GI Lecture 2 Flashcards
Enteric Nervous System
The intrinsic innervation of the GI tract, mediated by the nerve plexuses of the gut wall. Begins in the esophagus and ends in the anus. Made up of 100 million neurons.
Extrinsic innervation of the gut
The gut innervated by the autonomic nervous system, with both parasympathetic and sympathetic systems involved.
Describe the flow of neural control of GI function.
Stimuli –> sensors (mechanical and chemical) –> ((optional) brain and spinal cord) –> enteric nervous system –> effectors (motility, secretion, blood flow)
Sympathetic nervous system
Part of the autonomic nervous system. Concerned with activities that, in general, arouse the body for physical activity; also called the thoracolumbar division.Mainly postganglionic in GI tract. Has inhibitory effects on GI tract.
Parasympathetic nervous system
Part of the autonomic nervous system. Concerned with activities that, in general, inhibit or oppose the physiological effects of the sympathetic nervous system. Mainly preganglionic in GI tract. Has excitatory effects on GI system.
Postganglionic
Cell body in one of autonomic ganglia
Preganglionic
Cell body in CNS
Myenteric plexus
A network of sympathetic and parasympathetic nerve fibers located in the muscularis tunic of the small intestine. Between the longitudinal and circular muscle layers.
Submucosal plexus
A network of sympathetic and parasympathetic nerve fibers located in the submucosa tunic of the small intestine. Under the mucosa.
Epithelium
The type of tissue that covers or lines all exposed body surfaces.
Parts of brain the receive signals from sensory neurons of the GI tract
Prevertebral ganglia, spinal cord, brain stem
Does the parasympathetic system innervate the epithelium directly?
No, it innervates the plexuses which in turn innervate the epithelium.
Does the sympathetic system innervate the epithelium directly?
Yes, it innervates the epithelium and the plexuses.
Does the enteric nervous system actually act autonomously?
The enteric nervous system can and does function autonomously, but normal digestive function requires communication between the intrinsic system and the CNS.
What happens when the myenteric plexus is stimulated?
- Increased tonic contraction of the gut wall
- increased intensity and rate of contractions
- increased velocity of excitatory waves along the gut, causing more rapid peristaltic waves.
What are the main functions of the submucosal plexus?
Help control LOCAL secretion, absorption and submucosal muscle contraction. Receives sensory info from sensory neurons in GI tract.
What are the main functions of the myenteric plexus?
Controls motility of GI tract and controls muscle activity along the LENGTH of the gut.
Do autonomic fibers directly innervate structures in the GI tract?
No, but they modulare functions of the neurons in the enteric nervous systems.
Describe innervation via the parasympathetic nervous system.
Mainly preganglionic and cholinergic.
Nerve fibers terminate on the ganglion cells in enteric nerve plexuses and act on nicotinic ACh receptors.
No direct innervation on effector cells, but ganglion cells innervate the smooth muscle and secretory cells in the gut wall.
What main parts do we associate with the parasympathetic nervous system and the GI tract?
Medulla oblongata (dorsal vagal complex)
Vagal nerves - extends from brainstem (medulla oblongata) all the way to colon
Pelvic nerves - from sacral spinal cord to stimulate the hind gut
Sacral spinal cord
What main parts do we associate with the sympathetic nervous system and the GI tract?
Medulla oblongata
Thoraco-lumbar region
Superior cervical ganglion
Prevertibral ganglia
What are the three prevertebral ganglia?
Celiac, superior mesenteric, inferior mesenteric
Describe innervation via the sympathetic nervous system.
Preganglionic fibers arise from spinal cord (T5-L2) and terminate in prevertebral ganglia. Neurotransmitter is ACh and acts on nicotinic receptors.
Postganglionic fibers terminate in the end organs. Neurotransmitter is norepinephrine or epinephrine.
How would sensory neurons of the gut be stimulated?
- irritation of gut mucosa
- gut distension
- presence of chemicals
What happens, in general terms, when these sensory neurons are activated?
Excitation or inhibition of intestinal movements or secretion. Transmits sensory signals from the gut (acidic, osmolarity, stretch, pain) to the CNS.
Where are the cell bodies of the sensory gut neurons?
ENS or in the dorsal root ganglia of spinal cord
What are the three general types of GI reflexes?
- Reflexes that are integrated entirely within the ENS.
- Reflexes from the gut to the prevertebral sympathetic ganglia and back to the GI tract.
- Reflexes from the gut to the spinal cord or brain stem and back to the GI tract.
Reflexes that are integrated entirely within the ENS
Include reflexes that control GI secretion, peristalisis, mixing contractions, local inhibitory effects.
Reflexes from the gut to the prevertebral sympathetic ganglia and back to the GI tract
Includes:
- Gastrocolic reflex - signals from the stomach cause evacuation of the colon
- Enterogastric reflex - signals from the colon and small intestine to inhibit stomach motility and secretion
- Colonoileal reflex - signals from the colon to inhibit emptying of ileal contents
Reflexes from the gut to the spinal cord or brain stem and back to the GI tract
Includes:
- Vago-vagal reflexes - from the stomach and duodenum to the brainstem and back to control gastric motor and secretory activity
- Pain reflexes - general inhibition of gut
- Defecation reflexes - travel from colon and rectum to the spinal cord and back to produce the powerful colonic, rectal, and ab contractions required for defecation.
What are the gastrointestinal peptides?
Hormones, paracrines and neurocrines
Are peptides released within the GI tract cells?
Yes, some are, but hormones and transmitters from outside the GI tract also regulate its function.
