ACTIVITY OF SPINAL INTERNEURONS AND THEIR EFFECTS ON FOREARM MUSCLES DURING VOLUNTARY WRIST MOVEMENTS IN THE MONKEY

We studied the activity of 577 neurons in the C-6-T-1 spinal cord of t hree awake macaque monkeys while they generated visually guided, isome tric flexion/extension torques about the wrist. Spike-triggered averag ing of electromyographic activity (EMG) identified the units' correlat ional linkages with less than or equal to 12 forearm muscles. One hund red interneurons produced changes in the level of average postspike EM G with onset latencies consistent with mono- or oligosynaptic connecti ons to motoneurons; these were classified as premotor interneurons (Pr eM-INs). Most PreM-INs (82%) produced postspike facilitations in forea rm muscles. Earlier spike-related features, often beginning before the trigger spike, were seen in spike-triggered averages from 72 neurons. Postspike effects were present in one muscle for 64% of the PreM-INs. Neurons with divergent linkages to larger ''muscle fields'' usually g enerated postspike effects in synergistic muscles. Fifty eight percent of the PreM-INs had postspike effects in flexor muscles only and 29% in extensor muscles only. Postspike effects were distributed relativel y evenly among the primary flexor and extensor muscles studied. The me an percent change in EMG level from baseline and the mean onset latenc ies for postspike facilitations and postspike suppressions were simila r. PreM-INs exhibited a variety of response patterns during the genera tion of isometric wrist torque. The response patterns and output effec ts of 24% of the PreM-INs were consistent with a strict reciprocal org anization of flexor and extensor muscle control. For another 60% of th e PreM-INs, there was a congruent relation between activity and output effects for only one direction of torque production. These neurons we re active for both flexion and extension torques, including 37 neurons that exhibited bidirectional increases in discharge rate. The relativ ely small number of postspike suppressions observed suggests that inhi bitory interneurons were silent when their target muscles were recruit ed. Compared with premotor neurons in the motor cortex, the red nucleu s and the C-8-T-1 dorsal root ganglia, spinal PreM-INs affected flexor muscles in greater proportions and had smaller muscle fields. The mag nitudes of postspike facilitations were similar in all premotor popula tions. Bidirectional activity, common for PreM-INs, was rare for corti comotoneuronal and premotor dorsal root ganglion cells, which discharg e only for torques in their preferred direction.