Response of developing rat fast muscles to partial denervation.

The changes of motor unit size following partial denervation of the extensor digitorum longus muscles in rat neonates (at five to six days) and later in development (at 18-20 days) were studied. Extensor digitorum longus muscle is innervated mainly by axons from L4 ventral ramus and to a lesser extent by axons from L5 ventral ramus. In neonates the motor units in extensor digitorum longus are large, and they become restricted to their adult size during the first two weeks of life. Six to 10 weeks after removing the major input to extensor digitorum longus, i.e. L4 ventral ramus at five to six days, the motor unit sizes of axons in the remaining L5 ventral ramus decrease from their expanded neonatal territory to their adult smaller size. In spite of partial denervation the motor units remain small throughout the animal's life and the denervated muscle fibres do not become "occupied" by sprouts from the remaining axons of L5 ventral ramus motor nerves. Partial denervation of extensor digitorum longus muscles at 18-20 days by section of the L4 ventral ramus leads to the expected two- to three-fold increase in the size of motor units of L5 ventral ramus. These results are taken to show that fast motor units of neonatal rats are unable to maintain their enlarged peripheral field, while later in development their axons can sprout and occupy an expanded peripheral field.