BISPECTRAL ANALYSIS OF COMPLEX PATTERNS OF SYMPATHETIC-NERVE DISCHARGE

Bispectral analysis was used to demonstrate quadratic nonlinear coupli ng (i.e., phase locking) of different frequency components in inferior cardiac sympathetic nerve discharge (SND) of urethan-anesthetized cat s. The complex patterns of SND analyzed included mixtures of 1) the ca rdiac-related and 10-Hz rhythms, 2) the 10-Hz rhythm and irregular 2- to 6-Hz oscillations, and 3) the 10-Hz rhythm and a lower frequency no n-cardiac-related rhythm near 4 Hz. In some cases, the bicoherence fun ction (normalized bispectrum) showed no phase locking of these frequen cy components. Cases of nil bicoherence are equated with linear superp osition of frequency components, which implies the existence of multip le and noninteractive central circuits. Increased complexity of SND wa s observed in other cases, as evidenced by significant phase locking o f different frequency components with or without frequency locking. Fr equency locking (higher frequency rhythm is a multiple of lower) was c onfirmed by constructing Lissajous orbital plots showing covariation o f voltages in selectively filtered bands of SND. We equate frequency l ocking with nonlinear coupling of the central generators of different sympathetic nerve rhythms and phase locking without frequency locking possibly with nonlinearities arising at levels below noncoupled centra l rhythm generators.