Sterling P, Freed M A, Smith R G
Department of Anatomy, University of Pennsylvania, Philadelphia 19104.
J Neurosci. 1988 Feb;8(2):623-42. doi: 10.1523/JNEUROSCI.08-02-00623.1988.
Photoreceptors connect to the on-beta ganglion cell through parallel circuits involving rod bipolar (RB) and cone bipolar (CB) neurons. We estimated for a small patch in the area centralis of one retina the 3-dimensional architecture of both circuits. This was accomplished by reconstructing neurons and synapses from electron micrographs of 189 serial sections. There were (per mm2) 27,000 cones, 450,000 rods, 6500 CBb1, 30,300 RB, 4100 All amacrines, and 2000 on-beta ganglion cells. The tangential spread of processes was determined for each cell type, and, with the densities, this allowed us to calculate the potential convergence and divergence of each array upon the next. The actual numbers of cells converging and diverging were estimated from serial sections, as were the approximate numbers of chemical synapses involved. The cone bipolar circuit showed narrow convergence and divergence: 16 cones----4 CBb1----1 on-beta 1 cone----1 CBb1----1.2 on-beta This circuit is thought to contribute significantly to the on-beta cell's photopic receptive field because the CBb1 has a center-surround receptive field whose center diameter is greater than the spacing between adjacent CBb1s. Consequently, the receptive fields of the CBb1s converging on a beta cell are probably largely concentric and thus mutually reinforcing in their contributions to the on-beta. The rod bipolar circuit showed a wider convergence and divergence: 1500 rods----100 RB----5 AII----4 CBb1----1 on-beta 1 rod----2 RB----5 AII----8 CBb1----2----2 on-beta The 1500 rods converging via this circuit account for the spatial extent of the beta cell's dark-adapted receptive field. This convergence also accounts for the ganglion cell's maintained discharge, which is thought to arise from about 6 quantal "dark events" per second. This many dark events would appear in the ganglion cell if each rod in the circuit contributed 0.004 dark events per second, and this is close to what has been measured in monkey rods (Baylor et al., 1984). Divergence in this circuit serves to expand the number of copies of the quantal signal (1 rod----8 CBb1) and so to engage large numbers of chemical synapses that provide amplification. Reconvergence at the last stage (8 CBb1----2 on-beta) may reduce (by signal averaging) the synaptic noise that would otherwise accumulate along the pathway.
光感受器通过涉及视杆双极（RB）和视锥双极（CB）神经元的平行回路与on-beta神经节细胞相连。我们估计了一个视网膜中央凹小区域内这两种回路的三维结构。这是通过从189个连续切片的电子显微照片重建神经元和突触来完成的。每平方毫米有27000个视锥细胞、450000个视杆细胞、6500个CBb1、30300个RB、4100个无长突细胞和2000个on-beta神经节细胞。确定了每种细胞类型突起的切向扩展范围，并结合密度，使我们能够计算每个阵列在下一级的潜在会聚和发散情况。从连续切片中估计了会聚和发散的实际细胞数量，以及所涉及的化学突触的大致数量。视锥双极回路显示出狭窄的会聚和发散：16个视锥细胞→4个CBb1→1个on-beta；1个视锥细胞→1个CBb1→1.2个on-beta。该回路被认为对视锥细胞的明视觉感受野有显著贡献，因为CBb1具有中心-周边感受野，其中心直径大于相邻CBb1之间的间距。因此，会聚到一个beta细胞上的CBb1的感受野可能在很大程度上是同心的，从而在对on-beta的贡献中相互增强。视杆双极回路显示出更宽的会聚和发散：1500个视杆细胞→100个RB→5个AII→4个CBb1→1个on-beta；1个视杆细胞→2个RB→5个AII→8个CBb1→2个on-beta。通过该回路会聚的1500个视杆细胞构成了beta细胞暗适应感受野的空间范围。这种会聚也解释了神经节细胞的持续放电，据认为这是由每秒约6个量子“暗事件”引起 的。如果回路中的每个视杆细胞每秒贡献0.004个暗事件，那么在神经节细胞中就会出现这么多暗事件，这与在猴视杆细胞中测量到的结果很接近（贝勒等人，1984年）。该回路中的发散作用是扩大量子信号的副本数量（1个视杆细胞→8个CBb1），从而涉及大量提供放大作用的化学突触。在最后阶段的再会聚（8个CBb1→2个on-beta）可能会（通过信号平均）减少否则会沿通路积累的突触噪声。
