Differences in synaptic size in the superficial and deep layers of the molecular layer of the cerebellar cortex of the cat. An electronmicroscopic and autoradiographic study.

In a previous study observations in semithin sections of E-PTA-stained cerebellar cortex of the cat revealed differences in size of synaptic grids between the molecular and granular layer (Van der Want et al. 1984). In addition, synaptic size differences were observed between superficial and deep levels in the molecular layer. The present study was an attempt to analyze synapses in ultrathin sections of the cerebellar cortex with special emphasis on size differences of distinct types of synapses at different levels in the molecular layer. Climbing fibers were identified by means of anterograde transport of 3H-leucine injected in the inferior olive and parallel fibers were identified on account of fine structural criteria. Synaptic profiles were measured semi-automatically in the neuropil of the cerebellar cortex at the supra-Purkinje level and the subpial level. Measurements of the trace- and chordlength were obtained from random sections. The frequency distribution of the true diameters of the synapses was reconstructed with a discrete "unfolding"-procedure. The overall diameter at the superficial level was 390.2 +/- 1.5 nm, at the deep level 406.6 +/- 1.5 nm. Climbing fibers exhibited mean values of 431.9 +/- 4.7 and 461.3 +/- 4.1 nm at these levels and parallel fiber terminals mean values of 370.7 +/- 2.9 and 395.8 +/- 3.0 nm. The frequency distributions showed remarkable and statistically significant differences compared with the overall distributions observed at the superficial and the deep levels respectively. The frequency distributions of synaptic diameters at the superficial and deep levels also differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)