8.6 The Large Intestine

Question 1

What are some components of faeces?

Answer 1

• Mucus
• Alive/dead bacteria
• Unabsorbed biliary products
• Sloughed epithelial cells

Question 2

What are the pouches of the small intestine called? Why do they exist?

Answer 2

• Haustra
• Exist because thin band of longitudinal muscle in LI (taeniae coli) are slightly shorter than underlying smooth muscle

Question 3

What is the main histological difference between the small and large intestine?

Answer 3

No villi on large intestine (makes sense: surface area is decreasing)

Question 4

What are the pacemakers of the GI tract? What mixing movements are they involved in?

Answer 4

• Interstitial cells of cajal
• Involved in segmentation and haustral contractions

Question 5

Are haustral contractions propulsive? Are they fast/slow?

Answer 5

They are non-propulsive and slow

Question 6

Describe mass movements of the colon

Answer 6

• Occur ~3 times a day in response to a meal
• Large segments of LI contract orally to anally, haustrations are lost
• Moves contents 1/3 to 3/4 way through colon

Question 7

Describe the intrinsic innervation of the large intestine

Answer 7

Concentrated in myenteric plexus beneath taeniae coli

Question 8

Describe the extrinsic, sympathetic innvervation of the LI

Answer 8

• Proximal: Superior mesenteric ganglion
• Distal: inferior mesenteric
• Rectum and anal canal: hypogastric plexus

Question 9

Describe the extrinsic, parasympathetic innvervation of the LI

Answer 9

• Proximal: Vagus nerve
• Distal: Pelvic nerve

Question 10

Describe the gastrococlic reflex

Answer 10

Gastrin release and extrinsic innervation upon food ingestion cause colon motility and mass movements

Question 11

How much volume enters the colon? How much is defecated?

Answer 11

500ml enters, 350mL leaves, 150mL faeces remains

Question 12

Describe the relationship between the internal and external anal sphincters and the anal canal

Answer 12

• Mass movement
• Internal anal sphincter relaxes
• Anal canal opens
• External anal sphincter (skeletal muscle -> voluntary) relaxes

Question 13

Short reflex of defecation

Answer 13

Myenteric plexus -> contraction of rectum and sigmoid colon, relaxation of internal sphincter

Question 14

What dictates external sphincter relaxation and sensation to defecate?

Answer 14

Cerebral cortex

Question 15

Voluntary ways of assisting defecation

Answer 15

• Abdominal contractions
• Exhalation against a closed glottis
• Pelvic floor relaxes

Question 16

Overcoming the defecation reflex

Answer 16

• Voluntarily contract the external sphincter
• Urge subsides as stretch receptors accomodate stimulus
• Internal sphincter regains tone, anal canal closes

Question 17

Basic mechanism of constipation

Answer 17

Delayed transit -> more h20 absorbed into body -> hard dry faeces

Question 18

Hirchsprung’s disease

Answer 18

Absence of enteric nervous system in distal colon -> increased tone, narrow lumen. Proximal area becomes enlarged

Question 19

How are salt and water absorbed in the colon?

Answer 19

1. Sodium is actively absorbed into interstitial space (Na+/K+)
2. Cl- follows down electrochemical gradient
3. Water follows osmotic gradient

Question 20

Which cells secrete mucus in the intestines?

Answer 20

Goblet cells

Question 21

What is the function of protective alkaline mucus of the large intestine?

Answer 21

• Neutralises acids from fermentation
• Lubricates for faeces

Question 22

What can cause an increase in colonic mucus secretion?

Answer 22

• Mechanical/chemical stimulation
• Mediated by short reflexes and parasympathetic innervation

Question 23

Describe the mechanism of inflammatory bowel disease

Answer 23

1. Faulty inner mucus layer
2. Bacteria reach epithelium
3. Inflammation
   (Repeat/worsen)

Question 24

Why is the environment of the colon permissive to anaerobic bacteria

Answer 24

• Slow-moving
• No antibacterial secretions

Question 25

What are some common effects of colonic bacteria in terms of mineral absorption and vitamin synthesis?

Answer 25

• Synthesise vitamin K
• Promote absorption of calcium, zinc, magnesium

Question 26

Draw a diagram of colonic fermentation

Answer 26

[]

Question 27

True or false: microbiota produce enzymes that break down nutrients that cannot be hydrolysed by host enzymes

Answer 27

True

Question 28

What does colonic fermentation release?

Answer 28

• SCFAs (energy for colonocytes)
• Gases (e.g. CO2 and methane)

Question 29

How can colonic fermentation influence diarrhoea?

Answer 29

Some products can be implicated in diarrhoea

Question 30

Is most gas produced by fermentation expelled through the anus?

Answer 30

No. Most is absorbed through the mucosa, the remainder is farted out

Question 31

Which two bacteria account for 90% of food poisoning cases

Answer 31

• Salmonella
• Campylobacter