Grant A C, Lettvin J Y
Department of Applied Biological Sciences, Massachusetts Institute of Technology, Cambridge 02139.
Brain Res. 1991 Sep 27;560(1-2):106-21. doi: 10.1016/0006-8993(91)91221-l.
We have studied the outer neuropil layers in frog tectum where the unmyelinated optic nerve fibers terminate. At any point in the neuropil an extracellular microelectrode records several different visually evoked electrical transients, distinct by size and shape. When classified by shape alone, each transient falls into one of 3 distinct classes. Some of these transients are binocularly driven, as originally described by Finch and Collett. The aggregate of the receptive fields of all the elements recorded at a single point defines a multiunit receptive field (MURF). Each MURF is characteristically oval, and divided into 3 sections along its long axis. Each section represents the aggregate of the receptive fields associated with one class of transient; i.e. transients belonging to only one specific class can be evoked by stimulating that part of the visual field corresponding to the appropriate section of the MURF. All of the MURFs mapped by recording in a single tectum are radially arranged in visual space about a central point, or 'visual pole'. Several conclusions are made. First, the two larger types of transient are generated postsynaptically by electrically active dendritic elements, specifically the beaded dendritic appendages of tectal neurons. The smallest type of transient is of presynaptic origin. Second, these tectal elements have a local and global anatomical order across the tectum, which accounts for both the tripartite structure of the MURFs and their radial arrangement about a visual pole. Third, since the large transients are of postsynaptic origin, genuine recordings of single retinal ganglion cell (RGC) activity can be made only in the optic nerve or retina itself. Fourth, information is conveyed over the unmyelinated optic nerve fibers at pulse rates as high as 80/s and is transsynaptically effective at such rates. Finally, the electrically active tectal dendritic elements, with their highly organized spatial arrangement, are an important component of the frog's visual processing apparatus, instead of being merely relays or repeaters.
我们研究了蛙视顶盖中无髓鞘视神经纤维终止的外层神经毡层。在神经毡的任何一点，细胞外微电极都能记录到几种不同的视觉诱发电瞬变，它们在大小和形状上各不相同。仅按形状分类时，每个瞬变可归入3个不同类别之一。其中一些瞬变是双眼驱动的，正如芬奇和科利特最初所描述的那样。在单个点记录的所有元件的感受野总和定义了一个多单位感受野（MURF）。每个MURF的特征是椭圆形，沿其长轴分为3个部分。每个部分代表与一类瞬变相关的感受野总和；即，通过刺激与MURF相应部分对应的视野区域，只能诱发属于一个特定类别的瞬变。通过在单个视顶盖中记录所绘制的所有MURF在视觉空间中围绕一个中心点或“视极”呈放射状排列。得出了几个结论。第一，两种较大类型的瞬变是由电活动的树突状元件在突触后产生的，特别是视顶盖神经元的串珠状树突附属物。最小类型的瞬变起源于突触前。第二，这些视顶盖元件在整个视顶盖中具有局部和整体的解剖学顺序，这既解释了MURF的三分结构，也解释了它们围绕视极的放射状排列。第三，由于大的瞬变起源于突触后，因此只能在视神经或视网膜本身进行单个视网膜神经节细胞（RGC）活动的真实记录。第四，信息以高达80次/秒的脉冲频率通过无髓鞘视神经纤维传递，并且以这样的频率进行突触传递是有效的。最后，电活动的视顶盖树突状元件及其高度有组织的空间排列是青蛙视觉处理装置的重要组成部分，而不仅仅是中继器或重复器。
