Introduction

The complex organization of the adult cerebellar network is a product of precisely timed and spatially coordinated developmental events (Miale and Sidman, 1961; Fujita, 1967; Larramendi, 1969; Altman, 1982; Goffinet, 1983; Altman and Bayer, 1985a,1985b). Cerebellar Purkinje cells are generated in the cerebellar primordium around embryonic day (E) 12 and migrate to the surface before birth in the mouse (Miale and Sidman, 1961). Around postnatal day (P) 3, Purkinje cells start to disperse in a monolayer and soon afterwards they receive synaptic contacts from afferent axons. The advent of the interaction with migrating granule cells accelerates a profuse synaptogenesis with Purkinje cell dendrites, which grow into the characteristic Purkinje dendritic trees by P12 (Larramendi, 1969; Altman, 1982). Neurons of the deep cerebellar nuclei are generated about a day before Purkinje cells, Golgi cells toward the end of gestation, whereas stellate and basket cells are produced during the first postnatal week, and granule cells during the first two weeks of postnatal life (Miale and Sidman, 1961; Fujita, 1967; Altman, 1982).

Normally, only Purkinje cells project axons outside the cerebellar cortex toward the deep cerebellar nuclei (Eccles et al., 1967; Ito, 1984). All of the remaining cortical neurons are interneurons, functioning to modulate Purkinje cell activity. Purkinje cells are also modulated by afferent olivocerebellar climbing fibers (see also Chapter 2). Mossy fibers indirectly affect Purkinje cell activity through the mediation of the granule cell parallel fibers, which establish synapses on Purkinje dendrites. The axons of the deep nuclei neurons transmit impulses outside the cerebellum, toward postcerebellar targets that include the ventrolateral nucleus of the thalamus, the red nucleus, and the vestibular nuclei (Thach et al., 1992).