Plasticity in the distribution of the red nucleus output to forearm muscles after unilateral lesions of the pyramidal tract

It has been hypothesized that the magnocellular red nucleus (RNm) contribut es to compensation for motor impairments associated with lesions of the pyr amidal tract. To test this hypothesis, we used stimulus triggered averaging (StTA) of electromyographic (EMG) activity to characterize changes in moto r output from the red nucleus after lesions of the pyramidal tract. Three m onkeys were trained to perform a reach and prehension task. EMG activity wa s recorded from 11 forearm muscles including one elbow, five wrist, and fiv e digit muscles. Microstimulation (20 mu A at 20 Hz) was delivered througho ut the movement task to compute StTAs. Two monkeys served as controls. In a third monkey, 65% of the left pyramidal tract had been destroyed by an ele ctrolytic lesion method five years before recording. The results demonstrat e a clear pattern of postlesion reorganization in red nucleus-mediated outp ut effects on forearm muscles. The normally prominent. extensor preference in excitatory output from the RNm (92% in extensors) was greatly diminished in the lesioned monkey (59%). Similarly, suppression effects, which are no rmally much more prominent in flexor than in extensor muscles (90% in flexo rs), were also more evenly distributed after recovery from pyramidal tract lesions. Because of the limited excitatory output from the RNm to flexor mu scles that normally exists, loss of corticospinal output would leave contro l of flexors particularly weak. The changes in RNm organization reported in this study would help restore function to flexor muscles. These results su pport the hypothesis that the RNm is capable of reorganization that contrib utes to the recovery of forelimb motor function after pyramidal tract lesio ns.