Role of ventrolateral medulla in reflex cardiovascular responses to activation of skin and muscle nerves

Central neuronal circuits mediating reflex cardiovascular responses to skin and muscle nerve stimulation were studied in rats under urethan anesthesia. Responses of right rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM) cardiovascular neurons to stimulation of contralateral skin and muscle afferent fibers were investigated. Stimulation of the tibial (muscle) nerve excited 19 (86%) of 22 CVLM neurons and inhibited 18 (82%) of 22 RVLM neurons. Stimulation of the sural (skin) nerve excited 20 (91%) of the 22 RVLM neurons but did not affect the firing rate of any of the 22 CVLM neurons. Electrolytic lesions of the CVLM abolished the depressor responses induced by stimulation of the tibial nerve without affecting the pressor response caused by sural nerve stimulation. Similarly, reversible blockade of the CVLM by microinjection of γ-aminobutyric acid or CoCl₂ abolished the depressor response to stimulation of the tibial nerve without affecting the pressor response induced by sural nerve stimulation. These results suggest that vasodepressor responses to muscle nerve activation are mediated by a neuronal inhibitory pathway to the RVLM relayed through the CVLM.