[Interdigestive motility and secretion of the gastrointestinal tract]

In the fasting state, the digestive tract is not inactive but displays periodic motor and secretory activity. Each gastrointestinal motility cycle can be divided into specific phases of activity: a period of inactivity (phase I), followed by a longer period of irregular activity (phase II) and by a brief, conclusive period (phase III) of maximal, rhythmic contractions, the migrating motor complex (MMC), which migrates from the gastroduodenal region to the distal small bowel within one motility cycle. A short, inconstant transition segment between phases III and I is termed phase IV. Lower esophageal, gallbladder and sphincter of Oddi motilities are also linked to this functional cycle. In contrast, colonic motility follows independent patterns. Closely correlated to upper gastrointestinal motility are secretory activities: Output rates of gastric, pancreatic and bile secretions increase and decrease periodically, in concert with upper gastrointestinal motor activities. The physiologic role of periodic interdigestive activity is not fully understood. It is probably important for regular mechanical and enzymatic cleansing ot the gastrointestinal lumen and may serve to remove indigestible and/or foreign material, to prevent bacterial overgrowth or pathologic activation of pancreatic enzymes, and to revert duodeno-gastric and ceco-ileal refluxes. Control of the interdigestive cycle and mechanisms of motor/secretory coupling have been only partly uncovered; however, recent findings suggest that interactions between vagal and intrinsic cholinergic pathways and gastrointestinal hormones may play pivotal regulatory roles.