Describe the secretory route of hormones
Endocrine cell of the GI tract -> secretion -> portal circulation -> liver -> systemic circulation -> target cell
Describe the secretory route of paracrines
Endocrine cell of the GI tract -> diffusion -> target cell
Describe the secretory route of neurocrines
Neuron of the the GI tract -> action potential -> target cell
What are the functions that gastrointestinal peptides regulate?
- Contraction and relaxation of smooth muscle wall and sphincters
- Secretion of enzymes for digestion
- Secretion of fluid and electrolytes
- Trophic (growth) effects on the tissues of GI tract
- Sometimes secretion of other GI peptides
Give an example of a GI peptide regulating another GI peptide
Somatostatin inhibits secretion of all GI hormones
Acetylcholine (ACh)
A neurocrine that regulates contraction of smooth muscle, relaxation of sphincters, increased salivary, gastric, and pancreatic secretions.
Norepinephrine
A neurocrine involved in relaxation of smooth muscle, contraction of sphincters, increased saliva secretion.
Substance P
A neurocrine co-released with ACh and amplifies ACh action
Vasoactive intestinal peptide (VIP)
A neurocrine that inhibits smooth muscle contraction and stimulates secretion and vasodilation
Serotonin (5-HT)
A neurocrine with diverse motor and sensory functions
Nitric oxide
A neurocrine that relaxes smooth muscle activity
Where are hormones produced?
Endocrine cells in the mucosa or submucosa of the stomach, small intestine, or pancreas.
Why are hormones released?
Response to nervous activity and chemical and mechanical signals coincident with food ingestion.
What are the important GI hormones?
gastrin, CCK, secretin, GIP, motilin, Ghrelin
Gastrin
A GI hormone produced by G-cells in the antrum region of the stomach.
GRP
Gastrin releasing peptide
Directly stimulates G-cells
Amino acids, small peptides, GRP released from enteric neurons
Directly inhibits G-cells
Somatostatin release from the D cell
Stimulates D-cells
Acid and enteric neurons.
Stimulates enteric neurons
Vagus nerve and gastric distention
Stimulates vagus
Gastric distension.
What does gastrin do?
Stimulates acid secretion and mucosal growth. The parietal cell that it affects secretes HCl into the stomach.
Cholecystokinin
Aka CCK, a GI hormone that is secreted by I cells in the duodenum and jejeunum. Homologous to gastrin.
What activates CCK release?
- Fats -> fatty acids and monoglycerides -> CCK release
2. Proteins -> peptides and amino aicds -> CCK release
What are the actions of CCK?
Stimulates gallbladder contraction and relaxation of sphincter of Oddi.
What does CCK have a negative effect on?
Gastric emptying
What does CCK have a positive effect on?
Pancreatic enzyme secretion, pancreatic growth, pancreatic fluid secretion, bile secretion from gallbladder.
Secretin
A GI hormone that is secreted by the S-cells of the duodenum and is homologous to glucagon.
How is secretin stimulated?
- gastric acid secretion -> duodenal pH secretin release
2. fatty acids -> secretin release
What are the actions of secretin?
Overall leads to neutralization of acidity in the small intestine. Increase pancreatic HCO3- secretion, hepatic HCO3- secretion, and pancreatic growth. Decreases gastric acid secretion.
Glucose-dependent insulinotropic peptide
Aka GIP, secreted by K-cells of the duodenum and jejunum. Homologous to secretin and glucagon.
What stimulates GIP release?
Fatty acids, monoglycerides, glucose, amino acids.
What are the actions of GIP?
Increase insulin release and decrease gastric acid secretion. Inhibition of gastric secretion only occurs if higher than physiological concentrations are used.
Motilin
A GI hormone, 22-amino acid linear peptide, released cyclically from the gut when fasting. It is responsible for stimulating a specific pattern of GI motility known as the migrating motor complex.
What is the migrating motor complex?
Waves of electrical activity that sweep through the intestines in a regular cycle during fasting
Ghrelin
A GI hormone mainly released from the stomach and pancreas during fasting, and peaks just before meals.
What does ghrelin stimulate?
Vagal afferents that trigger the release of signals in the solitary nucleus and hypothalamus that promote food intake. Include orexins and neuropeptide Y.
How is ghrelin suppressed?
Leptin, that is released from adipose tissue.
What peptides reduce food intake?
CCK and PYY, which are released from the intestine in response to nutrients and trigger vagal afferent pathways to reduce good intake. Also modulated by higher brain areas.
Where is gastrin produced?
Antrum to jejunum, but mostly antrum.
Where is CCK produced?
Duodenum to Illeum, but mostly duodenum and jejunum.
Where is secretin produced?
Duodenum to Illeum, but mostly duodenum and jejunum.
Where is GIP produced?
Duodenum and jejunum.
Where is motilin produced?
Duodenum and jejunum.
Where is ghrelin produced?
Fundus to Colon, but mostly in stomach regions.
Has a positive effect on mucosal growth
Gastrin
Has a positive effect on acid secretion
Gastrin
Has a negative effect on acid secretion
Secretin
Has a positive effect on pancreatic enzymes, gallbladder, pancreatic growth, and pancreatic HCO3-
CKK
Has a negative effect on gastric emptying
CKK
Has a positive effect on pancreatic HCO3, hepatic HCO3, pancreatic growth.
Secretin
Gas a positive effect on insulin.
GIP
Somatostatin
A paracrine secreted by D‐cells in gastric mucosa. Acidity of antral region stimulates somatostatin release. Also released by neurons in the enteric nervous system. Inhibits the release of gastrin, histamine and inhibits gastric acid secretion from parietal cells.
Histamine
A paracrine secreted from the enterochromaffin cells in the stomach. Acts on H2 receptors on parietal cells. Together with gastrin and ACh it stimulates acid secretion by the gastric parietal cells. It is also secreted from mucosal mast cells and has a role as an immune mediator
