GI Physiology Flashcards
layering of the GIT (detailed)
- epithelial cells
- muscularis mucosa
- circular muscle
- longitudinal muscle
- submucosal plexus (Meissner plexus) and myenteric plexus
Meissner (submucosal) and myenteric plexus
= the enteric nervous system of the GIT; integrate and coordinate the motility, secretory, and endocrine functions of the GIT
autonomic nervous system (ANS) of the GIT
comprises both extrinsic and intrinsic nervous system
extrinsic innervation of the GIT
- parasympathetic nervous system
2. sympathetic nervous system
efferent fibers
carry sensory information from the brainstem and spinal cord to the GIT
afferent fibers
carry sensory information from chemoreceptors and mechanoreceptors in the GIT to the brainstem and spinal cord
function of the parasympathetic nervous system on the GIT
usually excitatory
parasympathetic nerves of the GIT
- vagus nerve
2. pelvic nerve
anatomy of the PNS of the GIT
preganglionic parasympathetic fibers synapse in the myenteric and submucosal plexuses; cell bodies in the ganglia of the plexuses then send information to the smooth muscle, secretory cells, and endocrine cells of the GIT
innervations of the vagus nerve
- esophagus
- stomach
- pancreas
- upper large intestine
vagovagal reflexes
= reflexes in which both afferent and efferent pathways are contained in the vagus nerve
innervations of the pelvic nerve
- lower large intestine
- rectum
- anus
function of the sympathetic nervous system on the GIT
usually inhibitory
sympathetic nerves of the GIT
fibers originating in the spinal cord btwn T8 and L2
anatomy of the SNS of the GIT
preganglionic sympathetic adrenergic fibers leave the vertebral ganglia and synapse in the myenteric and submucosal plexus–>
- direct postganglionic adrenergic innervation of blood vessels and some smooth muscle cells
- cell bodies in the ganglia of the plexus then send information to the smooth muscle, secretory cells, and endocrine cells of the GIT
intrinsic innervation (= enteric nervous system) functions
coordinates and relays information from the parasympathetic and sympathetic nervous system to the GIT; controls most functions of the GIT, especially motility and secretion, even in the absence of extrinsic innervation
MOA of the intrinsic nervous system
uses local reflexes to relay information within the GIT
plexuses of the intrinsic nervous system of the GIT and the functions they control
- myenteric plexus (Auerbach plexus)–>motility of GI smooth muscle
- submucosal plexus (Meissner plexus)–>secretion and blood flow; receives sensory information from chemo and mechanoreceptors in the GIT
gastrin summary: family site of secretion stimulus for secretion inhibition actions
- family: gastrin-CCK
- site of secretion: G cells of the stomach (antrum)
- stimulus for secretion: = a meal
1. small peptides and amino acids
2. stomach distension
3. vagus nerve via GRP (gastrin-releasing peptide) - inhibited by
1. acid in the stomach
2. somatostatin - actions of gastrin
1. increase gastric acid secretion
2. stimulate growth of gastric mucosa
CCK summary: family site of secretion stimulus for secretion actions
- family: gastrin-CCK
- site of secretion: I cells of the duodenum and jejunum
- stimulus for secretion
1. small peptides and amino acids
2. fatty acids and monoglycerides - actions of CCK
1. contraction of the gallbladder and relaxation of the sphincter of Oddi–>secretion of bile
2. increases pancreatic enzyme and bicarbonate secretion
3. increases growth of the exocrine pancreas and gallbladder
4. inhibits gastric emptying
secretin summary: family site of secretion stimulus for secretion actions
- family: secretin-glucagon
- site of secretion: S cells of the duodenum
- stimulus for secretion
1. acid in the duodenum
2. fatty acids in the du odenum - actions of secretin = coordinated to decrease the amt of acid in the SI lumen
1. increase biliary bicarb secretion
2. increase pancreatic bicarb secretion
3. decrease gastric acid secretion
GIP (glucose-dependent insulinotropic peptide) summary: family site of secretion stimulus for secretion actions
- family: secretin-glucagon
- site of secretion: duodenum and jejunum
- stimulus for secretion
1. fatty acids
2. amino acids
3. oral glucose - *NOTE: GIP is the ONLY GI hormone whose secretion is stimulated by carbohydrates (oral glucose)
- actions of GIP
1. decrease gastric acid secretion
2. increase insulin secretion
four “official” GI hormones
- gastrin
- CCK
- secretin
- GIP
most potent stimuli for gastrin secretion
- phenylalanine
2. tryptophan
location of gastrin’s biologic activity
the four C-terminal amino acids
anatomic feature of gastrin-secreting tumors
hypertrophy and hyperplasia of the gastric mucosa
Zollinger-Ellison syndrome
= gastrinoma; occurs when gastrin is secreted by non-beta cell tumors of the pancreas
structure of CCK
contains 33 amino acids
homologous to gastrin (the 5 C-terminal amino acids are the same inCCK and gastrin)
location of CCK biologic activity
C-terminal heptapeptide (which contains the sequence for the same 4 C-terminal amino acids of gastrin)
do triglycerides stimulate release of CCK?
