Questions: Gastrointestinal Tract Anatomy and Motility

Question 1

If the vagus nerve (which carries parasympathetic signals to the gut) is completely severed, which of the following best describes the effect on GI motility?

• A. Peristalsis stops completely because the central nervous system can no longer coordinate muscle contractions
• B. Only segmentation is preserved; peristalsis requires vagal input and ceases entirely
• C. Peristalsis and most motility continues, because the enteric nervous system governs gut motility intrinsically
• D. The stomach continues functioning but small intestinal motility is permanently lost

Answer: (C)

The enteric nervous system — centered on Auerbach's myenteric plexus — contains approximately 100 million neurons and can coordinate peristalsis entirely independently of the central nervous system. The gut is called the 'second brain' precisely because it does not require vagal or spinal input to maintain its intrinsic motor programs. Vagotomy may alter motility patterns (reducing gastric acid secretion, for example), but it does not abolish peristalsis. The misconception that the CNS controls all gut motility overlooks the enteric nervous system's substantial autonomy.

Question 2

In the small intestine during active digestion, ring contractions form and relax repeatedly at fixed locations without producing net movement of chyme toward the colon. What is the primary function of this pattern?

• A. It propels chyme rapidly toward the colon to maximize throughput during peak digestion
• B. It mixes chyme with digestive enzymes and maximizes contact between nutrients and the absorptive mucosa
• C. It is a preparatory phase that builds pressure before peristaltic waves begin
• D. It generates the pressure needed to force chyme through the ileocecal valve

Answer: (B)

This pattern is segmentation — the dominant motility mode of the small intestine during digestion. Segmentation contractions chop and stir chyme in place, mixing it with pancreatic enzymes and bile and bringing digested nutrients into close contact with the brush-border absorptive surface. Segmentation is optimized for absorption efficiency, not transport speed. Net aboral movement comes later via peristalsis. Chyme spends 3–5 hours in the small intestine, with segmentation ensuring thorough mixing throughout that time.

Question 3

The enteric nervous system can coordinate peristalsis without any input from the brain or spinal cord.

• T. True
• F. False

Answer: True

The myenteric (Auerbach's) plexus runs the entire length of the gut and governs the ascending excitatory reflex (ACh and substance P contracting the circular muscle behind a bolus) and the descending inhibitory reflex (VIP and nitric oxide relaxing muscle ahead of it). This oral-to-anal polarity — the peristaltic reflex — is maintained entirely within the enteric nervous system, which is why gut motility persists after spinal cord injury or vagotomy.

Question 4

Peristalsis is a uniform wave of contraction that squeezes the GI tract with equal force along its entire length, moving contents by sheer pressure from behind.

• T. True
• F. False

Answer: False

Peristalsis is a coordinated reflex, not a uniform squeeze. The muscle segment behind a bolus contracts (driven by ACh and substance P from ascending interneurons in the myenteric plexus) while the segment ahead simultaneously relaxes (via VIP and nitric oxide from descending interneurons). This coupled contraction-relaxation creates a pressure differential that propels the bolus forward. Without the relaxation ahead, the bolus would face resistance rather than an open pathway — the coordination is essential to efficient propulsion.

Question 5

Explain why the stomach has a third (oblique) muscle layer that the esophagus and small intestine lack, and what unique mechanical function this enables.

Model answer: The oblique muscle layer, combined with the circular and longitudinal layers, allows the stomach to perform grinding retropulsion — a churning motion that breaks food into fine chyme (particles smaller than ~2 mm) suitable for passage through the pyloric sphincter. The esophagus only needs to transport boluses (two layers suffice), and the small intestine's job is absorption via segmentation and gentle peristalsis (also achievable with two layers).

The stomach's mechanical task is qualitatively different from any other GI segment: it must physically break down solid food particles while mixing them with gastric acid and pepsin. The three-layer musculature generates the complex retropulsive churning that accomplishes this. The pyloric sphincter then acts as a size filter — releasing only particles smaller than ~2 mm into the duodenum — which explains why the stomach must reduce particle size before transport can proceed.