PROPERTIES OF SYMPATHETIC REFLEXES ELICITED BY NATURAL VESTIBULAR STIMULATION - IMPLICATIONS FOR CARDIOVASCULAR CONTROL

1 To study the properties of vestibulosympathetic reflexes we recorded outflow from the splanchnic nerve during natural vestibular stimulati on in multiple vertical planes in decerebrate cats. Most of the animal s were cerebellectomized, although some responses were recorded in cer ebellum-intact preparations. Vestibular stimulation was produced by ro tating the head in animals whose upper cervical dorsal roots were tran sected to remove inputs from neck receptors; a baroreceptor denervatio n and vagotomy were also performed to remove visceral inputs. 2. The p lane of head rotation that produced maximal modulation of splanchnic n erve activity (response vector orientation) was measured at 0.2-0.5 Hz . The dynamics of the response were then studied with sinusoidal (0.05 - to 1-Hz) stimuli aligned with this orientation. 3. Typically, maxima l modulation of splanchnic nerve outflow was elicited by head rotation s in a plane near pitch; nose-up rotations produced increased outflow and nose-down rotations reduced nerve discharges. The gains of the res ponses remained relatively constant across stimulus frequencies and th e phases were consistently near stimulus position, like regularly firi ng otolith afferents. Similar response dynamics were recorded in cereb ellectomized and cerebellum-intact animals. 4. The splanchnic nerve re sponses to head rotation could be abolished by microinjections of the excitotoxin kainic acid into the medial and inferior vestibular nuclei , which is concordant with the responses resulting from activation of vestibular receptors. 5. The properties of vestibulosympathetic reflex es recorded from the splanchnic nerve support the hypothesis that the vestibular system participates in compensating for posturally related changes in blood pressure.