Rainey W T, Ulinski P S
J Comp Neurol. 1982 Aug 1;209(2):208-23. doi: 10.1002/cne.902090207.
Nucleus rotundus is the primary thalamic recipient of projections from the optic tectum in pond turtles. Although the projection of the retina to the optic tectum is known to be topographically organized, earlier studies suggest that the tectorotundal projection is not topographically organized. Three types of analyses are used in this paper to characterize the organization of the projection of the optic tectum to nucleus rotundus. First, large iontophoretic injections of horseradish peroxidase into the optic tectum anterogradely fill axons with reaction product after the use of a cobalt-enhanced diaminobenzidine procedure. These preparations show that shafts of axons in the tectothalamic tract give rise to thinner, primary collaterals that enter nucleus rotundus from its caudolateral aspect and form sparsely branching arbors within the nucleus. Very thin secondary collaterals branch from these collateral bear terminal collaterals with frequent varicosities. Although the total size of such arbors is unknown, the evidence suggests that each arbor is large in relation to the size of nucleus rotundus. Thus, injection sites restricted to central tectum label axons throughout nucleus rotundus. Second, subtotal lesions of the tectum produce degeneration throughout nucleus rotundus in silver degeneration preparations. Finally, analysis of electron microscopic degeneration material indicates that tectal boutons are distributed along the full lengths of the dendrites of rotundal neurons, but not on their somata. These boutons form asymmetric synaptic junctions and contain round synaptic vesicles. In view of the relatively large size of the dendritic fields of rotundal neurons, these data suggest that the tectorotundal projection is both strongly convergent on individual neurons and strongly divergent from single tectorotundal axons. This type of organization is consistent with physiological evidence that rotundal neurons have receptive fields that cover at least one-half of the contralateral visual field and often include the entire hemifield. It seems unlikely that nucleus rotundus can be involved in neuronal transactions that preserve detailed spatial information, but it may be involved in processing information on other visual parameters such as stimulus velocity or color.
圆核是池龟中视顶盖投射的主要丘脑接受区。虽然已知视网膜到视顶盖的投射是按拓扑结构组织的，但早期研究表明，顶盖到圆核的投射并非按拓扑结构组织。本文采用三种分析方法来描述视顶盖到圆核投射的组织特征。首先，在使用钴增强二氨基联苯胺程序后，将大量辣根过氧化物酶离子导入视顶盖，顺行地使轴突充满反应产物。这些标本显示，顶盖丘脑束中的轴突干发出较细的初级侧支，这些侧支从圆核的后外侧进入圆核，并在核内形成稀疏分支的树突。非常细的次级侧支从这些侧支分支出来，并带有频繁曲张的终末侧支。虽然这种树突的总体大小未知，但证据表明，每个树突相对于圆核的大小来说是较大的。因此，局限于中央顶盖的注射部位会标记整个圆核中的轴突。其次，在银染退变标本中，顶盖的次全损伤会导致整个圆核发生退变。最后，对电子显微镜下退变物质的分析表明，顶盖终扣沿圆核神经元树突的全长分布，但不在其胞体上。这些终扣形成不对称的突触连接，并含有圆形突触小泡。鉴于圆核神经元树突野相对较大，这些数据表明，顶盖到圆核的投射在单个神经元上既强烈汇聚，又从单个顶盖到圆核的轴突强烈发散。这种组织类型与生理学证据一致，即圆核神经元的感受野覆盖至少对侧视野的一半，并且通常包括整个半视野。圆核似乎不太可能参与保留详细空间信息的神经元活动，但它可能参与处理其他视觉参数的信息，如刺激速度或颜色。
