VISCERAL PERCEPTION Visceral perception: sensory transduction in visceral afferents and nutrients

The possible mechanisms that may be involved in nutrient detection in the wall of the gastrointestinal tract are reviewed. There is strong functional and electrophysiological evidence that both intrinsic and extrinsic primary afferent neurones mediate mechano-and chemosensitive responses in the gastrointestinal tract. This review focuses on the extrinsic afferent pathways as these are the ones that convey information to the central nervous system which is clearly necessary for perception to occur. SUMMARY Nutrients, especially carbohydrates and lipids, in the intestinal lumen are known to inhibit gastric function. It has been postulated that enteroendo-crine cells (EC) in the intestinal mucosa respond to changes in luminal contents by releasing neu-roactive mediators, such as cholecystokinin (CCK) and serotonin (5-HT), which in turn activate specific receptors on primary afferent nerve terminals. Both vagal and spinal afferent pathways have been shown to be involved in intestinal feedback inhibition of gastric emptying precipitated by glucose. The response is mediated by 5-HT3 receptors and also by sodium-glucose cotransporter1 (SGLT-1) expressed by enterochro-maffin cells. Inhibition of gastric emptying and acid secretion induced by long chain fatty acids is thought to involve a vagal pathway, which is acti-vated by CCK release. It is thought that the central nervous system (CNS) may respond either to the particular pattern of gastrointestinal motility or to impulse frequency. Research is underway to investigate how these impulses are handled by second and third order central neurones