Differential patterns of spinal sympathetic outflow involving a 10-Hz rhythm

Time and frequency domain analyses were used to examine the changes in the relationships between the discharges of the inferior cardiac (CN) and verte bral (VN) postganglionic sympathetic nerves produced by electrical activati on of the midbrain periaqueductal gray (PAG) in urethan-anesthetized, baror eceptor-denervated cats. CN-VN coherence and phase angle in the 10-Hz band served as measures of the coupling of the central oscillators,rs controllin g these nerves. The 10-Hz rhythm in CN and VN discharges was entrained 1:1 to electrical stimuli applied to-the PAG at frequencies between 7 and 12 Hz . CN 10-Hz discharges were increased, and VN 10-Hz discharges were decrease d when the frequency of PAG stimulation was equal to or above that of the f ree-running rhythm. In contrast, stimulation of the same PAG sites at lower frequencies increased, albeit disproportionately, the 10-Hz discharges of both nerves. In either case, PAG stimulation significantly increased the ph ase angle between the two signals (VN 10-Hz activity lagged CN activity); c oherence values relating their discharges were little affected. However, th e increase in phase angle was significantly more pronounced when the 10-Hz discharges of the two nerves were reciprocally affected. Importantly, parti alization of the phase:spectrum using the PAG stimuli did not reverse the c hange in CN-VN phase angle. This observation suggests that the increase in the CN-VN phase angle reflected changes in the phase relations between coup led oscillators in the brain stem rather than the difference in conduction times to the two nerves from the site of PAG stimulation. In contrast to th e effects elicited by PAG stimulation, stimulation of the medullary lateral tegmental field induced uniform increases in the IO-Hz discharges of the t wo nerves and no change in the CN-VN phase angle. Our results demonstrate t hat changes in the phase relations among coupled brain stem 10-Hz oscillato rs are accompanied by differential patterns of spinal sympathetic outflow. The reciprocal changes in CN and VN discharges produced by PAG stimulation are consistent with the pattern of spinal sympathetic outflow expected duri ng the defense reaction.