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.

The Posterior Nervous System of C. elegans

D. Hall

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