Ritter A M, Frank E
Department of Neurobiology, University of Pittsburgh School of Medicine, BST W1413, 3500 Terrace Street, Pittsburgh, Pennsylvania 15261, USA.
J Neurobiol. 1999 Dec;41(4):471-81. doi: 10.1002/(sici)1097-4695(199912)41:4<471::aid-neu3>3.0.co;2-3.
Muscle sensory neurons, called Ia afferents, make monosynaptic connections with functionally related sets of motoneurons in the spinal cord. Previous work has suggested that peripheral target muscles play a major role in determining the central connections of Ia afferents with motoneurons. Here, we ask whether motoneurons can also be influenced by their target muscles in terms of the monosynaptic input they receive from Ia afferents, by transplanting thoracic motoneurons into the lumbosacral spinal cord so that they innervate foreign muscles. Three or four segments of thoracic neural tube from stage 14-15 chicken embryos were transplanted to the lumbosacral region of stage 16-17 embryos, and electrophysiological recordings were made from transplanted motoneurons after the embryos had reached stage 38-40. Transplanted thoracic motoneurons innervated limb muscles and received monosynaptic inputs from Ia afferents. These connections were not random: Most of the connections were formed between Ia afferents and motoneurons projecting to the same muscle (homonymous connections). Few aberrant connections were found although the anatomical distribution of afferents in the transplant indicated that they had ample opportunity to contact inappropriate motoneurons. We conclude that although peripheral target cues are not sufficient to respecify an already committed motoneuron (turn a thoracic motoneuron into a lumbosacral motoneuron), they do provide sufficient information for Ia afferent input to be functionally correct.
肌肉感觉神经元，即Ia传入神经元，与脊髓中功能相关的运动神经元组建立单突触连接。先前的研究表明，外周靶肌肉在决定Ia传入神经元与运动神经元的中枢连接方面起主要作用。在此，我们通过将胸段运动神经元移植到腰骶脊髓，使其支配外来肌肉，来探究运动神经元从Ia传入神经元接收的单突触输入方面，其是否也会受到靶肌肉的影响。将14 - 15期鸡胚的三到四个胸段神经管移植到16 - 17期胚胎的腰骶区域，并在胚胎发育到38 - 40期后，对移植的运动神经元进行电生理记录。移植的胸段运动神经元支配肢体肌肉，并从Ia传入神经元接收单突触输入。这些连接并非随机形成：大多数连接是在Ia传入神经元与投射到同一肌肉的运动神经元之间形成的（同名连接）。尽管移植中传入神经元的解剖分布表明它们有充足的机会接触不适当的运动神经元，但发现的异常连接很少。我们得出结论，虽然外周靶标线索不足以重新指定一个已经定型的运动神经元（将胸段运动神经元转变为腰骶段运动神经元），但它们确实为Ia传入输入在功能上的正确性提供了足够的信息。
