How are coordinated movements possible? In terrestrial vertebrates, spinal motoneurones control movements of the limbs and the trunk. Motor units by the thousands can in theory provide almost infinite degrees of freedom, which may be good for flexibility but makes for computational complexity. It would not be surprising if billions of neurones participated in premotor processing, but if so wouldn't motor chaos reign in such a system? We presume it does not, but why not? A key factor seems to be the intrinsic response properties of the spinal motoneurones themselves. These properties should allow motoneurones to fire when they ought to and hold fire at other times. And when they fire, intrinsic properties make them fire the way they should, in terms of frequency and duration.
