Computer-assisted stereological and quantitative morphological approaches were used to analyse cerebellar glomeruli of the “simple type” in serial ultrathin sections. It was found that, of the total volume (110–200 μm3) of the glomeruli studied, 53% was occupied by granule cell dendrites, 34% by mossy terminal and 13% by Golgi axons. None of the four analysed glomeruli contained Golgi cell dendrites. The mossy terminals that were studied received, on the average, 53 granule cell dendrites. All of the dendrites originated from different granule cells and all made synaptic contacts with mossy terminal. However only about 60% of granule cell dendrites made synapses with Golgi axons. The surface of the mossy terminals occupied by synaptic junctions, was found to be 5.4–5.5%. Each granule cell dendrite emitted 3–5 terminal protrusions (“dendritic digits”). Each digit receives one or more synaptic contact from either the mossy terminal (67% of all digits), or from Golgi axon varicosities (25%). Only about 8% of all digits were contacted synaptically by both types of axonal terminals. All of the dendritic digits that were observed made synaptic connections. Each digit was, on the average, connected by symmetric attachment plaques to 4 neighbouring digits. Three-dimensional reconstructions of mossy terminal and some of contacting granule cell dendrites demonstrated that the dendrites curved around the central mossy terminal and were much longer than expected from earlier Golgi-impregnation studies. In addition to mossy terminals and Golgi axons, an axon terminal of small calibre that contained large, empty, spheroid vesicles were occasionally observed. These terminals, which are most likely the axonal varicosities of ascending parallel fibers, made synaptic contacts exclusively with granule cell dendrites at the periphery of the glomeruli.
The results demonstrate that, in the rat cerebellum, there is a high degree of convergence of granule cells at a glomernius (53 to 1); and that there is a rich inhibitory input to about 60% of all granule cell dendrites. It is also shown that the main postsynaptic targets, for both mossy and Golgi axons, are the dendritic digits. The presence of synaptic contacts between parallel-fiber-like varicosities and granule cell dendrites may be an additional source of excitation within the glomerulus.