NO! because triglycerides cannot cross intestinal cell membranes; only fatty acids and monoglycerides stimulate CCK release
secretin structure
27 amino acids; homologous to glucagon (14 of the 27 amino acids in secretin are the same as glucagon)
location of secretin’s biologic activity
all of the amino acids
GIP structure
42 amino acids; homologous to secretin and glucagon
HINT about GIP actions
THINK: it’s homologous to glucagon and glucagon causes insulin release (check)
five candidate GI hormones
- motilin
- pancreatic polypeptide
- glucagon-like peptide-1 (GLP-1)
- leptin
- ghrelin
cells that secrete the 5 candidate hormones
cells of the GIT
motilin function
- increase GI motility
2. involved in interdigestive myoelectrical complexes
pancreatic polypeptide function
inhibit pancreatic secretions
GLP-1 functions
stimulate insulin secretion by the pancreatic beta-cells
target cell of GLP-1
pancreatic beta cells
leptin function
decrease appetite
ghrelin function
increase appetite
paracrines
= substances released from endocrine cells of the GI mucosa that diffuse over short distances to act on target cells
GI paracrines
- somatostatin
2. histamine
somatostatin summary: location of secretion stimulus for secretion inhibition actions
- location of secretion = throughout the GIT
- stimulus for release = acid in the lumen of the SI
- inhibition = vagal stimulation (because the vagus wants to increase acid for digestion)
- actions of somatostatin
1. inhibits release of all GI hormones
2. inhibits gastric acid secretion
histamine summary:
location of secretion
actions
- location of secretion: mast cells of the GI mucosa
- actions of histamine
1. increase gastric acid secretion directly and by potentiating the effects of gastrin and vagal stimulation
neurocrines
= substances synthesized in neurons of the GIT moved by axonal transport down the axons and released by action potentials in the nerves; then diffuse across the synaptic cleft to the target cell
GI neurocrines
- vasoactive intestinal peptide (VIP)
- neuropeptide Y
- nitric oxide (NO)
- GRP (bombesin)
- enkephalins
vasoactive intestinal peptide (VIP) structure
28 amino acids; homologous to secretin (whose function it is to DECREASE acid in the duodenal lumen)
VIP summary:
location of release
actions
- location of release = neurons in the mucosa and smooth muscle of the GIT
- actions of VIP
1. relaxation of GI smooth muscle (including the lower esophageal sphincter)
2. stimulates pancreatic bicarb secretion (similar to secretin)
3. inhibits gastric acid secretion (similar to secretin)
secretion of pancreatic islet cell tumors
VIP; presumed to mediate pancreatic cholera
GRP (bombesin) summary:
location of release
action
- location of release = vagus nerves that innervate the G cells
- action of GRP
1. stimulates the release of gastrin from G cells (–>increase gastric acid secretion and stimulate growth of GI mucosa)
two enkephalins
- met-enkephalin
2. leu-enkephalin
enkephalin summary:
location of release
actions
- location of release. = nerves in the mucosa and smooth muscle of the GIT
- actions of enkephalins
1. stimulate contraction of GI smooth muscle, particularly the lower esophageal, pyloric, and ileocolic sphincters
2. inhibit intestinal secretion of fluids and electrolytes
