The normal and aberrant development of synaptic structures between parallel fibers and Purkinje cell dendritic spines.

Under normal circumstances the dendritic spines of the cerebellar Purkinje cell differentiate in association with the parallel fibers formed by the descending granule cells. In the adult these elements form the most frequent synapse of the cerebellar cortex. In certain conditions, however, the granule cells are destroyed before they form the parallel fibers. Nevertheless, unattached dendritic spines are found which are complete with submembranous densities. Their cytochemical reactions and their morphology in both thin section and after freeze fracture are indistinguishable from normal spines except for the fact that they are unattached to any presynaptic elements. Examples of the formation of unattached presynaptic endings have also been observed. We conclude, therefore, that at least in some instances, pre- and postsynaptic terminals may form without benefit of the direct one-to-one influence of their synaptic mates.