Anatomy of the Myenteric Plexus

Enteric Nervous System (ENS)

• Located between longitudinal and circular smooth muscle layers
• Provides major nerve supply to GI tract wall
• Controls GI tract motility
• Pattern control of segmentation and peristalsis is largely automatic
• Involves pacemaker cells and local reflex arcs between enteric neurons in same or different plexuses (or organs)

Submucosal Plexus (Plexus of Meissner)

• Found within the submucosa
• Motor neurons supply secretory cells of the mucosal epithelium, controlling secretions of organs of the GI tract

Control of the Digestive System

• Interneurons interconnect neurons of the myenteric and submucosal plexuses
• Sensory neurons are chemoreceptors or stretch receptors that trigger reflexes
• Neurons of the NS can function independently, but are subject to regulation by the autonomic nervous system
• Reflexes can be intrinsic (short reflexes) or extrinsic (long reflexes), involving the CNS as an integration center
• Short and long reflexes can occur simultaneously

GI Tract Anatomy

• Composition of saliva:
  • 97-99.5% water, hypoosmotic
  • Slightly acidic to neutral pH
  • Electrolytes: Na+, K+, Cl-, PO43-, and HCO3
  • Digestive enzymes: salivary amylase and lingual lipase
  • Proteins: mucin, lysozyme, defensins, and IgA
  • Metabolic waste: urea and uric acid
• Control of salivation:
  • Intrinsic glands secrete saliva continuously
  • Food ingestion activates extrinsic glands
  • Salivation is mainly controlled by the parasympathetic division of the ANS
  • Dehydration inhibits salivation

Movements of the Small Intestine

• Mixing contractions: segmentation, important for mixing and absorption
• Propulsive contractions: responsible for the majority of chyme movement
• Migrating motility complexes: remaining amount of chyme is removed

Motility

• Mixing contractions (segmentation):
  • Localized contractions mix chyme with intestinal secretions and bring it into repeated contact with intestinal absorptive epithelium
  • Under the control of slow waves and excitatory activity of the nervous system
• Propulsive movements:
  • Peristaltic waves propelled by enteric nervous system, autonomic nervous system, and hormones
  • Arrival at the ileocecal valve prevents the return of fecal content into the small intestine
• Migrating motility complexes (MMCs):
  • Important for the removal of remaining chyme from the small intestine

GI Tract Layers

• Mucosa: most inner, moist epithelial layer that lines the lumen of the alimentary canal
  • Epithelium, lamina propria, muscularis mucosae
  • Functions: secretion of mucus, absorption of end products of digestion, protection against infectious disease
• Submucosa: dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves
• Muscularis externa: circular muscle and longitudinal muscle, plexus, responsible for segmentation and peristalsis
• Serosa: outermost layer, protective, replaced by adventia in esophagus

Oral Cavity

• Involves ingestion and initial digestion of food
• Accessory organs: teeth, salivary glands, and tongue
• Epithelium of gum and hard palate is slightly keratinized for protection against abrasion
• Releases antimicrobial peptides: defensins

Pharyngeal-Esophageal Phase

• Larynx pulled upward and respiration briefly inhibited
• Epiglottis swings backward over the opening of the larynx
• Tongue blocks off mouth to prevent passage of food into the trachea
• Pharyngeal muscles contract and the upper esophageal sphincter relaxes
• Rapid peristaltic wave to allow food processing into the esophagus