Functional properties of primate spinal interneurones during voluntary hand movements

The activity of cervical spinal interneurones (INs) was recorded in monkeys performing alternating hand movements. The contribution of INs to voluntary movement was determined by their response patterns during ramp-and-hold wrist movements and their postspike effects on forelimb muscle activity. Most INs were active during both flexion and extension, in contrast to the unidirectional activity of muscles and corticomotoneuronal cells. When recorded during performance of an instructed delay task, the activity of many INs was modulated during the delay period between the instruction cue and the subsequent go signal. Thus, spinal INs, like cortical neurones, participate in earliest stages of preparation for movement. The modulation of peripheral input to spinal INs was tested during an instructed delay task. The monosynaptic responses to electrical stimulation of a cutaneous nerve decreased during active movement, probably due to presynaptic inhibition. These results provide new insights into the role of spinal INs in preparation and execution of voluntary movement.