Gastrointestinal Tract Innervation

• The gastrointestinal tract (GIT) is subject to neural regulation.

Innervation of GIT

• Intrinsic innervation is local.
• Extrinsic innervation comes from higher brain centers.
• The myenteric/Auerbach plexus is one type of intrinsic innervation.
• The submucosal/Meissner's plexus is another type of intrinsic innervation.
• Parasympathetic and sympathetic nervous systems are types of extrinsic innervation of the GIT.

Myenteric Plexus

• The myenteric plexus is located in the wall of the gut, starting in the esophagus and extending to the anus.
• It lies between the longitudinal and circular muscle layers.
• This plexus controls gastrointestinal movements and secretion.
• The myenteric plexus consists of interconnected neurons along the entire length of the GIT.
• Stimulation leads to:
  • Increased tonic contraction of the gut wall.
  • Enhanced intensity and slight rate increase of rhythmic contractions.
  • Increased velocity of excitatory wave conduction.
  • More rapid movement of peristaltic waves.

Submucosal Plexus

• The submucosal plexus is located in the submucosa.
• It mainly controls gastrointestinal secretion and local blood flow.
• Neurotransmitters involved include:
  • Acetylcholine (Ach)
  • Norepinephrine (NE)
  • ATP
  • 5-HT
  • Dopamine
  • Cholecystokinin
  • Substance P
  • VIP
  • Somatostatin
  • Bombesin
  • Metenkephalin
  • Leuenkephali

Higher Center Innervation

• Extrinsic sympathetic and parasympathetic fibers connect to both the myenteric and submucosal plexuses.
• The enteric nervous system can function independently of extrinsic nerves.
• Stimulation by the parasympathetic and sympathetic systems greatly affects gastrointestinal functions.
• Sensory nerve endings in the gastrointestinal epithelium or gut wall send afferent fibers to the enteric system's plexuses and to:
  • Prevertebral ganglia
  • Spinal cord
  • Vagus nerves (to the brain stem)
• These sensory nerves can initiate local reflexes within the gut wall.

Autonomic Control: Parasympathetic

• Cranial parasympathetic nerve fibers supply the:
  • Mouth
  • Pharyngeal regions
  • Alimentary tract
  • Esophagus
  • Stomach
  • Pancreas
  • First half of the large intestine
• Sacral parasympathetics originate in the 2nd, 3rd, and 4th sacral segments of the spinal cord.
  • They pass through the pelvic nerves to the distal half of the large intestine and anus.
• The sigmoidal, rectal, and anal regions have a better parasympathetic supply.
  • Plays a major role in defecation reflexes.

Sympathetic Innervation

• Sympathetic innervation originates in the spinal cord between segments T-5 and L-2.
• Preganglionic fibers run through sympathetic chains to the celiac and mesenteric ganglia, then becoming postganglionic.
• Sympathetic fibers essentially innervate the entire gastrointestinal tract.
  • Their effect is generally inhibitory.
• Inhibition occurs:
  • Directly via secreted norepinephrine (NE) on intestinal tract smooth muscle.
  • Indirectly through an inhibitory effect of NE on the neurons of the enteric nervous system.

Afferent Sensory Nerve Fibers

• Sensory nerves are stimulated by:
  • Irritation of the gut mucosa
  • Excessive distention of the gut
  • Specific chemical substances in the gut

Gastrointestinal Reflexes

• Reflexes integrated entirely within the gut wall's enteric nervous system influence:
  • Secretion
  • Peristalsis
  • Mixing contractions
  • Local inhibitory effects
• Reflexes from the gut to the prevertebral sympathetic ganglia and back include:
  • The gastrocolic reflex, where stomach signals cause colon evacuation
  • Enterogastric reflexes, where colon and small intestine signals inhibit stomach motility and secretion
  • The colonoileal reflex, where colon reflexes inhibit emptying of ileal contents into the colon
• Reflexes from the gut to the spinal cord or brain stem and back include:
  • Reflexes from the stomach and duodenum to the brain stem, using the vagus nerves to control gastric motor and secretory activity
  • Pain reflexes that generally inhibit the entire GIT
  • Defecation reflexes traveling from the colon and rectum to the spinal cord, producing colonic, rectal, and abdominal contractions

Migrating Motor Complex (MMC)

• Involves electrical and motor activity in gut smooth muscles.
• Occurs during the interdigestive stage.
• Cycles last 90-100 minutes each.
• Movement rate is 5 cm/min from oral to aboral site.
• Phases include:
  • Quiescent period (no activity)
  • Period of irregular activity
  • Period of regular activity
• Initiated by motilin.
• Gastric secretion, bile flow, and pancreatic secretion increase.
• Food abolishes the MMC by inhibiting motilin.

The gastrointestinal tract is regulated by intrinsic and extrinsic neural pathways. Intrinsic innervation includes the myenteric and submucosal plexuses. Extrinsic innervation is provided by the parasympathetic and sympathetic nervous systems, enabling coordinated control of gastrointestinal functions.