Interaction between mossy fibre and climbing fibre responses in Purkinje cells

A major problem in cerebellar physiology relates to the manner in which information from mossy fibres and climbing fibres is integrated in the efferent neurones of the cerebellar cortex, the Purkinje cells. Recent findings by Ekerot and Oscarsson [8] and by Ito, Sakurai and Tongroach [13] indicate that the important interaction between mossy fibres and climbing fibres takes place in the dendrites rather than in the Purkinje cell somata. It has been demonstrated that impulses in climbing fibres evoke not only the so-called "complex spikes" in the somata but also plateau-like depolarizations in the distal dendrites which may have durations of hundreds of ms. The plateau potentials are presumably produced by a voltage dependent calcium conductance increase which, under physiological conditions, is triggered off exclusively by the large synaptic potentials generated by climbing fibre impulses. The spread of the plateau potentials to the distal dendrites and their long duration would facilitate spatial and temporal interaction between the mossy fibre and climbing fibre inputs. The findings suggest two kinds of interaction. (1) The duration of the plateau potentials in individual dendritic branches is modulated by the local mossy fibre/parallel fibre input. Thus, it might be postulated that dendritic branches act as independent integrators of mossy fibre and climbing fibre inputs. (2) It has been suggested that interaction between mossy fibres and climbing fibres forms the basis of learning processes in the cerebellum mediated through plastic changes in the synapses between parallel fibres and Purkinje cell dendrites. The plateau potentials would influence parallel fibre synapses by increasing the intradendritic calcium which, in turn, would lastingly depress the sensitivity of the postsynaptic receptors of the parallel fibre synapses that are activated in conjunction with the climbing fibres.