Function of the colon

Question 1

Discuss motility in the colon.

Answer 1

• Bands of longitudinal muscle (teania coli) contract forming haustra.
• This is called segmental propulsion, which is very slow and allows time for additional Na+ and fluid absorption (Colonic salvage).
• The slow movement dries out the chyme, producing faeces.
• Several times a day, mass movement will occur, creating peristaltic movement that force the chyme/ faeces towards the rectum.
• Factors that increase intestinal motility can cause diarrhoea, by minimising formation of the haustrae and increasing mass movement.

Question 2

What is a defaecation reflex?

Answer 2

• An Intrinsic reflex mediated by the local enteric nervous system in the rectal wall. To be effective it usually must be fortified by parasympathetic defecation reflex.

Question 3

Describe the defaection reflex.

Answer 3

• Distention of the rectum causes the internal anal sphincter to relax & initiate peristaltic movement which further pushes faeces into rectum
• This local (enteric) effect is reinforced by the PNS which initiates stronger contractions via the pelvic nerves
• At the same time an afferent signal is sent centrally which stimulates the urge to defecate
• In response to the urge to defaecate (defaecation reflex) the external anal sphincter is voluntarily constricted until appropriate conditions are present
• Relaxation of the external anal sphincter sphincter, coupled with contraction of the rectum and sigmoid colon & Valsava Manoeuvre results in defecation.

Question 4

What is the ultimate goal of the GIT?

Answer 4

• Getting bulk nutrients into the body.
• This occurs in the small and large intestines.

Question 5

What is secretion and what is its function?

Answer 5

• The process whereby our body produces mucus, buffers, hormones and enzymes.
• These facilitate lubrication, digestion, motility & absorption through the tract.

Question 6

Describe the absorption that occurs in the GIT.

Answer 6

Question 7

Draw a cell transport diagram.

Answer 7

• Draw rectangle
• Orientate long side (r - basolateral side/ blood, l- apical/ luminal side
• Draw generic transporter (Na+/k+)
• Draw the Na+/k+ atpase pump in the basal lateral side. BLOOD SOKI- blood/ basolateral sodium out K+ in.
• Know their concentration extracellularly/ intracellularly.

Question 8

What are the transport mechanism in the colon?

Answer 8

• Absorption (& secretion) of Water by the colon are passive processes driven by the absorption & secretion of electrolytes & solutes.

Question 9

What is the first of the three transport processes?

Answer 9

1. Electroneutral NaCl Absorption (same process as occurs in small intestine).
   - An osmotic gradient is established across the intestinal epithelium that simultaneously drives the movement of water across tight junctions.

Question 10

What is the second transport process?

Answer 10

2. Fluid Absorption in the Colon driven by SCFAs.
   - Examples of SCFA: Acetate, propionate, and butyrate.
   - SMCT= sodium-monocarboxylate transporter.

Question 11

What is the third transport process?

Answer 11

2. Fluid Absorption in the Colon driven by absorption of Na+.
   - An osmotic gradient is established across the intestinal epithelium that simultaneously drives the movement of water across tight junctions.

Question 12

Discuss the secretion of Cl- in the large and small intestines.

Answer 12

• Most of intestinal secretory flow of fluid into the intestinal lumen is driven by the active secretion of Cl-.
• In the Crypts of Lieberkuhn (intestinal crypt/gland), chloride is secreted by the cystic fibrosis transmembrane regulator (CFTR).

Question 13

How does Cholera affect Cl- secretion?

Answer 13

• Cholera toxin from the bacterium Vibrio Cholerae activates the CFTR causing excessive NaCl and water secretion, producing as much as 20 L/day of watery stool.

Question 14

Discuss the colonisation of cholera in depth.

Answer 14

• Cholera toxin is released from bacteria in the infected intestine and binds to enterocytes by interacting with the GM1 ganglioside receptor on the intestinal cell,
• This triggering endocytosis of the toxin.
• Cholera toxin modifies the G-protein Gsα causing G protein to be locked in its GTP-bound form, thereby continually stimulating adenylate cyclase to produce cAMP.
• High levels of cAMP leads to prolonged activation of CFTR causing active secretion of Cl- into lumen of small intestine
• Cl- efflux accompanied by Na+ efflux & water follows the electrolytes
• This isotonic secretion of water & electrolytes depletes the vascular space while maintaining plasma osmolarity.

Question 15

What is the treatment for Cholera?

Answer 15

• Oral Rehydration Therapy
• Antibiotic Tetracycline
• GM= Ganglioside1 receptor
• Anti-diarrhoea: enkephalins, which regulate
  intestinal secretion by acting directly on enterocytes. Enkephalins bind to the opioid receptors on enterocytes, which act through G proteins to inhibit the stimulation of cAMP synthesis induced by cholera toxin, thereby directly controlling ion transport