Fetcho J R
Physiology Department, State University of New York, Buffalo 14214.
J Comp Neurol. 1990 Sep 15;299(3):283-98. doi: 10.1002/cne.902990303.
As part of an attempt to understand the spinal control of the segmented axial musculature in goldfish, commissural spinal interneurons that are electronically coupled to the Mauthner axon (M-axon) were studied with intracellular recording and staining to examine their morphology, segmental relationships, and functional role. Prior studies suggested that these cells might mediate the crossed inhibition that blocks excitation of motoneurons on one side of the body during an escape bend to the opposite side. Simultaneous intracellular recordings from a M-axon, a commissural interneuron coupled to it, and a presumed primary motoneuron show that: (1) the interneurons produce monosynaptic, Cl(-)-dependent IPSPs in contralateral motoneurons, (2) the interneurons are responsible for the short latency, crossed spinal inhibition in the M-cell network, and (3) more than one interneuron terminates on each postsynaptic cell. Reconstructions of interneurons from wholemounts show that they form a fairly homogeneous morphological class of cells. Each one is unipolar, with an axon that crosses the cord and then usually bifurcates into a short, thin ascending branch and a thicker, longer descending one. Neighboring interneurons have overlapping terminal arbors consistent with the physiological data showing convergence of interneurons onto the same postsynaptic cell. The interneurons showed little relationship with body segments as defined by ventral roots. Their axons usually straddled segmental boundaries, with terminals typically occupying parts of two adjacent segments. Thus the functional unit of these cells is probably not a segment or a complete group of segments, but instead includes only parts of two adjacent segments. The presence of interneurons like these suggests that the overt peripheral segmentation of trunk musculature is not necessarily reflected in the organization of neurons that control those segments. A consideration of some functional characteristics of the activation of overlapping, serially repeated arrays of interneurons by descending pathways leads to the conclusion that the high conduction velocity of the M-axon, and the large size and short longitudinal extent of the axons of the inhibitory interneurons promote a strong, brief inhibition that is appropriate for the production of an escape turn that has a rapid bend to one side.
作为了解金鱼分节轴肌脊髓控制机制的一部分，研究人员采用细胞内记录和染色方法，对与莫特纳尔轴突（M轴突）存在电耦合的连合脊髓中间神经元进行了研究，以考察它们的形态、节段关系及功能作用。先前的研究表明，这些细胞可能介导交叉抑制，即在向对侧逃避弯曲时，阻止身体一侧运动神经元的兴奋。同时对一个M轴突、与其耦合的一个连合中间神经元以及一个假定的初级运动神经元进行细胞内记录，结果显示：（1）中间神经元在对侧运动神经元中产生单突触、Cl⁻依赖性抑制性突触后电位（IPSPs）；（2）中间神经元负责M细胞网络中潜伏期短的交叉脊髓抑制；（3）每个突触后细胞上有不止一个中间神经元终止。对整装标本中中间神经元的重建显示，它们形成了一类形态相当一致的细胞。每个细胞都是单极的，其轴突穿过脊髓，然后通常分为一个短而细的上升分支和一个较粗、较长的下降分支。相邻的中间神经元具有重叠的终末分支，这与生理数据显示中间神经元汇聚到同一个突触后细胞上相符。中间神经元与由腹根定义的身体节段关系不大。它们的轴突通常跨越节段边界，终末通常占据两个相邻节段的部分区域。因此，这些细胞的功能单位可能不是一个节段或一整个节段组，而仅包括两个相邻节段的部分区域。这类中间神经元的存在表明，躯干肌肉明显的外周分节不一定反映在控制这些节段的神经元组织中。考虑下行通路激活重叠、串联重复的中间神经元阵列的一些功能特性后得出结论，M轴突的高传导速度以及抑制性中间神经元轴突的大尺寸和短纵向范围，促进了一种强烈、短暂的抑制，这对于产生向一侧快速弯曲的逃避转向是合适的。
