The enteric nervous system: region and target specific projections and neurochemical codes.

The goal of this report is to summarise the current knowledge on the projection pathways of enteric neurones innervating the muscle and mucosa in different regions of the gut. Combination of neuronal tracing, immunohistochemical and electrophysiological methods has allowed researchers to gain insight into the enteric hardwiring of specific target tissue in the gut. A polarised innervation pattern of the circular muscle was demonstrated for the stomach fundus/corpus and the ileum with descending pathways being primarily nitrergic while ascending pathways were primarily cholinergic. This characteristic hardwiring is thought to set in part the functional basis for peristalsis. A similar polarised innervation pathway was found for the enteric innervation of the mucosa in the stomach and large intestine but not in the small intestine. In both the stomach (myenteric neurones) and in the proximal and distal colon (submucosal neurones), ascending pathways to the mucosa are primarily cholinergic while descending pathways are primarily non-cholinergic. In the colon, results suggest that activation of both pathways induces a cross potentiation of cholinergic and vasoactive intestinal polypeptidergic mediated secretion. Furthermore, a large population of myenteric neurone s projecting to the mucosa in the small and large intestine are probably intrinsic primary afferent neurones sensitive to mechanical as well as chemical stimuli.