Abstract
With its regular, almost crystal-like structure, the fly optic lobe represents a particularly beautiful piece of nervous system, which consequently has attracted the attention of many researchers over the years. While the anatomy of the various cell types had been known from Golgi studies for long, their visual response properties could only recently be revealed thanks to the advent of cell-specific driver lines and genetically encoded indicators of neural activity. Furthermore, dense EM reconstruction of several columns of the fly optic lobe now provides information about the synaptic connections between the different cell types, and RNA sequencing sheds light on the transmitter systems and ionic conductances used for communication between them. Together with the molecular tools allowing for blocking and activating individual, genetically targeted cell types, the fly optic lobe can soon be one of the best-understood visual neuropils in neuroscience. In this review, we summarize what we have learned so far, and discuss the major difficulties that keep us from a complete understanding of visual processing in the fly optic lobe.
Highlights
Flies are known for their big facet eyes and use vision as their most prominent sensory system
We summarize what we have learned so far, and discuss the major difficulties that keep us from a complete understanding of visual processing in the fly optic lobe
The optic lobe consists of four parts: the lamina, located most distally, receiving input from the outer photoreceptors R1-6; the medulla, where the axons of the two inner photoreceptors R7/R8 as well as those of the lamina neurons terminate; and the lobula and the lobula plate, which are both rotated against the medulla by 90 around the vertical body axis and receive input from the medulla in parallel
Summary
Flies are known for their big facet eyes and use vision as their most prominent sensory system. Looking at the Golgi gestalt of visual interneurons in different groups of flies, Buschbeck and Strausfeld [7] found an astonishing degree of similarity between them: despite being separated for 200–300 million years in evolution [8], horse flies, hover flies, robber flies, long-legged flies, tsetse flies, blow flies and their likes all contain a set of neurons which, despite some speciesspecific variations, reveal a similar arborization pattern and stratification specificity in their dendritic and axonal branching depth within the neuropil layers of the optic lobe This points to an important functional relevance of this highly conserved cell type diversity. This parallels the circuit design in the vertebrate retina [20], where visual signals are split into an www.sciencedirect.com
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