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Cerebellar Cortical Plasticity and Cutaneous Receptive Fields

Henrik Jörntell & Carl-Fredrik Ekerot

Section for Neuroscience, Dept Experimental Medical Sciences, Lund University , Sweden

In Purkinje cells (PCs) and molecular layer interneurons (Ints) in the forelimb area of the C3 zone, parallel fiber (PF) receptive fields are small and sharply defined and the location of the PF receptive field is specific to the climbing fiber CF receptive field. However, the specific relationship between the PF and CF receptive fields is different and functionally opposite in the two cell types. Our interpretation of these findings was that several processes of PF synaptic plasticity, which were unknown at the time, had been at play. In PCs, the CF-dependent LTD of PF input was not sufficient to explain the small, specific PF receptive fields since LTD would merely be expected to create a ‘hole’ in the PF receptive field in and around the skin area corresponding to the CF receptive field. The PF receptive fields of PCs could only be established if there was also a process for potentiating PF synapses postsynaptically. In Ints, in which the PF receptive field more or less equalled the CF receptive field, there should be a process that potentiated PF synapses activated simultaneously with the CF, and most likely also a postsynaptic LTD process would be required to depress PF synapses activated from surrounding skin areas. In a subsequent paper, we showed that the PF receptive fields in both cell types could be changed fundamentally and reversibly by using direct stimulation of PFs with or without conjunctive CF activation, thereby giving support to the existence of the proposed forms of synaptic plasticity. In the present study, we have been using skin stimulation to obtain a more specific activation of PFs. By using the induction protocols we previously deduced, we show that specific skin areas can be added and removed from the PF receptive fields of PCs and Ints. In contrast, the receptive fields of individual mossy fibers and CFs remain small and have a normal location. Hence, the present results suggest that PF synapses on both PCs and Ints are highly plastic in a bidirectional manner, and by using relatively simple combinations of skin stimulation, the PF receptive fields can be specified at will.