Abstract
Detailed knowledge of both synaptic connectivity and the spatial proximity of neurons is crucial for understanding wiring specificity in the nervous system. Here, we volumetrically reconstructed the C. elegans nerve ring from legacy serial-sectioned electron micrographs at two distinct time points: the L4 and young adult. The new volumetric reconstructions provide detailed spatial and morphological information of neural processes in the nerve ring. Our analysis suggests that the nerve ring exhibits three levels of wiring specificity: spatial, synaptic and subcellular. Neuron classes innervate well defined neighborhoods and aggregate functionally similar synapses to support distinct computational pathways. Connectivity fractions vary based on neuron class and synapse type. We find that the variability in process placement accounts for less than 20% of the variability in synaptic connectivity and models based only on spatial information cannot account for the reproducibility of synaptic connections among homologous neurons. This suggests that additional, non-spatial factors also contribute to synaptic and subcellular specificity. With this in mind, we conjecture that a spatially constrained, genetic model could provide sufficient synaptic specificity. Using a model of cell-specific combinatorial genetic expression, we show that additional specificity, such as sub-cellular domains or alternative splicing, would be required to reproduce the wiring specificity in the nerve ring.
