Lecture 24: Introduction to GI Physiology and Motility

Question 1

what is the overall function of the GI tract?

Answer 1

• obtain nutrients required for growth and energy
• replace fluid and salts lost in urine, feces, sweating, breathing

Question 2

what does the CNS, ENS and hormones do?

Answer 2

regulates motility and secretion

Question 3

what does motility do?

Answer 3

• carries out mechanical digestion
• aids chemical digestion
• aids absorption

Question 4

what does secretion do?

Answer 4

• aids mechanical digestion
• essential for chemical digestion
• aids absorption

Question 5

what is the path food takes in the GI tract?

Answer 5

• food is ingested
• undergoes mechanical digestion
• which essential for chemical digestion
• which is essential for absorption
  -which leads to excretion of waste

Question 6

what happens to the conditions of the intestinal lumen?

Answer 6

conditions of intestinal lumen are regulated

Question 7

what do receptors in the GI tract wall respond to?

Answer 7

• stretch (food in lumen)
• change in composition such as pH, osmolarity, amino acids, sugars, fats

Question 8

what are the effectors?

Answer 8

smooth muscle and glands

• reflexes stimulated by receptors stimulate smooth muscle contraction and glands secretion

Question 9

what type of regulation is GI function under?

Answer 9

nervous and hormonal regulation

Question 10

what is the nervous regulation of GI function?

Answer 10

• central nervous system
• enteric nervous system

Question 11

how does the central nervous system regulate GI function?

Answer 11

• 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

Question 12

how does the enteric nervous system regulate GI function?

Answer 12

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

Question 13

what is the hormonal regulation of GI function?

Answer 13

• GI tract is the largest endocrine organ in the body
• has endocrine and paracrine functions
• critical hormones are: gastrin, ghrelin, secretin, cholecystokinin

Question 14

what are tonic contractions?

Answer 14

sustained contractions for minutes to hours e.g. sphincters

Question 15

what are phasic contractions?

Answer 15

waves of contraction and relaxation over seconds (e.g. peristalsis)

Question 16

define motility/motility pattern?

Answer 16

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)

Question 17

define movement/propulsion?

Answer 17

motility patterns that specifically move or propel GI contents along the tract

Question 18

what is the basis of GI motility?

Answer 18

smooth muscle which is spontaneously active
- meaning it contracts without external input
- strength of contraction regulated by nervous and hormonal input

Question 19

what is the frequency of contraction property in each region?

Answer 19

stomach - 3/min
duodenum - 12/min
ileum - 9/min

Question 20

what are the features of the fasting motility pattern?

Answer 20

migrating motor complex
- 4 hours after a meal
- repeats every 2 hours until eat again
- housekeeping role

Question 21

what are the features of the feeding motility pattern?

Answer 21

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)

Question 22

what is peristalsis?

Answer 22

• 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

Question 23

what is segmentation?

Answer 23

Alternate patterns of contraction to push food together and apart

Question 24

what are the features of chewing?

Answer 24

• 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

Question 25

what is swallowing?

Answer 25

• rapid transfer of material from mouth to stomach
• initiated at will, proceeds reflexively

Question 26

what are the functions of the stomach? where do these occur?

Answer 26

Storage - fundus and body of stomach
Mechanical digestion - antrum
Mixing (retropulsion) - antrum
Controled delivery to duodenum - pyloric sphincter

Question 27

What happens to the stomach during fasting? (has some repeated info)

Answer 27

• 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

Question 28

What happens in the stomach during feeding? (some repeated info)

Answer 28

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

Question 29

what happens in gastric emptying?

Answer 29

• 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

Question 30

what factors affect gastric emptying?

Answer 30

• size of meal (larger meal = faster emptying)
• composition of meal (fluids faster than solids)
• fats slow gastric emptying (fats difficult to digest)

Question 31

What happens in small intestine?

Answer 31

digestion and absorption

Question 32

what is the function of small intestine motility?

Answer 32

• mixing with secretions from pancreas, biliary system and intestine
• controlled movement/propulsion
• exposure of products of digestion to absorptive surfaces

Question 33

what are the motility patterns in the small intestine between and after meals?

Answer 33

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

Question 34

what is the motility in colon/large intestine

Answer 34

• 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