Defenselike patterns of spinal sympathetic outflow involving the 10-Hz andcardiac-related rhythms

Frequency- and time-domain analyses were used to compare the effects of sti mulation of the defense region of the midbrain periaqueductal gray (PAG) on the 10-Hz and cardiac-related discharges of sympathetic nerves with differ ent cardiovascular targets. In baroreceptor-denervated cats anesthetized wi th 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 susta ined decrease in vertebral sympathetic nerve (VN) 10-Hz activity but increa sed the 10-Hz discharges of the inferior cardiac (CN) and renal (RN) nerves . In baroreceptor-innervated cats, VN cardiac-related activity was initiall y unchanged by high-frequency (25-Hz) PAG stimulation, or it increased alon g 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 ac tivity usually was reduced below control level. At this time, the increases in CN and RN cardiac-related discharges were largely sustained. The cardia c-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 1 0-Hz rhythm is different in mechanism and character from that involving the cardiac-related rhythm.