Colon Motility and Defaecation Reflex

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Questions and Answers

What primary function do enterocytes serve in the intestinal epithelium?

Secrete water and electrolytes (correct)

Absorb salts

Secrete mucus

Produce bile

What effect does cholera toxin have on intestinal fluid secretion?

It promotes absorption of water

It stimulates CFTR to increase NaCl and water secretion (correct)

It blocks chloride secretion entirely

It decreases NaCl and water secretion

How do sodium ions enter the gut lumen from enterocytes?

They are exchanged for potassium ions

They follow chloride ions through channels (correct)

They are absorbed with bicarbonate ions

They are secreted actively by enterocytes

What is the primary mechanism for HCO3- absorption in the small intestine?

Facilitated diffusion after formation in the cells Signup and view all the answers

What maintains sodium levels in enterocytes, allowing for effective absorption from the gut lumen?

Na+/K+ pump in the basolateral membrane Signup and view all the answers

What characterizes segmentation in the colon?

Haustrations occur at a rate of 2-6 per minute. Signup and view all the answers

How often do mass movements typically occur in the colon?

1-3 times a day. Signup and view all the answers

What reflex triggers mass movement in the colon?

Gastrocolic reflex. Signup and view all the answers

What is the approximate duration of mass movements in the colon?

15 minutes. Signup and view all the answers

Which mechanism helps expose colonic contents to the mucosa?

Segmentation. Signup and view all the answers

How does colonic motility compare to small intestinal motility?

Colonic transit is slower than small intestinal transit. Signup and view all the answers

What mechanism primarily propels feces into the rectum?

Mass movement. Signup and view all the answers

What role do haustrations play in colonic function?

They facilitate mixing and absorption. Signup and view all the answers

What initiates peristalsis during the intrinsic reflex of defaecation?

Rectal distension Signup and view all the answers

What is the primary role of the extrinsic reflex in defaecation?

To stimulate stretch receptors Signup and view all the answers

Which of the following is true about the control mechanisms during defaecation?

Voluntary relaxation of the external anal sphincter is necessary. Signup and view all the answers

How does cholera toxin affect the intestines?

Causes increased secretion of salt and water Signup and view all the answers

What is the total fluid volume ingested and secreted daily in the digestive system?

10 liters Signup and view all the answers

Where is chloride secretion primarily conducted in the digestive tract?

In enterocytes of both small intestine and colon Signup and view all the answers

What role do stretch receptors play during defaecation?

They convey the sense of rectal fullness to the brain. Signup and view all the answers

What is the function of the crypt of Lieberkühn?

Secretion of digestive enzymes Signup and view all the answers

What role does the Na+/K+ pump play in ion absorption?

It generates a negative charge that aids Cl- absorption. Signup and view all the answers

What effect do bacterial toxins have on the colon's absorptive mechanisms?

They do not affect absorption through sodium/sugar co-transporters. Signup and view all the answers

What is the effect of HCO3- secretion in the distal colon?

It results in net HCO3- secretion. Signup and view all the answers

How can diarrhea potentially lead to hypokalaemia?

By stimulating K+ secretion due to negative luminal charge. Signup and view all the answers

What stimulates the secretion of salt and water in the context of bacterial toxin presence?

Cystic fibrosis transmembrane regulator activity. Signup and view all the answers

Which ion has a luminal concentration limited to 25 mMoles/Litre due to potential difference across the colon wall?

K+ Signup and view all the answers

What is a consequence of HCO3- accumulation in the colon?

Secretion of K+ due to a change in potential. Signup and view all the answers

Which transport mechanism is NOT present in the distal colon?

Na+/H+ exchanger Signup and view all the answers

Study Notes

Colon Motility

Segmentation contractions occur in the colon, called haustrations, moving the contents back and forth for absorption.
Peristalsis in the colon is slow and weak, unlike the small intestine.
Mass movements propel feces towards the rectum, stimulated by hormonal and neurological reflexes.
Mass movements have the following features:
- occur 1-3 times each day
- last approximately 15 minutes
- triggered by gastrocolic and duodenocolic reflexes
- slow down after meals, allowing for absorption
- lead to a loss in haustration
Colon transit time takes approximately 18-24 hours, much longer compared to the small intestine.

