Gut Mechanics Flashcards
describe the outermost layer of gut
- longitudinal muscle
- contraction shortens gut
- innervated by myenteric/Auerbach’s plexus
describe the middle layer of gut
- circular muscle
- contraction closes luminal space
- innervated by both myenteric and submucosal plexi
describe the innermost layer of gut
- submucosal muscle
- contraction moves mucosal folds (plicae)
- innervated by submucosal plexus
role of myenteric/Auerbach’s plexus vs. submucosal/Meissner’s plexus
myenteric/Auerbachs: tone and rhythm
submucosal/Meissner’s: mucosal secretion
describe sphincters
- thickened areas of longitudinal and circular muscle
- regulate movement of food b/w gut sections
- have resting tone and increase luminal pressure
what is the basal electrical rhythm (BER)?
rhythmic depolarization of 10-15 mV every 3-20s in the gut smooth muscle
-starts in the longitudinal muscle -> myenteric plexus -> circular muscle
what causes the BER?
- pacemaker type ion channels
- interstitial cells of Cajal
- Na+/K+ pumping
how does the BER spread through a specific layer?
gap junctions
what is the contractile strength of gut smooth muscle proportional to?
AP frequency and number
describe parasympathetic extrinsic innervation of the gut
- secretomotor function
- vagus/sacral cord -> cholinergic preganglionics -> synapse in myenteric plexus -> cholinergic postganglionics -> excite gut muscles
describe sympathetic extrinsic innervation of the gut
- excitatory to sphincters
- inhibitory to plexus, vessels, smooth muscles
- thoracic/lumbar cord -> cholinergic preganglionics -> synapse in ganglion away from target -> noradrenergic postganglionic -> target
describe NANC inhibitory nerves of the gut
- alpha receptors excite gut smooth mm.
- beta receptors inhibit gut smooth mm.
- not blocked by adrenergic antagonists
- VIP peptidergic part
- ADP, ATP purinergic part
describe the intrinsic innervation of the gut
-myenteric and submucosal plexi connected via branching interneurons
what modulates the enteric nervous system?
ANS
what senses stretch in the gut?
intramuscular array (IMA) - stretch-sensitive neurons in the muscle layers
what modulates the enteric nervous system?
ANS
describe local reflex
- gut distension -> smooth mm. contraction
- ex: secondary peristalsis in lower esophagus
describe short reflex
- coordination of movement in ganglia outside the gut but not in the CNS
- ex: antrum distension -> stretch receptors -> DRG -> noradrenergic neurons -> constrict sphincters
describe peristalsis and its fxn
- fxn: move food in aboral direction - local reflex initiated by distension and myenteric reflexes
- above bolus: contraction of circular mm. (ACh) + relaxation of longitudinal mm.
- below bolus: contraction of longitudinal mm. + relaxation of circular mm. (NO, VIP)
describe long reflex
- involves the CNS
- ex: vagal sensory nerve -> NTS -> DVMN -> vagal efferents w/ motor signals (like w/ gastric peristalsis)
describe rhythmic segmentation and its fxn
- rhythmic contraction/relaxation of the circular mm. while adjacent segments relax
- fxn: to mix chyme
- most frequent small intestine movement
- distension increases the FOC via long and local reflexes
describe tonic contraction and its fxn
-sustained contraction w/ intermittent relaxation (relaxation by local reflex) -> facilitates movement b/w compartments
what activates deglutition?
it is a reflex activated by food contacting pressure receptors in the pharynx
what coordinates deglutition?
swallowing center in medulla
describe the process of deglutition
- soft palate elevates (closes nasal cavity)
- stop respiration
- close glottis
- UES and LES relax
- peristaltic movement in pharynx pushes bolus into epiglottis and over glottis into esophagus
what prevents reflex of food back into the pharynx?
UES
describe primary peristalsis
- begins in striated muscle and moves distally over entire esophagus length
- controlled by medullary swallowing center
describe secondary peristalsis
- due to stretch of esophagus
- long reflex in upper esophagus and local reflex in lower esophagus
what is gastro-esophageal reflux?
esophageal pressure is subatmospheric, while intra-abdominal pressure is above atmospheric -> if pressure below exceeds pressure above, get reflux
-fxn of LES mostly (also some secondary peristalsis)
what is receptive relaxation and how is it mediated?
