2 - GI Tract Structure and Regulation Flashcards
Objectives: Describe the roles of the extrinsic and enteric nervous systems in the fuction of the GI tract
Extrinsic - Parasympathetics / Sympathetics
Vagus N. vs Pelvic N.
Vagovagal Reflex
- Extrinsic Nervous System (Autonomic)
- Parasympathetic
- Vagus Nerve: Esophagus, stomach, small intestine, upper colon
- Pelvic Nerve: Descending/Sigmoid colon, rectum, anal canal
- Synapse w/enteric/intrinsic system
-
Vagovagal Reflex:
- Info from receptors in mucosa/smooth muscle relayed to CNS via Vagus N.
- Triggered respponse relayed back via same, Vagus N.
- Sympathetic
- Nerve fibers synapse outside GI tract, some innervate directly
- Parasympathetic
Objectives: Describe the roles of the extrinsic and enteric nervous systems in the fuction of the GI tract
Enteric Nervous System
Composition / Purpose
Myenteric Plexus
Submucosal Plexus
- Composition:
- Motor neurons, sensory neurons, interneurons
- Myenteric / Submucosal Plexi form networks
- Purpose:
- Relays info to and from gut via extrinsic
- Myenteric Plexus
- Throughout GI tract; controls motility
- Submucosal Plexus
- Intestines; controls secretion
Objectives: Describe the roles of the extrinsic and enteric nervous systems in the fuction of the GI tract
System control - Extrinsic vs Enteric
- Extrinsic Control:
- Esophagus
- Stomach
- Defecation
- Enteric Control: Intestines
- Small Intestine
- Large Intestine
Objectives: Descrive the vagovagal reflex
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Hormones: Gastrin
Zollinger-Ellison syndrome
- Action:
- (+) HCl secretion by parietal cells of stomach
- Release:
- G-Cells in Stomach
- Regulation:
- (+) Protein Digestion, Stomach Distension, Vagal Stimulation
- Zollinger-Ellison Syndrome:
- Hypersecretion of gastric acid
- Due to continuous release of Gastrin from gastrinoma in small intestine/pancreas
- Symptoms: Duodenal ulcers, diarrhea, steatorrhea
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Hormones: CCK
-
Actions: Digestion!
- (+) Gallbladder contraction
- (+) Pancreatic enzyme secretion
- (+) Potentiation w/pancreatic bicarbonate
- (-) Gastric Emptying
- (+) Satiety
- Secretion:
- I Cells of proximal small intestine
- Regulation: Digestion Products / Neural Input
- (+) Small peptides, amino acids, fatty acids, monoglcerides
- (+/-) CCK-Releasing Peptide (CCK-RP)
- Paracrine cells within epithelium
- (+/-) Monitor Peptide
- Secreted by pancreas
- No Digestion Occuring:
- CCK-RP / Monitor degraded by Trypsin
- Digestion Occuring:
- No digestion by Trypsin; continued stimulation of I-Cell (+ CCK)
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Hormones: Secretin
- Action:
- (+) Bicarbonate and water secretion in liver
- (+) Bile production
- (-) Gastric acid secretion by parietal cells
- Secretion:
- S-Cells of proximal small intestine
- Regulation:
- (+) Acid in proximal small intestine
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Hormones: Glucose Dependent Insulinotropic Peptide (GIP)
Hormones: Motilin
Hormones: Guanylin
- GIP
- Action: (+) Insulin release from pancreas
- Reason oral glucose insulin drive > IV glucose insulin drive
- Secretion: K cells in prox. small intestine
- Regulation: Fatty acids, glucose, amino acids
- Action: (+) Insulin release from pancreas
- Motilin
- Action: (+) Migrating myoelectric complexes
- Secretion: M-Cells in stomach/small intestine
- Regulation: (-) Eating blocks release
- Guanylin
- Action: Increases fluid secretion by increasing Cl- secretion
- Secretion: Intestines
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Paracrines: Somatostatin
Paracrines: Histamine
Paracrines: Serotonin
- Somatostatin
- Action:
- (-) Gastric Acid Secretion, Gastrin release
- (-) All GI hormone release
- Secretion: D-cells in GI tract
- Regulation: (+) Acid in lumen
- Action:
- Histamine
- Action:
- (+) Gastric Acid secretion
- (+) Potentiates Gastrin, ACh
- Secretion: ECL cells; acid secreting portion of stomach
- Action:
- Serotonin
- Action:
- (+) Intestinal fluid, mucus secretion
- (+) Gut motility
- Secretion: Enteric Neurons, ECL cells
- Action:
Objectives: Describe the different GI peptides, their actions, sites of release, and what regulates their release
Types: Hormones, Paracrines, Neurocrines
Neurocrines: Vasoactive Intestinal Peptide (VIP)
Neurocrines: Gastrin-releasing Peptide (GRP)
Neurocrines: Enkephalins
- VIP
- Action: (+) Intestinal, Pancreatic Secretion, (+) Relaxation of smooth muscle
- Secretion: Nerves in mucosa, smooth muscle of GI tract
- Clinical: Pancreatic Islet Cell Tumor; cholera/watery diarrhea syndrome
- GRP (Gastrin Releasing Peptide)
- Action: (+) Gastrin release
- Secretion: Nerves in gastric mucosa
- Regulation: Vagal stimulation
- Enkephalins:
- Action: (+) Contraction of GI smooth muscle (lower esophaeal, pyloric, ileocecal sphincters); inhibit secretion of fluid and electrolytes
- Opiates useful in treating diarrhea
- Secretion: Nerves in mucosa, smooth muscle of GI tract
*
- Action: (+) Contraction of GI smooth muscle (lower esophaeal, pyloric, ileocecal sphincters); inhibit secretion of fluid and electrolytes
Objectives: Describe slow waves and how they influence contractions in the GI tract
Basic Electrical Rhythm (BER)
Generation
Phases
Frequency
- BER:
- Periodic changes in resting membrane potential of smooth muscle cells
- GI Muscles
- Generation: Interstitial cells of Cajal, spread via gap junctions
- NOT Action Potentials
- Phases:
- Depolarization: Ca2+ influx
- Plateau: Ca2+ influx
- Repolarization: K+ efflux
- Frequency:
- Influenced by neural / hormonal inputs
Objectives: Describe slow waves and how they influence contractions in the GI tract
Cholinergic Neurons/Parasympathetics
Adrenergic Neurons/Sympathetics
Stretching smooth muscle
- Cholinergic Neurons/Parasympathetics
- Membrane potential more positive
- Produces action potentials, increase contractions
- Adrenergic Neurons/Sympathetics
- Membrane potential more negative
- Inhibits action potentials, decrease contractions
- Stretching can increase action potentials
- Slow wave frequency sets max frequency of contraction of given part of GI tract
