8.6 The Large Intestine Flashcards
What are some components of faeces?
- Mucus
- Alive/dead bacteria
- Unabsorbed biliary products
- Sloughed epithelial cells
What are the pouches of the small intestine called? Why do they exist?
- Haustra
- Exist because thin band of longitudinal muscle in LI (taeniae coli) are slightly shorter than underlying smooth muscle
What is the main histological difference between the small and large intestine?
No villi on large intestine (makes sense: surface area is decreasing)
What are the pacemakers of the GI tract? What mixing movements are they involved in?
- Interstitial cells of cajal
- Involved in segmentation and haustral contractions
Are haustral contractions propulsive? Are they fast/slow?
They are non-propulsive and slow
Describe mass movements of the colon
- 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
Describe the intrinsic innervation of the large intestine
Concentrated in myenteric plexus beneath taeniae coli
Describe the extrinsic, sympathetic innvervation of the LI
- Proximal: Superior mesenteric ganglion
- Distal: inferior mesenteric
- Rectum and anal canal: hypogastric plexus
Describe the extrinsic, parasympathetic innvervation of the LI
- Proximal: Vagus nerve
- Distal: Pelvic nerve
Describe the gastrococlic reflex
Gastrin release and extrinsic innervation upon food ingestion cause colon motility and mass movements
How much volume enters the colon? How much is defecated?
500ml enters, 350mL leaves, 150mL faeces remains
Describe the relationship between the internal and external anal sphincters and the anal canal
- Mass movement
- Internal anal sphincter relaxes
- Anal canal opens
- External anal sphincter (skeletal muscle -> voluntary) relaxes
Short reflex of defecation
Myenteric plexus -> contraction of rectum and sigmoid colon, relaxation of internal sphincter
What dictates external sphincter relaxation and sensation to defecate?
Cerebral cortex
Voluntary ways of assisting defecation
- Abdominal contractions
- Exhalation against a closed glottis
- Pelvic floor relaxes
Overcoming the defecation reflex
- Voluntarily contract the external sphincter
- Urge subsides as stretch receptors accomodate stimulus
- Internal sphincter regains tone, anal canal closes
Basic mechanism of constipation
Delayed transit -> more h20 absorbed into body -> hard dry faeces
Hirchsprung’s disease
Absence of enteric nervous system in distal colon -> increased tone, narrow lumen. Proximal area becomes enlarged
How are salt and water absorbed in the colon?
- Sodium is actively absorbed into interstitial space (Na+/K+)
- Cl- follows down electrochemical gradient
- Water follows osmotic gradient
Which cells secrete mucus in the intestines?
Goblet cells
What is the function of protective alkaline mucus of the large intestine?
- Neutralises acids from fermentation
- Lubricates for faeces
What can cause an increase in colonic mucus secretion?
- Mechanical/chemical stimulation
- Mediated by short reflexes and parasympathetic innervation
Describe the mechanism of inflammatory bowel disease
- Faulty inner mucus layer
- Bacteria reach epithelium
- Inflammation
(Repeat/worsen)
Why is the environment of the colon permissive to anaerobic bacteria
- Slow-moving
- No antibacterial secretions
