Low sensitivity of dorsal spinocerebellar neurons to limb movement speed

This paper reports the effect of limb movement speed on dorsal spinocerebellar tract (DSCT) activity recorded while the cat hindlimb was passively moved through two types of foot trajectories (figure eight and step cycle) at different speeds. While nearly all the DSCT neurons sampled (151/159; 94.5%) were significantly modulated by the direction of foot movement in these trajectories, they were only modestly influenced by movement speed. We quantified the speed effect and also accounted for intrinsic cell variability by computing a variability index (VI) between pairs of responses to trajectories made either at the same or at different speeds. The distribution of same-speed VIs across cells indicated a mean variability of about 10% over a trajectory cycle, whereas the two-speed distributions indicated a mean change of about 25% for a two- to fourfold change in movement speed. We also examined the relative contribution of movement speed to the activity of each DSCT cell by means of a multivariate regression model that also included as predictors the position, movement direction, and interactions between movement and position. We found that 28 of 103 (27.2%) neurons were not sensitive to movement speed. The rest were modulated in varying degrees by changes in speed, and the speed modulation depended on limb position for most of them (54/75). Overall, DSCT speed sensitivity resembles the 0.3-power relationship used to describe the velocity sensitivity of muscle spindles for large muscle stretches. We examined this by recording muscle spindle activity during these passive foot trajectories and found that their speed sensitivity was within the range observed for the DSCT and explained by the 0.3-power law. In total, movement speed accounted for about 15% of the variance in DSCT activity across cells, while the directional component of movement accounted for about 45%. The results suggest a separate processing of sensory information about the two components of movement velocity: namely, its direction and magnitude.