An analysis of HRP-filled basket cell axons in the cat's cerebellum. I. Morphometry and configuration.

Basket cell axons terminate directly on the Purkinje cell body in the mammalian cerebellum and thus are in a position to have a direct influence on this neuron. In the present study, individual basket cells were intracellularly filled with horseradish peroxidase and the morphological characteristics and distribution of their axons were analyzed and quantified. The horizontally directed axon of basket cells preferentially distributes toward one side of the cell body, and is usually directed toward the base of the folium. In their course, the axons give rise to three types of axonal specializations including descending collaterals that form the complex endings around the axon hillocks and somata of Purkinje cells (i.e. the pinceau), beaded tendrils that arise from the descending collaterals and distribute to the Purkinje cell and upper granule cell layer, and beaded tendrils that arise directly from the horizontal axon to enter the molecular layer. In some respects, basket cells comprise a heterogeneous population of neurons on the basis of the length of their axon, as well as the number, distribution, and morphological characteristics of the axonal collaterals. The major finding of this study is that a single basket cell does not form a pinceau around the axon hillock of every Purkinje cell located along the course of its horizontal axon. Further, the pinceaux that are formed vary in their complexity. These findings suggest that the classic concept of off-beam inhibition mediated by basket cells is more complex than previously described.