The fiber composition of the abdominal vagus of the rat.

The present study provides a LM and EM inventory of the fibers of the rat abdominal vagus, including dorsal and ventral trunks and the five primary branches. Whole mounts (n = 15) were prepared to characterize the branching patterns. A set of EM samples consisting of both trunks and all branches (i.e. dorsal and ventral gastric, dorsal and accessory celiac, and hepatic) were then obtained from each of six additional animals. A complete cross-sectional montage (x 10000) was prepared from each sample. All axons were counted, and greater than 10% of them were evaluated morphometrically. The means of unmyelinated axon diameters for each of the five branches were similar (0.75-0.83 microns). However, the shapes of the fiber size distributions, as summarized by their skew coefficients, revealed that the two gastric branches differed significantly from the two celiac branches; furthermore, the hepatic size distribution differed from all others. Most of the myelinated fibers (85%) in all branches were less than 2.6 microns in diameter and had sheath widths between 0.1 and 0.5 micron. The gastric branches, however, also contained a few larger myelinated fibers with sheath widths as great as 0.85 micron. Whole mounts revealed fibers which were not of supradiaphragmatic origin within all five vagal branches; these adventitial bundles were traced along the perineurium between adjacent branches. The sum of the fibers in the five branches (26930) was 21% more than the number counted in the parent trunks (22272); this excess probably reflects the adventitial fiber content. The whole mounts also showed that a large and regularly positioned paraganglion was associated with the dorsal branches. The structural profiles observed (i.e. unmyelinated and myelinated fibers size distributions, presence of extrinsic fascicles, glomus tissue content, etc.) differentiate the vagal branches into three morphologically distinct sets: a gastric pair, a celiac pair, and a hepatic branch. The fiber counts, when considered with observations of the numbers of efferents and adventitial fibers in the nerve, suggest that the percentage of efferent fibers is much higher than in all the widely accepted estimates found in the literature: efferent fibers may represent over a quarter of the total number of fibers.