10-4. Neuronal circuit for VOR adaptation

Vestibular mossy fiber input. The flocculus has long been known to be that part of the vestibulocerebellum that receives the projection of primary vestibular afferents in the form of mossy fibers (Brodal, 1972). This notion, however, was reexamined subsequently in various species. In rabbits, the presence of primary vestibular afferents to the flocculus was negated (Gerrits et al., 1989). Later, however, it was shown to engage 2% of the primary vestibular afferents, whereas 64%–89% of these afferents were shown to project to the nodulus and uvula (Barmack et al., 1993). In cats, the primary vestibular projection to the flocculus was described as being substantial but less pronounced than that to the nodulus and uvula (Korte and Mugnaini, 1979). In monkeys, the same projection to the flocculus was measured to be 6% (Nagao et al., 1997a), and in rats the value was 10% (Osanai et al., 2000). This relative paucity of primary vestibular afferents to the flocculus, however, could be supplemented by secondary projections from vestibular nuclei (Barmack, 2003). Head velocity-sensitive neurons have indeed been found in vestibular nuclei (Scudder and Fuchs, 1992). Some mossy fiber terminals in the monkey flocculus and ventral paraflocculus were shown to respond primarily to vestibular stimuli (Lisberger and Fuchs, 1978; Noda and Warabi, 1987; Markert et al., 1988). These observations support the idea that primary and/or secondary mossy fiber inputs convey vestibular signals to the flocculus.