The localization of the first synapse in the carotid sinus baroreceptor reflex pathway and its alteration of the afferent input

Summary In 78 anaesthetized cats the lower brain stem was explored with microelectrodes for localization of the secondary neurones of the carotid sinus primary afferents. By means of recording single unit activities and evoked potentials (E.P.) by stimulation of the carotid sinus nerve (CSN) these neurones were located exclusively in the nucleus tractus solitarius (NTS) at the level of the obex.The E. P. was recorded with both ipsilateral and contralateral CSN stimulation.During repetitive stimulation of the CSN the frequency transmission across the synapse showed a sharp frequency limitation. With 10 Imp/sec the amplitude of the E.P. was only 50% of the maximal amplitude obtained with 1 Imp/sec. This frequency limitation is also manifested in the frequency effectiveness of the whole baroreceptor reflex measured as blood pressure reduction. It showed a maximum with frequencies of 20–30 Imp/sec, while stimulation of the secondary neurones in the NTS showed an increase in blood pressure reduction up to the tested frequencies of 120 Imp/sec.The E.P. was tested in the different phases of the respiratory cycle. No difference in amplitude or latency between the E.P. was noted.The relevance of these findings to the arrangement of the central neural substrates for the cardiovascular control is discussed.This work was supported by the Deutsche Forschungsgemeinschaft.A summary of these studies was presented at the 34. Tagung der Deutschen Physiologischen Gesellschaft [46].