Response properties of spinal interneurons in awake, behaving primates

This paper reviews studies on spinal interneurons in awake, behaving monkeys inspired by the work of Prof Patrick D. Wall. Early studies documented the sensory responses of spinal interneurons in unanesthetized monkeys to natural cutaneous and proprioceptive stimulation. More recently, cervical interneurons 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 surprising contrast to the unidirectional activity of muscles and corticomotoneuronal cells). Premotor interneurons were identified by post-spike effects in spike-triggered averages of forelimb muscle activity. The cells' post-spike effects were generally congruent with their activity in their preferred direction, although many fired during components of movement when their output effects would seem inappropriate. In an instructed delay period task many interneurons showed preparatory delay period activity, much like cortical neurons. Other studies tested the excitability of corticospinal axons to electrical stimulation and demonstrated both post-spike and task-related modulations in excitability. Together, these studies suggest that many behavioral functions of spinal interneurons remain to be revealed by recording their activity in awake, behaving animals.