EFFECTS ON WRIST AND DIGIT MUSCLE-ACTIVITY FROM MICROSTIMULI APPLIED AT THE SITES OF RUBROMOTONEURONAL CELLS IN PRIMATES

1. The purpose of this study was to use the techniques of spike- and stimulus-triggered averaging (SpTA and StTA, respectively) to examine the output organization of individual rubromotoneuronal (RM) cells in relation to clusters of neighboring cells. SpTA of electromyographic (EMG) activity in awake monkeys reveals the target muscles of an individual recorded neuron, whereas StTA reveals the target muscles of the neuronal aggregate activated by the stimulus. 2. Three questions were of particular interest. First, does the pattern of poststimulus facilitation (PStF) across forearm muscles match the pattern of postspike facilitation (PSpF)? Second, does the output of RM cell aggregates tested with StTA favor forearm extensor muscles, as reported for individual RM cells in the companion paper? Third, how do RM poststimulus effects compare with corticomotoneuronal (CM) poststimulus effects? 3. Microstimuli were applied at the sites of 37 RM cells, identified by SpTA of EMG activity in awake monkeys performing an alternating wrist movement task. 4. The magnitudes of PStF at 5, 10, and 20-mu-A were, respectively, 4.3, 10.1, and 13.7 times greater than PSpF of the same muscles, reflecting activation, by the stimulus, of multiple RM cells. RM cell PStF was weaker than CM PStF. 5. The onset latency of poststimulus suppression (PStS) exceeded that of PStF. For example, at 20-mu-A the difference was 2.6 ms, comparable with the difference between PSpF and postspike suppression (PSpS). 6. The patterns of poststimulus effects on forearm flexor and extensor muscles were categorized in the same manner as post-spike effects. Three major patterns were observed: 1) pure facilitation, 2) reciprocal suppression, and 3) cofacilitation of extensors and flexors. 7. The profile of PStF across synergist muscles was broadly similar to that of PSpF. At 83% of sites, the muscle with the greatest PSpF was also the muscle with the greatest PStF. At 30% of sites (11 of 37), the set of muscles with PStF (muscle field) exactly matched the set with PSpF. Overall, the level of discrepancies in SpTAs versus StTAs computed at the same RM cell sites was 27%. Most of these discrepancies could be attributed to muscles with the weakest effects. 8. The fact that the poststimulus muscle fields at many RM cell sites matched the postspike fields at the same sites, even though the poststimulus effects were greater in magnitude and were mediated by more cells, suggests clustering of RM cells with similar target motoneuron pools. However, the degree of match was poorer than at CM cell sites, indicating greater intermixing of cells with different muscle fields. 9. The striking extensor preference of RM cells revealed with SpTA was also clearly evident in the StTA data. At 5, 10, and 20-mu-A, extensors were predominantly facilitated at 80, 95, and 94% of RM cell sites, respectively. Even at sites where individual RM cells facilitated flexor muscles, PStF was stronger in extensor muscles. Just as the magnitudes of flexor and extensor PSpF were not significantly different, neither were the magnitudes of flexor and extensor PStF at 5-mu-A. However, at 10 and 20-mu-A, the magnitude of PStF of extensors was 71 and 128%, respectively, greater than that of flexors. 10. On the basis of these findings, we conclude that RM cells with similar muscle fields tend to form functional output modules (clusters). The majority of RM cell output modules concerned with forearm muscles are directed toward facilitation of extensors. However, within this largely extensor-oriented output region are interspersed a smaller number of flexor RM cells.