Effects of spinal section and of positive-feedback excitatory reflex on sympathetic and heart rate variability

The sympathetic outflow appears to be capable of displaying a rhythmicity synchronous with cardiovascular Mayer's waves even after spinal-section. To test the hypothesis that spinal sympathetic low frequency (LF) oscillation can be enhanced during sympathetic excitation, we recorded cardiac sympathetic nerve activity (SNA), R-R interval, arterial pressure, and ventilation in 9 unanesthetized decerebrate-vagotomized cats before and after C1 spinal section. LF and high frequency (HF) components were detected in the variability of SNA, R-R interval, and systolic arterial pressure both before and after spinal section. In this latter condition, a significant coherence between LFSNA and LFR-R was present in 5 animals, whereas HFSNA and HFR-R were correlated in 4 animals. During an excitatory sympathetic spinal reflex elicited by aortic constriction, the efferent sympathetic firing was markedly enhanced (from 7 +/- 2 to 33 +/- 7 spikes/s); concomitantly, the powers of both LFSNA and HFSNA were also increased. Coherence between LFSNA and LFR-R became significant in all cases, whereas HFSNA and HFR-R became correlated in 6 animals. In 3 animals, the reflex sympathetic excitation was no longer elicitable after interrupting a vast contingent of sympathetic afferents by means of thoracic dorsal root section, We report for the first time that: LF and HF oscillations are detectable in SNA, R-R interval, and systolic arterial pressure variabilities of decerebrate-vagotomized spinal cats and that an excitatory spinal reflex is capable of increasing the power of both SNA spectral components.