gut motility Flashcards
what are the types of cells found in the wall of the GI tract?
interstitial cells of cajal
smooth muscle cells
enteric neurons
what do ICCs do?
spontaneously generate electrical slow waves –> phasic muscle contractions
how are the electrical waves of ICCs transmitted through smooth muscle
gap junctions
what is the advantage of smooth muscle being arranged as a functional syncytium?
allows waves to spread over large distances
what are the two plexuses of the GI system and what do they control?
Myenteric plexus – controls movement of the gut
Submucosal plexus – controls secretion, absorption and mucosal function
what does the ENS control?
GI motility, local blood flow and trans mucosal movement of fluids
what neurones does the ENS contain?
sensory
interneurons
motor
what modulates the ENS?
extrinsic nerves
hormones
explain the intrinsic innervation of the gut
parasympathetic - vagus and pelvic nerves
sympathetic - prevertebral ganglia (greater splanchnic nerve, lumbar colonic nerve, hypogastric)
describe which system controls each part of the GI tract?
oesophagus - CNS stomach - myogenic, ENS and CNS small intestine - ENS large intestine - ENS and myogenic rectum and anus - ENS and CNS
what is the migrating motor complex?
large contraction every 90-120 minutes which occurs during hunger
where does the MMC originate?
stomach - vagus dependent
small intestine - vagus independent
what are the three phases of the MMC?
Phase I + II: build-up of irregular contractions
Phase III: high amplitude, big contraction
what is the function of the MMC?
clears undigested material
prevents bacterial growth
gives sensation of hunger
what are the three phases of food intake and when do they occur?
cephalic - prepares GI tract for food.
gastric - when food enters the stomach
intestinal - when food has been digested
what occurs in the cephalic phase
MMCs abolished. saliva, gastric acid, pancreatic acid secreted.
what is the gastric phase responsible for and what triggers it?
satiation, early digestion, gastric emptying
triggered by mechanical effect
what triggers the intestinal phase and what occurs in it?
triggered by chemoreceptors in the small bowel
signals to stomach to stop eating - decreases appetite
explain how the primary peristaltic wave occurs?
when bolus enters the oesophagus it hits the striated muscle
triggers vagus nerve –> primary peristaltic wave
when does a secondary peristaltic wave occur in the oesophagus?
when the bolus moves down and gets stuck, triggering stretch receptors which trigger the ENS
alongside peristalsis of the oesophagus, how is food movement into the stomach aided?
shortening of the longitudinal muscle
what is the proximal area of the proximal area of the stomach?
funds and body
what is receptive relaxation?
reflex in which the fundus relaxes to accommodate food triggered by the vago-vagal reflex
what id adaptive relaxation?
the reflex which reinforces the reflective relaxation reflex. inhibitory NTS (NO) is released from the ENS so stomach dilates to accommodate food
why may pressure build up in the stomach?
gas bubble forming from swallowing air
how is pressure build up in the stomach relieved?
increased pressure picked up by the stretch receptors signals to vagus causes TLOSRs Burp reduces pressure inside stomach Avoids early satiety
which has a thicker wall - distal or proximal stomach?
proximal
what are the 3 phases of propelling food into the duodenum?
Phase 1: propulsion
Phase 2: emptying
Phase 3: Retropulsion - large particles are sent back into the antrum
why is gastric emptying of liquids exponential?
no solids to grind or liquefy - no lag time
how does the volume of liquid affect the gastric emptying time?
larger volume = emptied faster
why do large solid particles suffer a lag phase?
need sufficient grinding first
what does the rate of gastric emptying depend on?
physical properties
neuronal and hormonal feedback
nutritional content
explain the duodenal and jejunal brake?
duodenum detects nutrients, osmolality and HCl levels
CCK hormone is released to slow gastric emptying when too many nutrients enter the duodenum
vagal afferents signal the motor nuclei which signal vagal efferents to reduce opening of pyloric sphincter, reduce contractions in antrum and enhance relaxation
explain the ileal brake
Fats usually digested and absorbed in the duodenum
Fats reach ileum
Feedback slows gastric emptying and induces satiety
what is the ileal brake mediated by?
peptide YY, Glucagon-like peptide-1, oxyntomodulin
name some of the GI conditions associated with motility
Gastro-oesophageal reflux: failure to clear acid/dysfunctional lower oesophageal sphincter.
Early satiety/nausea: incomplete gastric accommodation.
Neuropathy: diabetes
Dysrhythmia: nausea (gastroparesis)
Obesity: poor feedback control.
what kind of receptors do the enteric sensory neurons contain?
mechanical and chemical sensitive receptors
what are the mechanical and chemical stimuli of peristalsis?
Mechanical stimuli – stretch, mucosal deformation
Chemical stimuli – substances from endocrine cells (e.g. 5-HT), nutrients and low pH
what do the sensory neurons of the GI tract do?
detect intraluminal stimuli
initiate peristalsis, plus increased secretion & vascular flow
how is information from sensory neurons transmitted to motor neurons?
Via interneurons
ACh - secreted by excitatory nerves to the ascending wave of peristalsis
NO - secreted by inhibitory neurons to the descending wave of peristalsis
what are the functions of the ascending colon?
fermentation - rich in bacteria
absorption - of water and ions
what is the advantage of haustra in the ascending colon?
increase surface area
what are the movements of the ascending colon and what do they do?
propulsion and retropulsion - churn and slow transit
segmentation - mixes content and increases exposure to mucosa
what controls movement in the colon and rectum?
ICCs, local mediators (5-HT) and local environment
what occurs in the transverse colon?
absorption
relatively rapid transmit
what occurs in the descending colon?
storage, slow, partly involuntary transit
how does the colon decide which way to go?
interstitial cells, ENS and extrinsic nervous system
explain how the conscious urge to defecate is triggered
contents distend rectum
pressure increases passively (increases further by active contraction)
accompanied by relaxation of internal anal sphincter and contraction of external sphincter
contents continue to enter rectum
triggers conscious urge to defecate
what normally presents defecation?
tone of internal anal sphincter & puborectalis
mechanical effects of acute anorectal angle
how is defecation initiated?
puborectalis muscle & external anal sphincter relax
intraabdominal pressure increased - facilitated by squatting
what is water needed for?
Digestion and absorption – hydrolysis is main chemical reaction
Facilitates propulsive movement of intestinal contents
By liquefying intestinal contents, allows digested nutrients to be brought into close contact with absorptive microvilli surface
name some conditions of the colon
pseudo-obstruction
diarrhoea
slow transit constipation - adult females
chronic constipation - more common in elderly
faecal incontinence
