Worm Breeder's Gazette 5(2): 22
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.
By taking advantage of the mass of connectivity data that John White, Eileen Southgate, and Nichol Thomson have been accumulating over the years, we have been able to draw the following, simplified, figure of how the microtubule cells (the touch cells) synapse onto the ventral cord interneurons. Although the microtubule cells synapse onto other neurons, we will only deal with the circuit involved in locomotion [See Figure 1] The particularly striking feature of this connectivity is each interneuron receives chemical synapses from one but not both sets of microtubule cells and either electrical synapses or no input from the other set. Since stimulating the anterior and posterior cells produces opposite behaviors, it seems very likely that the chemical and electrical synapses have opposing effects. The simplest model assumes that the electrical synapses are excitatory (+) and the chemical synapses are inhibitory (-). All the pluses and minuses in the diagram are deduced from the above considerations. (One other simplifying, and possibly wrong, assumption has been made: a given cell always makes the same type of chemical synapse.) As can be seen, all the connections can be assigned as either excitatory or inhibitory except the complex synapses made by AVA (especially the AVA-PVC connections). Obviously, things such as timing control have not been taken into account. Still, it is surprising that so much of the network seems amenable to this treatment. The model makes a number of predictions in addition to the assignment of inhibitory and excitatory synapses. First, it says that PVC and AVD interneurons are equivalent, but are connected differently. Perhaps they will share the same neurotransmitter, or have similar lineage origins. AVB interneurons, however, should be quite different from PVCs and AVDs. Secondly, the model predicts that the posterior microtubule cells can only function through the PVCs. We have lasered the PVCs and have found that the resulting animals are touch insensitive in the tail but not in the head (These animals move normally both backward and forward.) Finally, the anterior cells should function through synapses to either the AVBs or the AVDs. Animals in which the AVDs have been killed are touch insensitive (in the head only) as young larvae but not as adults. (As in the experiment with the PVCs, the animals move normally at all ages. ) The change in touch sensitivity can be explained by noting that the synapses are only made by AVM, a cell that appears later in the animal's development. Thus, it is likely that in the AVD-less larvae are touch insensitive because the AVM-AVB have not yet formed. To test this, we have killed AVM and the AVDs. These animals are touch insensitive (in the head) as larvae and adults (killing only AVM has negligible affect on touch sensitivity). Since the touch sensitivity of the AVD-less adults is the same as in untreated animals but the touch sensitivity of animals with AVM but not ALML and ALMR is only partial, it seems that AVM, by its gap junctions to ALML and ALMR, connects these cells to the AVBs. We have not looked yet at animals in which the AVAs or the AVBs have been killed. All comments on this model are encouraged.