Two distinct mechanisms generate the respiratory modulation in fibre activity of the rat cervical sympathetic trunk.

Preganglionic multifibre activity was recorded in the cervical sympathetic trunk of vagotomized Wistar rats and analysed for its respiratory modulation. The aim of the study was to investigate whether both a central and a reflex component of respiratory modulation are observed in sympathetic activity of rats as was previously demonstrated in cats. For this purpose, sympathetic activity was summed over several hundred cycles (i) with respect to phrenic nerve discharge as an indicator for central respiration and (ii) with respect to tracheal pressure as an indicator for artificial ventilation. As a consequence of vagotomy both cycles were desynchronized. Sympathetic activity which was analysed exhibited a central respiratory modulation with a minimum during inspiration and a broad peak during expiration. Additionally, in the activity of 44/49 filaments a ventilation-related modulation was seen in parallel with the falling phase of the blood pressure waves accompanying artificial ventilation. The analysis of the latter rhythm during central apnoea and after complete sino-aortic denervation proved its reflex origin and its independence of the central respiratory modulation. We conclude that in rats respiratory modulation of sympathetic activity is caused by two distinct mechanisms, one being of central and one being of reflex, probably baroreceptor, origin.