L20 - Large intestine physiology

Question 1

What are the 3 main movements involved in the large intestine?

Answer 1

1. Haustral contractions (segmentation)
2. Mass movements (peristalsis)
3. Defecation Reflex

Question 2

Haustral contractions

Answer 2

• Segmentation - mixing + churning of faeces (entering from ileum to cecum - ascending colon)
• Triggered by distension - stretch of the intestinal wall
• Occurs primarily in the ascending colon + transverse colon
• Increase contact of faeces with mucosal surface - absorption of water + electrolytes
• Very limited chemical digestion
• Only bacterial enzymes present

Question 3

Mass movements

Answer 3

• Propulsion - pushing faeces towards the rectum by peristalsis
• Slow, giant peristaltic contractions - occurs 3-4 times a day (during or after meals)
• Triggered by stretch ( distention) + irritants to the mucosal surface + gastrocolic reflex
• Gastrocolic reflex => increased motility in colon in response to stretch in stomach + byproducts of digestion in small intestine
• Primarily occurs in transverse colon + descending colon

Question 4

What is Defecation Reflex?

Answer 4

• Triggered by stretch in sigmoid colon + rectum
• Stimulation of sensory fibres which travel to spinal chord (CNS)
• Stimulation of parasympathetic motor fibres which travel to myenteric plexus on large intestine - release of ACh
• Stimulates contraction of sigmoid colon + rectum
• Stimulates relaxation of the internal anal sphincter (smooth muscle - involuntary - innervated by ANS)
• Contraction of external anal sphincter (skeletal muscle - voluntary - innervated by somatic NS) until relaxes when on the toilet

Question 5

What are the causes of constipation?

Answer 5

• Idiopathic - bad diet (low fibre, low water intake)
• Gender (women more susceptible)
• Psychiatric (e.g. depression)
• Disease (e.g. megacolon)
• Latrogenic (e.g. morphine, laxative abuse)

Question 6

What are the causes of diarrhoea?

Answer 6

• Dietary indiscretion - mild food poisoning
• Disease
• Drug-induced
• Travel

Question 7

What are the causes of IBS?

Answer 7

• Irritable Bowel Syndrome - abnormal GI motility
• Psychological factors - stress/anxiety
• Luminal compounds which may irritate bowel - food allergens, bile acids
• Altered serotonin levels