SUPRASPINAL AND SEGMENTAL INTERACTIONS

For the most part descending systems evoke movements through spinal in terneurons interposed in reflex pathways. The advantage of this arrang ement is that it ensures an integration of descending commands and pro prioceptive and other exteroceptive feedback during the production of purposeful movement. It has also become clear that spinal reflex pathw ays can be reorganized during movement add that this could profoundly modify the effects of supraspinal commands on motor output. Recent exp eriments illustrate the existence and regulation of intrinsic motoneur on membrane currents that can dramatically change how motoneurons resp ond 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 exerte d.