van Mier P, ten Donkelaar H J
Department of Anatomy and Embryology, University of Nijmegen, The Netherlands.
J Neurosci. 1989 Jan;9(1):25-37. doi: 10.1523/JNEUROSCI.09-01-00025.1989.
This study presents direct evidence that in Xenopus laevis embryos ipsi- and contralaterally descending reticulospinal fibers from the caudal brain stem project to the spinal cord, where they directly contact primary motoneurons. At stage 30, occasional contacts between primary motoneurons and descending axons are present. These contacts are possibly already functional since presynaptic vesicles were sometimes observed. Furthermore, the physiological data obtained in this study suggest that reticulospinal neurons in the caudal brain stem are involved in the central generation of early swimming. The first ingrowth of reticulospinal axons was observed in the rostral spinal cord after application of HRP to the caudal brain stem of stage 27/28 embryos. By stage 32, many supraspinal axons could be found in the spinal cord at the level of the 12/13th myotome, near the time of the first rhythmic swimming. Both lamellipodial and varicose growth cones were found. Intracellular recordings from the brain stem and extracellular recordings from the myotomal muscles in curarized embryos around stage 30 revealed neurons in the caudal brain stem which were active during early fictive swimming. After intracellular staining with Lucifer yellow neurons with descending axons were found in the brain-stem reticular formation. These reticulospinal neurons showed "motoneuron-like" phasic activity, producing one spike each swimming cycle. Rhythmically occurring spikes with swimming periodicity were superimposed on a sustained depolarization level of some 5-30 mV. Reticulospinal neurons in the brain stem resemble descending interneurons in the spinal cord by their morphology, projection pattern, and activity during early swimming. Reticulospinal neurons and descending interneurons might therefore form one continuous population of projecting interneurons with a different location but a similar function. In support of this we propose that the embryonic brain-stem reticular formation forms part of the swimming pattern generator.
本研究提供了直接证据，表明在非洲爪蟾胚胎中，来自尾端脑干的同侧和对侧下行网状脊髓纤维投射到脊髓，并在那里直接与初级运动神经元接触。在第30期时，初级运动神经元与下行轴突之间偶尔会出现接触。由于有时能观察到突触前囊泡，这些接触可能已经具备功能。此外，本研究获得的生理学数据表明，尾端脑干中的网状脊髓神经元参与早期游泳的中枢产生过程。在将HRP应用于第27/28期胚胎的尾端脑干后，在脊髓头端观察到网状脊髓轴突的首次长入。到第32期时，在第12/13肌节水平的脊髓中可以发现许多脊髓上轴突，此时接近首次出现节律性游泳的时间。发现了片状伪足和曲张生长锥。在第30期左右的箭毒化胚胎中，对脑干进行细胞内记录以及对肌节肌肉进行细胞外记录，结果显示尾端脑干中的神经元在早期虚拟游泳期间处于活跃状态。在用荧光黄进行细胞内染色后，在脑干网状结构中发现了具有下行轴突的神经元。这些网状脊髓神经元表现出“运动神经元样”的相位活动，每个游泳周期产生一个动作电位。具有游泳周期性的节律性动作电位叠加在约5 - 30 mV的持续去极化水平上。脑干中的网状脊髓神经元在形态、投射模式以及早期游泳期间的活动方面与脊髓中的下行中间神经元相似。因此，网状脊髓神经元和下行中间神经元可能形成一个连续的投射中间神经元群体，它们位置不同但功能相似。为此我们提出，胚胎脑干网状结构是游泳模式发生器的一部分。
