Death of motorneurons during the postnatal loss of polyneuronal innervation of rat muscles

A study was made of the decline in the number of motor neurons and axons of the brachial spinal cord of the rat during postnatal development. The injection of horseradish perodidase (HRP) into the biceps muscle showed that it was innervated by motor neurons located in the dordolateral position of the lateral mototr coluimn in segments C5 and C6; HRP injections into the triceps muscle showed that it was innervated by mototr neurons located in the ventrolateral posotion of the lateral motor coumn in segments C7 and C8. There was no change in the position of these motor neurons pools between birth and maturity, However, these was a decline in the number of neurons in each pool during the postnatal period; over 35% of the neurons presents at birth had disappeared by maturity. This loose of neurons was uniform throughout the restrocaudal extent of each pool. It was accompanied by a similar percentage loss in the number of axons in a ventral root at the brachial level (C8). Electrophjysiological measurements showed that the disappearance of motor neurons was accompanied by a loss in the polyneuronal innervation of synaptic sites in the biceps muscle. The possibility that a decrease in the number of neurons contributes to the loss of polyneorunal innervation is discussed.