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Aortic depressor nerve unmyelinated fibers in spontaneously hypertensive rats.

The objective of the present study was to compare the morphology of the unmyelinated fibers in the aortic depressor nerves (ADN) of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). In anesthetized rats, the ADN was identified by its spontaneous activity synchronous with the arterial pulses. Thin sections of the proximal and distal segments of the ADN were analyzed by electron microscopy, and a morphometric study of the unmyelinated fibers and Schwann cells was performed. The proximal segments of WKY and SHR ADN contain an average of 335 +/- 68 and 130 +/- 14 unmyelinated fibers, respectively (P < 0.05), and the distal segments contain an average of 337 +/- 46 and 242 +/- 77 unmyelinated fibers, respectively (P < 0.05). The distribution of the diameters of unmyelinated fibers was unimodal for both strains, with the histogram from the SHR significantly shifted to the left. Because the unmyelinated fibers play a role in the tonic inhibition of the medullary vasomotor centers, especially in the presence of hypertension, the morphological differences observed in the ADN from SHR may account, at least in part, for the blunted baroreflex of SHR.