Neural Control of GI Motility & Pharmacological Correlations

Neural Control of GI Motility & Pharmacological Correlations

Sections

Overview
Extrinsic system
Interaction of the extrinsic and intrinsic systems
Pharmacological correlations

Overview

Intrinsic control

The Enteric Nervous System (ENS) is intrinsic to the GI wall and runs the length of the GI tract.

The ENS Primarily coordinates local activity in the digestive tract via two key nerve plexuses:

• The submucosal plexus (aka Meissner's plexus)
• The myenteric plexus (aka Auerbach's plexus)

Extrinsic control

Parasympathetic innervation stimulates digestion: it stimulates GI motility and the secretion of hormones and digestive juices; remember its tagline is "Rest and Digest".

Sympathetic nervous system inhibits digestive activity. Remember its tagline is "fight or flight" – neither of which have anything to do with digestion.

Intrinsic System

Anatomy of the digestive tract, from inside to outside:

• The GI lumen
• The mucosal layer
  • Epithelial layer
  • Lamina propria
  • Muscularis mucosae
• The submucosal layer
  • The submucosal plexus (Meissner's plexus) lies within the outer portion of this layer.
• The smooth muscle (muscularis externa) layer.
  • The inner, circular layer.
  • The myenteric plexus (Auerbach's plexus) lies in between the inner, circular and outer, longitudinal layers.
  • The outer, longitudinal layer.
    The different orientations of the inner, circular and outer, longitudinal muscle layers allow us to distinguish them.
• The adventitia/serosa layer; it's serosa within the abdominal cavity.

Extrinsic system

Parasympathetic nervous system

Cranial nerve 10 (the vagus nerve) innervates the gut; it innervates the upper 2/3 of the GI tract (ie, the foregut and midgut). It is this wandering nature of the vagus nerve all the way to the gut that give it its name "vagus," which is Latin for "wandering."

Spinal neurons S2 to S4 of the intermediolateral cell column of the sacral spinal cord innervate pelvic splanchnic nerves, which innervate the gut; they innervate the lower 1/3 of the GI tract (ie, the hindgut).

Sympathetic nervous system

Originates from the T5 to L2 neurons of the intermediolateral cell column.

Abdominopelvic splanchnic nerves innervate prevertebral ganglia, which innervate the GI tract.

Interaction of the extrinsic and intrinsic systems

Extrinsic neuronal input innervates the myenteric plexus

We show the extrinsic neuronal input converge on a neuron in the myenteric plexus; then it innervates a neighboring neuron, which extends into the submucosa to innervate a neuron of the submucosal plexus, which then innervates the muscular layer of the mucosa to activate or inhibit GI motility.

Pharmacological correlations

Neurotransmitters

Acetylcholine INcreases GI motility (it's the major parasympathetic neurotransmitter – the "D" in the "SLUDS" acronym stands for defecation or diarrhea.

Norepinephrine DEcreases GI motility (it's the major postganglionic sympathetic neurotransmitter).

Opioid peptides DEcrease GI motility.

Serotonin INcreases GI motility – serotonin syndrome causes diarrhea.

Drugs

Amitriptyline decreases circulating acetylcholine, so it decreases GI motility.

Donepezil increases circulating acetylcholine, so it increases GI motility.

Beta-blockers decrease circulating norepinephrine, so they increase GI motility.

Venlafaxine increases circulating norepinephrine, so it decreases GI motility.

Morphine is an opioid agonist, so it decreases GI motility.

Naltrexone is an opioid antagonist, so it increases GI motility.

Quetiapine decreases circulating serotonin, so it decreases GI motility.

Citalopram increases circulating serotonin, so it increases GI motility.