Phase-specific sensory representations in spinocerebellar activity during stepping: evidence for a hybrid kinematic/kinetic framework

The dorsal spinocerebellar tract (DSCT) provides a major mossy fiber input to the spinocerebellum, which plays a significant role in the control of posture and locomotion. Recent work from our laboratory has provided evidence that DSCT neurons encode a global representation of hindlimb mechanics during passive limb movements. The framework that most successfully accounts for passive DSCT behavior is kinematics-based having the coordinates of the limb axis, limb-axis length and orientation. Here we examined the responses of DSCT neurons in decerebrate cats as they walked on a moving treadmill and compared them with the responses passive step-like movements of the hindlimb produced manually. We found that DSCT responses to active locomotion were quantitatively different from the responses to kinematically similar passive limb movements on the treadmill. The differences could not be simply accounted for by the difference in limb-axis kinematics in the two conditions, nor could they be accounted for by new or different response components. Instead, differences could be attributed to an increased relative prominence of specific response components occurring during the stance phase of active stepping, which may reflect a difference in the behavior of the sensory receptors and/or of the DSCT circuitry during active stepping. We propose from these results that DSCT neurons encode two global aspects of limb mechanics that are also important in controlling locomotion at the spinal level, namely the orientation angle of the limb axis and limb loading. Although limb-axis length seemed to be an independent predictor of DSCT activity during passive limb movements, we argue that it is not independent of limb loading, which is likely to be proportional to limb length under passive conditions.