Aspects of peripheral motor system development

The adult motor system is precisely connected, topographically and functionally. This order is reached during embryonic development through a sequence of mechanisms which increasingly resolve the adult patterns of connectivity. First is axon guidance, which gives rise to a pattern of axon outgrowth leading to an approximation of the adult projections. Second there is a period of motoneuron death, which eliminates motoneurons which have projected to inappropriate regions of the limb, and, it is conjectured, motoneurons which are functionally less well connected in terms of the behaviour of the whole system. Third there is a period of retraction of axon terminals which refines the pattern of connectivity from polyneuronal to mononeuronal innervation of the muscle fibres. The patterns of retraction may be determined by the same functional criteria as are used in the control of motoneuron death.