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Serial-section electron microscopy has been used to reconstruct the cellular architecture of the posterior nervous system of the nematode Caenorhabditis elegans. Each of 40 neurons in the tail of the adult hermaphrodite can be reproducibly and unambiguously identified by a set of morphological features, including cell body position, fiber geometry and size, and staining properties. A complete list of synapses has been assembled for 2 isogenic animals, and these lists are compared with a third isogenic animal reconstructed by White et al. (1986). The set of neurons and their pattern of synaptic interactions is simple and reproducible. Most of the cells are involved in sensory transduction or in local signal processing to relay signals via a few interneurons to motoneurons and thence to body muscles. Because the tail neurons are well separated and fairly reproducible in position, the hermaphrodite tail lends itself to laser-ablation studies of sensory processing (cf. Chalfie et al., 1985). Most of the synapses in the tail are concentrated in the preanal ganglion. Among the approximately 150 synapses there, about 85% are dyadic chemical synapses. The dyadic synapses are involved in reproducible patterns that have several interesting features. Most neurons synapse onto a few preferred pairs of target cells, in patterns that suggest a combinatorial model of synapse specification that may be open to genetic analysis. Furthermore, most dyadic contacts A->B,C fit a pattern in which the 2 postsynaptic partners are involved elsewhere in unidirectional synapses B -> C. Thus, the dyadic synapse may serve to diverge sensory signals into parallel pathways, which then reconverge. This divergence/reconvergence pattern eventually directs processed sensory signals to the ventral cord interneurons PVCL and PVCR. About 80-90% of the synapses fall into repeated classes of synapses. Many of the remaining synapses are widely scattered and irreproducible from one animal to the next. Some of these contacts may be developmental mistakes reflecting a degree of "noise" in synapse specification (Waddington, 1957).
Received Feb. 19, 1990; revised Aug. 16, 1990; accepted Aug. 17, 1990.
This work was supported by a Sloan Foundation Neurosciences grant to R.L.R.,
by U.S. Public Health Service Grants NS09654 and NSI3749 to R.L.R., NSI5909
to D.H.H., NS07512 to M. V.L. Bennett, and by U.S. Public Health Service Training
Grant GM02031 to D.H.H. We wish to thank John White, John Sulston,
Nicol Thomson, and Lois Edgar for extensive cooperation in the identification of
cells and for sharing many results with us before publication. We also thank
Randle W. Warc for an introduction to serial-section electron microscopy. Finally,
we are indebted to M. V. L. Bennett for his continuing interest in these
Correspondence should be addressed to David H. Hall, Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461.
a Present address: Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260.
Copyright (c) 1991 Society for Neuroscience 0270-6474/91/010001-22$03.00/0
Web adaptation, Thomas Boulin, for Wormatlas, 2002