The gastrointestinal (GI) tract displays complex motor patterns when nutrients are present in the lumen; even when excised from an organism and completely isolated in physiological saline.

This is demonstrated in an experiment with an isolated intestine similar to the image below.

If the image doesn't show, it should not affect how you answer the question. Imagine a piece of isolate gut as you have seen in the lecture recordings.

The following observations were made from this experiment:

An isolated intestine is capable of reflex contractile behavior.
Perfusing saline through the lumen of the isolated intestine produced no reflex motility.
Perfusing the lumen with nutrient content is able to evoke reflex contractions.
The observed contractile behavior is blocked by application of tetrodotoxin (TTX) to the organ bath. TTX blocks voltage-dependent sodium channels.

Based on the observations above, which TWO of the following statements are CORRECT?

SELECT ALL THE CORRECT OPTIONS.

1 mark will be awarded for each correct answer. Incorrect answers will results in deduction of -1 mark (you will not score lower than zero).

Choose wisely.

Question

The gastrointestinal (GI) tract displays complex motor patterns when nutrients are present in the lumen; even when excised from an organism and completely isolated in physiological saline.

This is demonstrated in an experiment with an isolated intestine similar to the image below.

If the image doesn't show, it should not affect how you answer the question. Imagine a piece of isolate gut as you have seen in the lecture recordings.

The following observations were made from this experiment:

An isolated intestine is capable of reflex contractile behavior.   
Perfusing saline through the lumen of the isolated intestine produced no reflex motility. 
Perfusing the lumen with nutrient content is able to evoke reflex contractions. 
The observed contractile behavior is blocked by application of tetrodotoxin (TTX) to the organ bath. TTX blocks voltage-dependent sodium channels.

Based on the observations above, which TWO of the following statements are CORRECT?

SELECT ALL THE CORRECT OPTIONS.

1 mark will be awarded for each correct answer. Incorrect answers will results in deduction of -1 mark (you will not score lower than zero).

Choose wisely.

BlackTom AI · Accepted Answer

Tetrodotoxin blocks voltage-gated sodium channels, so the fact that TTX abolishes the nutrient-evoked contractions shows the response is mediated by action-potential–dependent neurons rather than direct smooth muscle activation.

1) Relevant knowledge points and concepts
- Enteric nervous system (ENS) contains intrinsic sensory (afferent) neurons and excitatory motor neurons that control circular muscle contraction (via acetylcholine, tachykinins).
- Intrinsic primary afferent neurons (IPANs) or mucosal sensory mechanisms detect luminal chemicals (or via enteroendocrine/EC cell release of mediators like 5-HT) and initiate local reflexes.
- TTX blocks neuronal Na+ channels and therefore action potentials; smooth muscle contractility depends mainly on Ca2+ channels and would not be abolished by TTX if directly activated.
- An isolated intestine in physiological saline is disconnected from the CNS (vagus) and sympathetic ganglia.

2) Step-by-step reasoning
- Observation: nutrient perfusion in lumen evokes contractions, saline does not. This implies a chemical stimulus in the lumen is required to trigger the response.
- Observation: isolated gut (no extrinsic nerves) still responds → circuit is intrinsic to the intestinal wall (ENS).
- Observation: TTX in the bath blocks contractions → response requires neuronal action potentials (TTX-sensitive), so effect is neurally mediated, not direct smooth muscle activation.

3) Calculations
- None required.

4) Why the two correct answers are correct
- “There are sensory neurons within the intestinal wall that can detect nutrients within the lumen”: supported because a luminal nutrient signal produces an enteric reflex while saline does not, and this occurs in the isolated gut, indicating mucosal/intrinsic sensory detection.
- “There are excitatory motor neurons supplying the circular muscle within the ENS”: supported because the reflex causes contractions of circular muscle and is TTX-sensitive, indicating excitatory enteric motor neurons are mediating contraction.

5) Why other options are incorrect
- Direct activation or inhibition of smooth muscle by nutrients is unlikely because TTX abolished responses (if direct, TTX would not block).
- CNS/vagal or sympathetic ganglia involvement is impossible in the completely isolated preparation.
- Secretomotor activation of chloride movement is a separate epithelial secretory pathway not demonstrated by these motility observations and is not required to explain TTX-sensitive contractions.

6) Conclusion
The evidence supports the two ENS-neuronal statements. Final answer confirmed: “There are excitatory motor neurons supplying the circular muscle within the ENS” and “There are sensory neurons within the intestinal wall that can detect nutrients within the lumen.”

类似问题

Why might a common pharmaceutical treatment for depression cause dysregulation of the gastrointestinal tract? 为什么一种常见的抗抑郁药物会导致胃肠道功能紊乱？

Why might a common pharmaceutical treatment for depression cause dysregulation of the gastrointestinal tract?

The enteric nervous system is sometimes called:

In the gastrointestinal tract, between the longitudinal and circular muscle layers, what cells would you expect to find?

The submucous plexus and the myenteric plexus are components of the Blank ______.

Imagine you are digesting a meal. Which division of your nervous system would be the most activated?

Which of the following most accurately characterises the enteric nervous system (ENS) and its interaction with the rest of the nervous system?

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