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Connectome of Caenorhabditis elegans. Behavior is determined by the connections between an organism's nerve cells. Networks of neurons, which in mammals reach an astounding degree of complexity, choreograph our simplest reflexes and our most profound insights. While exploration of neural networks in the mammalian brain is in its early stages, the complete set of neural connections—the connectome—has been almost entirely mapped for the nematode C. elegans. With only 302 neurons, 6393 chemical synapses, 890 ga junctions, and 1410 neuromuscular junctions, this simple animal provides a window into the operations carried out by a complete nervous system. In this image each point represents a neuron and the thickness of each line reflects the number of contacts between the neurons (from 1–37). Gap junctions are red, chemical synapses are blue, and neuromuscular junctions are magenta; all neuromuscular junctions are directed at one idealized point in the graph. For simplicity, directionality and neuron type are not shown. (Reproduced, with permission, from Eduardo Izquierdo, with data from White, JG, Southgate E, Thomson JN, Brenner S. 1986. The structure of the nervous system of the nematode Caenorhabditis elegans. Philosophical Transactions of the Royal Society B:Biological Sciences 314:1–340; and Varshney LR, Chen, BL, Paniaqua E, Hall DH, Chklovskii DB. 2011. Structural properties of the C. elegans neuronal network. PLoS Comput Biol 7:e1001066. doi:10.1371/journal.pcbi. 1001066)

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