Enteric Nervous System Overview

Enteric Nervous System

• The enteric nervous system (ENS) is a division of the autonomic nervous system, innervating the gastrointestinal tract (GIT), pancreas, and biliary system.
• The ENS functions intrinsically meaning it can operate independently, but communicates with the brain and central nervous system (CNS) for certain functions.
• The ENS is composed of sensory, motor, and interneurons that communicate with receptors and effectors through two main plexuses of ganglion cells and their associated fiber bundles:
  • Submucosal (Meissner's) plexus: Located in the inner layers of the stomach and small intestine, responsible for glandular secretion.
  • Myenteric (Auerbach's) plexus: Located between the circular and longitudinal muscle layers of the muscularis externa; extends from the esophagus to the anus.

Components of the ENS

• Sensory (afferent) neurons:
  • Associated with mechanoreceptors and chemoreceptors, detecting the presence and characteristics of food.
  • Mechanoreceptors respond to physical pressure; chemoreceptors detect chemical compounds.
• Interneurons:
  • Excitatory or inhibitory neurons, coordinating responses.
• Secretomotor (efferent) neurons:
  • Innervate effectors (secretory glands and smooth muscles).
  • Release acetylcholine (ACh) or different types of peptides as neurotransmitters.

Enteric Reflexes

• Short reflexes: Nervous components of these reflexes are contained within the gut; they do not require the CNS to function.
• Motility reflexes: Coordinate peristalsis (rhythmic movement of food).
  • Detect food boluses through mechanoreceptors and chemoreceptors.
  • Afferent neurons carry this information to interneurons in the ENS.
  • Excitatory interneurons stimulate smooth muscle contraction behind the bolus to propel food.
  • Inhibitory interneurons relax smooth muscle ahead of the bolus.
• Secretory reflex: Coordinate the release of chemicals (enzymes, mucus, acids) to aid in food digestion within the stomach and small intestine.
  • Detects food presence via chemoreceptors and baroreceptors.
  • Sensory neurons carry this information to interneurons in the submucosal plexus.
  • Interneurons stimulate secretomotor neurons that innervate exocrine glands.

Blood Flow Reflex

• Involves both plexuses (myenteric and submucosal).
• Detect presence of food using mechanoreceptors and chemoreceptors.
• Motor neurons release vasoactive neurotransmitters that dilate blood vessels supplying the G.I. tract to facilitate digestion.

Integration with the CNS

• The ENS can communicate with the brain.

• Actions involving this bidirectional communication are considered long reflexes.

• ENS function declines with age due to neuronal death.

ENS-Specific Disorders

• Chagas' disease: Insect-borne trypanosome parasite, killing ENS neurons, compromising function, and causing problems with digestion.
• Achalasia: Damage to ENS neurons in the esophagus and sphincter, leading to swallowing difficulties.
• Hirschsprung's disease: Congenital absence of ENS neurons in the distal colon, resulting in bowel paralysis and potential complications.

General Neurological Disorders Affecting the ENS

• Parkinson's disease: Pathogens may enter through the enteric nervous system, and travel to the brain, potentially causing nerve damage.
• Peripheral neuropathies: Damage to enteric nervous system nerves can impact digestion
• Myasthenia gravis: Condition impacting smooth muscle function in the gut that can potentially impact the ENS.

IBS (irritable bowel syndrome)

• Dysfunctional interaction between the gut and brain (ENS and CNS.)
• Often associated with anxiety and other mental health concerns.

Drugs Targeting the ENS

• Some drugs target opioid receptors within the ENS, such as those for diarrhea.
• Pain medications (e.g., morphine, codeine) may also activate opioid receptors in the GI tract, reducing gut activity and potentially leading to constipation.