Synaptic linkages between red nucleus cells and limb muscles during a multi-joint motor task.

The magnocellular red nucleus (RNm) becomes highly active when a monkey reaches to grasp an object. However, the only spike-triggered averaging studies of the RNm to date have been restricted to a simple wrist tracking paradigm and electromyographic (EMG) measurements of muscles of the forearm. We have now measured EMG signals from a large number of muscles throughout the shoulder, arm, forearm, and hand during a variety of tasks, including unconstrained reaching and grasping movements. Relations between these EMG signals and single-unit activity were assessed by on-line spike-triggered averaging and revealed significant post-spike effects among muscles of the shoulder and proximal arm, as well as intrinsic hand muscles. Although there remained a strong bias toward the extensor muscles of the forearm, as has been shown earlier, these results reinforce the importance of the RNm in the control of coordinated, whole-limb reaching movements.