Transport Along Alimentary tract Flashcards
What 2 mechanisms allow the transport of digested material from the gastric reservoir into the antral pump?
- Tonic contractions
2. Peristaltic waves (in gastric corpus region)
What are tonic contractions?
Contractions maintained from mins to hrs at a time
They can occur in the stomach and sphincters of GI tract
How does storage of food occur in the stomach?
The proximal stomach relaxes to store food at low pressure whilst it’s acted upon mechanically and by gastric acids + enzymes
Why is stomach emptying regulated?
Carefully regulated to ensure adequate:
- acidification and neutralisation
- action of enzymes
- mechanical breakdown
- and to avoid duodenal swamping
What is gastroparesis?
chronic (long-term) condition in which the stomach cannot empty itself of food in the normal way, causing food to pass through it slowly
Describe the sieving effect of the pyloric sphincter
only chyme of size 1-2 mm is allowed to pass
Large food particles are retained in the proximal part of the stomach;
What is the fate of large food particles retained in the proximal stomach?
antrum repulses them→ mixing and grinding
with digestive juices
What occurs during accommodation?
electrical activity is minimal , VIP / NO activity ↑= relaxation
Describe the events that occur during gastric emptying
Ach and (hormonal) Gastrin cause contraction & motility, ICC cause tonic contractions - slow wave of impulses travel through gut initiating contraction
What is gastric emptying dependent upon?
Propulsive force generated by the tonic contractions of proximal stomach
The stomach’s ability to differentiate types of meals ingested and their components
Gastric emptying depends on type of food we eat
What decreases rate of gastric emptying?
Fatty, hypertonic, acidic chyme in the duodenum, decrease the force and rate of gastric emptying
Describe how different foods pass through stomach
Liquids pass in spurts
Solids are broken down to 1-2 mm sizes
Large indigestible materials remain: cleared by MMC
What is the gastric emptying limiting factor?
particle size is limiting factor in the fed, but not the fasting state
Explain how liquids pass through stomach
Rapidly disperse, empty without lag time
Rate of emptying is influenced by nutrient content (nutrient-containing liquids retained longer)
Explain how solids pass through stomach
2 phases (lag time and linear phase); duration of lag time is related to size of particle
Liquid part emptied
Solid component retained in proximal stomach
Trituration of larger particles into smaller ones
(~60min for a typical solid-liquid meal)
What regulates passage of material through stomach into duodenum?
Pylorus regulates passage of material
What is trituration?
a form of reducing particle size or creating a homogenous solution through thorough mixing
How do fatty foods affect gastric emptying?
Liquefy at body temperature; float on top of liquid layer and empty slowly
Fats are potent inhibitors of gastric motor events and gastric emptying
What are indigestible solids?
Solids that are not emptied in immediate post-prandial period
MMC activity
What factors determine rate of gastric motility
Type of food eaten:
- carbohydrates > protein > fatty foods > indigestible solids
Osmotic pressure of duodenal contents:
- hyperosmolar chyme ↓ gastric emptying
Vagal innervation upon over-distension ↓ gastric motility
Hormones (somatostatin, secretin, CCK, GIP):
- inhibit emptying
Injury to intestinal wall and bacterial infections ↓ motility
What is the role of the intrinsic basic electric rhythm?
BER a rhythm of depolarisation-repolaristaion allowing smooth muscle cell to depolarise and contract rhythmically when exposed to hormonal signals
Explain how BER causes movement in the stomach
Stomach muscle cells produce electric depolarisations from resting potential
And move ripples towards the antrum
How is the fundus innervated?
The Fundus is under vagal excitatory control
What are the intestinal cells of cajal?
specialised pacemaker cells located in the wall of the stomach, small intestine, and large intestine producing BER that is transmitted to stomach smooth muscle
What factors decrease BER?
Sympathetic system → NA
Effects of NA or adrenaline on the membrane of GI smooth muscle
What causes the depolarisation-repolarisation of GI smooth muscle?
Depolarisation of GI smooth muscle is caused by calcium-sodium entry
Repolarisation of GI smooth muscles is caused by K+ efflux
What is decreased proximal stomach motor activity caused by?
Glucagon Cholecystokinin (CCK) Secretin VIP Somatostatin Duodenal distension, duodenal acid Gastrin-releasing peptide (GRP) - effect depends which receptor its acting on
What increases fundic contractions?
motilin
Outline the different controls of movement through the small intestine
Hormonal + nervous factors initiate & maintain peristalsis and mixing
Localised distention of duodenum
Cholecystokinin (CCK), gastrin and motilin increase intestinal motility (colonic motility)
Secretin decreases the activity
Which hormones can increase motility?
