BROAD DIRECTIONAL TUNING IN SPINAL PROJECTIONS TO THE CEREBELLUM

1. Spinocerebellar neurons that project in the dorsal spinocerebellar tract (DSCT) receive mono- and polysynaptic inputs from specific senso ry receptors in the hindlimb, and they project mossy fiber terminals t o the cerebellar vermis. We examined the functional organization of th ese neurons and found that it relates to whole-limb parameters like li mb posture and direction of limb movement. 2. We recorded the activity of 444 DSCT units during passive perturbations of the hind foot in an esthetized cats. The movements were either confined a single joint (th e ankle; 234 cells) or involved the entire hindlimb (210 cells). The c ells exhibited opposite responses for opposite directions of whole-lim b movement, but a variety of response patterns for opposite directions of movement at one joint. We interpret the result to imply that the p opulation encodes information about the whole limb rather than single joints. 3. Most of the 78 neurons recorded during passive limb placeme nts (63%) responded to changes in limb length and also changes in limb orientation. In fact, the activity of most of the cells was broadly t uned with respect to the direction of passive limb movements generated by moving the hind foot in the sagittal plane. Changes in unit activi ty could be described by a cosine tuning function with respect to foot positions (72% of responses) and directions of foot movement (50%). 4 . The similarity of this behavior to that of neurons in the motor cort ex and cerebellar nuclei recorded during voluntary movements is consis tent with a common neural code to represent the sensorimotor parameter s of limb movement.