (a) intrinsic myenteric nervous system circuits, and (b) reflexes from the pharynx transmitted via vagal afferent fibers to the medulla, then back through glossopharyngeal and vagal efferents

Muscle Types and Innervation:

• Upper 1/3 of esophagus (pharyngeal wall included): striated muscle - controlled by CN IX and X (skeletal nerve impulses)
• Lower 2/3 of esophagus: smooth muscle - controlled by the vagus nerve via the myenteric (Auerbach's) plexus
• Even after vagotomy, the myenteric plexus can maintain secondary peristalsis

Lower Esophageal Sphincter (LES / Gastroesophageal Sphincter):

• Located in the lower ~3 cm of esophagus
• Normally tonically constricted at approximately 30 mmHg intraluminal pressure
• As a peristaltic wave approaches, "receptive relaxation" occurs ahead of it (via myenteric inhibitory neurons), allowing easy passage of bolus into the stomach
• Prevents gastroesophageal reflux (failure = achalasia if incomplete relaxation, or GERD if inadequate tone)

Neural Control of Swallowing

Swallowing Center

• Located in the medulla oblongata and lower pons (reticular formation)
• Key nuclei: Nucleus ambiguus (CN IX, X), dorsal motor nucleus of vagus (CN X), tractus solitarius (receives all sensory input from mouth/pharynx)
• Coordinates the entire sequential swallowing pattern
• Modulated by the cerebral cortex (which is why voluntary suppression or modification of swallowing is possible)
• Directly inhibits the respiratory center during the pharyngeal phase

Sensory Afferent Pathway:

• Trigger zone: Ring around the pharyngeal opening, with greatest sensitivity at the tonsillar pillars (anterior faucial pillar)
• Sensory receptors also on: soft palate, tongue base, posterior pharyngeal wall
• Central ganglia:
  • CN V - Gasserian ganglion
  • CN IX - Inferior (Andersch) and superior (Ehrenritter) ganglia
  • CN X - Inferior (jugular) and superior (nodose) ganglia
• Impulses travel via CN V (trigeminal) and CN IX (glossopharyngeal) → tractus solitarius in medulla

Motor Efferent Pathway:

Summary Table

Clinically Relevant Points (for exam scoring)

• Aspiration risk increases when laryngeal elevation is impaired - cricopharyngeal opening is limited, pressure drop during deglutition is slower, and the transient negative pressure in the cricopharyngeal sphincter does not occur (Scott-Brown's, p. 944)
• Achalasia: failure of LES to relax - food accumulates in esophagus
• Laryngeal penetration (barium above true vocal cords) can occur in healthy individuals; aspiration is defined as material passing below the true vocal cords into the trachea
• The epiglottis is NOT essential for airway protection in all cases - patients without an epiglottis can swallow safely
• FEES (Fiberoptic Endoscopic Evaluation of Swallowing) and Modified Barium Swallow are the principal investigations for dysphagia

• Guyton & Hall Textbook of Medical Physiology (p. 788-790)
• KJ Lee's Essential Otolaryngology (p. 672-673)
• Cummings Otolaryngology Head and Neck Surgery
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery (p. 944)

Stroboscopy in ENT (Laryngeal Stroboscopy / Videostroboscopy)

Definition and Principle

Equipment

1. Strobe light source - synchronized to the patient's F0 (detected via a microphone or contact sensor at the neck)
2. Rigid 70° or 90° telescope (oropharyngeal) - gives higher image quality, preferred for most adults
3. Flexible nasopharyngoscope - used when the rigid scope cannot be tolerated, or for connected speech evaluation
4. Video camera and monitor - for real-time visualization and recording
5. Microphone/frequency detector - picks up the patient's F0 to synchronize the strobe

Vocal Fold Anatomy Relevant to Stroboscopy (Body-Cover Model)

Parameters Assessed on Stroboscopy

1. Symmetry

• Refers to whether both vocal folds mirror each other in their movements
• Asymmetry suggests a difference in mass, tension, or innervation between the two folds
• Unilateral lesions, paresis, or scar characteristically produce asymmetric vibration

