COUPLED OSCILLATORS ACCOUNT FOR THE SLOW RHYTHMS IN SYMPATHETIC-NERVEDISCHARGE AND PHRENIC-NERVE ACTIVITY

Phase-locked slow rhythms in sympathetic nerve discharge (SND) and phr enic nerve activity (PNA) are generally thought to arise from a common brain stem ''cardiorespiratory'' oscillator. The results obtained in vagotomized and baroreceptor-denervated cats anesthetized with pentoba rbital sodium do not support this view. First, partial coherence analy sis revealed that the discharges of pairs of sympathetic nerves remain ed correlated at the frequency of the central respiratory cycle after mathematical removal of the portion of these signals common to PNA. Th e residual coherence suggests that the slow rhythm in SND is dependent on central mechanisms in addition to those responsible for rhythmic P NA. Second, the rhythms in SND and PNA became coupled in a 2:1 relatio nship during either moderate systemic hypocapnia or hypercapnia. Third , the slow rhythm in SND was maintained when rhythmic PNA was eliminat ed during extreme hypocapnia. Fourth, during extreme hypercapnia, cohe rence of the rhythms in SND and PNA was drastically reduced. These res ults suggest that the slow rhythms in SND and PNA arise from separate oscillators that are normally coupled.