What is the physiology of swallowing?

Physiology of Swallowing

Normal swallowing requires the coordinated activity of the muscles of the mouth, pharynx, larynx, and esophagus, which are innervated by the central and peripheral nervous systems to ensure safe passage of food from the oral cavity to the stomach while protecting the airway. 1

Phases of Swallowing

Swallowing is divided into four sequential phases:

1. Oral Preparatory Phase:

   • Food and saliva are masticated to form a bolus
   • The soft palate lifts to close off the nasopharynx
   • The tongue manipulates the bolus against the hard palate 1

2. Oral Propulsive Phase:

   • The tongue propels the bolus posteriorly under positive pressure into the pharynx
   • This phase is under voluntary control 1

3. Pharyngeal Phase:

   • Duration: 1-2 seconds
   • Rapid sequence of protective airway events:
     • Hyoid bone and larynx move upward and forward
     • Vocal folds move to midline
     • Epiglottis folds over arytenoid cartilages
   • The tongue pushes backward and downward to propel the bolus
   • Pharyngeal walls contract in a progressive wave 1

4. Esophageal Phase:

   • Upper esophageal sphincter opens
   • Bolus passes into proximal esophagus
   • Upper esophageal sphincter closes to prevent regurgitation
   • Peristaltic wave moves bolus through esophagus 1

Neurological Control of Swallowing

Swallowing is controlled by a complex neural network:

• Swallowing Center: Located in the rhombencephalon (brainstem) 2, 3

• Two Key Medullary Regions:

  • Dorsal region: Nucleus of the solitary tract and adjacent reticular formation - initiates and programs swallowing
  • Ventral region: Reticular formation surrounding nucleus ambiguus - distributes swallowing excitation to motoneurons 3, 4

• Cranial Nerves Involved:

  • Trigeminal (V) - sensory input and motor control of mastication
  • Facial (VII) - taste and facial muscle control
  • Glossopharyngeal (IX) - sensory input from pharynx
  • Vagus (X) - motor control of pharynx and larynx
  • Hypoglossal (XII) - tongue movement 3

Developmental Aspects of Swallowing

• Infant Swallowing:

  • Consists of suck reflex followed by oral-pharyngeal and esophageal phases
  • Infants can breathe and swallow simultaneously (ability lost with maturity)
  • Superior larynx position and shorter pharyngeal length 1

• Developmental Milestones:

  • First 3 months: Tongue acts as piston for sucking
  • 7 months: Development of biting
  • 10-12 months: Development of chewing
  • 5-8 years: Adult pattern of muscle activation during swallowing 1

Protective Mechanisms

• Airway Protection:

  • Vocal fold adduction
  • Laryngeal elevation
  • Epiglottic folding
  • Coordinated breathing pause during swallowing 1

• Laryngeal Chemoreflex:

  • In adults: Hypochloremic or strongly acidic solutions contacting laryngeal epithelium trigger cough
  • In infants: Water in larynx causes apnea and increased swallowing (not cough) 1

Swallowing Disorders (Dysphagia)

Dysphagia is defined as abnormal swallowing due to impaired coordination, obstruction, or weakness affecting swallowing biomechanics 1. It can lead to:

• Aspiration pneumonia
• Malnutrition
• Dehydration
• Reduced quality of life 1, 5

Common neurological conditions associated with dysphagia include:

• Stroke (affects ~50% of patients)
• Parkinson's disease
• ALS (30% at diagnosis, virtually all as disease progresses)
• Multiple sclerosis (>33% of patients)
• Traumatic brain injury
• Dementia 1, 5

Functional Polarization of Swallowing

An important feature of the swallowing network is functional polarization:

• Activity of proximal portions of the swallowing tract inhibits that of distal portions
• This involves inhibitory connections between interneurons
• Creates "time-lag lines" responsible for the series of delays in the swallowing sequence 2, 3

Understanding the complex physiology of swallowing is essential for diagnosing and treating swallowing disorders, which can significantly impact morbidity, mortality, and quality of life in affected patients.