Excitatory and inhibitory medullary nuclei projecting to spinal cardioacceleratory neurons in the cat

To identify medullary nuclei projecting to spinal sympathetic neurons cardioacceleratory sites in the right intermediolateral nucleus (ILN) in spinal segment T2 were electrically stimulated and the right medulla was explored for evoked antidromic activity. In 20 chloralosed, vagotomized cats evoked responses were observed at 264 histologically identified sites. The responses had a mean latency of 1.5 msec, followed the stimulus at 300 Hz and persisted after 90-150 sec of asphyxia and after 20 mg/kg of sodium pentobarbital iv. Stimulation of 79 sites from which evoked responses were recorded elicited cardioacceleration at 17 sites in the lateral reticular nucleus and the nucleus parvocellularis, and cardiac slowing at 27 sites in the raphe nuclei, the paramedian reticular nucleus and the nucleus medullae oblongatae centralis. These results demonstrate the function of discrete medullary nuclei projecting with either an excitatory or an inhibitory input to ILN cardioacceleratory neurons. In addition, as all these nuclei except the raphe nuclei have been implicated previously as sites of termination of cardiovascular afferents these results provide information on the functional pathways involved in the sympathetic component of cardiovascular reflexes.