[Increase in the efficiency of parallel fiber synapses on Purkinje cells of the frog after simultaneous activation of climbing and parallel fibers].

Ability of the Purkinje cells to long-term plasticity changes after joint stimulation of parallel and climbing fibres has been studied in the isolated cerebellum-medulla oblongata preparation of frog. 18 neurons have been chosen on the ground of the presence of clearly identifiable monosynaptic response to parallel fibre stimulation and stability of the background activity within two hours. These neurons belonged to three distinct categories: those having the clear climbing fibre response, those having the climbing fibre-like response and those without the climbing fibre response. The neurons of the first group showed a significant increase of the ability to respond to parallel fibre stimulation after joint stimulation of both types of fibres. The stimulating current strength which was necessary for the Purkinje cell firing index 1/2 was reduced after conditioning procedures in this neuron group to 0.7 of its original value. There were no substantial changes in parallel fibre stimulation efficiency in two other neuron groups.