RESPONSE PATTERNS AND POSTSPIKE EFFECTS OF PREMOTOR NEURONS IN CERVICAL SPINAL-CORD OF BEHAVING MONKEYS

Most of our information about spinal neurons has been derived from exp eriments with anesthetized or surgically reduced preparations. To inve stigate these neurons under normal behavioral conditions, we recorded the activity of single afferent units in cervical dorsal root ganglia and of single interneurons in the cervical spinal cord of macaque monk eys, as they generated alternating flexion and extension torques about the wrist. Spike-triggered averages of rectified electromyographic ac tivity were used to identify ''premotor'' (PreM) units associated with correlated postspike effects in active muscles. In addition to postsp ike effects, some spike-triggered averages showed early increases in a verage muscle activity, which were attributed to synchronous discharge s in other PreM units. In recordings of peripheral afferents, 49% of t he task-related dorsal root ganglia units produced postspike facilitat ion (PSF) of at least one forearm muscle, with a mean PSF latency of 5 .8 +/- 0.3 ms (SE). The PSF amplitude was measured as the mean percent increase (MPI): the average increase of the PSF as a percentage of th e prespike baseline mean. PreM afferent units produced PSF with an ave rage MPI of 4.6 +/- 0.3%. In a study of cervical interneurons, about 1 3% (72/562) of the task-related cells showed postspike effects. These PreM interneurons had a mean PSF latency of 7.2 +/- 0.3 ms and a mean MPI of 4.6 +/- 0.2%. The MPI values for spinal neurons were similar to the MPIs reported for rubromotoneuronal and corticomotoneuronal cells . PreM neurons usually facilitated a subset of the coactivated muscles , called the unit's ''muscle field.'' The PreM afferents facilitated a n average of 46% of the synergistically coactivated muscles, while Pre M interneurons facilitated an average of 37%. These are comparable wit h the percentage of muscles facilitated by corticomotoneuronal (40%) a nd rubromotoneuronal (50%) cells. During the step-tracking task the mo nkeys generated ramp-and-hold torques about the wrist. The PreM affere nts typically became active during either flexion or extension of the wrist, although a few were bidirectionally active. The most common res ponse pattern in PreM afferents was a tonic discharge, followed by pha sic and phasic-tonic discharge. The most common patterns exhibited by PreM interneurons were tonic and phasic-tonic responses. PreM afferent units began to discharge on average 51 +/- 13 ms before activation of their target muscle. This early onset supports our hypothesis that th ese PreM afferents arose from muscle spindles, which is also consisten t with their short-latency PSF and their responses to perturbations th at stretched their target muscles. The results reveal some salient dif ferences between the discharge properties of dorsal root ganglia neuro ns, spinal interneurons, and supraspinal PreM cells in the motor corte x and red nucleus. All four PreM populations include tonic, phasic-ton ic, and phasic cells, but in significantly different proportions. Most PreM afferents resembled corticomotoneuronal cells in being active on ly with their target muscles, unlike rubromotoneuronal cells and spina l PreM interneurons, which tended to exhibit more bidirectional discha rges.