DISTRIBUTION AND CHARACTERISTICS OF POSTSTIMULUS EFFECTS IN PROXIMAL AND DISTAL FORELIMB MUSCLES FROM RED NUCLEUS IN THE MONKEY

We used stimulus-triggered averaging (StTA) of electromyographic (EMG) activity to investigate two major questions concerning the functional organization of the magnocellular red nucleus (RNm) for reaching move ments in the macaque monkey. The first is whether the clear preference toward facilitation of extensor muscles we have reported in previous studies for distal (wrist and digit) forelimb muscles also exists for proximal muscles (shoulder and elbow). The second question is whether distal and proximal muscles may be cofacilitated from RNm suggesting t he representation of functional muscle synergies for coordinated reach ing movements. Two monkeys were trained to perform a prehension task r equiring multijoint coordination of the forelimb. EMG activity was rec orded from 24 forelimb muscles including 5 shoulder, 7 elbow, 5 wrist, 5 digit, and 2 intrinsic hand muscles. Microstimulation (20 mu A at 2 0 Hz) was delivered throughout the movement task. From 137 microstimul ation sites in the RNm, a total of 977 poststimulus effects was obtain ed including 733 poststimulus facilitation effects (PStF) and 244 post stimulus suppression effects (PStS). Of the PStF effects, 58% were obt ained from distal muscles; 42% from proximal muscles. Digit muscles we re more frequently facilitated (35%) than the wrist, elbow, or shoulde r muscles (20, 24, and 18%, respectively). The intrinsic hand muscles were infrequently facilitated (3%). At all joints tested, PStF was mor e common in extensor muscles than flexor muscles. This extensor prefer ence was-very strong for shoulder (85%), wrist (85%), and digit muscle s (94%) and weaker for elbow muscles (60%). Of the PStS effects, 65% w ere in distal muscles and 35% in proximal muscles, interestingly, the flexor muscles were more frequently inhibited from RNm than extensor m uscles. At 72% of stimulation sites, at least two muscles were facilit ated. The majority of these sites (61%) cofacilitated both proximal an d distal muscles. At the remaining sites (39%), PStF was observed in e ither the proximal (17%) or distal muscles (22%). Facilitation most of ten involved combinations of shoulder, elbow, and distal muscles (30%) or shoulder and distal muscles (26%). Only rarely were intrinsic hand muscles part of the total muscle synergy. Our results show that the R Nm 1) controls both proximal and distal muscles but the strength of in fluence is biased toward distal muscles, 2) preferentially controls ex tensor muscles not only at distal forelimb joints but also at proximal joints, and 3) output zones cofacilitate synergies of proximal and di stal muscles involved in the control of forelimb reaching movements.