2. Regularity (Periodicity)

• Describes how consistent the cycle-to-cycle vibration is
• In stop mode: regular vibration = image appears static; irregular vibration = image quivers
• Irregularity suggests: paresis, neuromuscular disease (e.g., spasmodic dysphonia), stiff lesions (cyst, carcinoma), or edematous lesions

3. Glottal Closure Pattern

• Describes the degree and shape of vocal fold contact at the most closed instant
• Assessed at most comfortable pitch and loudness (MCPL)
• Closure patterns:
  • Complete closure - normal in men
  • Posterior gap - most common normal pattern in women
  • Anterior gap / Spindle-shaped gap - normal variant in older adults; also seen in scar, sulcus vocalis, atrophy
  • Hourglass gap - characteristic of bilateral vocal nodules or unilateral lesion with contralateral reactive change
  • Incomplete closure (full length) - vocal fold paralysis, scar, severe atrophy

4. Amplitude of Vibration

• Lateral excursion of the vocal fold from midline during the open phase
• Reduced amplitude: suggests stiffness (scar, malignancy, post-radiation change)
• Exaggerated amplitude: seen in flaccid paralysis or edema (Reinke's edema)

5. Mucosal Wave

• The traveling wave that propagates from the inferior to superior surface during each vibratory cycle - a direct marker of cover pliability
• Absent or reduced mucosal wave = stiff cover (scar, post-surgical fibrosis, carcinoma, sulcus vocalis, radiation changes)
• Normal mucosal wave essentially rules out deep invasion by carcinoma
• This is the single most diagnostically useful stroboscopic parameter

6. Adynamic Segments

• Portions of the vocal fold that fail to vibrate while adjacent areas vibrate normally
• Indicates focal stiffness - very suspicious for submucosal lesion, cyst, scar, or carcinoma

7. Phase Closure (Open Quotient)

• Refers to the proportion of each vibratory cycle during which the glottis is open
• Normal: glottis is open for approximately two-thirds of one cycle at MCPL
• Closed phase generally decreases with higher pitch

8. Vertical Closure Level

• Normal: both vocal folds meet at the same horizontal plane
• Abnormal vertical level difference: suggests vocal fold paralysis or arytenoid dislocation

9. Vocal Fold Edges

• Shape of the free edge of the vocal fold
• Straight, smooth edge = normal
• Irregular, notched, or bowed edges suggest sulcus, scar, or prior phonosurgery

Clinical Applications / Indications

1. Dysphonia evaluation - the single most important investigation for any persistent voice change
2. Differentiation of benign lesions - vocal nodules vs. polyps vs. cysts vs. sulcus vocalis (all have characteristic patterns)
3. Assessment of vocal fold paralysis/paresis - asymmetric mucosal wave, abnormal closure, adynamic segments
4. Mucosal pliability assessment - distinguishing superficial mucosal disease from deep/submucosal lesion
5. Suspected early carcinoma - absent or markedly reduced mucosal wave suggests submucosal invasion, prompting biopsy
6. Pre- and post-phonosurgery assessment - baseline documentation and monitoring of tissue healing
7. Professional voice users - identify subtle lesions before the voice complaint becomes severe
8. Reinke's edema - characteristic large-amplitude, slow, undulating mucosal wave
9. Spasmodic dysphonia - irregular, aperiodic vibration pattern with abnormal adductory/abductory spasms

Stroboscopy changes the diagnosis in 30-47% of patients diagnosed by flexible endoscopy alone, and provides additional diagnostic detail in a further 32%. (Cummings Otolaryngology)

Limitations

• Not reliable when vibration is severely aperiodic (irregular F0)
• Requires patient cooperation - sustained phonation for adequate assessment
• The slow-motion image is a composite reconstruction, not true continuous recording of a single cycle (contrast with high-speed videoendoscopy)
• Rigid scope requires adequate mouth opening and gag reflex control
• Does not show subglottic or tracheal pathology

Comparison: Stroboscopy vs. High-Speed Videoendoscopy

Summary

• Cummings Otolaryngology Head and Neck Surgery (Chapter 54, 58)
• KJ Lee's Essential Otolaryngology (p. 918)