  C. elegans is the only animal for which a detailed neural connectivity
diagram has been constructed. However, synaptic polarities in this diagram, and
thus, circuit functions are largely unknown. Here, we deciphered the likely
polarities of 7 pre-motor neurons implicated in the control of worm's
locomotion, using a combination of experimental and computational tools. We
performed single and multiple laser ablations in the locomotor interneuron
circuit and recorded times the worms spent in forward and backward locomotion.
We constructed a theoretical model of the locomotor circuit and searched its
all possible synaptic polarity combinations and sensory input patterns in order
to find the best match to the timing data. The optimal solution is when either
all or most of the interneurons are inhibitory and forward interneurons receive
the strongest input, which suggests that inhibition governs the dynamics of the
locomotor interneuron circuit. From the five pre-motor interneurons, only AVB
and AVD are equally likely to be excitatory, i.e. they have probably similar
number of inhibitory and excitatory connections to distant targets. The method
used here has a general character and thus can be also applied to other neural
systems consisting of small functional networks.
