Questions: Gastrointestinal Motility and Sphincter Coordination

Question 1

A patient has achalasia caused by selective degeneration of inhibitory motor neurons at the lower esophageal sphincter. What is the expected physiological consequence?

• A. The LES becomes hypotonic and fails to close properly, allowing gastric acid to reflux into the esophagus
• B. The LES cannot relax during swallowing because the unopposed excitatory drive keeps it tonically contracted, blocking food passage
• C. The LES loses its tonic contraction because inhibitory neurons are needed to maintain basal sphincter tone
• D. Peristalsis in the esophageal body ceases because the LES and esophageal body share the same innervation

Answer: (B)

Sphincters maintain tonic CLOSURE through ongoing excitatory drive and open by releasing inhibition — specifically through inhibitory motor neurons releasing nitric oxide and VIP. When inhibitory neurons degenerate, the LES loses the ability to relax on command. The excitatory drive (acetylcholine) continues, keeping the sphincter shut. When a patient swallows, the normal inhibitory relaxation signal is absent, the LES stays closed, and food cannot pass. This is achalasia: a failure of relaxation, not a failure of closure. Option A describes GERD (LES insufficiency), which is the opposite problem.

Question 2

A person's stomach makes loud gurgling sounds between meals, even though they are not hungry. What physiological mechanism is responsible?

• A. Acid secretion in the empty stomach produces gas that creates sounds as it moves
• B. The migrating motor complex — a wave of coordinated contractions sweeping the GI tract during fasting — produces the sounds
• C. Segmentation contractions in the small intestine continue between meals to maintain intestinal tone
• D. The ileocecal valve periodically opens and closes, creating turbulence that generates audible sounds

Answer: (B)

The migrating motor complex (MMC) is the fasting motility program — a wave of contractions sweeping from stomach to terminal ileum roughly every 90–120 minutes, driven by the hormone motilin. Its function is housekeeping: clearing residual content, cells, and bacteria. The stomach gurgling between meals is the sound of MMC contractions in the gastric antrum and small intestine. When you eat, motilin is suppressed and the MMC stops. This is why you stop hearing those sounds after a meal, even though the gut is doing more mechanical work during digestion.

Question 3

GI sphincters, like the rest of the gut smooth muscle, are normally relaxed at rest and contract when stimulated to prevent inappropriate passage of content.

• T. True
• F. False

Answer: False

This is backwards. Sphincters maintain TONIC CLOSURE at rest — they are normally contracted. They RELAX to allow passage in response to appropriate neural signals. This is the opposite of ordinary GI smooth muscle, which is relaxed between contractions and contracts to propel content. The LES stays closed by tonic excitatory tone and opens by inhibitory neurons releasing nitric oxide and VIP. This design makes functional sense: a sphincter's job is to prevent passage by default and open only when signaled — not to close in response to activation.

Question 4

The enteric nervous system can coordinate peristalsis and sphincter function without any input from the brain or spinal cord, making it functionally semi-autonomous.

• T. True
• F. False

Answer: True

The ENS contains roughly 500 million neurons in two layers (myenteric and submucosal plexuses) and can orchestrate the full range of GI motility — peristalsis, segmentation, the MMC, and sphincter regulation — entirely on its own. Experiments on isolated gut segments deprived of vagal and sympathetic input demonstrate this autonomy. The vagus nerve and sympathetic fibers modulate the ENS (increasing or decreasing activity, affecting secretion) but do not drive the fundamental motor programs. This is why GI motility largely continues in spinal cord injury patients and why the gut is sometimes called 'the second brain.'

Question 5

Why does Hirschsprung's disease cause a functional bowel obstruction in the affected segment, and which specific neural element is absent?

Model answer: Hirschsprung's disease involves congenital absence of ganglionic cells (enteric neurons) in a segment of colon, most often the rectosigmoid region. Without local enteric neurons, there are no inhibitory motor neurons to release nitric oxide and VIP — the signals that allow the smooth muscle to relax. The affected segment therefore remains in sustained tonic contraction. Content cannot pass through a segment that cannot relax. The aganglionic segment acts as a rigid, non-propulsive obstruction, causing the proximal colon to dilate massively as content accumulates.

This pathology illustrates the general principle of GI motor control: the absence of inhibition is as pathological as the absence of excitation. Normal propulsion requires coordinated relaxation ahead of the bolus (receptive relaxation) and contraction behind it — the 'law of the intestine.' Without inhibitory neurons, the segment ahead cannot relax to accommodate the approaching content. Hirschsprung's treatment is surgical resection of the aganglionic segment, allowing normal ganglionated bowel to reach the anus.