Neural Regulation of the Gastrointestinal Tract

• Neural regulation of the gastrointestinal tract (GIT) consists of intrinsic and extrinsic innervation

Intrinsic Innervation

• Refers to the local nervous system within the gut wall
• Includes the myenteric/Auerbach plexus and the submucosal/Meissner's plexus
• Lies in the wall of the gut, beginning in the esophagus, and extending all the way to the anus
• Controls gastrointestinal movements and secretion

Myenteric/Auerbach Plexus

• Located between the longitudinal and circular muscle layers
• Primarily controls gastrointestinal movements

Submucosal/Meissner's Plexus

• An inner plexus located in the submucosa
• Primarily controls gastrointestinal secretion and local blood flow

Myenteric Plexus Stimulation Effects

• Increased tonic contraction of the gut wall
• Increased intensity of rhythmical contractions
• Increased rate of rhythmical contractions
• Increased velocity of conduction of excitatory waves along the gut wall leading to more rapid peristaltic waves

Submucosal Plexus Function

• Controls function within the inner wall
• Facilitates local intestinal secretion, local absorption, and local contraction of the submucosal muscle
• Operates through various neurotransmitters like Acetylcholine (Ach), Substance P, Norepinephrine (NE), Vasoactive Intestinal Peptide (VIP), ATP, Somatostatin, 5-HT, Bombesin, Dopamine, Metenkephalin, Cholecystokinin, and Leuenkephalin

Extrinsic Innervation

• Refers to higher-level control from the parasympathetic and sympathetic nervous systems
• Connects to both the myenteric and submucosal plexuses
• Enteric nervous system can function independently of extrinsic nerves
• Stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit gastrointestinal functions

Higher Center Innervation Sensory Feedback

• Sensory nerve endings in the gastrointestinal epithelium or gut wall send afferent fibers to the enteric system plexuses
• Information is also relayed to the prevertebral ganglia, spinal cord, and brain stem via the vagus nerves

Autonomic Control: Parasympathetic

• Cranial parasympathetic fibers supply the mouth, pharyngeal regions, alimentary tract, esophagus, stomach, and pancreas, with lesser supply to the first half of the large intestine
• Sacral parasympathetics originate in the S2-S4 segments of the spinal cord
• Travel through the pelvic nerves to supply the distal half of the large intestine and all the way to the anus
• Sigmoidal, rectal, and anal regions have significant parasympathetic innervation for defecation reflexes

Sympathetic Innervation

• Originates from the spinal cord segments T5-L2
• Preganglionic fibers synapse in the celiac and various mesenteric ganglia, with postganglionic fibers innervating essentially all of the gastrointestinal tract
• Primarily inhibitory to gastrointestinal activity
• Caused by the direct effect of secreted Norepinephrine (NE) on intestinal tract smooth muscle
• Inhibitory effect of NE on the neurons of the entire enteric nervous system.

Afferent Sensory Nerve Fiber Stimuli

• Irritation of the gut mucosa
• Excessive gut distention
• Presence of specific chemical substances in the gut

Gastrointestinal Reflexes

• Reflexes integrated entirely within the gut wall control secretion, peristalsis, mixing contractions, and local inhibitory effects
• Reflexes from the gut to the prevertebral sympathetic ganglia and back regulate various functions
  • Signals from the stomach cause colon evacuation (gastrocolic reflex)
  • Signals from the colon and small intestine inhibit stomach activity (enterogastric reflexes)
  • Reflexes from the colon inhibit emptying of ileal contents into the colon (colonoileal reflex)
• Reflexes from the gut to the spinal cord or brain stem and back:
  • Reflexes from the stomach and duodenum to the brain stem to control gastric motor and secretory activity (via vagus nerve)
  • Pain reflexes cause general inhibition of the entire gastrointestinal tract
  • Defecation reflexes travel from the colon and rectum to the spinal cord, producing contractions needed for defecation

Migrating Motor Complex (MMC)

• Electrical and motor activity in gut smooth muscles during the interdigestive stage, cycles lasting 90-100 minutes
• Rate of movement is 5cm/min from oral to aboral site
• Phase 1: Quiescent period with no activity
• Phase 2: Period of irregular activity
• Phase 3: Period of regular activity
• Initiated by motilin
• Gastric secretion increases, along with bile flow and pancreatic secretion .
• Food abolishes MMC by inhibiting motilin

Explore the neural control of the gastrointestinal tract (GIT), focusing on intrinsic innervation. Learn about the myenteric (Auerbach's) and submucosal (Meissner's) plexuses and their roles in controlling gastrointestinal movements, secretion, and local blood flow. Understand how stimulating the myenteric plexus affects gut wall contraction, rhythm, and peristaltic wave velocity.