Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Abstract

Research on animals as well as humans has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body. Elimination of vestibular inputs attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. However, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers (VCF) and a decrease in hindlimb vascular resistance, but an excitation of most upper body VCF accompanied by an increase in upper body vascular resistance. This study tested the hypothesis that neurons in the RVLM whose projections extended to lower thoracic spinal levels would respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminated in the rostral thoracic cord. The data failed to support our hypothesis: the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.