Innervation of Fetal and Neonatal Muscle Fiber Types

研究者将检查发生在新生大鼠肌肉中的突触重排（“突触消除”）。长期目标是了解这种重排在产生成人神经支配中的定性和定量作用。该项目有两个具体目标。1)确定单个运动神经元如何将它们的突触分配到出生时存在的纤维类型。细胞内染料注射将用于标记由单个运动神经元支配的纤维；这些纤维的类型将使用针对肌球蛋白重链异构体的抗体来确定。这些结果将揭示突触重排是否产生了出生一周后发现的纤维类型的选择性神经支配，或者这些连接是否在出生时就已经具有选择性。在新生儿肌肉神经再生的早期阶段，还提出了对纤维类型的神经支配的检查，以确定在这些肌肉中看到的最终选择模式是否是突触重排的结果。2)确定在胚胎发育过程中，慢速和快速运动神经元是否支配相同的纤维。存在于新生儿比目鱼肌中的快纤维直到妊娠第20天以后才产生。因此，在这个时候，快速运动神经元必须要么支配慢纤维，要么在它们开始突触发生之前等待快速纤维的产生。在妊娠第20天，通过测定支配比目鱼肌的运动神经元数量来区分这些选择。

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.