Lecture 24: Introduction to GI Physiology and Motility Flashcards
what is the overall function of the GI tract?
- obtain nutrients required for growth and energy
- replace fluid and salts lost in urine, feces, sweating, breathing
what does the CNS, ENS and hormones do?
regulates motility and secretion
what does motility do?
- carries out mechanical digestion
- aids chemical digestion
- aids absorption
what does secretion do?
- aids mechanical digestion
- essential for chemical digestion
- aids absorption
what is the path food takes in the GI tract?
- food is ingested
- undergoes mechanical digestion
- which essential for chemical digestion
- which is essential for absorption
-which leads to excretion of waste
what happens to the conditions of the intestinal lumen?
conditions of intestinal lumen are regulated
what do receptors in the GI tract wall respond to?
- stretch (food in lumen)
- change in composition such as pH, osmolarity, amino acids, sugars, fats
what are the effectors?
smooth muscle and glands
- reflexes stimulated by receptors stimulate smooth muscle contraction and glands secretion
what type of regulation is GI function under?
nervous and hormonal regulation
what is the nervous regulation of GI function?
- central nervous system
- enteric nervous system
how does the central nervous system regulate GI function?
- coordinates activity over long distances
- parasympathetic nervous system stimulates motility and secretion
- sympathetic nervous system inhibits motility and secretion
- modulates activity of the enteric nervous system
how does the enteric nervous system regulate GI function?
ENS consists of
- submucosal plexus which regulates secretion
- myenteric plexus which regulates motility
- involved in local reflexes (peristalsis and segmentation)
- completely self contained in GI tract
what is the hormonal regulation of GI function?
- GI tract is the largest endocrine organ in the body
- has endocrine and paracrine functions
- critical hormones are: gastrin, ghrelin, secretin, cholecystokinin
what are tonic contractions?
sustained contractions for minutes to hours e.g. sphincters
what are phasic contractions?
waves of contraction and relaxation over seconds (e.g. peristalsis)
define motility/motility pattern?
any pattern of contraction or relaxation of GI tract smooth muscle
- some motility patterns are a combination of two or more simpler patterns (e.g. retropulsion)
- sometimes relaxation can be a motility pattern (e.g. storage in stomach)
define movement/propulsion?
motility patterns that specifically move or propel GI contents along the tract
what is the basis of GI motility?
smooth muscle which is spontaneously active
- meaning it contracts without external input
- strength of contraction regulated by nervous and hormonal input
what is the frequency of contraction property in each region?
stomach - 3/min
duodenum - 12/min
ileum - 9/min
what are the features of the fasting motility pattern?
migrating motor complex
- 4 hours after a meal
- repeats every 2 hours until eat again
- housekeeping role
what are the features of the feeding motility pattern?
Storage
- occurs in stomach and colon
- relaxation of smooth muscle allows volume to increase without a change in pressure
Propulsion/Movement
- occurs in esophagus, stomach, small and large intestine
- peristalsis
Mixing
- Stomach (retropulsion)
- Small and large intestine (segmentation)
what is peristalsis?
- bolus of food arrive in digestive system
- circular muscle contract behind bolus
- longitudinal muscles head of bolus contract
- contraction in circular muscle layer pushes bolus forward
what is segmentation?
Alternate patterns of contraction to push food together and apart
what are the features of chewing?
- reduces size of food
- allows ingestion and reduces size
- mixes food with saliva for taste
- under voluntary control (skeletal muscle)
- involuntary control of strength, frequency and rhythm
what is swallowing?
- rapid transfer of material from mouth to stomach
- initiated at will, proceeds reflexively
what are the functions of the stomach? where do these occur?
Storage - fundus and body of stomach
Mechanical digestion - antrum
Mixing (retropulsion) - antrum
Controled delivery to duodenum - pyloric sphincter
What happens to the stomach during fasting? (has some repeated info)
- Stomach shrinks to 50ml volume
- activation of migrating motor complex:
- occurs 4 hours after meal, continues every 2 hours until eat
- 2 hours of inactivity
- 50 mins uncoordinated activity, 10 mins coordinated activity
- has housekeep function:
- removes residual secretions
- removes undigested material
- promotes epithelial cell turnover
What happens in the stomach during feeding? (some repeated info)
Storage
- receptive relaxation and gastric accommodation
- nervous regulation by vagus nerve
- increase in volume with minimal change in pressure
- occurs in fundus and body of stomach
Propulsion/movement
- Peristalsis
- initiated in greater curvature and spreads to antrum
- 3 contractions per minute
- first 60 minutes following meal is gentle
- 60-300 minutes is more intense activity
Mixing and mechanical breakdown
- retropulsion in stomach
- this is a combination of peristalsis and closure of the pyloric sphincter
what happens in gastric emptying?
- gastric emptying is a result of motility
the rate of gastric emptying matches the digestive capacity of intestine
- combination of peristalsis and periodic opening/relaxation of the pyloric sphincter
- regulated by feedback from the duodenum
what factors affect gastric emptying?
- size of meal (larger meal = faster emptying)
- composition of meal (fluids faster than solids)
- fats slow gastric emptying (fats difficult to digest)
What happens in small intestine?
digestion and absorption
what is the function of small intestine motility?
- mixing with secretions from pancreas, biliary system and intestine
- controlled movement/propulsion
- exposure of products of digestion to absorptive surfaces
what are the motility patterns in the small intestine between and after meals?
between meals is migrating motor complex
after meals
- main motility pattern is segmentation for mixing, exposure to absorptive surfaces and movement
- there is some peristalsis for propulsion
what is the motility in colon/large intestine
- storage of faeces
- large periods of inactivity
- segmentation for exposure to absorptive surfaces and absorption of fluid and salt from faeces
- mass movement 1-2 times a day following meals by a peristaltic wave which drives faeces into rectum to initiate defecation
