Distribution of climbing fibres on cerebellar Purkinje cells in X‐irradiated rats. An electrophysiological study.

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Abstract
The distribution of climbing fibers on cerebellar Purkine 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 fibers, the response was all-or-none in character in the other cells and in all Purkinje cells recorded in normal animals. In the neurones exhibiting the former type of response, no collision was seen along the afferent fibers during interaction experiments between just-threshold juxtafastigial and maximal olivary stimulations, whereas a collision always occurred when all-or-none responses were recorded. These results show that in X-irradiated rats, the majority of Purkinje cells have a multiple innervation by 2-4 climbing fibers, instead of the one-to-one relationship seen normally. Input resistances and total electrotonic lengths of Purkinje cells were measured in normal and treated rats. Mean values for these 2 parameters were higher than normal in multiply innervated cells. Mean time course and mean current for reversal of the post-synaptic potential elicited in Purkinje cells by stimulation of the climbing fibers were nearly the same in mono- and in multiply innervated neurones. In multiply innervated cells, time courses and currents for reversal were independent of the size of the response or varied slightly with it, suggesting that the climbing fibers involved innervated territores whose electrotonic distance from the recording site were either the same or slightly different. Interactions between 2 all-or-none steps of the graded post-synaptic potential evoked in multiply innervated cells by justafastigial and olivary stimulations revealed either a very weak or a very marked shunting effect between synapses of the 2 climbing fibers involved. The over-all distribution of climbing fiber synapses on multiply innervated Purkinje cells is probably not grossly abnormal and 2 fibers contacting a given cell can probably be either intermingled on the same dendrites, or segregated on distinct dendritic branches. The existence of a strong competition among climbing fibers innervating each Purkinje cell during development is not suggested, at least when granule cells are absent.