Lec 19 GI Motility Flashcards
What 4 parts of GI are skeletal muscle?
- pharynx
- upper esophageal sphincter
- upper 1/3 of esophageal body
- external anal sphincter
What 4 parts of GI are smooth muscle?
- lower 2/3 esophagus
- stomach
- small and large intestine
- internal anal sphincter
Is esophagus skeletal or smooth muscle?
- upper 1/3 skeletal
- lower 2/3 esophagus
Are the internal/external anal sphincters skeletal or smooth?
- external anal sphincter is skeletal
- internal anal sphincter is smooth
What are slow waves in GI?
- constant background rhythm
- propagate down GI tract
- control timing of phasic contractions
- each organ has characteristic slow wave frequency
What is spike activity in GI?
- occurs when threshold for action potential is reached
- rapid depolarization results in contraction
How does frequency of electrical activity differ in stomach, small intestine, colon? [which shortest/longest]?
stomach = slow activity
colon = medium
small intestine = fastest
What is path of smooth muscle contraction
depolarization –> threshold –> spike activity –> Ca influx –> muscle contraction
What 4 things regulate GI contractions
- intrinsic properties of muscle
- extrinsic hormones,nerves
- intrinsic neurons
- electrical pacemaker potentials [interstitial cells of cajal]
What type of muscle contractions are there? what do they do?
- contractions: some increase or decrease transit
- tonic contractions: promote [gallbladder empties bile] or impede [sphincters] transport
- phasic contractions: peristaltic [propulsive], anti-peristaltic [vomiting], segmenting [mixing]
What produce tonic contractions?
- mainly storage organs of GI tract: gastric fundus, gallbladder, colon
What kind of pressure for tonic contractions? how much energy?
- exert relatively low pressure for prolonged periods
- require less energy
What causes phasic contractions?
- cell membrane depolarizes, causes inward flux of extracellular Ca
- cell repolarized by outward flow of K+
How do phasic contractions alter intestinal transit?
- can impair or accelerate
difference tonic vs phasic contraction
phasic: rapid contraction and relaxation
tonic: slow and sustained contraction
4 examples of phasic contractions?
- peristaltic contractions
- segmenting contractions
- contractions during intestinal housekeeping [sweep intestine/GI clear of debri]
- spasm
What are examples of phasic contractions that occur in retrograde direction and impair transit?
- vomiting reflex
- to and fro movement of intraluminal contents in large intestine
What 2 major types of contraction occur in small intestine
- peristalsis
2. segmentation
What type of contraction is peristaltic contraction [tonic or phasic]?
phasic contraction
What does peristalsis do?
- slow movement
- forms proximal to distal gradient
- orderly contraction and relaxation of circular muscle moving along GI tract
What does segmentation do in small intestine?
- major contractile activity of small intestine
- contracts circular smooth muslce
What is the major contractile activity of the small intestine?
segmentation
segmentation vs peristalsis
- peristalsis has specific directionality
- segmentation does not
Where is peristalsis found?
- in esophagus and gastric antrum
- also in intestine
What 2 places ONLY have peristalsis [no other normal pattern of phasic contraction]
- esophagus
- gastric antrum
What controls peristalsis?
- switch or gate that commands neurons
- serotonin [5-HT] plays key role
- contraction mediated by ACh containing neurons
- substance P may contribute to contraction
- NO mediates distal relaxation
What 4 things substances mediate peristalsis?
- serotonin
- ACh containing neurons [contraction]
- substance P [contraction]
- NO [distal relaxation]
What 1 things mediates distal relaxation in peristalsis?
NO
What initiates vomiting [reverse peristalsis]?
central vomiting center in medulla
What is mech/pathway of vomiting?
- epiglottis closes
- massive sympathetic discharge
- forceful anti-peristaltic contractions beginning in duodenum
- progress up through stomach
- relaxation of lower esophageal sphincter
What 4 things than activate vomiting?
- gastric receptors
- cortical input from learned or aversive response
- vestibular irritation
- activation of chemoreceptor in chemoreceptor trigger zone of medulla
What are sphincters?
- narrow zones of tonically contracted muscle
What 2 functions of sphincters?
- retard retrograde flow
- regulate forward flow of materials
What type of contractions in sphincters?
- they are tonically contracted with superimposed phasic contractions
What are 3 examples of sphincters?
- pylorus
- ileocecal valve
- anal sphincter
Does forward flow across sphincter occur with muscle contraction or relaxation?
relaxation!
What effect does proximal and distal distention cause on sphincters?
proximal distension –> sphincter relaxation
distal distension –> sphincter contraction
What two sphincters are under voluntary control?
- the two striated muscle sphincters
- – UES [upper esophageal sphincter]
- – EAS [external anal sphincter]
Which kind of muscle [smooth or skeletal] are each sphincter?
- all smooth EXCEPT
- – UES [upper esophageal], EAS [external anal] are both skeletal
What does NO do to GI sphincters? Via what type of neurons?
- relaxes them
- via nonadrenergic, noncholinergic [NANC] neurons
What does VIP do to GI sphincters? Via what neurons?
- vasoactive intestinal polypeptide [VIP] is co-localized in NANC neurons
- interacts with NO
What two peptides act to relax GI sphincters
- NO
- vasoactive intestinal polypeptide [VIP]
Where is the swallowing reflex coordinated? What 2 nerves carry info?
- the medulla
- fibers in vagus and glossopharyngeal carry info between GI and medulla
What is the pathway of events in swallowing?
