Defenselike patterns of spinal sympathetic outflow involving the 10-Hz and cardiac-related rhythms.

Frequency- and time-domain analyses were used to compare the effects of stimulation of the defense region of the midbrain periaqueductal gray (PAG) on the 10-Hz and cardiac-related discharges of sympathetic nerves with different cardiovascular targets. In baroreceptor-denervated cats anesthetized with urethan, PAG stimulation at frequencies equal to or higher (up to 25 Hz) than that of the free-running 10-Hz rhythm produced an immediate and sustained decrease in vertebral sympathetic nerve (VN) 10-Hz activity but increased the 10-Hz discharges of the inferior cardiac (CN) and renal (RN) nerves. In baroreceptor-innervated cats, VN cardiac-related activity was initially unchanged by high-frequency (25-Hz) PAG stimulation, or it increased along with that in the CN and RN. Later, during high-frequency PAG stimulation, when the rise in blood pressure approached its peak, VN cardiac-related activity usually was reduced below control level. At this time, the increases in CN and RN cardiac-related discharges were largely sustained. The cardiac-related discharges of the three nerves were unaffected by PAG stimulation at frequencies just below or just above that of the heartbeat. We conclude that the defenselike pattern of spinal sympathetic outflow involving the 10-Hz rhythm is different in mechanism and character from that involving the cardiac-related rhythm.