- The Enteric Nervous System (ENS):
- Is intrinsic to the GI wall.
- Runs the length of the GI tract.
- 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)
- 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.
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.
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
- Show the extrinsic neuronal input converge on a neuron in the myenteric plexus.
- Show it then innervate 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.
- 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.
- 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.