Shoichiro Nosaka

Central vagal organization in rats: An electrophysiological study

Abstract The organization of the vagal nuclei was studied electrophysiologically in chloralose-anesthetized rats by analyzing the field potentials and unitary responses evoked in the nuclei by stimulation of the cervical vagus nerve. The rostral part of the nucleus commissuralis yielded only a long-latency response to stimulation of this nerve, suggesting that this region receives projections solely of nonmyelinated afferent fibers. In the nucleus tractus solitarius the stimulation elicited both short-latency and long-latency responses, indicating converging projections of myelinated and nonmyelinated afferents. A long-latency response was recorded diffusely within n. commissuralis and n. tractus solitarius of the contralateral side, whereas a short-latency response was restricted to a midline area, the caudal n. commissuralis, and the most medial part of n. tractus solitarius adjacent to it. These observations also suggest a difference in projections of myelinated and nonmyelinated afferents. Two types of motor neurons were identified in the dorsal vagal nucleus by antidromic activation: one with B-fiber axons and the other with C-fiber axons. C-Fiber motor neurons were characterized by the large positivity of the spike and the presence of an inflection in the rising phase of the spike, presumably between the initial segment and somatodendritic components. The latter component was readily blocked by repetitive stimulation. In the nucleus ambiguus, stimulation of the vagus nerve produced the earliest antidromic response of A-fiber motor neurons accompanied by multiple orthodromic responses of short and long latencies. Electrolytic lesions of the dorsomedial medulla oblongata abolished all potentials in n. ambiguus except the antidromic one, indicating that all the orthodromic responses were generated via the vagal sensory nuclei sinuated dorsomedially.

Vagus cardioinhibitory fibers in rats.

In chloralose-urethane anesthetized rats, compound spike potentials provoked in the cervical vagus nerve with electrical stimulation of the central cut end were found to consist of three major groups, A, delta-B and C. A remarkable cardioinhibition was observed on repetitive stimulation of the vagus nerve with an intensity which gave rise to the delta-B spike potential group. However, when the stimulus intensity was increased further beyond the level where the delta-B potential group had reached the maximum, the potency of cardioinhibition continued to be reinforced with the development of the C potential group. Selective activation of C fibers by anodal block of conductions along A and delta-B fibers was still associated with a considerable degree of cardioinhibition. These findings indicate that activities of both C fibers and delta-B fibers contribute to vagal cardioinhibition in rats and the view that the vagal cardioinhibition is mediated by B fibers, although valid in cats, can not be held applicable to all species of mammalians.