Serotonin and insulin can increase motility
Which hormone decreases gastric motility?
glucagon
Describe the 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 and hormones
Describe the regulation of gastric emptying elicited from the stomach
Pyloric sphincter can contract in response to antral or duodenal rhythm (e.g. fatty acids in duodenum → contraction of pylorus; liquids in the antrum may be squirted into the duodenum via pylorus
Describe regulation of gastric emptying from the small intestine
Contraction of middle antrum elicits a descending inhibitory reflex → relaxation of pylorus (NO/VIP)
Duodenal stimuli e.g. presence of acidic chyme (HCl) or oleic acid induces ascending excitatory reflex
→ contractions of pylorus (increased tone) – prevents duodenogastric reflux.
What is segmentation?
(mixing contractions): stationary contraction & relaxation
What is propulsive peristalsis?
in stomach (3 waves/min) Migrating Motor Complex Mass movements (evacuation)
What are the phases of motor activity in the GI?
Phase I: quiescence/quiet period
Phase II: irregular propulsive contractions
Phase III: burst of uninterrupted phasic contractions (peristaltic rush) - rumbling when hungry / fasting
Where is segmentation initiated?
Originates in the pacemaker cells (ICC)
What is the function of segmentation?
Segmentation causes the slow migration of chyme towards ileum
mixes chyme with pancreatic secretions, which aids digestion
Contraction of circular muscles in the digestive tract; slows down progression of chyme through the GIT which makes it available for absorption
What is peristalsis?
Peristalsis is a propagating contraction of successive sections of circular smooth muscle preceded by relaxation / dilation
Explain the movement of muscles causing peristalsis
Rhythmic contractions of longitudinal muscles in GIT
Ach associated with circular muscle contraction
Longitudinal muscle on outer gut
Circular are inner facing
What is the role of peristalsis?
Peristaltic (propulsive) contractions spread the food out allowing digestive enzymes to mix with it, but primarily push the food towards the anus (global movement)
What is MMC?
Migrating Motor Complex: - Highly organised motor activity - Cyclically recurring sequence of events - Occurs between meals when stomach / intestine is “empty” - Starts in lower portion of stomach
When does MMC occur?
During phase III
Burst of high frequency, large amplitude contractions that migrate along the length of the intestine and die out
Interval between phase IIIs is 90-120min
What are the functions of MMC?
“Intestinal housekeeper”
Indigestible residues moved out of stomach by large contractions + wide opening of pyloric sphincter during phase III
Removes dead epithelial cells by abrasion
Prevents bacterial overgrowth
Prevents colonic bacteria entering small intestine
Occurs after digestion + absorption of a meal (empty stomach)
What is MMC regulated by?
Smooth muscle cells of stomach can produce “slow waves”
Contractions coordinated by ENS by pacemaker cells (interstitial cells of Cajal)
Initiated by vagus nerve in upper tract
Some evidence for cyclical secretion of the hormone motilin from stomach & duodenum
What inhibits release of motilin?
Feeding
What motor activity occurs in the small intestien after feeding?
Mixing contractions: segmentation, mixes and stirs contents with enzymes, prevents unstirred layer formation
Peristaltic contractions (slow waves): these move the contents in an oral to anal direction (law of the gut)
How is motor activity in small intestine regulated?
local reflex mediated via ENS but can be enhanced or suppressed by extrinsic innervation (i.e. parasympathetic/sympathetic); ↑sympathetic and ↑ parasympathetic inhibit and stimulate motility, respectively
What decreases small intestine motility?
Pain and fear ↓ motility
When does storage occur in small intestine?
Storage, particularly whilst water is absorbed from contents
Explain what happens in the small intestine after a meal
Intensive mixing and slow movement of waste and indigestible material aborally
Contains “fermenting chambers” which allow the hydrolysis of fibre and indigestible nutrients
→Faeces formation
Slow propagating-slow aboral flow
Explain the peristalsis occurring in small intestine aborally
slow in large intestine in comparison to small intestine; moves contents towards the anus; distension initiates contraction
Describe the mass movement in small intestine
powerful contraction of mid-transverse colon that sweeps colon contents into rectum (responsible for colonic evacuation)
Describe the motility of the large intestine
Large amplitude
long duration
slow propagating velocity
Describe the muscular structure of the large intestine
The teniae coli are three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end
What causes diarrhoea and constipation?
Disorders of motility, fluid secretion and absorption are important in the pathogenesis of diarrhoea and constipation
What is diarrhoea?
frequent (>3x/day) discharge of liquid faeces
What is constipation?
difficulty/some constraint in opening/emptying bowels (hard faeces)
