Distribution of climbing fibres on cerebellar Purkinje cells in X-irradiated rats

The distribution of climbing fibres on cerebellar Purkinje cells was studied with intracellular recordings in X-irradiated and normal rats. In the treated rats, multiple steps in the post-synaptic potential were elicited in 57% of the Purkinje cells by graded stimulation of the climbing fibres, the response was all - or - none in the other cells and in all Purkinje cells recorded in normal animals. In the former type of response, no collision was seen along the afferent fibres during interaction stimulation experiments whereas a collision always occurred when all -or - none responses were recorded. Input resistances and total electrotonic lengths of Purkinhe cells were higher than normal in treated rats. Mean time course and mean current for reversal of the post-synaptic potential elicited in Purkinje cells by stimulation of the fibres were nearly the same in mono - and in multiply innervated neurones. Interactions between the two all - or - none steps of the graded post-synaptic potential evoked in multiply innervated cells revealed either a very weak or a very marked shunting effect between synapses of the two fibres involved. These results show that in X-irradiated rats, the majority of Purkinje cells have a multiple innervation by two to four climbing fibres, instead of the one - to -one relationship. Two fibres contacting a given Purkinje cell can be either intermingled on the same dendrites, or segregated on distinct dendritic branches. A strong competition among fibres innervating each Purkinje cell during development is not indicated. (UK)