An analysis of HRP-filled basket cell axons in the cat's cerebellum. II. Axonal distribution.

The axons of basket cells course horizontally within the lower molecular layer of the cerebellar cortex and give rise to three types of axon collaterals. One collateral distributes around the somata and axon hillocks of Purkinje cells, forming a complex pericellular basket-like formation or "pinceau". The other two axonal specializations have the form of beaded tendrils, but differ in their origin and laminar distribution. In the previous paper, the morphometry and configuration of these axonal branches were described. In the present account, their synaptic relationships are analyzed based on serial section analysis and computer reconstructions. Our data indicate that: (1) Purkinje cells receive descending collaterals from more than one basket cell; (2) the contribution of the descending collaterals to individual pinceau is not uniform; Purkinje cells located close to the basket cell of origin receive a greater number of descending collaterals when compared to Purkinje cells located more distally; (3) few synaptic junctions are formed by the descending collaterals within the pinceaux; (4) beaded tendrils make synaptic contacts with the somata and dendrites of Purkinje cells, and (5) the horizontal axon forms synaptic junctions with the dendritic shafts and spiny branchlets of Purkinje cells. Functional considerations of these synaptic relationships are discussed in the light of the classic concept of off-beam inhibition mediated by basket cells.