Motor cortical modulation of feline red nucleus output: cortico-rubral and cerebellar-mediated responses.

Abstract

The neocerebellum receives an input from the motor cortex, and its output modulates the activity of rubrospinal and corticospinal tract neurons. In a previous study (Larsen and Yumiya, 1979) we examined the organization of the input from the motor cortex to the cerebellum and found that the discharge of cerebellar nuclear neurons, which were driven by passive movement of a limb segment in one direction, was suppressed by stimulation of cortical sites from which movement was evoked in the opposite direction. The purpose of the present study was to examine the organization of the output from the cerebellum to the red nucleus. Red nucleus neurons were characterized by their sensory input with natural stimulation and their motor output with movements evoked by microstimulation in unanesthetized cats. A receptive field was identified in 152 of 184 neurons, 82 of which were driven by passive movement of one or two limb segments. Microstimulation at the rubral recording sites evoked movements in the direction opposite to the passive movement. The response of rubral neurons to motor cortical microstimulation was examined in post-stimulus time histograms. A short-latency facilitation presumably mediated by the corticorubral projection, was termed the early component of the response. A longer latency response presumably mediated by the cerebellum and termed the late component consisted primarily of suppression. Twenty-eight (70%) of 40 of the tested rubral neurons driven by passive movement of a limb segment in one direction responded (with the early and/or late component) to stimulation of the cortical site from which movement was evoked in the opposite direction. By contrast, only three (17%) of 17 tested neurons responded to stimulation of the cortical site from which movement was evoked in the same direction as the passive movement. Therefore, the cerebellar-mediated suppression was found in rubral neurons, which have a target similar to neurons in the cortical region from which the suppression was evoked. Based on this and other studies, a model is proposed in which the cerebellum mediates negative feedback to the motor cortex.