small and large intestine

Question 1

1. Compare and contrast the mucosal surface of the small intestine and large intestine

Answer 1

• Small intestine: longer (in length); much larger surface area; contains folds, villi, microvilli, and crypts/glands. Site of nutrient and water/electrolyte absorption.
  o Anatomy: circular folds on luminal surface  folds divided into crypts and villi  enterocytes of villi have fingerlike projections called microvilli.
  o Villi: contain goblet cells and absorptive enterocytes.
  o Crypts: contain secretory enterocytes, endocrine cells, and stem cells.
  o Microvilli: aka brush border: surface contains digestive enzymes and transporters.
  o Duodenum: also contains specialized glands for secreting bicarbonate and mucus (Brunner’s glands).
• Large intestine: shorter (in length); much smaller surface area; contains folds, microvilli, and crypts/glands; does not contain villi (thus the smaller surface area). Site of water/electrolyte absorption only (no nutrient absorption).
  o Anatomy: overlying layer of circular smooth muscle, contracted at certain locations  haustra-formation. Longitudinal smooth muscle concentrated into 3 bands. Muscle pattern allows for mixing and propulsive movements of large intestine.
  o No villi.

Question 2

2. Describe the mixing movements (haustrations) and propulsive movements (mass movement) in the colon.

Answer 2

• Mixing movement (haustration):
  o Contraction of circular smooth muscle + longitudinal muscle  exaggeration/bulging of haustra = “non-propulsive segmentation”.
• Propulsive movement/mass peristalsis:
  o Occurs ~3x a day (usually following a meal).
  o Distension in colon  coloncolonic reflex = contraction at site of distension + relaxation distal to site of distension  propulsion of food towards anus.
  o Note: contraction at site of distension  “smoothing out” of haustra.

Question 3

3. Describe how gastroenteric, coloncolonic, gastrocolic, duodenocolic, and defecation reflexes mediate motility in the small and large intestine.

Answer 3

• Gastroenteric:
  o Influences motility of small intestine.
  o Distension of stomach  initiation of peristaltic movement in small intestine.
• Coloncolonic:
  o Influences motility of large intestine.
  o Look at #2.
• Gastrocolic & duodenocolic:
  o Influences motility of large intestine.
  o Distension in stomach and duodenum  signal through extrinsic autonomic nerves  initiate mass movement.
  o Mass movement also initiated by irritation of colonic mucosa.
• Defecation:
  o Influences motility of large intestine/rectum.
  o Chyme enters distal colon  initiation of defecation reflex.
  o Subdivision of 2 reflexes:
   Intrinsic reflex: distension in distal colon  signal through enteric nerves of myenteric plexus  peristaltic wave initiated in descending colon, sigmoid colon, and rectum  feces forced towards anus. Also, internal anal sphincter relaxes in response to NO and VIP.
   Parasympathetic defecation reflex:
  • Intrinsic reflex is weak.
  • Reinforced through signals to spinal cord and back via parasympathetic nerves (parasympathetic defecation reflex).

Question 4

4. Describe the three distinct anatomical features that gives the small intestines its large surface area of approximately 200 m2.

Answer 4

• Folds, villi, and microvilli (look at #1 for details).

Question 5

5. Define the cellular make-up of the mucosal surface of the small intestines and the cellular secretions.
6. Describe the distinguishing features of Brunner’s glands and crypts of Lierberkühn.

Answer 5

• Cells of small intestine: absorptive enterocytes and goblet cells (surface of villi); secretory enterocytes, endocrine cells, and stem cells (surface of crypts).
• Secretions:
  o Brunner’s glands of duodenum:
   Contains:
   Epithelial cells: secrete mucus and HCO3- (both for protection of duodenal epithelium).
  o Crypts of Lieberkuhn and villi of small intestines:
   Contain:
  • Goblet cells: secrete mucus (lubrication/protection).
  • Secretory enterocytes: secrete H20 and electrolytes (absorption).
  • Absorptive enterocytes: express peptidases, sucrase, maltase, lactase, lipase, and transporters (digestion and absorption of nutrients.

Question 6

7. Compare and contrast the two types of movements in the small intestines [mixing (segmentation) and propulsive (peristaltic) contractions and describe how these activities are regulated by neuronal and hormonal stimuli.

Answer 6

• Mixing (segmentation):
  o Induced by distension.
  o Contraction upstream and downstream of distension.
• Propulsion (peristalsis):
  o Induced by many factors (look at regulators below).
  o Contraction at site of distension, relaxation downstream of distension.
• Note: a third important movement type in small intestine: migratory motor complex (same as the one described in GI Motility section).
• Regulation:
  o Neuronal:
   Acetylcholine: stimulate contraction
  o Hormonal:
   VIP: inhibit contraction.
   NO: inhibit contraction.

Question 7

8. Describe the general distribution of normal microbiota throughout the gastrointestinal tract.

Answer 7

• Stomach: none – a few bacteria (pH is too low for survival of many species).
• Jejunum: none – a few more bacteria.
• Ileum: greater number of bacteria than stomach and ileum.
• Cecum/colon: largest number of bacteria.

Question 8

9. List the factors that contribute to the growth of intestinal microbiota, and key functions of intestinal microbiota.

Answer 8

• Factors contributing to growth:
  o GI secretions (saliva, acid, fluid/electrolytes, bile).
  o Mucosal immunity: influences which organisms can survive.
  o Intestinal motility: the greater the motility, the lower the chances of organisms colonizing.
  o Pharmacological agents (antibiotics, cancer-chemotherapeutics, immunosuppressants).
• Key functions:
  o Conversion of primary bile acids to secondary bile acids.
  o Deconjugation of compounds conjugated in liver (ex: drugs, bile salts, etc.): for example, oral contraceptives include estrogen which is conjugated by the liver but un-conjugated by the bacteria in the gut. This allows for re-uptake of estrogen, allowing us to prescribe the lowest dose of estrogen necessary to see the effects we want.
  o Nutrient salvage: bacteria can break down some nutrients that our body can’t, facilitating their uptake.
  o Suppression of pathogenic organisms: very important!

Question 9

10. Know and understand the following pathophysiological conditions: irritable bowel syndrome, inflammatory bowel disease, and Hirschprung’s disease.

Answer 9

• Irritable bowel syndrome (IBS):
  o What it is: chronic disorder; causes abdominal pain/bloating/distension/cramps and diarrhea/constipation/both.
  o Causes: unknown.
  o Note: treatment is symptomatic; patients with constipation-predominant IBS should be encouraged to work out regularly.
• Inflammatory disease (IBD):
  o What it is: ulcerative colitis + Crohn’s disease; inflammation of GI tract.
  o Causes: unknown.
  o Disease process: malabsorption (specifically of vitamin K and D), impaired water and electrolyte uptake  dehydration.