Modulation of dorsal spinocerebellar responses to limb movement. II. Effect of sensory input.

Dorsal spinocerebellar tract (DSCT) neurons receive converging sensory inputs from muscle, skin, and joint receptors and their cerebellar projection is a product of the spinal sensory processing of movement-related information. We concluded earlier that DSCT activity relates to global rather than to local parameters of hindlimb postures and movement, specifically to a kinematic representation of the limb endpoint. The waveforms of principal components (PCs) derived from an ensemble of DSCT movement responses were found to correlate with either the waveform of the limb axis length or orientation trajectories. It was not clear, however, whether these global representations resulted from neural processing or from biomechanical factors. In this study, we perturbed the limb biomechanical factors by decoupling limb geometry from endpoint position during passively applied limb trajectories patterned after a step cycle. We used two types of perturbations: mechanical constraints that limited joint rotations and electrical stimulation of hindlimb muscles. We found that about half of the 89 cells studied showed statistically different response patterns during the perturbations. We compared the PCs of the altered responses with the PCs of the control responses, and found two basic results. With the joint constraints, >85% of the total variance in both control and changed responses was accounted for by the same five PCs that were also observed in the earlier study. The differences between altered and control responses could be fully accounted for by changes in the PC weighting, suggesting a modulation of global response components rather than an explicit representation of local parameters. With the muscle stimulation, only the first and third PCs were the same for the control and altered responses. The second PC was modified, and additional PCs were also required to account for the altered responses. This suggests that the stimulus parameters were specifically represented in the responses. The changes induced by both types of perturbation affected primarily the weighting or waveform of the second PC, which relates to the limb axis length trajectory. The results are consistent with the suggestion that information about limb orientation and length may be separately modulated.