PROJECTIONS OF INDIVIDUAL PURKINJE-CELLS OF IDENTIFIED ZONES IN THE FLOCCULUS TO THE VESTIBULAR AND CEREBELLAR NUCLEI IN THE RABBIT

The rabbit flocculus can be divided into five zones (zones 1, 2, 3, 4, and C2) with the use of acetylcholinesterase histochemistry. The proj ections of individual Purkinje cells in these zones to the vestibular and cerebellar nuclei were studied by using biocytin as an anterograde tracer. The zones were physiologically identified in terms of the Pur kinje cell complex spike modulation occurring in response to optokinet ic stimulation. In zones 1 and 3 neurons respond best to rotation abou t a horizontal axis that is close to perpendicular to the ipsilateral anterior semicircular canal, whereas in zones 2 and 4 neurons respond best to rotation about the vertical axis. Complex spike activity in zo ne C2 is unresponsive to optokinetic stimulation. Collectively, Purkin je cells of zone 1 projected to the ventral dentate nucleus, dorsal gr oup y, and superior vestibular nucleus; Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus; Purkinje cells of zone 3 projected to dorsal grou p y, ventral group y, and the superior vestibular nucleus; and Purkinj e cells of zone C2 projected to the interposed posterior nucleus and d orsal group y. Some of the labeled Purkinje cell axons branched and in nervated two nuclei. Branching axons from zone 1 either innervated bot h the ventral dentate nucleus and the superior vestibular nucleus or b oth dorsal group y and the superior vestibular nucleus. Branching axon s from zones 2 and 4 innervated both the magnocellular and the parvice llular parts of the medial vestibular nucleus. Branching axons from zo ne 3 innervated both dorsal group y and the superior vestibular nucleu s, or both ventral group y and the superior vestibular nucleus. Branch ing axons from zone C2 innervated both the interposed posterior nucleu s and dorsal group y. Some of the target nuclei of the floccular Purki nje cell axons (e.g., dorsal group y and interposed posterior nucleus) project to the part of the inferior olive that, in turn, projects to the corresponding floccular zone, thus completing a closed pathway con sisting of the inferior olive, the cerebellar cortex, and the cerebell ar and vestibular nuclei. Other target nuclei (e.g., superior vestibul ar nucleus and medial vestibular nucleus) do not project back to the o livary subnuclei that innervate the flocculus and are part of an open olivofloccular pathway. An individual Purkinje cell thus can innervate a nucleus in the closed pathway as well as a nucleus in the open path way. To summarize, the present study illustrates that floccular Purkin je cells located in discrete zones are part of different pathways invo lved in the regulation of specific components of compensatory eye move ments. (C) 1994 Wiley-Liss, Inc.