Enteric Nervous System Overview

Study Notes

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.

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.

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.

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.

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.

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.

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.

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.