Interrelations of basket cell axons and climbing fibers in the cerebellar cortex of the rat

Summary An analytical study was undertaken with both electron microscopy and the rapid Golgi method in order to clarify the interrelations of climbing fibers, basket cell axons, and Purkinje cell dendrites. The two fibers are readily distinguished in electron micrographs by means of their differing content of microtubules and neurofilaments, the packing density of synaptic vesicles, and the disposition of their synaptic junctions on the Purkinje cell dendrite. Climbing fibers are generally thin and contain many microtubules. They give off attenuated collaterals, whose rounded varicosities are densely packed with vesicles and which form en passant synapses with clusters of thorns projecting from the major Purkinje dendrites. In contrast, basket axons are relatively thick and contain many neurofilaments. By means of slight dilatations containing loosely aggregated vesicles, the axon and its collaterals form numerous synapses en passant with the smooth dendritic shafts and the perikaryon of the Purkinje cell. Climbing fibers and basket cell axons run along parallel with each other but without forming axo-axonic synapses as they ascend over the surface of the Purkinje dendrites. Both fibers form especially elaborate intertwined festoons at the branching points of the major dendrites. The kinds of synapses found are described in detail, and the functional implications are discussed.The hypothesis is developed that the dendritic thorn is a device for isolating the subsynaptic membrane from electrical events in the rest of the dendrite at the cost of reducing the effectiveness of the synapse. This principle is incorporated in the Purkinje dendrite—parallel fiber synapses, in which an individual fiber can be expected to have little importance. The disadvantage of using thorns as postsynaptic surfaces can be mitigated by clustering them and increasing the number of thorns contacted by each presynaptic terminal. This method is utilized at the junctions between the climbing fiber and the Purkinje dendrite to produce one of the most powerful excitatory synapses known. It is furthermore suggested that the elaborate plexus of climbing fibers and basket cell axons synapsing in the crotches of branching dendrites is strategically located to control the flow of information in the Purkinje cell dendritic tree.Supported by U.S. Public Health Service Research Grant NS03659 and Training Grant NS05591 from the National Institute of Neurological Diseases and Stroke.Postdoctoral trainee in Anatomy under Training Grant GM906 from the National Institute of General Medical Sciences.