Worm Breeder's Gazette 2(2): 33
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Two young adult C. elegans have been serially sectioned and reconstructed from the tail tip forward through the anterior end of the pre-anal ganglion. Thirty-nine neurons can be identified in the tail, twelve cells in each lumbar ganglion, twelve cells in the pre- anal ganglion, and three cells in the dorso-rectal ganglion. Each cell in the tail can be reproducibly identified on the basis of a set of morphological features, including cell body position, fiber projections, fiber size, and cytoplasmic appearance. Eleven neurons in each lumbar ganglion are bilaterally homologous. Many lumbar cells have sensory dendrites in the tail. Two pairs of lumbar cells which lack sensory dendrites are prominent interneurons in the synaptic interactions of the tail. Virtually all synaptic contacts in the tail are found in the pre-anal ganglion. Most synapses involve lumbar fibers and fibers from cells whose cell bodies lie anterior to the reconstructed region. Pre-anal ganglion cells themselves are relatively minor participants in these synaptic interactions. A complete connectivity matrix has been constructed for both animals, involving about 150 synapses in each case. Certain ceIls make repeated contacts with one another (up to thirteen contacts) in both animals. Other instances of non-reproducible synapses are found, usually involving one contact in one animal and none in the other. No self-synapses are observed, but sensory cells frequently synapse onto their bilateral homologues. Homologously paired cells make similar sets of synaptic contacts. One class of reciprocal synapse formation is found. Eighty per cent of the contacts are dyadic, with one pre-synaptic cell and two post-synaptic ones. Ten per cent of the contacts are triadic; the remaining ten per cent are apparently conventional synapses with a single post-synaptic element. Each dyadic synapse generally involves three different types of neurons - none homologous to another - such that A- B/C. Each type of pre-synaptic neuron (A) contacts only a few preferred pairs of fibers (B, C). Most dyadic contacts are involved in multiple routes of information flow, such that A- B/C and, elsewhere B-C. The formation of dyadic synapses appears to follow strict rules which may reflect important factors in the development of the nervous system. Most synaptic Interactions can be included in a simple wiring diagram by which information flows from sensory cells through multiple routes to converge on a pair of interneurons which project forward into the ventral cord. Positional information is used to identify three pairs of interneurons which are important both in ventral cord synaptic patterns and in the synaptic interactions of the pre-anal ganglion.