AN ANALYSIS OF HRP-FILLED BASKET CELL AXONS IN THE CATS CEREBELLUM .2. AXONAL DISTRIBUTION

The axons of basket cells course horizontally within the lower molecul ar layer of the cerebellar cortex and give rise to three types of axon collaterals. One collateral distributes around the somata and axon hi llocks of Purkinje cells, forming a complex pericellular basket-like f ormation or ''pinceau''. The other two axonal specializations have the form of beaded tendrils, but differ in their origin and laminar distr ibution. In the previous paper, the morphometry and configuration of t hese axonal branches were described. In the present account, their syn aptic relationships are analyzed based on serial section analysis and computer reconstructions. Our data indicate that: (1) Purkinje cells r eceive descending collaterals from more than one basket cell; (2) the contribution of the descending collaterals to individual pinceau is no t uniform; Purkinje cells located close to the basket cell of origin r eceive a greater number of descending collaterals when compared to Pur kinje cells located more distally; (3) few synaptic junctions are form ed by the descending collaterals within the pinceaux; (4) beaded tendr ils make synaptic contacts with the somata and dendrites of Purkinje c ells, and (5) the horizontal axon forms synaptic junctions with the de ndritic shafts and spiny branchlets of Purkinje cells. Functional cons iderations of these synaptic relationships are discussed in the light of the classic concept of off-beam inhibition mediated by basket cells .