The rearrangement of synaptic connections ("synapse elimination") occurring in neonatal rat soleus and extensor digitorum longus muscles will be examined. The proposed project has four long term objectives. (1) To understand the relationship between synapse elimination and the generation of motor units composed of a single type of muscle fiber. The development of different types of muscle fibers in new-born muscle will be determined using ATPase histochemistry and the fibers innervated by single motor neurons at different postnatal ages ascertained using the technique of glycogen depletion. Since muscle fiber types are already differentiated at a stage when these fibers are still multiply innervated, this study will show how homogeneous motor units are created during postnatal development. (2) To investigate the role of neuromuscular activity in determining which synapses are displaced from muscle fibers. The activity of a portion of the innervation of rat soleus muscle will be increased by stimulation and it will be determined whether the more active motor neurons are favored over the less active ones in maintaining their inputs in muscle fibers. The results of these experiments should give some indication of where (the synapse or the muscle fiber) activity is important in synapse elimination. (3) To examine muscles during the period of synapse elimination to determine whether physiological changes in terminals precede their elimination. Intracellular records will be made of the end-plate potentials obtained by repetitively stimulating single motor neurons. (4) To determine whether the competition for elimination of motor nerve terminals is affected by the number of competing motor neurons. Synapse elimination will be compared in rat strains having differing numbers of soleus motor neurons and a constant number of soleus muscle fibers. The results of this study will further knowledge of the development of synaptic connections in muscle.