GI Motility - Fan 2/18/16 Flashcards
role of organ in digestion/motility:
mouth
chewing/mastication
role of organ in digestion/motility:
esophagus
swallowing
role of organ in digestion/motility:
stomach
digestive period: receptive relaxation, accomodation, gastric emptying
interdigestive period: MMC (migrating myoelectric complexes)
role of organ in digestion/motility:
small intestine
digestive period: segmentation (for optimal digestion/abs)
interdigestive period: peristalsis (MMC)
role of organ in digestion/motility:
large intestine
haustral shuttling (storage of feces)
mass movement (defecation)
role of organ in digestion/motility:
rectum
defecation
organs involved in function:
mechanical digestion/breaking food down into smaller particles
mouth
stomach
organs involved in function:
storage
stomach
small intestine
organs involved in function:
mixing of luminal contents
mouth
stomach
small intestine
organs involved in function:
housekeeping
stomach, small intestine: MMC
large intestine: defecation
“aboral”
downward
onward
orthograde
resident and transit time through GI tract
- mouth/esoph: seconds
-
stomach/sm int: DIGESTIVE PERIODS
- stomach: 2-4h
- sm intestine: 2-4h
-
stomach/sm int: INTERDIGESTIVE PERIOD
- empty stomach/sm int: migrating myoelectric complex cycles (approx 1.5h/cycle) for 10-18 hours
-
large intestine: 42-52 hours (8-12h min)
- defecation: seconds-minutes
SWALLOWING
phases
organs involved
- oral phase: involuntary or voluntary control
- pharyngeal phase: involuntary reflex
- esophageal phase: involuntary reflex
mouth, pharynx, esophagus, stomach, resp system - actions coordinated by CNS
key events of swallowing
- pharynx rapid sequential contraction
- upper eso sphincter relaxes, then contracts
- esophageal slow peristaltic wave
- lower eso sphincter relaxes, then contracts
- fundus and body of stomach relax: happens early on in swallowing, in prep to receive bolus
what happens when primary eso peristalsis fails?
if something gets stuck in esophagus, secondary eso peristalsis takes over, starting a contractile/peristaltic wave just above the stuck bolus (not all the way up at the upper eso sphincter!)
LES overshooting
lower eso sphincter contraction aims to prevent reflux
general mechanisms to prevent reflux
- high tone of lower eso sphincter
- secondary eso peristalsis (acitvation of chemoreceptors)
- pinching of LES by diaphragm
- 2 reflexes
- LES contracts in response to gastric pressue increase and abd pressure increase
*
mechanisms to prevent reflux: infants
only have pinching of LES by diaphragm as anti-reflux mechanism
- have to be careful squeezing babies, also have to burp them to decrease the pressure/volume of stomach post meal
mechanisms to prevent reflux: pregnant women
only have secondary peristalsis as anti-reflux mechanism
- LES tone knocked out by hormonal changes
- diaphragm elevated due to pregnancy
- gastric/abd pressure constantly elevated due to preg: hard to activate the pressure reflexes
problems with LES contraction
insufficient/weak: GERD
- heartburn
excessive/strong: esophageal achalasia
- overactive, excitatory neurons
- abnormal muscle overgrowth
- difficulty swallowing
roles of stomach (and its parts) in gastric motility
DIGESTIVE PERIOD
- proximal stomach: storage
- receptive relaxation (during swallowing)
- accomodation (significant increase in volume without huge wall tension increase due to elasticity of sm muscle)
- distal stomach: mixing/grinding
- mixing, size reduction, emptying via contraction
INTERDIGESTIVE PERIOD
- housekeeping
gastric contractions during digestive period
serve the function of mixing, size reduction, and emptying via…
- propulsion: mixing/size red, emptying 0-3.7x/min
- antral systole: grinding, shearing, retropulsion for net effect of mixing/size red
control of gastric emptying
promoted by
- proximal stomach tone
- distal stomach peristalsis
inhibited by
- pyloric sphincter tone
- duodenal contraction
gastric emptying: stomach
- food enters stomach, triggers stomach distension, stretch receptors hit vagal afferent-CNS-vagal efferents which come through and release Ach and gastrin
- also, peptides/a.a.s trigger gastrin secretion
- in total, stomach contraction triggered
gastric emptying: duodenum
- presence of acid → secretin production
- presence of FAs, a.a.s/peptides → CCK production
- presence of glucose → GIP production
all three of these neuroendrocine secretions will have joint effect of…
- duodenal contraction
- inhibition of gastrin secretion [either directly or through somatostatin]
contraction of the duodenum, in turn, triggers stretch receptors in duodenum for joint effect of…
- duodenal contraction
- inhibition of stomach contraction
interdigestive period: “housekeeping”
migrating myoelectric complexes [as compared to digestive motility]
- stronger
- intermittent/in waves [vs. constant]
- pylorus is open [vs. closed or close to it]
- sm intestine undergoes peristalsis [vs segmentation - slow, nonperistaltic]
- fx: remove undigestible material and prevent bacterial growth [vs. optimize digestions/abs]
- mediated by motilin
pyloric contraction
pylorus kept small during digestive period because you don’t want everything emptying into small intestine all at once
- clinically: dumping syndrome
small intestinal motility
(muscles and functions)
- muscularis externa: intraluminal mixing and propulsion via segmentation to optimize digestion/abs. also housekeeping via MMC
- muscularis mucosae, villus muscle: facilitate absorption and lymph flow
segmentation
aka slow movement
main fx: separation and recombo of chyme boluses
effects two means of digestion
