The effect of reducing the peripheral field on motoneurone development in the rat

The tibialis anterior and extensor digitorum longus muscles of the rat were reduced in size either by crushing the sciatic nerve or by removing part of the muscle tissue during the first postnatal week. Four to 6 weeks later the number and size of the motoneurones supplying these muscles were assessed using retrograde transport of horseradish peroxidase. The pattern of synaptic connections in the muscles supplied by these motoneurones was examined 3-46 weeks after the initial operation using a combined silver cholinesterase stain. The number of labelled motoneurones was not reduced after nerve crush but was reduced to some extent after partial muscle removal. The distribution of motoneurone sizes, however, was altered by both procedures in that the largest motoneurones became smaller. In the muscle both procedures affected synaptic organization. In the case of sciatic nerve crush at 5-6 days the incidence of muscle fibres with more than one endplate and endplates contacted by more than one axon terminal was higher than in normal adult muscles. When part of the muscle was removed, the predominant feature was the persistence of a high incidence of free sprouting nerve fibres. We therefore conclude that reduction of the peripheral field during the postnatal period does affect the development of some motoneurones.