Supraspinal and segmental interactions.

For the most part descending systems evoke movements through spinal interneurons interposed in reflex pathways. The advantage of this arrangement is that it ensures an integration of descending commands and proprioceptive and other exteroceptive feedback during the production of purposeful movement. It has also become clear that spinal reflex pathways can be reorganized during movement and that this could profoundly modify the effects of supraspinal commands on motor output. Recent experiments illustrate the existence and regulation of intrinsic motoneuron membrane currents that can dramatically change how motoneurons respond to descending commands. To understand how movements are controlled we must, therefore, understand both the activity of supraspinal motor systems and the spinal substrate upon which these commands are exerted.