Questions and Answers
What mechanism triggers mass movements in the colon?
What is the primary function of segmentation contractions in the colon?
What is the typical duration of mass movements in the colon?
Which of the following accurately describes the impact of cholera toxin on colon function?
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Which electrolytes are primarily absorbed in the large intestine?
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What role does the internal anal sphincter play in the defaecation reflex?
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What triggers the entry of Na+ ions into colon cells for absorption?
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The defaecation reflex is primarily controlled by which of the following?
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What is the primary secretory mechanism for chloride ions in the intestinal crypts?
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What role do enteric bacteria play in the colon?
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Where does segmentation (haustrations) occur most frequently in the colon?
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Mass movements in the colon are typically triggered by which reflexes?
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How many times per day do mass movements typically occur in the colon?
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Which part of the nervous system controls the voluntary relaxation of the external anal sphincter during defecation?
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What triggers the defecation reflex?
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Conscious control over defecation is absent in which of the following cases?
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How much fluid is absorbed by the large intestine per day?
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Where is the majority of fluid absorbed in the gastrointestinal tract?
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Which of the following is NOT secreted in the gastrointestinal tract?
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What is the primary route for salt absorption in the ileum?
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In the large intestine, which bacterial toxin inhibits salt and water absorption?
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Which channel is responsible for chloride secretion in the colon?
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What is the consequence of cholera toxin activating the CFTR channel?
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How much watery stool can be produced per day as a result of cholera toxin’s effects on the large intestine?
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Segmentation contractions in the colon are also known as:
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How often do mass movements in the colon typically occur?
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Compared to the small intestine, colon transit is:
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Rectal distension causes peristalsis in which parts of the colon?
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Rectal distension and the subsequent relaxation of the internal anal sphincter are controlled by which of the following?
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If there are lesions in the extrinsic nerves or spinal cord, defecation is:
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Conscious control of the external anal sphincter during defecation is mediated by:
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In the small intestine (ileum), the Na+/K+ pump is located in the:
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In the ileum, Na+ enters the cells through:
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In the colon, NaCl absorption via Na+/H+ and HCO3-/Cl- exchangers is:
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In the colon, compared to the small intestine, which of the following is absent in the Na+ absorption process?
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Bacterial toxins stimulate:
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Diarrhea caused by bacterial toxins can be treated with oral solutions containing sodium and glucose because:
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Cholera toxin activates which of the following channels, leading to excessive NaCl and water secretion?
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Cholera toxin causes excessive secretion of water and NaCl, resulting in:
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Study Notes
1. Colonic Motility
-
Segmentation (Haustrations):
- Location: Throughout the colon (from the caecum to the sigmoid colon).
- Contractions occur in small segments (~2.5 cm), almost occluding the lumen.
- Frequency increases from:
- 2/min in the caecum to
- 6/min in the sigmoid colon.
-
Peristalsis:
- Location: Throughout the colon.
- Weak, slow movements that help propel contents over a long period.
-
Mass Movements:
- Location: Typically start in the transverse colon and move towards the rectum.
- Occur 1-3 times/day, usually after meals.
- Triggered by:
- Gastrocolic reflex (stomach stretch after meals).
- Duodenocolic reflex (duodenal stretch after food enters the small intestine).
- Propels fecal matter into the rectum.
2. Defecation Reflex
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Rectal Distension:
- Location: Rectum.
- Triggers peristalsis in the descending and sigmoid colon, and rectum.
- Causes relaxation of the internal anal sphincter (involuntary).
-
Conscious Control:
- Location: External anal sphincter (striated muscle).
- Controlled by the pudendal nerves, enabling voluntary relaxation of the external sphincter during defecation.
- Valsalva maneuver increases intra-abdominal pressure to aid defecation.
-
Fullness Perception:
- Location: Sensory signals travel from rectal stretch receptors to the brain, conveying the urge to defecate.
3. Fluid and Electrolyte Absorption
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Volumes of Secretion and Absorption:
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Fluid secretions occur in the:
- Stomach (gastric juice),
- Liver (bile),
- Pancreas (pancreatic juice),
- Small intestine (intestinal juice).
-
Absorption Sites:
- Small Intestine (Ileum): Absorbs most of the fluid (~8.5 L/day).
- Large Intestine (Colon): Absorbs the remaining fluid (~1.4 L/day).
- Excreted in Feces: About 0.1 L/day.
-
-
Water and Electrolyte Reabsorption:
-
Small Intestine (Ileum):
- Location: Ileum (last part of the small intestine).
- Uses a Na+/K+ pump in the basolateral membrane for sodium absorption.
- Na+ enters cells with amino acids and sugars (primary route for salt absorption).
- This mechanism remains functional even in the presence of bacterial toxins.
-
Large Intestine (Colon):
- Location: Colon.
- Electrolyte exchange (Na+/H+ and HCO3-/Cl- exchangers) similar to the small intestine, though no amino acid or sugar carriers are present.
- Na+ is absorbed by a different Na+ carrier.
- NaCl absorption is inhibited by bacterial toxins in this area.
-
4. Colon Secretions & Chloride Secretion Mechanism
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Chloride Secretion:
- Location: Intestinal crypts of the colon.
- Cl- is secreted via the CFTR (Cystic Fibrosis Transmembrane Regulator) channel.
-
Impact of Cholera Toxin:
- Location: Colon.
- Cholera toxin (from Vibrio cholerae) activates the CFTR channel, resulting in excessive NaCl and water secretion.
- This can produce up to 20 L/day of watery stool (leading to severe diarrhea).
- Toxin stimulates secretion of salt and water while inhibiting absorption.
- However, sodium and glucose co-transport systems are unaffected, allowing for treatment with oral rehydration solutions containing sodium and glucose.
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Description
This quiz explores the intricate functions of the large intestine, including colonic motility, haustrations, and mass movements. It covers fluid secretion and absorption, as well as electrolyte reabsorption mechanisms and the impact of cholera toxin. Perfect for anyone looking to deepen their understanding of gastrointestinal physiology.
