Medullary relay neurons in the carotid-body chemoreceptor pathway of cats

Central terminations of the carotid body chemoreceptor afferents were localized by recording field potentials and unit activity evoked by electrical stimulation of the carotid sinus nerve, and unit activity during chemical stimulation of the chemoreceptors. Areas where evoked responses with short latency could be recorded and which contained neurons that responded to carotid body excitation were located in two regions of the medulla, about the level of the obex: a dorsal region which included the nucleus tractus solitarius and the reticular formation just below this nucleus; and a ventrolateral region which included the nucleus ambiguus and the ventrolateral reticular formation around that nucleus. The evoked field responses in the two regions were similar. In these two regions, the only neurons which increased their firing both to NaCN and a decreased PIO2 had a respiratory, bursting activity which was phase locked with phrenic nerve firing. The authors' findings suggest that there is a direct synaptic input from carotid body chemoreceptor afferents onto medullary respiratory neurons.