Organization and distribution of the rat subdiaphragmatic vagus and associated paraganglia

This experiment analyzed the organization of the rat abdominal vagus. To spare delicate tissues and preserve positional information, untrimmed blocks of the subdiaphragmatic viscera (N = 22) were fixed, impregnated by using a pyridine‐silver protocol, and double embedded. Each block was sectioned transversely at 7 μm, and a section every 70 μm from the diaphragm to the cardia was analyzed. The features of the section were traced and digitized for computer reconstruction. Included in the measurements were sizes and locations of bundles, fascicles, and paraganglia. The anterior and posterior vagi were consistently distinctive in size, distribution, cross‐sectional shape, and paraganglionic content. In the most common pattern (41% of animals), the anterior trunk coursed longitudinally on the ventral surface of the esophagus, giving off at successively more distal levels the hepatic branch, the accessory coeliac branch and then the bundles of the anterior gastric branch. The posterior trunk separated into a coeliac branch and a posterior gastric branch, each consisting of numerous bundles, in the most distal quarter of the esophagus. Fifty‐nine percent of all animals exhibited one or more significant variations in vagal organization (e.g., double primary trunks—41%, supernumerary branches—18%, or atypical branching sequences—9%). Four to 14 vagal paraganglia (mean = 8 ± 1; equivalent to 32/rat, corrected for sampling) were found in each animal, and no branch was consistently devoid of paraganglia. Ninety‐four percent of the paraganglia were located at nerve branch points. Some of the larger paraganglia contained at their central poles one to six neurons with soma diameters ranging from 14 to 22 μm.