Response properties of spinal interneurons in awake, behaving primates

This paper reviews studies on spinal interneurons in awake, behaving monkey s inspired by the work of Prof Patrick D. Wall. Early studies documented th e sensory responses of spinal interneurons in unanesthetized monkeys to nat ural cutaneous and proprioceptive stimulation. More recently, cervical inte rneurons were documented in monkeys performing an active step-tracking task . During alternating wrist movements, most task-related interneurons showed bidirectional activity, firing during both flexion and extension (in surpr ising contrast to the unidirectional activity of muscles and corticomotoneu ronal cells). Premotor interneurons were identified by post-spike effects i n spike-triggered averages of forelimb muscle activity. The cells' post-spi ke effects were generally congruent with their activity in their preferred direction, although many fired during components of movement when their out put effects would seem inappropriate. In an instructed delay period task ma ny interneurons showed preparatory delay period activity, much like cortica l neurons. Other studies tested the excitability of corticospinal axons to electrical stimulation and demonstrated both post-spike and task-related mo dulations in excitability. Together, these studies suggest that many behavi oral functions of spinal interneurons remain to be revealed by recording th eir activity in awake, behaving animals. (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.