Functional Organization of Cerebellar Modules Controlling Forelimb Movements: Climbing and Mossy Fibre Input and Motor Output via the Red Nucleus and Motor Cortex

Sammanfattning: A modular organization has recently been described for the cerebellar control system comprising the forelimb areas of the C1/C3/Y zones in the cerebellar cortex and nucleus interpositus anterior (NIA) of the cat. A module is a group of cells in NIA which receives homogenous climbing fibre input via Purkinje cells that belong to the same set of microzones in the C1/C3/Y zones. The organization of input and output of these modules was investigated in order to elucidate their specific roles in motor control. Specific convergence patterns between muscle and cutaneous afferents to climbing fibres in the C3 zone were found. Thus, in the projection of the two modalities to climbing fibres, convergence was found mainly between wrist dorsiflexor muscles and cutaneous receptive fields (cut.rfs) on the dorsal and distal ulnar parts of the forelimb, between ventral flexors and ventral cut.rfs, between triceps and proximal ulnar and dorsal cut.rfs and between biceps and proximal radial cut.rfs. Convergence between muscles acting across the same joints was extensive, whereas convergence between muscles acting across different joints was less extensive. In the C3 zone, mossy fibre units were found to terminate in cluster-like aggregates of units with mainly similar cut.rfs. Clusters were apparently larger in width than microzones and adjacent clusters overlapped considerably. Accordingly, beneath the climbing fibre microzones a heterogeneous set of mossy fibres were found but the least common denominator of their receptive fields was that they covered an area of the skin that closely corresponded to the cut.rfs of the climbing fibre microzone. The movements evoked via the rubrospinal tract by electrical microstimulation of NIA were of a multi-joint character already at threshold stimulation. At least the movement component evoked at the joint just proximal to the proximal extent of the module’s climbing fibre receptive field showed a specific relationship with the receptive field. Such movement components consisted of wrist dorsiflexion for modules with ventral cut.rfs on the paw, wrist ventral flexion for modules with dorsal cut.rfs on the paw, elbow flexion for modules with ulnar cut.rfs on the forearm and elbow extension for modules with radial cut.rfs on the forearm. The topographical organization and maximal amplitudes of the responses in the motor cortex evoked from NIA were specifically related to the cut.rfs of modules. Modules with distal radial and ventral cut.rfs had the largest projection amplitudes whereas other modules with distal cut.rfs had intermediate projections and modules with proximal ulnar and dorsal cut.rfs had only weak projections. Overall topographical differences were found for the cortical projections of these three types of modules and in addition, even fine grain differences in receptive fields between modules could be related to topographical differences in their cortical projections. Input to the motor cortex from the forelimb skin differed from that from cerebellar modules both in being located to a largely separate termination area and in having a complex pattern of temporal spread over the cortical surface. The present study above all emphasizes a high degree of connectional specificity for the cerebellar modules, indicating that they may represent the ’building blocks’ of cerebellar motor control. The organizational features found also provide new restraints on the number of conceivable ways in which the cerebellar neuronal circuitry may operate.