Transport along alimentary tract Flashcards
Transport of digesta from gastric reservoir into antral pump is caused by 2 mechanisms. What are they?
tonic contractions
peristaltic waves in region of gastric corpus/body
What are tonic contractions?
these are contractions that can last from mins to hrs
Where does tonic contractions occur?
occurs in sphincters of the tract
What happens in storage of food?
proximal stomach relaxes to store food at low pressure whilst it’s acted upon by acids, enzymes and mechanically
What happens in emptying?
this is carefully regulated to ensure adequate acidification, neutralisation, action of enzymes, mechanical breakdown and to avoid swamping of duodenum -> swamping of duodenum can lead to duodenum ulcer
What is one disorder associated with gastric motility/emptying?
Gastric stasis (gastroparesis)
What is gastric emptying dependent upon?
- propulsive force generated by tonic contractions of proximal stomach
- Stomach’s ability to differentiate type of meals ingested and their components
What are the determinants of rate of gastric motility?
- type of food eaten ; carbohydrates > protein> fatty foods> indigestible solids
- osmotic pressure of duodenal contents: hyperosmolar chyme decreases gastric emptying
- vagal innervation upon over-distension decreases gastric motility
- hormones (somatostatin, CCK, GIP (gastrin inhibitory peptide) : inhibit emptying
- injury to intestinal wall + bacterial infections decreases motility
Describe the myogenic control of gastric motility?
intrinsic basis electric rhythm (BER) -> BER allows smooth muscle cell to depolarise and contract rhythmically when exposed to hormonal signals
Stomach muscle cells produce electric depolarisation from resting potential and move ripples towards the antrum
BER -> rhythm of depolarisation -repolarisation
Describe the neurohormonal control of gastric motility?
following mediate a decrease in fundic motor activity: CCK, secretin, VIP, somatostatin, duodenal distension, duodenal acid -> GRP -> Motilin increase fundic contractions
What are factors that increase intestinal motility (colonic motility)?
CCK, gastrin + motilin
How does secretin affect intestinal motility?
secretin inhibits intestinal motility
What is feedback control of gastric emptying?
gastric emptying is regulated by negative feedback systems e.g. antral over-distension: Vago-vagal reflex, duodenal over distension + chemical stimulation: vago-vagal reflex + hormones
How does pyloric sphincter respond in response to antral or duodenal rhythm?
it contracts
fatty acids in duodenum cause contraction of the pylorus
Describe how gastric motility + emptying is regulated?
regulation is elicited from small intestine
Pyloric activity is modulated by antral inhibitory + duodenal excitatory reflexes:
ascending excitatory reflex causes pyloric contractions + increases tone. Presence of acidic chyme is important
Descending inhibitory reflex causes pyloric relaxation -> NO/VIP
Describe motility in intestine?
- Segmentation (mixing contractions): stationary contraction & relaxation
- Peristalsis (propulsive): in stomach (3 waves/min)
- Migrating Motor Complex (MMC) -> tend to be sweeping and tends to cleanse
- Mass movements (evacuation)-> associated with getting rid of undigested material
What are 3 phases of motor activity?
Phase 1: quiescence/quiet period
Phase 2: irregular propulsive contractions
Phase 3: burst of uninterrupted phasic contractions (peristaltic rush)
Where does segmentation originate in?
originates in the pacemake cells (ICC)
What is segmentation?
tends to be more of churning actions
it’s division + subdivision of chyme, bringing chyme in contact with intestinal walls
it causes slow migration of chyme towards ileum
What does regulation of peristalsis require?
requires neural reflexes
- are excitatory motor neurones (important in contraction) + inhibitory motor neurones (important in relaxation)
What is peristalsis?
- Peristalsis is propagating contraction of sucessive sections of circular smooth muscle preceded by relaxation/dilation
- Is produced by coordinated contraction + relaxation of muscle cells
What is migrating motor complex?
is highly organised motor activity
cyclically recurring sequence of events
occurs between meals when stomach/intestine are ‘empty’
Only phase 3 is of interest
burst of high frequency, large amplitude contractions that migrate along length of intestine + die out
Where does migrating motor complex start?
starts in lower portion of stomach
Describe the control of migrating motor complex?
smooth muscle cells of stomach can produce slow waves
contractions are coordinated by enteric nervous system by pacemaker cells (interstital cells of Cajal)
initiated by vagus nerve in upper tract
What is function of migrating motor complex?
discourages overgrowth of bacteria, cleanses gut + removes dirt cells
Describe motor activity in small intestine in fed state?
- mixing contractions: segmentation, mixes + stirs contents with enzymes, prevents unstirred layer formation
- peristaltic contractions (slow waves): these move the contents in an oral to anal direction (law of gut) -> local reflex mediated via ENS but can be enhanced or suppressed by extrinsic innervation (parasympathetic/sympathetic)
How does an increase of sympathetic activity affect motility?
it inhibits motility
How does an increase parasympathetic activity affect motility?
it stimulate motility
How does pain and fear affect motility?
pain and fear decrease motility
Describe storage in large intestine?
Intensive mixing + slow movement of waste + indigestible material aborally
contains fermenting chambers which allow hydrolysis of fibre + indigestible nutrients -> faeces formation
What are 3 things that occur in motility of large intestine?
- Segmental or haustral contractions - mix contents/ key role for taenia coli longitudinal muscle
- Peristalsis: slow in large intestine in comparison to small intestine, moves contents towards anus; distension initiates contraction
- Mass movement: powerful contraction of mid-transverse colon that sweeps colon contents into rectum (responsible for colonic evacuation)
What are features of motility in large intestine?
intensive mixing
fermentation
slow propagating- slow aboral flow
