Stimulation of otolith irregular fibers produces a rostro-caudal gradient in activity in the vestibular nuclear complex (VNC), but not the vestibulocerebellum (VeCb)

The vestibular system is important for posture, balance, motor control, and spatial orientation. Each of the vestibular end organs have specialized neuroepithelia with both regular and irregular afferents. In otolith organs, the utricle and saccule, afferents most responsive to linear jerk (jerk - derivative of acceleration) are located in the striola and project centrally to the vestibular nuclear complex (VNC) as well as the uvula and nodulus of the vestibulocerebellum (VeCb). The pattern of central neuronal activation attributed to otolith irregular afferents is relatively unknown. To address this gap, c-Fos was used as a marker of neuronal activity to map the distribution of active neurons throughout the rostro-caudal extent of the VNC and VeCb. Immunohistochemistry for c-Fos was performed to assess activation of VNC and VeCb neurons in response to a linear jerk stimulus delivered in the naso-occipital plane. Activated neurons were distributed throughout the VNC, including the lateral vestibular nucleus (LVe), magnocellular medial vestibular nucleus (MVeMC), parvocellular medial vestibular nucleus (MVePC), spinal vestibular nucleus (SpVe), and superior vestibular nucleus (SuVe). Notably, after stimulation, the MVePC exhibited the greatest number of c-Fos labeled nuclei. Significant increases in c-Fos labeling were found in mid-rostrocaudal and caudal regions of the VNC in the LVe, MVe, and SpVe. Additionally, c-Fos labeling was observed across all regions of the VeCb after jerk stimulation. Significant increases in the number of labeled nuclei were found throughout the rostro-caudal extent of the nodulus and uvula. However, jerk stimulated increases in activity for the paraflocculus were restricted to the caudal VeCb. The distribution of neuronal activity suggests that regions receiving the greatest direct otolith input exhibit the most substantial changes in response to otolith derived, irregular fiber stimulation.