- relaxation of proximal stomach in response to bolus in esophagus
- long reflex via vagal receptors in mouth, esophagus, stomach
- increases volume of stomach to prepare for food load
how/where does peristaltic contraction begin?
starts in the middle body of stomach and moves distally, gaining speed and strength b/c of an increase in BER frequency
what induces peristaltic contraction?
pH and stretch-sensitive afferents via vagal reflex
how does gastrin affect peristaltic contraction?
- directly increases BER
- stimulates action potentials
- causes release of ACh from myenteric neurons
what happens as the peristaltic wave approaches the terminal antrum?
pyloric sphincter contracts and closes -> some chyme fluid passes through (gastric sieving), but most fluid and solids are forced back (retropulsion)
what is the function of retropulsion?
- better mixing
- mechanical disruption of solids
describe rates of gastric emptying for liquids and solids
- liquids empty readily
- solids pass more slowly
what affects the rate of gastric emptying?
chemical composition of chyme:
- high carb -> few hours
- high protein -> more slowly
- high fat -> takes the longest
major functions of small intestine motility (3)
- mix chyme w/ digestive secretions
- expose chyme to mucosal surface for digestion/absorption
- move chyme aborally
what are two ways in which the small intestine facilitates fat exposure and absorption?
- submucosal muscle contractions to change plicae
- villar shortening (stirs water layer next to mucosa)
interdigestive phase:
- where does this happen?
- what happens?
- what is the function of it?
- fasted stomach, small intestine
- migrating motility complex every 1-2h
interdigestive phase:
- where does this happen?
- what happens?
- fasted stomach, small intestine
- migrating motility complex every 1-2h (moves distally)
- phase I (60-70min) - only slow waves
- phase II (10-20min) - slow waves w/ or w/o spikes
- phase III (1-5min) - contractions w/ every slow wave
2 functions of the migrating motility complex
- remove residual chyme
- lower bacterial content
what controls the migrating motility complex?
motilin - circulating peptide from intestinal mucosa
describe the changes that happen when the digestive phase begins
- migrating motility complex stops
- contractions now part of segmentation and peristalsis
- random contractions - occur w/ only 1/3 of all slow waves
- net movement still aboral
what initiates transition to the digestive phase? (3 things)
- chyme in the lumen
- gastrin
- CCK
due to interaction of neural and hormonal elements acting at various sites along bowel
what regulates movement of chyme from ileum to colon?
ileocecal sphincter - normally closed due to circular muscle tone
describe the myenteric reflex arcs involved in regulating the ileocecal sphincter
- distention of ileum -> relaxation of sphincter
- distention of cecum -> contraction of sphincter
2 major functions of large intestine
- reabsorption of water and electrolytes
- evacuation of undigested food
large intestine motility: 4 types of movement
- haustral shuttling
- haustral propulsion
- multihaustral propulsion
- peristalsis (mass propulsion)
what is haustral shuttling? what interrupts it?
- annular contractions that cause haustra to appear
- move chyme back and forth -> reabsorption
- interrupted by eating
what is haustral propulsion?
adjacent haustra contract sequentially to move chyme distally
what is multihaustral propulsion?
simultaneous contraction of several haustra moving chyme distally
what is mass propulsion?
peristaltic wave moving chyme 30 cm or more
- doesn’t happen a lot
- occurs soon after a meal
what is the gastrocolic reflex?
ACh, gastrin, and CCK increase colonic movement soon after a meal
what moves feces into the rectum?
mass movement from the sigmoid colon
describe the defecation reflex
distention of rectum causes:
-internal anal sphincter relaxes
-external sphincter contracts first, then relaxes unless CNS override
then reflexive contractions of rectum/distal colon + increase in intra-abdominal pressure by contraction of chest/abs
(parasympathetic long arc)
how do VIP and norepi influence gut contraction?
blunts the BER and causes a big hyperpolarization, making contractions much less likely
what is achalasia?
food accumulates in esophagus, causing massive esophageal dilation - due to increased LES tension and failure to relax (low VIP)
-commonly seen in myasthenia gravis
what is LES incompetence? how do you treat it?
heart burn - treated with H2 receptor blockers or surgery
what is the rectosphincteric reflex?
- opening of anorectal angle hip flexure
- relaxation of puborectalis and pubococcygeus mm.
