Expiratory bulbospinal neurons of dogs. II. Laterality of responses to spatial and temporal pulmonary vagal inputs

Abstract

Pulmonary mechanoreceptors with vagal fibers produce a combination of excitation and inhibition in the majority of the expiratory bulbospinal (EBS) neurons of dogs. Both aspects of this transpulmonary pressure-dependent neuronal response appear to be slowly adapting and activated at low pressure levels, suggesting the involvement of the slowly adapting pulmonary stretch receptors (PSRs). The purpose of the present study was to determine the contribution of different afferent pathways to each of the response components and to characterize the spatial and temporal processing of ipsi-, contra-, and bilateral vagal afferent inputs by two types of EBS neurons. For this purpose low-intensity electrical stimulation of the intact, desheathed, vagus nerves was used in thiopental sodium-anesthetized paralyzed dogs. The phrenic neurogram was used to synchronize both ventilation and stimulation. During test respiratory cycles, pulse trains (4-5 s duration) were applied during the neural expiratory phase to each and both vagus nerves. The mean discharge frequency (Fn) during the stimulus period was obtained from cycle-triggered histogram data. Plots of Fn vs. stimulus strength and Fn vs. stimulus frequency suggest that inhibition of both type D and type A EBS neurons is mediated mainly by the ipsilateral vagus nerve, and that the excitation of type D neurons is mediated bilaterally. These conclusions are also supported by inflation responses obtained before and after unilateral vagotomies. Differences in latencies and spatial and temporal summation characteristics suggest the possible involvement of different 1) types of PSRs, 2) central pathways, and/or 3) synaptic mechanisms in the biphasic response of the caudal ventral EBS neurons to lung inflation.