The fiber composition of the abdominal vagus
Abstract
The abdominal branches of the vagus nerve are involved in vital digestive and metabolic reflexes, and in ingestive behavior, yet little is known about their structure in any species. The aim of the present study is to provide a light and electron microscopic inventory of the fibers found in the two trunks and five primary branches of the abdominal vagus of the rat. To obtain an overview of the branching patterns and the organization of intraneural bundles, fifteen nerve whole mounts were prepared. For analyzing fiber content, electron microscopic samples of both trunks and all primary branches were obtained from each of six rats. From each sample a complete cross-sectional electron microscopic (EM) montage (X10,000) was prepared. All fibers were counted, and more than 10% of them were morphometrically evaluated with an image analyzer. The mean sizes of unmyelinated axons in each of the branches were similar (0.75-0.83 $\mu$m). But the shapes of the size distributions, as summarized by the coefficient of skewness, revealed significant differences between the bilateral gastric branches and the two celiac branches; the hepatic branch size distribution differed from all others. Most of the myelinated fibers (85%) in vagal branches were less than 2.6 $\mu$m in diameter, and had myelin sheath widths between 0.1 and 0.5 $\mu$m. The gastric branches, however, consistently contained a few larger myelinated fibers with sheath widths as great as 0.85 $\mu$m. The whole mounts showed that a large and regularly positioned paraganglion was associated with the dorsal vagal branches. Whole mounts also revealed fiber bundles within the vagal branches which were not of supradiaphragmatic origin; these adventitial bundles were traced along the perineurium between the adjacent vagal branches. The sum of the fibers in the combined branches was 21% more than the number of fibers counted in the parent trunks; this excess probably reflects the adventitial fiber content. The results demonstrate a cytological diversity in the abdominal vagus, as well as a specificity among the branches. The verification of subdiaphragmatic adventitial bundles within the primary branches is particularly relevant for the design and interpretation of experiments.
Degree
Ph.D.
Advisors
Powley, Purdue University.
Subject Area
Neurology
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