Humphrey A L, Weller R E
Department of Neurobiology and Behavior, SUNY, Stony Brook 11794.
J Comp Neurol. 1988 Feb 15;268(3):448-68. doi: 10.1002/cne.902680312.
In the companion paper (Humphrey and Weller, '88), we demonstrated 2 physiologically different groups of X-cells (XL and XN) in the A-laminae of the cat lateral geniculate nucleus. In order to investigate their possible morphological correlates, we iontophoresed horseradish peroxidase intracellularly into physiologically identified XL- and XN-cells and examined their light microscopic appearance. The 11 HRP-labeled XL-cells constituted the smallest relay neurons in the A-laminae, and were similar morphologically. All had small somata (mean soma size = 236 micron2), very thin (less than 1.0 micron) axons, few primary dendrites, and narrow, sinuous distal dendrites, which usually formed trees that were oriented perpendicular to laminar borders. The dendrites could be smooth or display beadlike varicosities, hairlike appendages, and/or occasional complex stalked appendages, but their most consistent feature was numerous clusters of grapelike dendritic appendages located at or near dendritic branch points. The 14 labeled XN-cells were structurally more heterogeneous, and they included relay neurons and interneurons. Eight of 11 XN-relay cells differed markedly from the XL-cells. These XN-cells were multipolar neurons with medium to large somata (mean soma size = 365 micron2), small to medium-size axons (1.0-2.0 micron), numerous primary dendrites, and straight distal dendrites that formed radially symmetric trees. The dendrites of the cells were largely smooth, except for occasional spines and/or hairs, and they were devoid of grapelike and other complex appendages. The three other XN-relay neurons had morphologies either similar to XL-cells or intermediate between XL-cells and more simple, multipolar XN-relay cells, but two of these cells had larger somata and axons than most XL-cells. Finally, three XN-cells were intrageniculate interneurons, which possessed small somata (mean soma size = 174 micron2), fine sinuous dendrites covered with beadlike varicosities on stalked appendages, and no obvious axon. These results reveal that, despite minor overlap, there are marked structural differences between XL- and XN-cells. Among the relay cells, these differences relate to soma and axon diameter, dendritic orientation, and the presence or absence of grapelike dendritic appendages. Our finding that interneurons were strongly excited at short latencies by spot onset supports the hypothesis (Mastronarde, '87a; Humphrey and Weller, '88) that such interneurons provide the major inhibitory input to XL-cells, and that this input is important in generating the spot-induced early dips in XL-cell discharge.
在配套论文（汉弗莱和韦勒，1988年）中，我们证明了猫外侧膝状核A层中存在两组生理功能不同的X细胞（XL和XN）。为了研究它们可能的形态学关联，我们将辣根过氧化物酶通过离子导入法注入生理上已识别的XL细胞和XN细胞内，并检查它们在光学显微镜下的外观。11个经辣根过氧化物酶标记的XL细胞是A层中最小的中继神经元，在形态上相似。所有细胞都有小的胞体（平均胞体大小 = 236平方微米）、非常细（小于1.0微米）的轴突、很少的初级树突以及狭窄、蜿蜒的远端树突，这些树突通常形成垂直于层边界的树状结构。树突可以是光滑的，也可以有念珠状膨大、毛发状附属物和/或偶尔的复杂有柄附属物，但它们最一致的特征是在树突分支点或其附近有许多葡萄状树突附属物簇。14个标记的XN细胞在结构上更为多样，包括中继神经元和中间神经元。11个XN中继细胞中的8个与XL细胞有明显不同。这些XN细胞是多极神经元，胞体中等至大（平均胞体大小 = 365平方微米），轴突小至中等大小（1.0 - 2.0微米），有许多初级树突，远端树突笔直，形成放射状对称的树状结构。这些细胞的树突大部分是光滑的，偶尔有棘突和/或毛发，并且没有葡萄状和其他复杂附属物。另外三个XN中继神经元的形态要么与XL细胞相似，要么介于XL细胞和更简单的多极XN中继细胞之间，但其中两个细胞的胞体和轴突比大多数XL细胞大。最后，三个XN细胞是膝状核内中间神经元，它们有小的胞体（平均胞体大小 = 174平方微米），细的蜿蜒树突，在有柄附属物上覆盖着念珠状膨大，并且没有明显的轴突。这些结果表明，尽管有轻微重叠，XL细胞和XN细胞之间存在明显的结构差异。在中继细胞中，这些差异与胞体和轴突直径、树突方向以及葡萄状树突附属物的有无有关。我们发现中间神经元在光点开始时短潜伏期内被强烈兴奋，这支持了这样的假设（马斯托纳德，1987a；汉弗莱和韦勒，1988年），即这些中间神经元为XL细胞提供主要的抑制性输入，并且这种输入在产生光点诱导的XL细胞放电早期下降中很重要。
