The effects of partial denervation at birth on the development of muscle fibres and motor units in rat lumbrical muscle.

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Abstract
Two aspects of nerve-muscle development were studied in neonatal rats, the role of competition between motor neurons during the elimination of polyneuronal innervation, and the dependence of muscle fiber production upon the number of motor neurons innervating the muscle. Rat lumbrical muscles were partially denervated at birth by cutting the lateral plantar nerve. Many muscles remained innervated by a single motor axon from the sural nerve. These motor units developed in the complete absence of competition from other motor units. In the adult muscles the number of innervated muscle fibers was approximately the same as at birth (about 120 muscle fibers). In muscles that were totally denervated at birth, the normal post-natal production of muscle fibers was arrested. In partially denervated muscles, the production of new muscle fibers depended on the number of remaining motor units. The relationship between the total number of muscle fibers and the number of remaining motor units was fitted by a simple model. In the lumbrical muscle, the decrease in motor unit size that occurs during normal development can be accounted for entirely by competition between motor nerve terminals. The normal post-natal increase in the total number of muscle fibers depends on a trophic interaction between the muscle and its innervation.

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