Responses of neurons in the rostral ventrolateral medulla of the cat to natural vestibular stimulation.

To investigate the neural substrate of vestibulo-sympathetic reflexes, we studied the responses of neurons in the rostral ventrolateral medulla (RVLM) of decerebrate cats to natural stimulation of the labyrinth in vertical and horizontal planes. The RVLM is a major source of excitatory inputs to sympathetic preganglionic neurons. The animals used in these studies were baroreceptor-denervated and vagotomized and had a cervical spinal transection so that inputs from tilt-sensitive receptors outside of the labyrinth did not influence the units we recorded. Of the 38 neurons whose type of vertical vestibular inputs could be classified, the majority (27) received signals mainly from otolith organs. Only 4 of the neurons received inputs predominantly from vertical semicircular canals, and 7 were classified as having convergent inputs from otoliths and canals that were spatially aligned (2 cells) or misaligned (5 cells). In addition, only 2 of 68 neurons tested responded to sinusoidal horizontal rotations in a manner typical of brainstem neurons that receive inputs from the horizontal semicircular canals. Thus, the vestibular inputs to the RVLM appear to come mainly from otolith receptors. In labyrinthectomized cats, we were unable to locate neurons with responses to tilt similar to those of cells recorded in labyrinth-intact cats, confirming that the responses attributed to vertical vestibular inputs were produced by signals from the labyrinth. In animals whose semicircular canals had been rendered dysfunctional by plugging, we only recorded responses similar to those of neurons classified as having mainly otolith inputs in canal-intact animals, indicating that the dynamic behavior of these cells does not depend upon canal inputs. The presence of otolith inputs to the RVLM is consistent with the hypothesis that this region mediates vestibulo-sympathetic reflexes involved in correcting posturally-related changes in blood pressure.