Spinal neurons play a critical role in coordinating limb movements, but since they have usually been studied in anesthetized animals, little is known about their activity during normal voluntary movements. Moreover, they usually have been characterized with respect to sensory input, so virtually nothing is known about their output effects on different muscles. To investigate these issues, we recorded the activity of task-related interneurons in the C7-T1 cervical spinal cord of monkeys generating isometric flexion-extension torques about the wrist. Premotor (PreM) interneurons were identified by post-spike output effects in spike-triggered averages of EMG of forearm muscles. Most PreM interneurons fired during wrist responses in proportion to active torque. In contrast to corticomotoneuronal (CM) cells, which fire during either flexion or extension. but never both, most spinal interneurons showed some activity with both directions. CM cells tend to have larger muscle fields than spinal PreM interneurons, and more often exert reciprocal inhibitory effects on antagonists of their facilitated muscles. Thus cortical output neurons tend to control larger and more complex muscle fields. To determine whether interneurons were related to preparation for movement as well as to generation of muscle activity, interneurons were recorded during a task involving an instructed delay period between a cue that indicated the direction of the next movement and a """"""""go"""""""" signal that indicated the time to begin the movement. A significant number of interneurons changed their firing rates during the delay period. Sometimes the delay period activity occurred in the same direction as the activity associated with movement, which may simply represent a form of subthreshold preparatory activation. However, in many other cases the instructed delay period activity was distinctly different from their changes in rate during movement. Thus, spinal interneurons can show the same evidence of involvement in preparation to move as do cortical neurons.
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