GI: Regulation and Motility Flashcards
List GI regulatory mechanisms.
endocrine
paracrine
neural
Define enteroendocrine cells (EECs) and distinguish between open, closed, and ECL
EECs - gastric sensory cells that respond by secreting a peptide or hormone
open-type - apex is on the lumen and base is connected to a capillary
closed-type - no access to lumen
ECL = enterochromaffin-like cells; receive direct neural inpu
Describe endocrine regulatory mechanisms.
- stimulus activates EEC cell
- secreted hormone/peptide travels in circulation to a target cell that can be in the same organ, in a different organ, or outside the GI system
- triggers signal transduction
Describe paracrine regulatory mechanisms.
sensory cells (EECs) secrete a regulatory peptide or chemical messenger that diffuses through the ISF into nearby cells - no circulation involved
What are target cells for paracrine regulation?
- smooth muscle
- absorptive enterocytes
- secretory cells
- other EECs
Describe paracrine release of histamine.
ECLs => histamine => parietal cells => acid secretion
Describe paracrine release of serotonin.
enteric neurons/mast cells/ECLs => serotonin => regulates contractility of smooth muscle
Describe Celiac Disease
- allergic response to gliadin, component of gluten
- leads to inflammation
- reduces density and length of villi
- malabsorption
What are the 2 parts of GI neural regulation?
- extrinsic nervous system via the ANS (parasympathetic and sympathetic)
- enteric nervous system (within the muscularis externa and submucosal layers
Describe parasympathetic extrinsic innervation (which nerves)?
through the vagal and pelvic nerves
Vagal
- everything except distal colon and anorectal region
- mostly afferents that deliver sensory information to CNS; efferents carry out vasovagal reflexes
Pelvic
- distal colon and anorectal regions
Describe feature of parasympathetic extrinsic innervation.
- activation of GI
- acts on smooth muscle, secretory, and endocrine cells
- postganglionic can be cholinergic or peptidergic
Describe features of sympathetic extrinsic innervation.
- inhibition of GI (activation of sphincters)
- prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric)
- postganglionic adrenergic fibers synapse onto ENS or directly
Describe the interplay between extrinsic and intrinsic innervation.
extrinsic neurons synapse onto ENS ganglia, delivering information. ENS then carries out the effect and returns sensory information to the ANS
Describe the 2 plexuses of the enteric nervous system.
- myenteric = between muscularis externa
- submucosal = in submucosal layer
- linked by interganglionic fibers
- secretes neurotransmitters or neuromodulators
Which GI muscles are striated?
pharynx
upper esophagus
external anal sphincter
Describe the smooth muscle of the GI tract.
gap junctions between interstitial cells of Cajal facilitates low resistance and rapid spread of AP
Describe phasic contraction.
- periodic contractions followed by relaxation
- occurs in esophagus, antrum, and small intestine
- allows mixing and propulsion
Describe tonic contraction
- constantly contracted
- occurs in upper stomach/lower esophageal, ileocecal, and internal anal sphincter
Differentiate slow wave frequency in different regions of the GI tract.
stomach = 3/min duodenum = 12/min
Describe interstitial cells of Cajal.
located in the muscularis externae
pacemaker cells of the GI
What are slow waves?
oscillations in membrane potential that is not large enough to create an AP
What happens if an action potential occurs when a slow wave reaches threshold?
creates contraction
What contributes to GI tonic contraction?
slow waves below threshold create weak contractions
What contributes to GI phasic contraction?
when APs occur on top of a slow wave
What is unique about GI action potentials?
- slow waves
- APs are summed up to create longer contractions, instead of separate contractions
What controls amplitude and frequency of slow waves?
normal GI regulation
Describe segmentation contraction
- mixes the food up, NO MOVEMENT
- contraction occurs through the chyme (separating it)
- relaxation brings the chyme back together (mixing it)
Describe peristaltic contraction.
- movement of food
- orad contraction with simultaneous caudad relaxation pushes the food forward
Which neurotransmitters are involved in peristaltic contraction?
- orad contraction = ACh and Substance P
- caudad relaxation = VIP and NO
What is the difference between parasympathetic and sympathetic innervation?
- sympathetics synapse on ENS ganglia or directly on cells
- parasympathetics synapse onto the ENS ganglia
Describe gastrin as a hormone mediator (cells, stimulus, action)?
- stimulus: small peptides and AAs
- secreted by: G cells of gastric antrum epithelium
- action: increased parietal acid secretion, increased ECL histamine secretion, increased growth of gastric mucosa
Describe CCK (cholecystokinin) as a hormone mediator (cells, stimulus, action)?
- stimulus: fatty acids, hydrolyzed proteins
- secreted by: I cells of duodenum
- action: decreased gastric emptying and acid secretion, decreased food intake, increased pancreatic enzyme secretion, increased contraction of gallbladder
Describe secretin as a hormone mediator (cells, stimulus, action)?
- stimulus: protons in duodenum
- secreted by: S cells of duodenum
- action: increase pancreatic secretion of water and bicarb
Describe GIP (gluco-insulinotropic-peptide) as a hormone mediator (cells, stimulus, action)?
- stimulus: fatty acids and glucose
- secreted by: K cells of upper intestines
- action: increased insulin secretion from pancreatic B cells
Describe peptide YY as a hormone mediator (cells, stimulus, action)?
- stimulus: fatty acids, glucose, hydrolyzed protein
- secreted by: L cells of intestine
- action: decrease gastric emptying and acid secretion, decrease pancreatic secretion, decrease intestinal motility, decrease food intake
For the most part, what is the relationship between CCK and PYY?
opposite effects
CCK increases acid secretion
PYY decreases
Describe GLP1/GLP2 (proglucagon-derived peptide) as a hormone mediator (cells, stimulus, action)?
- stimulus: fatty acids, glucose, hydrolyzed protein
- secreted by: L cells of intestine
- action: glucose homeostasis, increased epithelial proliferation
Which 2 hormones are secreted by L cells?
PYY and GLP
Which hormones are only endocrine?
- gastrin
- GIP
Which hormones have both endocrine and paracrine functions?
- CCK
- secretin
- PYY
- GLP
Describe ACH neuromodulation.
- source: cholinergic neurons
- contraction of smooth muscle
- relaxation of sphincters
- increased salivary, gastric, and pancreatic secretions
==> pushes food forward
Describe NE neuromodulation.
- source: adrenergic neurons
- relaxation of smooth muscle
- contraction of sphincters
- increased salivary secretions
===> helps in peristalsis
Describe VIP (Vasoactive Intestinal Peptide) neuromodulation
- source: neurons
- relaxation of smooth muscle
- increased intestinal and pancreatic secretion
==> helps in peristalsis
Describe GRP/Bombesin (Gastrin-releasing peptide) neuromodulation.
- increased gastric secretion
Describe enkephalin neuromodulation
- contraction of smooth muscle
- decreased intestinal secretion
Describe neuropeptide Y neuromodulation
- relaxation of smooth muscle
- decreases intestinal secretion
Describe substance P neuromodulation
- co-secreted with ACh
- contraction of smooth muscle
- increased salivary secretion
==> helps with peristalsis
