Motility in the GI tract

Question 1

What 3 functions does motor activity of the GI tract perform?
What are the 2 types of muscle essential for these properties?

Answer 1

1. segmental contractions –> mixing increases digestion and absorption
2. peristaltic contractions –> caudal propulsion
3. act a reservoirs for holding luminal contents, made possible by sphincters

tonic and phasic muscle

Question 2

What are the differences between SI motility in the fed and fasted state?

Answer 2

Fed state = segmentation and peristalsis

Fasted state = migrating motor complex

Question 3

What is a migrating motor complex?
In humans MMC's occur at intervals of ..... to ...... minutes and consist of four distinct phases:
1. 
2. 
3. 
4.

Answer 3

In the fasted state the small intestine in relatively quiescent apart from synchronised, rhythmic changes in both electrical and motor activity which is termed the MMC.
90-120
1. prolonged quiescent period
2. increasing AP frequency and contractility
3. peak electrical/mechanical activity for a few minutes
4. declining activity merges into the next stage 1

Question 4

What is the role of the migrating motor complex? x3

Where do MMC’s originate? Where do they travel to?

Answer 4

1. propel particles greater than 2mm from the stomach into the duodenum
2. clears SI of residual content e.g undigested food, desquamated cells, intestensial and pancreatic secretions
3. stops colonic bacteria from migrating into the terminal ileum

Usually originate in the stomach —> distal ileum
25% initiate in duodenum and prox jejunum

Question 5

What is the major stimulant of the MMC pattern?

When and where is it released?

Answer 5

motilin - peptide synthesised in the duodenal mucosa and released just before initiate of phase 3 of the MMC cycle

Question 6

How is smooth muscle contraction controlled?

Answer 6

NT - ACh
Hormones e.g. motilin and somatostatin (stimulation), VIP (relax)
Stretch reflexes - produces reflex contraction by opening of Ca channels
ENS which is modulated by the ANS

Question 7

What is the ileogastric reflex?
What is the gastroileal reflex?
What are these mediated via?

Answer 7

• ileal distention leads to decreased gastric motility
• increased gastric activity causes increased ileal motility and increased movement of chyme through ileoceacal valve
• ENS

Question 8

What stimulates contraction and relaxation of ileocaecal sphincter? (obvious!)

Answer 8

stimuli proximal to sphincter cause relaxation

stimuli distal to sphincter cause contraction

Question 9

What is the taniae coli?
Where are they?
What do contractions result in?

Answer 9

• longitudinal muscle layers thickened to form 3 muscular bands which run the length of the colon
• in the large intestine
• contractions gather up the colon forming haustra

Question 10

What are the 6 functions of the colon?

What is the difference in function between the proximal and distal colon?

Answer 10

1. absorb water
2. absorb short chain fatty acids, SCFA, (product of carbohydrate fermentation)
3. storage
4. regulated release of faecal matter
5. environment for bacteria to synthesise vitamins
6. secretes mucus and ions

Proximal = site of absorption and bacterial fermentation
Distal = reservoir/storage function

Question 11

What are the two types of colonic motility?

What are their functions?

Answer 11

1. colonic rhythmic phasic contractions - short duration no propagation, long duration propagate over a short distance. Helps to propel semi-solid contents. Highly disorganised in space and vary widely in amplitude and duration.
2. colonic giant migrating contractions - large amplitude lumen occulting contractions propogates very rapidly, mass movements

Question 12

What is the major control of motility in the colon?

Answer 12

bulk –> distention

mediated via myenteric plexus

Question 13

What conditions occur more in Western populations whose diet is low in fibre?

Answer 13

constipation
diverticular disease
haemorrhoids
polyps
colon cancer
irritable colon
ulcerative colitis

Question 14

What are the 4 classifications of laxatives? How are they different?

Answer 14

1. stimulant laxatives - increase motility through chemoreceptors and myenteric nerve plexus
2. osmotic laxatives - draw water into bowel
3. faecal softener - non absorbable lubricants
4. bulk-forming laxatives - cause distention –> motility

Question 15

Defaecatio reflex:

1. Internal anal sphincter is ……….. ……….. ………. muscle. External anal sphincter is ………. ……….. muscle.
2. Faeces enter rectum and causes ……. due to mass …………
3. Intrinsic defaecation reflex is mediated via the …… …….
4. Peristalsis and relaxation of …… ……. …….
5. The parasympathetic defaecation reflex reinforces the internal anal sphincter. This is mediated by the …… …….. of the ……. …….
6. Result in …….. of peristaltic waves and …… of the internal anal sphincter
7. Defeacation can be inhibited by concious control over the ….. ….. ……

Answer 15

1. circular, smooth, involutary, striated voluntary
2. distention, peristalsis
3. myenteric plexus
4. internal anal sphincter
5. sacral segments, spinal cord
6. amplification, relaxation
7. external anal sphincter

Question 16

How long is the small intestine?
What is its diameter?
What are its 4 functions?

Answer 16

9ft
1.5in
stomach to colon
digestion, absorption, secretion, motility

Question 17

What stops the MMC? And what else does this initiate?

Answer 17

Feeding terminates MMC’s and initiates the appearance of the ‘fed motor pattern - segmentation/peristalsis

Question 18

What is the length and diameter of the colon?
What is the surface area compared to that of the SI?
Are there villi present?

Answer 18

last 4ft of GIT, 2.5in diameter
Surface area only 1/30th of the SI
No villi