Postnatal development of the adult pattern of motor axon distribution in rat muscle.

Establishment of the adult pattern of connections between motoneurones and their muscles is known to be achieved in stages. There is an initial guidance mechanism which in higher vertebrates directs the axons with considerable accuracy to their appropriate muscles. This is followed by a stage when a large proportion of neurones die. Then, after birth, the final modifications are brought about by elimination of redundant motoneuronal branches. We present here evidence that branch elimination does not occur at random from within a motoneurone's initial intramuscular distribution, but in such a way as to sharpen the segmento-topic relationship within a muscle that motoneuronal death had earlier provided.