- nasopharynx closes [breathing inhibited]
- laryngeal muscles contract to close glottis and elevate larynx
- peristalsis begins in pharynx, propels food bolus toward esophagus
- at same time: UES relaxes, allows food to enter
What 3 phases of swallowing? under voluntary or involuntary control?
- oral: voluntary
- pharyngeal: involuntary
- esophagus: involuntary
What occurs in esophageal phase of swallowing?
- UES relaxes to allow food to enter
- then UES contracts so food will not reflux to pharynx
- primary peristaltic contraction created pressure behind food bolus
- further peristalsis moves food along
- secondary peristaltic contraction clears esophagus of remaining food
What mediates lower esophageal sphincter relaxation in pharyngeal phase of swallowing?
- vagally mediated
What is secondary peristaltic contraction
- clears esophagus of remaining food
What is receptive relaxation?
- orad region of stomach relaxes in esophageal phase to allow food bolus to enter stomach
What is LES pressure at rest?
20 mmHg at rest, relaxes with swallow
What are the 2 organs of the stomach?
- fundus and body
2. antrum
What is function of fundus/corpus/body of stomach?
- receptive relaxation: accomodates increased gastric volume without increasing pressure gradient between proximal and distal stomach
- acts as reservoir
- regulates emptying of liquids
what is function of antrum of stomach?
- mixes and grinds food to 1 mm particles
- peristalsis/propulsion/retropropulsion
- regulates emptying of solids
How is antrum function mediated?
- vagally
how is receptive relaxation of fundus/corpus/body of stomach mediated?
- vasoactive intestinal polypeptide [VIP]
- NO
What is normal electrical activity of stomach? What is bradygastria? tachygastria?
normal = 3 cycles/min bradygastria = 0-2.5 cycles/min tachygastria = 4-9 cycles/min
What is the migrating motor complex [MMC]?
- the housekeeper of the gut
- kicks in when things are quiet between meals
- turned off during eating, turns on again following meal
- travels proximal –> distal
- electrical activity followed by muscular contraction
- time between pulses ~ 90 min
What are the interstitial cells of cajal?
- pacemaker of gut
- branching network that generate stimuli to surrounding muscle
- also mediate input from enteric motor neurons
What is a GIST tumor?
a benign proliferation of ICC cells
What kind of motility in small intestine during fasting vs fed?
fasting: MMC working, its cyclic and sweeps intestine clean of bacteria/residue
fed: segmental and peristalsis/propulsion promotes mixing and absorption of food
What 4 things promote upstream contraction in peristaltic reflex?
- Acetylcholine
- Tachykinins
- Gastrin
- Motilin
What 2 things promote downstream relaxation in peristaltic reflex?
- vasoactive intestinal polypeptide
- nitric oxide [NO]
What types of nerves are excitatory for peristalsis? what are inhibitory?
- cholinergic are excitatory
- NANC [non adrenergic non cholinergic] are inhibitory
What is 5HT?
neurotransmitter released from intrinsic nerves of ENS that helps sensory and motor nerves communicate
What are 4 functions of colon?
- absorption of water
- propulsion of contents
- storage of feces
- expulsion of feces
MAIN = absorption H2O
How is colonic motor activity characterizes?
- intermittent
- varies between segments
- temporal, spatial variation
Is colonic transit faster or slower than small intestine transit?
big intestine slower than small intestine
What are 2 main functions of cecum/ascending colon?
- reservoir [storage]
- mixing [segmenting]
Where is segmenting function of colon primarily?
ascending colon/cecum
What is function of low amplitude contractions? Amplitude? Frequency?
- transport fluid contents
- low amplitude: < 50 mmHg
- very frequent: >100/day
- associated with distension and flatus
Low or high amplitude contractions associated with flatus? defecation? distension?
flatus –> low amplitude
defecation –> high amplitude
distension –> low amplitude
What is function of high amplitude contractions? Amplitude? Frequency?
- mass movement [defecation]
- amplitude: > 100 mmHg
- infrequent: ~6/day
Where are propagating contractions primarily?
left colon [transverse, descending]
What is the gastrocolic reflex?
- distension of stomach increases mass movement in colon
- when you eat and stomach descends, via vagus triggers mass movement to clear out GI in prep for more food
- afferent mediated by parasympathetic [vagus]
- efferent mediated by CCK and gastrin
- this reflex used in toilet training
What 2 things involved in efferent part of gastrocolic reflex?
- CCK
- gastrin
What is path of defecation?
- propagation intraluminal contents to rectum
- sensation rectal fullness
- internal anal sphincter relaxes
- external anal sphincter contracts
- individual squats, anorectal angle straightens
- puborectalis muscle and EAS relax
- pelvic floor descends, anorectal angle straightens more
- after elimination tonic activity returns
Is internal or external sphincter more responsible for resting tone?
- internal: 70-80% resting tone
- external: 20-30% resting tone
What 4 anatomical things involved in fecal continence?
- internal anal sphincter
- external anal sphincter
- puborectalis muscle
- rectal curves and transverse rectal folds
What is function of puborectalis muscle in fecal continence?
- voluntary contraction of puborectalis creates anorectal angle
- sling muscle that sits above rectum
What is function of rectal curves and transverse rectal folds in fecal continence?
- delay passage of stool from sigmoid to rectum
What are 2 functional factors that must be intact to maintain fecal continence?
- anorectal sensation
- rectal compliance [adequate storage capacity]
What are 7 requirements/factors for continence
- anal sphincter integrity
- puborectalis integrity
- anal sensation intact
- rectum compliance [storage capacity] adequate
- nerve integrity maintained
- stool voume and consistency play a role
- psych motivation necessary
