Novo microcóspio decodifica o circuito do olho complexo

New Microscope Decodes Complex Eye Circuitry

ScienceDaily (Mar. 16, 2011) — The properties of optical stimuli need to be conveyed from the eye to the brain. To do this efficiently, the relevant information is extracted by pre-processing in the eye. For example, some of the so-called retinal ganglion cells, which transmit visual information to the brain via the optic nerve, only react to light stimuli moving in a particular direction. This direction selectivity is generated by inhibitory interneurons that influence the activity of the ganglion cells through their synapses.

Cells and synapses reconstructed from serial block face electron microscopy data. A single starburst amacrine cell (yellow, note synaptic varicosities) and two direction-selective ganglion cells (green). Even though there is substantial dendritic overlap with both cells, all connections (magenta) go to the right ganglion cell. 

(Credit: Copyright Kevin)

Using a novel microscopy method developed at the Institute, scientists from the Max Planck Institute for Medical Research in Heidelberg have now discovered that the distribution of the synapses between ganglion cells and interneurons follows highly specific rules. Only those dendrites that extend from the cell body of the amacrine cell in a direction opposite to the preferred direction of the ganglion cell connect with the ganglion cell.

The sensory cells in the retina of the mammalian eye convert light stimuli into electrical signals and transmit them via downstream interneurons to the retinal ganglion cells which, in turn, forward them to the brain. The interneurons are connected to each other in such a way that the individual ganglion cells receive visual information from a circular area of the visual field known as the receptive field. Some ganglion cells are only activated, for example, when light falls on the centre of their receptive fields and the edge remains dark (ON cells). The opposite is the case for other ganglion cells (OFF cells). And there are also ganglion cells that are activated by light that sweeps across their receptive fields in a particular direction; motion in the opposite (null-) direction inhibits activation.

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Wiring specificity in the direction-selectivity circuit of the retina

Kevin L. Briggman, Moritz Helmstaedter & Winfried Denk

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Nature 471, 183–188 (10 March 2011) doi:10.1038/nature09818

Received 20 October 2010 Accepted 10 January 2011 Published online 09 March 2011

Abstract
The proper connectivity between neurons is essential for the implementation of the algorithms used in neural computations, such as the detection of directed motion by the retina. The analysis of neuronal connectivity is possible with electron microscopy, but technological limitations have impeded the acquisition of high-resolution data on a large enough scale. Here we show, using serial block-face electron microscopy and two-photon calcium imaging, that the dendrites of mouse starburst amacrine cells make highly specific synapses with direction-selective ganglion cells depending on the ganglion cell’s preferred direction. Our findings indicate that a structural (wiring) asymmetry contributes to the computation of direction selectivity. The nature of this asymmetry supports some models of direction selectivity and rules out others. It also puts constraints on the developmental mechanisms behind the formation of synaptic connections. Our study demonstrates how otherwise intractable neurobiological questions can be addressed by combining functional imaging with the analysis of neuronal connectivity using large-scale electron microscopy.

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