Innervation of Fetal and Neonatal Muscle Fiber Types

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The investigator will examine the rearrangement of synapses ("synapse elimination") occurring in neonatal rat muscle. The long-term goal is to understand the qualitative and quantitative roles of this rearrangement in generating the adult innervation. The project has two specific aims. 1) To ascertain how single motor neurons distribute their synapses to the types of fibers present at the time of birth. Intracellular dye injection will be used to mark fibers innervated by a single motor neuron; the types of these fibers will be determined using antibodies directed against myosin heavy chain isoforms. These results will reveal whether synaptic rearrangement generates the selective innervation of fiber types found to exist a week following birth or whether these connections are already selective at birth. An examination of the innervation of fiber types during the early stages of reinnervation of neonatal muscle is also proposed in order to determine whether the final selective pattern seen in these muscles is the result of synaptic rearrangement. 2) To determine whether slow and fast motor neurons innervate the same fibers during embryonic developments. The fast fibers which are present in the neonatal soleus muscles are not generated until after the 20th day of gestation. Thus at this time, fast motor neurons must either be innervating slow fibers or alternatively awaiting the generation of fast fibers before they begin synaptogenesis. These alternatives will be distinguished by determining the number of motor neurons innervating soleus on the 20th day of gestation.