Chapter 27. Gastrointestinal Motility
In infants, defecation often follows a meal. The cause of colonic contractions in this situation is
B. increased circulating levels of CCK.
C. the gastrocolic reflex.
D. increased circulating levels of somatostatin.
E. the enterogastric reflex.
The correct answer is C. The gastrocolic reflex readies the colon for the contents of a new meal by triggering colonic and rectal contractions. It is triggered when the stomach is distended, and is assumed to be neutrally mediated, although it may be modified by humoral agents. CCK levels increase in the circulation after a meal but the hormone is not the cause of the reflex (rules out option B) whereas histamine and somatostatin released in the course of responding to a meal are paracrines, whose concentration in the circulation changes little, if at all (rules out options A and D). Somatostatin would in any case be expected to reduce motility. Finally, the enterogastric reflex slows gastric emptying when the meal is in the small intestine, rather than targeting the colon (rules out option E).
A patient who has undergone a partial gastric resection to remove a tumor reports to his primary care physician that he has experienced several episodes of nausea, cramping, dizziness, sweating and a rapid heart rate after ingesting sugary beverages. His symptoms are caused in part by
A. increased blood pressure.
B. increased secretion of acid.
C. increased secretion of CCK.
The correct answer is D. With gastric surgery, the ability to regulate the delivery of meal constituents from the stomach to the small intestine may be significantly impaired. Especially when large amounts of sweets are ingested, the sugar is delivered rapidly to the small intestine and the osmotic forces draw in large volumes of fluid from the interstitium, resulting in bowel distension, pain and nausea. The associated hypovolemia would result in low blood pressure (rules out option A) and tachycardia. The rapid absorption of simple sugars also markedly stimulates the release of insulin and inappropriately triggers glucose sequestration leading to hypoglycemia (rules out option E). Acid and CCK secretion are not markedly triggered by carbohydrates, and acid secretion would likely be reduced in this patient in any event since the gastric mucosal area is reduced and the stomach empties rapidly (rules out options B and C).