Factors that determine connectivity in the nervous system of Caenorhabditis elegans.

The nervous system of C. elegans is arranged as a collection of process bundles. Processes within bundles are generally unbranched and occupy defined positions relative to their neighbors. Small groups of processes are often closely associated together and run adjacent to one another for relatively long distances. We have defined the set of neurons that have processes adjacent to the processes of a given neuron as the neighborhood of that neuron. Synapses in C. elegans are made en passant between adjacent processes. Of the 1165 pairs of adjacent processes that were analyzed, 520 (45%) had synaptic contacts. The set of neurons that make synaptic contact with a given neuron is therefore, on average, 45% of that neuron's neighborhood. Neurons make synaptic contacts with fewer classes of partners than they have the potential for, as they are limited in their choice of partner to those that inhabit their neighborhood. Some classes of neurons have processes that make abrupt transitions from one neighborhood to another. There is usually some identifiable cue at the transition point, such as the termination of a closely associated process or a discontinuity at the junction of one process bundle with another. Neurons that inhabit more than one neighborhood have a more extended set of synaptic partners than those that are confined to a single neighborhood.