- luminal digestion: mixing of chyme with secretions (enzymes, alkaline fluid, bile)
-
membrane digestion: moving chyme to cell membranes toward…
- digestive enzymes: membrane digestion
- transporters/carriers: absorption
ECA
electrical control activity (unique to gut smooth muscle)
- aka: slow waves, basal electric rhythm
- origin: pacemaker cells (interstitial cells of Cajal, ICC)
- made spontaneously, constantly
- conducted to smooth muscle cells via gap jx
- modulated (not originated) by motor neurons in muscle and by endocrine secretions
frequency of ECAs in GI tract
stomach: 3.7 x/min
duodenum: 12 x/min
ileum: 10 x/min
colon: 3 x/min
ECA action potential events
- depolarization: Na, Ca influx
- plateau phase: Na, Ca influx; K efflux
- repolarization: K efflux
large ECAs with spike potentials cause muscle contraction = ERA: electric response activities
- excitatory neuroendocrine factors can lead to higher levels of Ca, spike potentials
modulation of ECAs by neuroendocrine factors
excitatory factors: Ach, gastin
inhibitory factors: norepi, nitric oxide
relationship between ECAs and ERAs
and ERA is an ECA that is strong/spiked enough to actually initiate contraction
- excitatory mediators increase ERA/ECA ratio
- inhibitory mediates decrease ERA/ECA ratio
neuroendocrine control of peristalsis
peristalsis involves circular and longitudinal muscle layers oppositely relaxing/contracting
-
constricted part of tract
- inhibitory neuroendocrine activity/relaxation of longitudinal m
- excitatory neuroendocrine activity/contraction of circular m
-
relaxed part of tract
- inhibitory neuroendocrine activity/relaxation of circular m
- excitatory neuroendocrine activity/contraction of longitudinal m
how is peristaltic motility coordinated?
interneurons
- presence of bolus triggers receptors that communicate with interneurons
- distension triggers baroreceptors
- content might also trigger osmoreceptors, chemoreceptors
- interneurons communicate with motor neurons
- proximal motor neurons: excitatory neuroendo: contraction
- distal motor neurons: inhibitory neuroendo: relaxation
generates a pressure gradient for the bolus to move down
describe the various contractile behaviors associated with physiological states of
- segmentation
- MMC
- diarrhea/vomiting
- segmentation: short distance propulsion
- MMC: intermed distance propulsion
- diarrhea/vomiting: long distance propulsion (opposite directions)
reflexive relaxation/contraction of ileocecal sphincter
relaxes/contracts based on distention of regions immediately proximal and distal
- distention in distal ileum leads to sphincter relaxation
- movement of chyme into cecum
- distention in proximal cecum leads to sphincter constriction
- prevents reflux
overview of large intestinal motility
- haustral shuttling (segmenting, nonperistaltic contractions) to slow fecal stream
- reabs of water and electrolytes (700mL water into cecum, all but 100-300 reabs)
- periodic peristalsis to move feces along
- rare mass movement to move stool into rectum
- defecation (100-300mL)
if defecation does NOT occur - get retrograde propulsion!
reflexes involved in defecation
- involuntary: stool distends rectum
- involuntary: reflexive contraction of rectum, relaxation of int anal sph, contraction of ext anal sph: movement of stool into anal canal
- brain receives signals of awareness, which then makes it your voluntary call to hold off (contract EAS) or defecate (relax EAS)
constipation factors and mechanisms:
diet low in insoluble fiber
- insufficient stim of baroceptors
- loss of benefical microbes that come with a high fiber diet
constipation factors and mechanisms:
insufficient fluid intake
- lack of baroceptor stimulation [low volume]
- dry feces
constipation factors and mechanisms:
excessive delay of defecation
- dry feces (excessive reabs?)
- low stim of baroceptors [low volume]
constipation factors and mechanisms:
narcotics
- inhibition of fluid secretion, leading to low stim of baroceptors [low vol]
- increased sphincter tone
constipation factors and mechanisms:
physical inactivity
- decreased baroceptor stim due to decreased food intake
- decreased GI neuroendo secretion (associated with exercise)
constipation factors and mechanisms:
antibiotics
- disruption of microbiome and associated ENS activities
constipation factors and mechanisms:
old age
combination of
- reduced food intake
- reduced ENS neuroendo activity, physical activity
gastroileal reflex
stimulatory
- entry of food into stomach → relaxation of ileocecal sph, emptying of ileum
gastrocolic reflex
duodenalcolic reflex
stimulatory
- entry of food into empty stomach or chyme into duodenum → increases aboral propulsive movement in colon
colonocolonic reflex
stimulatory
- distention in one part of colon → relaxation of another part
- ultimately favors aboral movement
duodenogastric reflex
inhibitory, physiological
- duodenal distension → delayed gastric emptying
ileogastric reflex
inhibitory, physiological
- distention of ileum → delayed gastric emptying
intestinointestinal reflex
inhibitory
- overdistention of one part of intestine → general inhibition of instestinal muscle activity
colonic-intestinal reflex
inhibitory
- overdistention of colon → general inhibition of intestinal muscle activity
peritoneo-intestinal reflex
inhibitory
- handling of GI tract during surgeries and/or intestinal irrigation → general inhibition of intestinal muscle activity
adynamic ileus
occurs due to
- obstruction
- bacterial overgrowth
- abdominal surgery
results in triggering of intestino-intestinal reflex, colonic-intestinal reflex, peritoneo-intestinal reflex
which is why you ask patients to fast until GI motility is regained!
