Studies on the origin and generation of sympathetic nerve activity

This paper summarizes the efforts of our laboratory to define the substrates and mechanisms for generation of one of the primary components in sympathetic nerve discharge (SND) of the cat, the 2- to 6-Hz rhythm. Using spike-triggered averaging, we have identified single brain stem neurons with naturally occurring activity correlated to the 2- to 6-Hz rhythm in SND. Such neurons were found in three regions--the rostral ventrolateral medulla (RVLM), medullary raphe (R) and medullary lateral tegmental field (LTF). RVLM neurons were inhibited in parallel to SND when carotid sinus pressure was raised while R neurons were excited. These observations suggest that RVLM neurons exert sympathoexcitatory (SE) actions while R neurons mediate sympathoinhibitory (SI) effects. Both SE and SI neurons were found in the LTF. Antidromic mapping revealed that the axons of RVLM-SE and R-SI neurons innervate the thoracic spinal intermediolateral nucleus. In contrast, LTF neurons do not project to the spinal cord. Rather, the axons of LTF-SE neurons project to the RVLM while those of LTF-SI neurons project to R. We hypothesize that the aforementioned cell groups comprise a network oscillator responsible for the 2- to 6-Hz rhythm in SND.