Harris A J, McCaig C D
J Neurosci. 1984 Jan;4(1):13-24. doi: 10.1523/JNEUROSCI.04-01-00013.1984.
Chronic paralysis of rat embryos during the last 4 to 6 prenatal days causes a diminution in skeletal muscle fiber numbers but inhibits motoneuron death. Consequently, as paralyzed muscles develop, an increased number of motoneurons attempts to form synapses at a reduced number of synaptic sites. Paralyzed muscle fibers receive their synapses at a single endplate, as in control muscles, but these endplates are hyperinnervated, with about twice the normal number of inputs. Counts of axons, synaptic inputs, and muscle units showed that motoneurons normally contact a maximum number of muscle fibers shortly before birth, and this number remains stable for several days postnatal until it finally is reduced to the adult number. The average motor unit size in paralyzed embryos at the time of birth was the same as in controls. We suggest that it is not necessary to postulate the existence of competition between embryonic nerve terminals in order to explain regulation of the number of muscle fibers initially contacted by a motoneuron. Motoneuron death was not immediately affected by paralysis, but paralysis "rescued" all motoneurons whose death normally would have occurred 24 hr or more after the time when paralysis was initiated, regardless of when this was. This implies that the peak period for determination to die is during embryonic day 14, when myotube formation is just beginning and no recognizable endplate structures are present in muscles. When paralyzed, motoneurons normally destined to die are capable of forming a normal number of functional nerve-muscle contacts.
在产前最后4至6天对大鼠胚胎进行慢性麻痹，会导致骨骼肌纤维数量减少，但会抑制运动神经元死亡。因此，随着麻痹肌肉的发育，越来越多的运动神经元试图在数量减少的突触部位形成突触。麻痹的肌纤维像对照肌肉一样，在单个终板处接受突触，但这些终板有超神经支配现象，其输入数量约为正常数量的两倍。对轴突、突触输入和肌肉单位的计数显示，运动神经元通常在出生前不久接触到最大数量的肌纤维，这个数量在出生后几天内保持稳定，直到最终减少到成年数量。出生时麻痹胚胎中的平均运动单位大小与对照相同。我们认为，为了解释运动神经元最初接触的肌纤维数量的调节，没有必要假定胚胎神经末梢之间存在竞争。运动神经元的死亡并没有立即受到麻痹的影响，但麻痹“挽救”了所有那些正常情况下在麻痹开始后24小时或更长时间就会死亡的运动神经元，无论麻痹何时开始。这意味着决定死亡的高峰期是在胚胎第14天，此时肌管形成刚刚开始，肌肉中没有可识别的终板结构。当发生麻痹时，正常情况下注定要死亡的运动神经元能够形成正常数量的功能性神经-肌肉接触。
