Red nucleus and motor cortex: parallel motor systems for the initiation and control of skilled movement

This study examines the differential contributions of motor cortex (MCx) and red nucleus (RN) neurons to the initiation of a targeted limb response and to the control of trajectory. These questions were assessed in two ways. First, by comparing the characteristics of task-related neuronal activity in MCx and RN. Second, by determining the changes in reaction time and trajectories produced by the reversible inactivation of corticospinal fibers in the crus cerebri (CSTc), the rubrospinal tract (RST) and the RN, using microinjections of lidocaine, gamma-aminobutyric acid, or muscimol. Neurons in forelimb areas of both MCx and RN were modulated in advance of forelimb force production. RN neurons more frequently had a phasic discharge pattern, while neurons in MCx more frequently had a tonic pattern. Whereas the modulation of most forelimb area neurons in MCx correlated with responses in a specific direction, the majority of RN neurons were non-directional. Reversible inactivation of CSTc, RN and RST prolonged reaction time. The normal stereotyped form of isometric force trajectories was unaffected by injections at any site. While CSTc inactivation resulted in hypometric responses, response amplitude was unchanged during RN and RST inactivation. We conclude that both MCx and RN contribute to response initiation, but that only MCx is involved in the proper scaling of targeted responses.