Baroreceptor inputs to the nucleus tractus solitarius in the cat: postsynaptic actions and the influence of respiration.

1. The postsynaptic action of carotid sinus nerve (SN), aortic nerve (AN), superior laryngeal nerve (SLN) and vagal nerve (VN) stimulation has been studied on neurones in the nucleus of the tractus solitarius (NTS) in vivo. 2. Three distinct patterns of postsynaptic responses were evoked by SN stimulation, an EPSP, an EPSP-IPSP sequence and an IPSP, observed separately in individual neurones. This diversity of response was represented in cells proven to receive baroreceptor input by inflation of a balloon-tipped catheter within the ipsilateral carotid sinus. 3. Virtually none of the neurones identified as baroreceptive exhibited pulse-related discharge. 4. A variety of influences to AN, SLN and VN stimulation were observed in neurones receiving baroreceptor afferent information. This wide convergence of input implies that this region of the brain stem is important in the integration of cardiovascular reflexes. 5. The hypothesis was tested that respiratory 'gating' of the baroreceptor reflex is produced by synaptic actions within the NTS. There was an absence of any modification of PSPs by lung inflation and by variations in the timing of the stimulation of the afferent nerves within the respiratory cycle. These observations indicate that respiratory modifications of the baroreceptor reflex must occur at later stages in the reflex pathway.