本期封面所示为在一块小鼠视网膜中重建的950个神经元中的7个以及它们的相互接触点（青色球，代表579,724个接触点中的112个），同时还有它们的接触矩阵（见168页）。本期Nature上的三篇论文用视网膜作为一个模型来从单个突触接触点的层面到远程树状相互作用的层面比对神经回路。Helmstaedter等人利用电子显微镜来比对由近1000个神经元组成的一个哺乳动物视网膜回路。这项工作显示了一种新型的视网膜双极神经元，并为已知的视觉计算提出了功能机制。另外两个小组研究了果蝇视觉系统（一个经典的神经计算模型）中视觉运动的检测。Takemura等人用半自动电子显微镜来重建果蝇光髓质的基本“连接组”（379个神经元中的8637个化学突触）。这些结果揭示了一个具有与方向选择性相一致的连线方案的候选运动检测回路。Maisak等人利用钙成像发现，T4 和T5神经元因应四个基本方向上的运动而被分成特定的子类群，并且分别是‘ON’和‘OFF’边缘所特有的。（生物谷 Bioon.com）
生物谷推荐的英文摘要
Nature doi:10.1038/nature12450
A visual motion detection circuit suggested by Drosophila connectomics
Shin-ya Takemura, Arjun Bharioke, Zhiyuan Lu, Aljoscha Nern, Shiv Vitaladevuni, Patricia K. Rivlin, William T. Katz, Donald J. Olbris, Stephen M. Plaza, Philip Winston, Ting Zhao, Jane Anne Horne, Richard D. Fetter, Satoko Takemura, Katerina Blazek, Lei-Ann Chang, Omotara Ogundeyi, Mathew A. Saunders, Victor Shapiro, Christopher Sigmund, Gerald M. Rubin, Louis K. Scheffer, Ian A. Meinertzhagen & Dmitri B. Chklovskii
Animal behaviour arises from computations in neuronal circuits, but our understanding of these computations has been frustrated by the lack of detailed synaptic connection maps, or connectomes. For example, despite intensive investigations over half a century, the neuronal implementation of local motion detection in the insect visual system remains elusive. Here we develop a semi-automated pipeline using electron microscopy to reconstruct a connectome, containing 379 neurons and 8,637 chemical synaptic contacts, within the Drosophila optic medulla. By matching reconstructed neurons to examples from light microscopy, we assigned neurons to cell types and assembled a connectome of the repeating module of the medulla. Within this module, we identified cell types constituting a motion detection circuit, and showed that the connections onto individual motion-sensitive neurons in this circuit were consistent with their direction selectivity. Our results identify cellular targets for future functional investigations, and demonstrate that connectomes can provide key insights into neuronal computations.
Nature doi:10.1038/nature12346
Connectomic reconstruction of the inner plexiform layer in the mouse retina
Moritz Helmstaedter, Kevin L. Briggman, Srinivas C. Turaga, Viren Jain, H. Sebastian Seung & Winfried Denk
Comprehensive high-resolution structural maps are central to functional exploration and understanding in biology. For the nervous system, in which high resolution and large spatial extent are both needed, such maps are scarce as they challenge data acquisition and analysis capabilities. Here we present for the mouse inner plexiform layer—the main computational neuropil region in the mammalian retina—the dense reconstruction of 950 neurons and their mutual contacts. This was achieved by applying a combination of crowd-sourced manual annotation and machine-learning-based volume segmentation to serial block-face electron microscopy data. We characterize a new type of retinal bipolar interneuron and show that we can subdivide a known type based on connectivity. Circuit motifs that emerge from our data indicate a functional mechanism for a known cellular response in a ganglion cell that detects localized motion, and predict that another ganglion cell is motion sensitive.
Nature doi:10.1038/nature12320
A directional tuning map of Drosophila elementary motion detectors
Matthew S. Maisak, Juergen Haag, Georg Ammer, Etienne Serbe, Matthias Meier, Aljoscha Leonhardt, Tabea Schilling, Armin Bahl, Gerald M. Rubin, Aljoscha Nern, Barry J. Dickson, Dierk F. Reiff, Elisabeth Hopp & Alexander Borst
The extraction of directional motion information from changing retinal images is one of the earliest and most important processing steps in any visual system. In the fly optic lobe, two parallel processing streams have been anatomically described, leading from two first-order interneurons, L1 and L2, via T4 and T5 cells onto large, wide-field motion-sensitive interneurons of the lobula plate1. Therefore, T4 and T5 cells are thought to have a pivotal role in motion processing; however, owing to their small size, it is difficult to obtain electrical recordings of T4 and T5 cells, leaving their visual response properties largely unknown. We circumvent this problem by means of optical recording from these cells in Drosophila, using the genetically encoded calcium indicator GCaMP5 (ref. 2). Here we find that specific subpopulations of T4 and T5 cells are directionally tuned to one of the four cardinal directions; that is, front-to-back, back-to-front, upwards and downwards. Depending on their preferred direction, T4 and T5 cells terminate in specific sublayers of the lobula plate. T4 and T5 functionally segregate with respect to contrast polarity: whereas T4 cells selectively respond to moving brightness increments (ON edges), T5 cells only respond to moving brightness decrements (OFF edges). When the output from T4 or T5 cells is blocked, the responses of postsynaptic lobula plate neurons to moving ON (T4 block) or OFF edges (T5 block) are selectively compromised. The same effects are seen in turning responses of tethered walking flies. Thus, starting with L1 and L2, the visual input is split into separate ON and OFF pathways, and motion along all four cardinal directions is computed separately within each pathway. The output of these eight different motion detectors is then sorted such that ON (T4) and OFF (T5) motion detectors with the same directional tuning converge in the same layer of the lobula plate, jointly providing the input to downstream circuits and motion-driven behaviours.
