1. The cerebellar uvula-nodulus receives vestibular projections from primary and secondary vestibular afferents as well as vestibularly related climbing fibers. It also receives visually related information from climbing fiber pathways. In this experiment we investigated how this information is mapped onto the uvula-nodulus. We studied the specificity, dynamics, and topographic distribution of climbing fiber responses (CFRs), simple spike responses, and mossy tiber terminal responses evoked by vestibular and optokinetic stimulation in rabbits anesthetized with α-chloralose. 2. Vestibularly evoked CFRs were found in the ventral uvula and nodulus. These responses were evoked during static roll tilt of the rabbit about a longitudinal axis and by sinusoidal oscillation about the longitudinal axis. Purely static responses were attributed to stimulation of the utricular otolith by the linear acceleration of gravity. CFRs that lacked a static component were attributed to activation of the semicircular canals. 3. Using a 'null technique' we showed that the canal-sensitive CFRs were caused by stimulation of the anterior or posterior semicircular canals. Of the CFRs classified as canal related, 96% could be attributed to stimulation of the vertical semicircular canals. 4. Increases in CFRs were correlated with decreases in simple spike responses in half the Purkinje cells from which we recorded. These climbing-fiber-induced pauses in simple spikes occurred during spontaneous climbing fiber discharge as well as during climbing fiber discharge evoked by vestibular stimulation. The duration of this pause was inversely proportional to the spontaneous level of simple spikes before the occurrence of a CFR. In the other half of the recorded population of Purkinje cells, vestibularly driven CFRs did not alter the simple spike responses. 5. Vestibularly and visually mediated CFRs were topographically represented on the surface of the uvula-nodulus. CFRs driven by ipsilateral otolithic inputs were distributed over the entire mediolateral surface of the uvula-nodulus. CFRs driven by the ipsilateral posterior semicircular canal were distributed in a sagittal strip ~1.5 mm wide, extending laterally from the midline of the nodulus. CFRs driven exclusively by horizontal, posterior → anterior optokinetic stimulation of the ipsilateral eye were distributed in a sagittal strip ~0.5 mm wide located 0.5-1.0 mm from the midline and restricted to the ventral nodulus. CFRs driven by the ipsilateral anterior semicircular canal were found in a sagittal strip ~1.0 mm wide extending 1.0-2.0 mm from the midline. 6. The sagittal, topographically arrayed climbing fiber strips effectively map a mediolateral gradient of possible postural responses based on vestibular and optokinetic information.
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