Pearson K G, Boyan G S, Bastiani M, Goodman C S
J Comp Neurol. 1985 Mar 1;233(1):133-45. doi: 10.1002/cne.902330108.
The G, B1, and B2 neurons are three prominent interneurons located in adjacent segmental ganglia in the central nervous system of locusts. Previous studies on the adult nervous system have shown that each of these cells has its own distinctive morphology and responsiveness to auditory input. Previous studies on the embryonic nervous system have described the lineage and development of one of these cells, the G neuron, in the mesothoracic (T2) segment. In this paper it is shown that the G, B1, and B2 neurons are segmental homologues in that they arise from equivalent lineages during embryogenesis in the T2, T3, and A1 segments, respectively. Each cell arises (along with its identified sibling neuron) from the division of the second ganglion mother cell of neuroblast 7-4. The segment-specific morphology of the G homologues was determined in the T3 and A1 segments between 60-70% of embryonic development, and their identity was established as the adult B1 and B2 neurons by comparing the distinctive cell-specific features of their morphology between embryo and adult. Although all three neurons display striking morphological differences, they all share certain structural features in common, including the location of their primary axons and neurites in specific tracts in the neuropil. By recording intracellularly from the main neurites of the G, B1, and B2 neurons, clear differences were found in the synaptic inputs each of the neurons receives and the synaptic outputs each makes. For example, G and B2, but not B1, receive direct monosynaptic input from the descending contralateral movement detector (DCMD) interneurons and from auditory afferents; B1, but not B2, connects directly to G; and B2, but not B1 or G, connects directly to flight motoneurons. The main conclusion from these observations is that lineally equivalent neurons in different segments can develop similar primary structures but quite different secondary morphologies and synaptic connections. How these segment-specific differences arise during embryogenesis remains unknown.
G神经元、B1神经元和B2神经元是蝗虫中枢神经系统中位于相邻节段神经节的三种主要中间神经元。以往对成年神经系统的研究表明，这些细胞中的每一个都有其独特的形态以及对听觉输入的反应。以往对胚胎神经系统的研究描述了其中一种细胞，即中胸（T2）节段的G神经元的谱系和发育。本文表明，G神经元、B1神经元和B2神经元是节段同源物，因为它们分别在T2、T3和A1节段的胚胎发生过程中起源于等效的谱系。每个细胞（连同其已确定的同胞神经元）都起源于神经母细胞7-4的第二个神经节母细胞的分裂。在胚胎发育的60%-70%期间，在T3和A1节段确定了G同源物的节段特异性形态，并通过比较胚胎和成年期之间其形态的独特细胞特异性特征，将它们确定为成年B1和B2神经元。尽管所有这三种神经元都表现出明显的形态差异，但它们都具有某些共同的结构特征，包括其主要轴突和神经突在神经纤维网特定束中的位置。通过从G神经元、B1神经元和B2神经元的主要神经突进行细胞内记录，发现每个神经元接收的突触输入和产生的突触输出存在明显差异。例如，G神经元和B2神经元，但不是B1神经元，接收来自对侧下行运动探测器（DCMD）中间神经元和听觉传入神经的直接单突触输入；B1神经元，但不是B2神经元，直接与G神经元相连；B2神经元，但不是B1神经元或G神经元，直接与飞行运动神经元相连。这些观察结果的主要结论是，不同节段中谱系等效的神经元可以发育出相似的初级结构，但二级形态和突触连接却大不相同。这些节段特异性差异在胚胎发生过程中如何产生仍然未知。
