The Innervation of the Gut of the Brown Trout (Salmo trutta)

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Abstract
1. The extrinsic and intrinsic innervation of the trout gut has heen examined by using methylene blue, osmium tetroxide, and silver staining techniques. 2. Left and right vagus nerves penetrate the oesophageal wall and run between the longitudinal and circular muscle-coats of the stomach, but do not reach the intestine. Branches of these nerves anastomose with each other and become incorporated into Auerbach'a plexus. Pericellular endings of vagal fibres are found about enteric neurones in the stomach, while some fibres end directly on the striated muscle-cells of the oesophagus. 3. A single, fine, anterior splanchnic nerve anastomoses along the major branches of the coeliaco-metenteric artery to supply both the stomach and intestine. Moat of the splanchnic fibres pass directly into the smooth muscle-coats, but some fibres run into the aubserous plexus. 4. Two posterior autonomic nerves provide separate innervation of the rectum. 5. Three main types of nerve-cells have been distinguished in the gut by their distribution as well as structure. (a) Small uni-, bi-, and multi-polar cells (10-15 µ in diameter) form the main body of Auerbach's plexus, lying between the muscle-coats. Throughout moat of the gut they are not arranged in true ganglia, since only from 1 to 3 neuronea occur at the nodes and many cells lie singly along the internodal connectives. (b) A smaller number of large multipolar neuronea (30-60 µ in diameter) lie half-embedded in the longitudinal muscle along the whole length of the gut, their principal axons passing down to lie between the muscle-coats in Auerbach's plexus. (c) A third cell type consisting mainly of large mono- or bi-polar cells (30-70 µ in diameter) contributes to Auerbach's plexus in the stomach only and is associated with the endings of vagus nerve-fibres. 6. Bundles of nerve-fibres pass down through the circular muscle-coat from Auerbach's plexus to join a loosely arranged plexus of nerve-fibres, the submucous plexus; but unlike Meissner's plexus of mammals, which occupies a similar region, no nerve-cells are present. 7. Nerve-endings, some of which may be sensory, are located at the bases of the mucous epithelial cells and are continuous with fibres forming a prominent subepithelial plexus from which fibres pass to both submucoue and Auerbach's plexusea. 8. A subaeroua plexus and ‘interstitial cell network’ are also present in the fish gut. 9. A quantitative study of nerve-cell densities in different regions of the trout gut revealed density peaks of 93 cells per square mm and 170 cells per square mm in the posterior cardiac stomach and anterior duodenum respectively. The ratio of small large neurone types in Auerbach's plexus is about 20 to 1 in all regions. The total number of nerve-cells in the whole gut was calculated to be about half a million.

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