Morphology of parallel fibres in the cerebellar cortex of the rat: an experimental light and electron microscopic study with biocytin.

Microinjections of biocytin have been made in the granular layer of the rat cerebellar cortex in order to label the axonal projections of a localised population of granule cells. Light microscopic techniques were used to determine the lengths of the parallel fibres and to measure the spacing and size of the fibre varicosities. Fibres were longest in the superficial one-third of the molecular layer, where mean overall length was 4.7 mm, and mean length decreased to 4.2 mm in the lower one-third of the molecular layer. We found no very short fibres but a small population deep in the molecular layer had a branch length of about one-half the average. Mean intervaricosity interval and varicosity size varied with distance from proximal to distal along the fibres. Mean intervaricosity interval was 3.7 microns within 250 microns of the fibre bifurcation points and progressively increased towards the distal ends, where the mean interval was 7.4 microns. Mean varicosity size was 0.82 microns 2 in this proximal region and decreased to 0.47 microns 2 about 1.2 mm distally. Mean intervaricosity interval on the ascending axons of the granule cells was 4.0 microns. Electron microscopy revealed that a high proportion (89%) of the parallel fibre varicosities formed synaptic junctions. The majority of the synapses (91%) were formed on Purkinje cell dendritic spines. Some varicosities also formed simultaneous synaptic contacts or double synapses with two spines. These double synapses occurred more frequently in the proximal region of the fibres (11%) than on the distal ends (2%). The length of the postsynaptic density also differed according to the location of the varicosities and the mean length at the proximal parallel fibre synapses was 0.59 microns compared with 0.38 microns at the distal synapses. It is concluded that a beam or bundle of parallel fibres originating from cells in a focal region of the granular layer will exert a graded synaptic influence on its target Purkinje cells, with the most powerful influence occurring on cells located around the proximal region of the fibres where they bifurcate and the weakest action being exerted on cells located at the distal end of the fibres.