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BOX 31.2 DIFFERENCES BETWEEN THE VESTIBULAR AND NONVESTIBULAR PORTIONS OF THE CEREBELLUM Phylogenetically, the vestibular and fastigial (medial) cerebellar nuclei predate the interpositus and dentate. Perhaps as a result, the vestibular and fastigial cerebellar circuits exhibit some distinctive properties compared to their relatively younger neighbors: 1. Unipolar brush cells are present in highest densities in the granule cell layer of the flocculonodular lobe. These excitatory neurons receive mossy fiber input, like granule cells, but synapse locally onto granule cells. Golgi cells feed back to unipolar brush cells with mixed glycinergic and GABAergic synapses, in contrast to the purely GABAergic feedback to granule cells (Dugue, Dumoulin, Triller, & Dieudonne, 2005). The role of these neurons in the vestibular circuit is unclear. 2. Olivary neurons in the dorsal cap of Kooy, to which both the vestibular and fastigial nuclei project, have different intrinsic electrical properties than those in the principal and accessory olive. Neurons in the dorsal cap exhibit an approximately linear relationship between injected current and resulting firing rate, tonically firing at up to 35 Hz during a current step. In contrast, neurons in the rest of the olive exhibit significant subthreshold oscillations, usually only fire a couple of spikes at a maximum of 10 Hz in response to depolarization, and are relatively insensitive to depolarizing current injection (Urbano, Simpson, & Llinas, 2006). 3. In a similar vein, the GABAergic feedback from the vestibular cerebellum to the dorsal cap of Kooy is phasic and time-locked to presynaptic firing. The comparable feedback from nonvestibular cerebellum to the principal and accessory olive is slow, aphasic, and appears to be mediated by asynchronous release of GABA (Best & Regehr, 2009). 4. The vestibular and fastigial nuclei contain two major types of premotor projection neurons: glutamatergic neurons that project to contralateral targets, and glycinergic neurons that project to ipsilateral targets. Premotor projection neurons in the interpositus and dentate are exclusively contralateral-projecting and glutamatergic (although the dentate nucleus contains in addition some nucleocortical glycinergic neurons of unknown role) (Bagnall et al., 2009; Shin et al., 2011; Uusisaari & Knopfel, 2010). 5. Some projection neurons in the vestibular nuclei synapse directly onto motor neurons, whereas projection neurons in the nonvestibular cerebellum influence behavior less directly via synapses onto thalamus, pons, medulla, and colliculi. As a result of these differences, it is not known whether computations carried out by one portion of the cerebellum are necessarily the same as those carried out by the other. The generally homologous structures of the vestibular and nonvestibular parts of the cerebellum may mask significant underlying dissimilarities in the nature of their information processing. This distinction may be related to the need for high-speed processing in the vestibular system, or perhaps to the relatively tight coupling between vestibular signaling and motor output. Martha Bagnall and Michael Mauk