Transport Along and Across GI Tract PART 1 Flashcards
What two mechanisms is transport from the gastric reservoir to the antral pump, mediated by?
- Tonic contraction
- Peristaltic waves in the gastric corpus
Name the area tonic contractions originate.
Name two areas tonic contractions can occur.
- Originate at top of antrum
- Stomach and sphincters in GI tract
Describe storage.
- Proximal stomach relaxes to store food at low pressure
- Food acted upon by acid, enzymes and mechanical degradation.
Describe emptying.
- Regulated to ensure adequate acidification/neutralisation
- Ensure enzyme action and mechanical breakdown
- Avoid duodenal swamping
What is gastroparesis?
- Abnormal emptying of food by stomach
- Low rate of passage of food
What is gastric emptying dependent upon?
- Propulsive force generated by tonic contractions of proximal stomach
- Stomach’s ability to differentiate types of meals ingested and their components
What factors decrease force and rate of gastric emptying?
Fatty, hypertonic, acidic chyme in the duodenum
Summarise the emptying of liquids.
- rapidly empty without lag time
- rate of emptying is influenced by the nutrient content (nutrient-containing liquids retained longer)
Summarise the emptying of solids. PART 1
- 2 phases (lag time and linear phase); duration of Phase 1 related to particle size
- Liquid part emptied and solid component retained in proximal stomach
Summarise the emptying of solids. PART 2
- trituration of larger particles to smaller ones
- pylorus regulates the passage of materal
Define trituration.
Reducing particle size or creating homogenous solution through thorough mixing
Summarise emptying of fatty foods
- liquefy at body temperature; float on top of liquid layer and empty at slow rate
- fats are potent inhibitors of gastric motor events i.e gastric emptying
Summarise emptying of indigestible solids.
- empties in immediate post-prandial period
- MMC acitivity
What is the order of highest rate of gastric motility to lowest?
carbohydrates > proteins > fatty foods > indigestible solids
List 3 determinants of the rate of gastric motility.
- type of food eaten
- osmotic pressure of duodenal contents
- hormones (somatostatin, secretin, CCK, GIP)
What factors can decrease or inhibit emptying?
- Hormones
- Hyperosmolar chyme
What decreases gastric motility?
- vagal innervation upon over-distension
- injury to intestinal wall and bacterial infections
What does ICC stand for?
Intestinal Cells of Cajal
What are the ICC and where are they located?
- Specialised pacemaker cells involved in myogenic control of motility
- Located in the wall of the stomach, small intestine, and large intestine.
What is special about the cell membrane of the ICC?
Rhythmic depolarisation and repolarisation constantly occurring in membranes
What is the result of the rhythmic depolarisation and repolarisation in the ICC membranes?
- Creates a slow wave- the BER
- This wave transmitted to smooth muscle cells.
What does BER stand for?
Basal electrical rhythm
What does BER determine?
Frequency of the contractions in the GI tract
What happens when the BER reaches its plateau point?
Smooth muscle contraction
What does the BER specificially allow the smooth muscle cells to do?
Depolarise and contract rhythmically when exposed to hormonal signals
What causes depolarisation of the GI smooth muscle?
What causes repolarisation of the GI smooth muscle?
Depolarisation - calcium-sodium entry.
Repolarisation - K+ efflux.
List some factors that will mediate a decrease in fundic motor activity.
- CCK
- secretin
- VIP
- somatostatin
- gastrin-releasing peptide (GRP)
- glucagon
List a factor that mediates an increase in fundic contractions.
MOTILIN
How is movement through the small intestine controlled?
- Hormonal and nervous factors initiate and maintain peristalsis and mixing
- Localised distention of the duodenum
Name some factors that increase intestinal motility.
Name some factors that decrease intestinal motility.
INCREASE: CCK, gastrin, and motilin
DECREASE: Secretin
Describe the feedback control of gastric emptying. PART 1
- Contraction of the middle antrum elicits a descending inhibitor reflex, causing pyloric sphincter relaxation (via NO/VIP).
Describe the feedback control of gastric emptying. PART 2
- Duodenal stimuli induces the excitatory reflex, leading to pyloric contractions
- Prevents duodenal-gastric reflux.
What can cause duodenal stimuli?
Prsence of acidic chyme
What can the pyloric sphincter contract in response to?
Antral/duodenal rhythm (e.g fatty acids in duodenum)
How does pyloric contraction affect the antrum?
Cause liquids in the antrum to be squirted into the duodenum
Describe the feedback control of gastric emptying. PART 3
- Gastro-gastric reflexes provide balance between gastric reservoir and antral pump.
- Distention of reservoir stimulates antral contractions.
- Distension of the antrum prolongs reservoir relaxation.
What are the different components of motility in the intestine? PART 1
- segmentation (mixing contractions): stationary contractions and relaxation
- peristalsis (propulsive): in the stomach
What are the different components of motility in the intestine? PART 2
- migrating motor complex
- mass movements (evacuation)
What are the phases of motor activity?
- PHASE 1: quiscence/ quiet period
- PHASE 2: irregular propulsive contractions
- PHASE 3: peristaltic rush
Where does segmentation originate?
ICC
How does segmentation influence chyme?
- Creates divisions of chyme, bringing chyme in contact with intestinal walls.
- Chyme is divided, subdivided and mixed with luminal contents and pushed back and forth. Slowly migrates to ileum
What is peristalsis and what is it used for?
- Propulsive contractions of successive sections of circular smooth muscle preceded by relaxation
- Spread the food out, allowing mixing with digestive enzymes
- Push the food towards the anus
What is segmentation and what is it used for?
- Mixing contractions primarily churn the food, but also propel it towards the anus
Describe the migrating motor complex (MMC) and when it occurs.
- Cyclically recurring sequence of events
- Occurs between meals, when the stomach/intestines are ‘empty’
- Starts in lower portion of stomach
Describe Phase 3 of motor activity.
- High frequency, large amplitude contractions that migrate along the length of the intestine and die out
What are the functions of the MMC? PART 1
- Allows movement of indigestible contents from stomach by large contractions followed by opening of the pyloric sphincter during Phase 3
- Removes dead epithelial cells by abrasion
What are the functions of the MMC? PART 2
- Prevents bacterial overgrowth
- Prevents colonic bacteria from entering the small intestine
Describe the control of the MMC through smooth muscle cells.
- Smooth muscle contracts
- Contractions are coordinated by ENS by ICC.
- Initiated by the vagus nerve in the upper tract.
Describe the control of the MMC through hormones
- Cyclical secretion of motilin from the stomach and duodenum.
- Inhibited by feeding
How do the ICC link to the GI smooth muscle cells?
Form synaptic connections with GI smooth muscle fibres
How long are the contractions in the ileum?
How long are the contractions in the jejunum/duodenum?
→ 8-9 contractions per min
→ 10-12 contractions per min
In terms of passage along the GI tract:
How do liquids pass?
How do solids pass?
How do large indigestible materials pass?
→ in spurts
→ Broken down into 1-2mm sizes
→ Cleared by MMC or vomiting
What do fatty acids in the duodenum cause?
Contraction of the pylorus
What happens during duodenal over distension and chemical stimulation?
→ Vago-vagal reflex
What does distension of the fundus cause?
→ Excitatory effects in the antrum
→ Antrum contracts
What does antral over-distension cause?
→ Inhibitory signals
→ Vago- vagal reflex
What control is the fundus under?
→ Vagal excitatory control
What do the stomach cells produce from resting potential?
→ Electrical depolarisations
→ Move ripples towards the antrum
How does gastric accommodation occur?
→ Decrease in cholinergic activity
→ VIP / NO promote accommodation
