A medical student is studying a patient who experiences nausea, cramping, and delayed gastric emptying during a period of intense psychological stress. What is the most likely physiological mechanism?
AParasympathetic (vagal) overdrive causing excessive gut contractions
BSympathetic activation suppressing GI motility by inhibiting the enteric nervous system and constricting splanchnic blood flow
CCCK release from duodenal stress receptors slowing gastric motility
DFailure of the migrating motor complex to initiate during waking hours
Sympathetic activation ('fight or flight') actively suppresses gut function — it inhibits motility, reduces secretion, and constricts splanchnic blood vessels to redirect resources to skeletal muscle and the brain. This is the opposite of the parasympathetic 'rest and digest' mode. Stress-induced GI symptoms are a direct consequence of the autonomic balance shifting toward sympathetic dominance. The vagus (parasympathetic) promotes gut function; stress suppresses it.
Question 2 Multiple Choice
When does gastric acid secretion begin relative to food arriving in the stomach?
AOnly after food reaches the stomach and distends its walls
BBefore food reaches the stomach — the sight, smell, or thought of food triggers vagal reflexes that start acid secretion
COnly when protein fragments contact parietal cells directly
DAfter the intestinal phase begins, triggered by CCK from the duodenum
This is the cephalic phase: anticipatory vagal reflexes stimulate gastric acid and enzyme secretion before food arrives. The brain prepares the stomach for the incoming meal. Parietal cells do not wait for food to physically arrive. Once food does arrive, the gastric phase (distension + protein → gastrin) amplifies what the cephalic phase started. Understanding the three-phase sequence is key: cephalic (anticipatory) → gastric (food in stomach) → intestinal (chyme in duodenum).
Question 3 True / False
The enteric nervous system can coordinate basic peristalsis even if all connections to the brain and spinal cord are severed.
TTrue
FFalse
Answer: True
The ENS — sometimes called the 'gut brain' — is a semi-autonomous network of 200–600 million neurons embedded in the gut wall. It contains sensory neurons, interneurons, and motor neurons capable of independently coordinating peristalsis in response to luminal distension. This functional independence is demonstrated by transplanted intestinal segments retaining peristaltic function despite no extrinsic innervation. The brain and autonomic nervous system modulate the ENS but are not required for basic motility.
Question 4 True / False
Peristalsis and segmentation both move food aborally (toward the anus) through the GI tract; they differ in whether they are driven by circular or longitudinal muscle.
TTrue
FFalse
Answer: False
Peristalsis is propulsive — it moves food forward (aborally) through a coordinated wave of contraction behind the bolus and relaxation ahead. Segmentation, by contrast, is rhythmic non-propagating contraction that chops and mixes luminal contents *without net forward movement*. Segmentation maximizes contact between nutrients and the absorptive mucosa. The distinction is not which muscle layer is used but whether the contractions propagate (peristalsis) or occur in place without net displacement (segmentation).
Question 5 Short Answer
Explain the functional difference between peristalsis and segmentation, and why both are necessary for effective digestion and absorption.
Think about your answer, then reveal below.
Model answer: Peristalsis is a propagating wave — circular muscle contracts behind food and relaxes ahead, generating net forward movement along the gut. Segmentation is non-propagating rhythmic contraction that mixes luminal contents with digestive enzymes and exposes nutrients to the absorptive surface without propelling them forward. Both are needed: peristalsis moves the meal from stomach to colon in a reasonable time, while segmentation ensures thorough mixing and maximizes the surface area contact necessary for absorption.
Without peristalsis, food would stagnate. Without segmentation, nutrients would pass through in a poorly mixed bolus with limited contact with the mucosa, resulting in poor absorption. The gut alternates between these modes depending on the local content and neural signals. The migrating motor complex adds a third mode — a powerful sweeping contraction between meals that clears residual debris, demonstrating that the gut's motility program is context-sensitive, not just a constant forward push.