Recent studies on intestinal vagal afferent innervation. Functional implications.

Data obtained during recent years have completely changed our understanding of the organization and function of visceral sensitivity. The digestive tract--and especially the small intestine--provides a very good example of this evolution. Classically, it was believed that the sensory innervation of the gut is supplied by both the vagus and the splanchnic nerves. The vagus nerves play the major role in the sensory innervation of the first part of the intestine (all layers of the bowel, except mesentery). These vagal sensory fibers are mostly nonmedullated and generally originate from free endings. Microelectrophysiological techniques have disclosed the existence of a great variety of receptors: mechanoreceptors, chemoreceptors, thermoreceptors and osmoreceptors in the intestinal area. Some receptors like glucoreceptors, are specific receptors since they do not respond to any stimuli other than glucose. Others, like osmosensitive receptors, behave as non-specific or multimodal receptors. The mechanism by which an identical structure (free ending) induces various sorts of signals (mechanical, thermal, chemical etc.) is not yet known, but several hypotheses have been proposed. The discovery of a varied and complex mass of information obtained from studies of the gut finally corroborates the behavioural and clinical data which suggest that the intestinal sensory innervation plays an important role in physiological regulation. Now it is possible to distinguish 3 main kinds of mechanisms involved according to whether they concern digestive motility, homeostasis or alimentary behaviour.