A defence-lire reaction: An emergent property of a system of coupled non-linear oscillators

1. The present paper deals with the central mechanisms responsible for the defence-like differential pattern of spinal sympathetic outflow elicited by electrical activation of the midbrain periaqueductal grey (PAG) in urethan e-anaesthetized, baroreceptor-denervated cats. The pattern is characterized by increases in the 10 Hz discharges of the inferior cardiac (CN) and rena l (RN) sympathetic nerves and a decrease in 10 Hz activity of the vertebral sympathetic nerve (VN) that supplies vasoconstrictor outflow to the foreli mb. 2, The model used to explain this pattern is based on the self-organizing p roperties of a system of coupled brainstem oscillators rather than the acti vation of point-to-point hard-wired connections leading to increases in sym pathetic outflow to some targets and decreases to others. 3, The bet that VN 10 Hz activity was inhibited by PAG stimulus frequencies equal to or just above, but not just below that of the free-running (contr ol) rhythm argues against a 'hard-wired' model. 4, The evidence supporting the hypothesis that the defence-like pattern is an emergent property of a system of coupled oscillators includes changes in the phase lag of VN 10 Hz activity relative to that in the CN, temporal co rrelation of the changes in phase angle and 10 Hz powers and the direct rel ationship between the magnitude of the change in phase angle and the degree to which PAG stimulation reciprocally affected the 10 Hz discharges of the CN and VN, 5, It is proposed that changes in phase angle reflect the reorganization of the coupling of 10 Hz oscillators and that such changes in state lead to d ifferential patterns of spinal sympathetic outflow.