Inhibition of brain stem neuronal activity by cardiac and pulmonary vagal afferent fibres in the cat.

Recordings have been made of the activity of 311 neurones in the nucleus tractus solitarius (NTS) and dorsal motor vagal nucleus (DMVN) which responded to stimulation of the cardiac or pulmonary vagal branches in the anaesthetized cat. Eighty-one neurones showed inhibitory effects to vagal branch or vagal trunk stimulation which were exhibited in three different ways. First, the responses of forty neurones evoked by stimulation of one vagal branch were reduced or abolished by stimulation of another vagal branch or the vagal trunk. Second, the spontaneous activity of six neurones, not excited by branch stimulation, was reduced or abolished by vagal branch stimulation, and third, thirty-five neurones showed evidence of reduced excitability to a second stimulus pulse applied to the same branch. The inhibitory effects were characterized by a long time course (mean 932 ms, range 4-10,000 ms), and the latency to onset suggests that both myelinated and non-myelinated afferents are capable of such actions. Inhibitory effects were observed on neuronal elements activated both synaptically, i.e. interneurones, and non-synaptically, i.e. efferent motoneurones or afferent fibres, although no pattern of branches mediating particular effects could be discerned. The possible physiological implications of these observations are discussed.