Homeodynamics versus homeostasis: periodicities superimposed on non-linear dynamic sympathetic tone generated in ventral medulla

Homeodynamics based on theories of complexity and chaos and its impact on mechanisms generating sympathetic activity are presented. Activity in rats cervical, lumbar and renal sympathetic nerves was analyzed. In time domain glutamate stimulation of neurons within medullary periambigual area (PAA) disturbed temporal pattern of respiratory-sympathetic synchronization. Divalént calcium antagonists, Co2+ and Mg2+, blockers of synaptic transmission, uncoupled respiratory oscillator and sympathetic activity. PAA neurons act as an interphase between different subsets of respiratory neurons and bulbospinal sympathoexcitatory neurons in rostral ventrolateral medulla (RVLM). In frequency domain sympathetic activity analyzed by FFT algorithm and power density spectra (PDS) exhibited periodicities at the range from 0.4 Hz to 7.5 Hz. Blockers of synaptic transmission microinjected bilaterally into RVLM reduced total power exhibited in PDS to low level of magnitude generated in spinal cord and increased total, yet non-synchronized sympathetic activity and arterial blood pressure. A two component hybrid model of generation of sympathetic activity was proposed: a tone-generating system confined mainly to intrinsic activity of RVLM pacemaker neurons responsible for chaos-like discharges and a second component-neuronal circuits superimposed on tone-generating neurons and shaping the pattern of PDS. Contribution of spinal cord oscillatory mechanism to overall power of sympathetic periodicities was discussed.