Nature:視網膜中的神經元(封面)

本期封面所示為在一塊小鼠視網膜中重建的950個神經元中的7個以及它們的相互接觸點（青色球，代表579,724個接觸點中的112個），同時還有它們的接觸矩陣（見168頁）。本期Nature上的三篇論文用視網膜作為一個模型來從單個突觸接觸點的層面到遠程樹狀相互作用的層面比對神經迴路。Helmstaedter等人利用電子顯微鏡來比對由近1000個神經元組成的一個哺乳動物視網膜迴路。這項工作顯示了一種新型的視網膜雙極神經元，並為已知的視覺計算提出了功能機制。另外兩個小組研究了果蠅視覺系統（一個經典的神經計算模型）中視覺運動的檢測。Takemura等人用半自動電子顯微鏡來重建果蠅光髓質的基本「連接組」（379個神經元中的8637個化學突觸）。這些結果揭示了一個具有與方向選擇性相一致的連線方案的候選運動檢測迴路。Maisak等人利用鈣成像發現，T4 和T5神經元因應四個基本方向上的運動而被分成特定的子類群，並且分別是『ON』和『OFF』邊緣所特有的。（生物谷 Bioon.com）

生物谷推薦的英文摘要

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.