Roberts A, Walford A, Soffe S R, Yoshida M
School of Biological Sciences, University of Bristol, Bristol BS8 1UG, United Kingdom.
J Comp Neurol. 1999 Aug 30;411(3):472-86. doi: 10.1002/(sici)1096-9861(19990830)411:3<472::aid-cne9>3.0.co;2-b.
Xenopus tadpole motoneurons make cholinergic synapses within the spinal cord. This excitation changes with longitudinal position and contributes to the excitation that controls motor activity and its longitudinal spread during swimming. To explore the anatomic constraints on this excitation, backfilling has been used to examine the anatomy and distribution of the whole population of spinal motoneurons, to define the extent of their central axons and to find where they make synapses. Motoneuron features show considerable variation but do not allow their separation into primary and secondary. Most motoneurons have descending central axons and it is likely that central synapses are made from these axons as longitudinal dendritic extent is very limited. Motoneuron density reaches a broad plateau over the mid-trunk region at 12-13 per 100 microm. Soma size does not change with longitudinal position, but the dorsoventral extent of the dendrites decreases caudally, whereas the central axon length increases. Motoneuron distribution data were used to estimate the longitudinal distribution of central motoneuron axons. This has a broad plateau at 12-14 per 100 microm over much of the trunk and only decreases significantly caudal to the anus. This distribution correlates with cholinergic excitation during swimming. Transmission electron microscopy of motoneurons backfilled with horseradish peroxidase was used to show that central motoneuron axons make en passant synapses with motoneuron dendrites and the dendrites of other unstained neurons. By using measures of synapse frequency and total dendrite length, trunk motoneurons are estimated to each receive 100-200 synapses.
非洲爪蟾蝌蚪的运动神经元在脊髓内形成胆碱能突触。这种兴奋性随纵向位置而变化，并有助于在游泳过程中控制运动活动及其纵向传播的兴奋性。为了探究这种兴奋性的解剖学限制，已采用反向填充法来检查整个脊髓运动神经元群体的解剖结构和分布，以确定其中枢轴突的范围，并找出它们形成突触的位置。运动神经元的特征显示出相当大的变异性，但无法将它们分为初级和次级。大多数运动神经元具有下行的中枢轴突，并且由于纵向树突范围非常有限，很可能这些轴突形成了中枢突触。运动神经元密度在躯干中部区域达到一个较宽的平台期，每100微米有12 - 13个。胞体大小不随纵向位置变化，但树突的背腹范围在尾部减小，而中枢轴突长度增加。运动神经元分布数据用于估计中枢运动神经元轴突的纵向分布。在躯干的大部分区域，每100微米有12 - 14个，有一个较宽的平台期，仅在肛门尾部显著下降。这种分布与游泳过程中的胆碱能兴奋性相关。用辣根过氧化物酶反向填充的运动神经元的透射电子显微镜显示，中枢运动神经元轴突与运动神经元树突以及其他未染色神经元的树突形成旁突触。通过测量突触频率和总树突长度，估计躯干运动神经元每个接收100 - 200个突触。
