Cerebellar influences on descending spinal motor systems

Abstract

The cerebellar nuclei, and to some extent the vestibular nuclei, mediate the ultimate result of cerebellar processing to the rest of the brain. Cerebellar output is directed to the diencephalon and to a score of brainstem regions. This chapter reviews the cerebellar nuclear projections to the brainstem areas that give rise to descending connections that can influence motor programming at spinal cord level, i.e., the reticulospinal, vestibulospinal, rubrospinal, tectospinal, and interstitiospinal pathways. In addition, cerebellar projections to other areas will be briefly considered. Although cerebellar output is structured by the modular internal organization of cerebellar circuitry and related olivocerebellar connections, it is concluded that the modular output, that is, output of individual cerebellar nuclei or parts thereof, still reaches many areas in the brainstem and diencephalon. In addition, multiple modules may converge their outputs indirectly to the same muscles. This suggests that multiple modules may each take part in different aspects of control of the same muscle or muscle group. Conversely, individual modules, due to the distributed nature of their outputs, may simultaneously affect several descending motor systems with the same ensuing goal. More detailed anatomical and physiological studies will be necessary to explore these possibilities.