normal gastro-duodenal motility interdigestive phase - migrating motor complex post-prandial phase -...
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NORMAL GASTRO-DUODENAL MOTILITY
Interdigestive phase
- Migrating motor complex
Post-prandial phase
- Gastric digestion - Emptying
POST-PRANDIAL MOTILITY
Reservoir capacity of the stomach
Trituration of food (gastric outlet resistance)
Gastric emptying of liquids and solids
Digestion and dispersion of food
Small intestinal feedback inhibition
RESERVOIR FUNCTION OF THE STOMACH
Swallowing and oesophageal distension induce a reflex relaxation of the fundus (adaptive relaxation)
Expansion occurs along the greater curvature, with little change in the antrum and the lesser curvature
The reflex is responsible for the accomodation of the stomach after ingestion and foods
ADAPTIVE RELAXATION OF THE STOMACH
Adaptive relaxation maintains the intraluminal pressure within a narrow range, even though the intragastric volume may fluctuate
Mechanisms involved: - Viscoelastic property of the smooth muscle - Stretch-induced modulation of the muscle tone through intramural / vagovagal reflexes
ADAPTIVE RALAXATION OF THE STOMACH
No changes in
intraluminal pression
Increased volume
TRITURATION OF FOOD
Mechanical process
Propulsion of content toward the gastric outlet by propagating ring contractions of the corpus and antrum
Retropulsion of content by the closure of the pylorus
Fragmentation of particles occurs as a result of the crushing of food against the narrow and rigid distal antrum
GASTRIC EMPTYING OF LIQUIDS AND PARTICULATE SOLIDS
Propulsion
The wave moves over the proximal third of the antrum
Distal antrum and pylorus are relaxing
Chyme is forced into the distal antrum
All the gastric contents are propelled into the antrum at the same time and at same rate
GASTRIC EMPTYING OF LIQUIDS AND PARTICULATE SOLIDS
Evacuation & Retropulsion
The wave travels over the distal part of the antrum
Chyme is evacuated through the relaxed pylorus accompanied with retropulsion
Subsequent wave moves over the gastric body driving digesta into the proximal antrum
Only liquids and smaller particles escape through the pylorus
GASTRIC EMPTYING OF LIQUIDS AND PARTICULATE SOLIDS
Retropulsion & Grinding
The wave moves over the terminal antrum with increasing velocity
The contractions of terminal antrum and pylorus enhance retropulsion and grinding and prevent obstruction of the pyloric opening
A propagative wave starts on the duodenal bulb
GASTRIC EMPTYING OF LIQUIDS AND PARTICULATE SOLIDS
Liquids and suspended particles (< 2 mm) leave the stomach early
Large particles are retained until they are broken down and partially digested
Undigested food is retained in the stomach until the resumption of phase III of the interdigestive MMC
GASTRIC EMPTYING TIME Response to food
• Determinants of the rates at which chyme is moved in post-prandial phase:
– Caloric density
– Mixture of specific nutrients
• Physiological mechanisms
– Overall rate of contractions
– Contractile force
– Length over which contractions spread
DIGESTION & DISPERSION OF FOOD
Chemical process
Splitting of starch and carbohydrate by parotid amylase
Emulsification of fat by lipase (lingual?)
Breakdown of protein by gastric pepsin
Mechanical activity allows the digestive juice to penetrate food particles
GASTRIC PATHOPHYSIOLOGY
• Low frequency of contractions and contractile force
GASTROPARESIS
• Abnormal direction of contractions and length over which contractions spread
FUNCTIONAL DYSPEPSIA
SMALL INTESTINAL FEEDBACK INHIBITION
Duodenal chemioreceptors exert a feedback control of gastric emptying
aminoacid receptors
glucoreceptors
lipid receptors (CCK release)
osmoreceptors
pH receptors
INTERDIGESTIVE MOTILITY
• Small, cycling migrating band of intense phasic contractions originating from a gastric pace-maker and migrating slowly over the length of the small bowel
• As one activity front arrives at the terminal ileum another begins in the stomach
• The cycle continues until interrupted by food
GASTROINTESTINAL MOTILITY Gastric myoelectrical pace-maker
Anatomical site
GASTROINTESTINAL MOTILITYMigrating motor complex (MMC)
• PHASE 1: Quiescence (45-50 min)
• PHASE 2: Irregular or random contractions ( 30-45 min)
• PHASE 3: High amplitude phasic contractions at the maximal frequency for the locus (5-15 min)
• PHASE 4: Decreasing contractions merging into phase 1
Migrating Motor Complex (MMC)
MMC originates in the stomach and LES and propagates through the intestine
Gastric fundus
Gastric antrum
Duodenum
Jejunum
Proximal ileum
Distal ileum
Minutes
MMC PHASE% Slow waves with spikes
III I II III I II III
GASTROINTESTINAL MOTILITY Migrating Motor Complex (MMC)
POTENTIAL CONTROL MECHANISMS
• Central nervous system
• Cyclic release of a chemical transmitter
• Enteric nervous system
GASTROINTESTINAL MOTILITY MMC: hormone regulation
MOTILIN
PANCREATIC POLIPEPTIDE
SOMATOSTATIN
Antral phase 3
Antral phase 3
Duodenal phase 3
Motilin
GASTROINTESTINAL MOTILITY MMC: Hormone regulation
MOTILINANTRUM Phase 2 Phase 3
ANTRUM Phase 3 Phase 4 DUODENUM Phase 1
Phase 2
Phase 3
SOMATOSTATIN
GASTRO-DUODENAL MOTILITY MMC: Central nervous system (CNS)
regulation
• It is probably not essential, since extrinsic denervation does not abolish cyclic activity
• This hypotesis has been tested in a variety of experimental transplantation models
GASTRO-DUODENAL MOTILITYMMC: hormone regulation
• MMC periodicity is related to cyclic fluctuations of motilin, PP, somatostatin
• Serum peaks of motilin and PP preceed somatostatin rise
GASTRO-DUODENAL MOTILITY MMC: Enteric Nervous System (ENS)
regulation
• It is considered to be the major control mechanism for interdigestive cycles of motility
• Phase 2 activity seems not influenced by ENS (reduction by truncal vagotomy abnormal in IBS patients)
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