Pallas S L, Finlay B L
Section of Neurobiology and Behavior, Cornell University, Ithaca.
Vis Neurosci. 1989;2(2):121-35. doi: 10.1017/s0952523800011986.
The formation of topographic maps requires not only that afferents synapse with the appropriate targets, but that the spatial relationships between the afferents be maintained. During development, in addition to the formation of the topographic map, the connectivity patterns responsible for the receptive-field properties of the target cells are being formed. The extent of interaction between these two processes is unknown. The present study addresses this question by manipulating afferent/target ratios during development, thus altering the topography of the map, and studying the effects of this alteration on the receptive-field properties of single target cells in the adult. Partial unilateral lesions of the superior colliculus (SC) were made in neonatal hamsters. These lesions result in a compression of the retinotopic map onto the remaining collicular fragment. Single cells were recorded from the superficial gray layer of the SC in the adult in response to visual stimuli. Receptive-field properties observed in lesioned animals were compared to those in normal animals and in sham operates. Receptive-field properties were largely unaffected by the change in the topographic map. There was no difference in the receptive-field size of single tectal cells of lesioned and unlesioned animals. Stimulus velocity and stimulus size tuning functions remained the same. This raises the possibility that, rather than the expected increase in convergence of retinal ganglion cells (RGC) onto single collicular cells, single SC cells receive input from ganglion cells representing the same amount of retinal area as in unlesioned animals. The excess ganglion cells created by the partial target removal would then project elsewhere and/or reduce their arbor within the SC. Regardless of the mechanism, it is clear from our results that circuitry in the retinotectal system of the hamster can compensate for conditions of increased afferent availability and thus maintain receptive-field properties.
地形图的形成不仅要求传入神经与合适的靶细胞形成突触，还要求维持传入神经之间的空间关系。在发育过程中，除了地形图的形成，负责靶细胞感受野特性的连接模式也在形成。这两个过程之间相互作用的程度尚不清楚。本研究通过在发育过程中操纵传入神经/靶细胞的比例来解决这个问题，从而改变地图的拓扑结构，并研究这种改变对成年个体中单个靶细胞感受野特性的影响。在新生仓鼠中对上丘（SC）进行部分单侧损伤。这些损伤导致视网膜拓扑图压缩到剩余的丘块上。在成年个体中，从SC的浅灰层记录单细胞对视觉刺激的反应。将损伤动物中观察到的感受野特性与正常动物和假手术动物中的进行比较。感受野特性在很大程度上不受拓扑图变化的影响。损伤和未损伤动物的单个顶盖细胞的感受野大小没有差异。刺激速度和刺激大小调谐功能保持不变。这就提出了一种可能性，即单个SC细胞接受来自视网膜神经节细胞（RGC）的输入，其代表的视网膜面积与未损伤动物相同，而不是如预期那样视网膜神经节细胞向单个丘细胞的汇聚增加。部分靶细胞移除所产生的多余神经节细胞随后会投射到其他地方和/或减少它们在SC内的分支。无论机制如何，从我们的结果可以清楚地看出，仓鼠视网膜顶盖系统中的神经回路可以补偿传入神经可利用性增加的情况，从而维持感受野特性。