Defaecation Reflex

Comprises intrinsic and extrinsic reflex mechanisms.
The intrinsic reflex:
- stimulated by rectal distension
- causes peristalsis and relaxation of the internal anal sphincter
- controlled by myenteric plexuses
- independent of the extrinsic nerves and spinal cord
The extrinsic reflex:
- activated by stretch receptors in the rectum, sending signals via the sacral parasympathetic nerves, which stimulates peristalsis and relaxes the internal anal sphincter
- the sympathetic nervous system has the opposite effect
Conscious control of defecation:
- regulated by the pudendal nerves innervating the external anal sphincter
- involves voluntary relaxation of the external sphincter and a Valsalva maneuver.
- this control is underdeveloped in infants and those with spinal cord injuries, nerve damage, and intellectual disabilities

Large Intestine Secretions

Responsible for the secretion of fluids and electrolytes.
The key secretory cells are the enterocytes.
The colon is a site of bicarb secretion.
The chloride secretion by enterocytes plays a key role:
- facilitated by the cystic fibrosis transmembrane regulator (CFTR)
- driven by the influx of sodium ions through channels between enterocytes.
Cholera toxin:
- acts on the CFTR
- causes excessive secretion of NaCl and water, resulting in significant diarrhea.
The ileum plays a key role in salt absorption:
- utilizes co-transporters with sugars and amino acids, coupled with the sodium/potassium pump to move sodium into cells

Fluid Volume in the Digestive System

Total ingested fluid: 2 liters daily
Other secretions:
- saliva: 1 liter
- gastric juice: 3 liters
- bile: 1 liter
- pancreatic juice: 1 liter
- intestinal juice: 2 liters
Overall digestive fluids: 10 liters per day
Small intestine absorption: 8.5 liters
Large intestine absorption: 1.4 liters
Feces excretion: 0.1 liters

HCO3- Absorption in the Ileum

Indirect absorption, similar to the kidney.
For each HCO3- destroyed in the lumen, a new HCO3- is formed in the cell and enters the blood
Cl- absorption is driven by the positive charge of the intercellular space due to the Na+/K+ pump.
H2O absorption is facilitated by the accumulation of HCO3-, Na+, and Cl-

Sodium Absorption in the Colon

Sodium and Chloride absorption are facilitated by Na+/H+ and HCO3-/Cl- exchangers.
These exchangers are inhibited by bacterial toxins.
The distal colon lacks the Na+/H+ exchanger, leading to net HCO3- secretion.
HCO3- secretion can reach a luminal concentration of 45 mMoles/Litre.
K+ secretion, due to the negative charge in the lumen created by the Na+/K+ pump, can reach 25 mMoles/Litre.

Bacterial Toxins & Intestinal Function

Bacterial toxins:
- stimulate chloride and water secretions by activating the CFTR in the small intestine and colon
- inhibit salt and water absorption by interfering with Na+/H+ and HCO3-/Cl- exchangers in the colon
This imbalance leads to diarrhea.
The toxins have no impact on absorption mediated by co-transporters in the ileum
Therefore, oral rehydration solutions containing sodium and glucose can be effective in managing diarrhea.

Diarrhoea, Hypokalemia & Metabolic Acidosis

Severe diarrhea leads to hypokalemia: excess potassium is lost in the stool
Diarrhea can also cause metabolic acidosis due to:
- loss of bicarbonate in the stool
- loss of buffering capacity in the gut
- potential shift in the electrolyte imbalance leading to a state of metabolic acidosis

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Description

Explore the complex processes of colon motility and the defaecation reflex in this quiz. Learn about segmentation contractions, peristalsis, and the factors influencing mass movements in the colon. Test your knowledge on the intrinsic and extrinsic reflex mechanisms involved in defaecation.