9-4. The Granule Cell-Golgi Cell Loop

Golgi cells have characteristically large divergence and convergence numbers. Each Golgi cell receives ~4,788 excitatory inputs to its dendrites in the molecular layer from parallel fibers and also ~228 mossy fiber terminals onto its descending dendrites (Pelionisz and Szentágothai, 1973). Each Golgi cell, in turn, extends a broadly branching axon to up to ~5,700 granule cells (cat; Palkovits et al., 1971). These mutual connections form a loop involving granule cells and Golgi cells. Marr (1969) emphasized the action of Golgi cells in regulating codon size. More recent researchers, however, have considered the contribution of Golgi cells to be more important for the generation of signals with temporal profiles, which are essential for determining the timing of movements. Unlike earlier models based on conduction delays in parallel fibers, arrays of elements with different time constants, or populations of elements oscillating at different frequencies, recent models assume that temporal coding emerges from the dynamics of those cerebellar circuits that include Golgi cells.